# Statistical Physics Modeling of Sorption Isotherms of Aluminum, Iron, and Indium on Tetraphenylporphyrin (H2TPP) and Tetrakis(4-tolylphenyl)porphyrin (H2TTPP): Phenomenological Investigation of Metalloporphyrins at the Molecular Level

Authors: Zainab Elqahtani; Mohamed Ben Yahia; Nada Alfryyan; Shatha Aldaghfag; Fatma Aouaini
Journal: Adsorption Science & Technology
 (2021)

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## Abstract

A quartz crystal adsorbent functionalized with two promising porphyrins (the 5,10,15,20-tetrakis(4-tolylphenyl)porphyrin and the 5,10,15,20-tetraphenylporphyrin) was applied for the investigation of the adsorption phenomenon of aluminum chloride, iron chloride, and indium chloride. The aim is to prove new insights about the appropriate adsorption materials for metalloporphyrin fabrication. The equilibrium isotherms were measured at five adsorption temperatures (from 290 to 330 K) through the microbalance (QCM) method. The discussion of the experimental observations indicated that the adsorption of the aluminum chloride and the iron chloride was performed via a monolayer process. On contrary, the participation of the chloride ions in the double-layer adsorption of the indium chloride was explained by the layer-by-layer process. Overall, the statistical physics modeling of the experimental curves indicated that the number of ions per adsorbent siten was found inferior to 1 for all the adsorption systems (multi-interaction process for the three ions). Interestingly, the physicochemical investigation of the three adopted models showed that the complexation mechanism of the tested porphyrins was an endothermic process since the two steric parameters (n and PM) increased with the rise of the temperature. The FeCl3 curves were discussed via a monolayer adsorption model which includes the parameters a and b (lateral interaction description), indicating the lowest stability of the formed iron-porphyrin complex. The energetic study showed that the adsorption energies ∣−ΔE1/2∣ of AlCl3 on H2TTPP and H2TPP are superior to 40 kJ/mol (chemical adsorption mechanism), whereas the adsorption mechanisms of FeCl3 and InCl3 took place via a physical process since they presented adsorption energy values lower than 40 kJ/mol.


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## Body

## 1. Introduction

In recent years, porphyrins have been studied as chemical sensor materials of various metals leading to the formation of metalloporphyrin complexes [1, 2]. The metal-porphyrin complexes have received increased interest in several biological and environmental processes such as stereochemistry study and molecular recognition [3–6]. Note here that the nature of the central ion has influenced the photophysical properties of porphyrins [7, 8]. For example, these tetrapyrrolic macrocycles are involved in the chlorophyll structure (magnesium-porphyrin complex) and the hemoglobin structure (iron-porphyrin complex), so they are known as pigments of life [8]. In addition, metalloporphyrin complexes were also used as ionophores in the development of potentiometric recognition sensors supported with their structural variety and coordination chemistry [7, 8]. In particular, the photosensitizing properties of aluminum(III)-porphyrin have promoted its use as potential ionophore of fluoride [9, 10]. Moreover, the aluminum(III)-porphyrin and the indium(III)-porphyrin have been potentially used as supramolecular building blocks [11] and numerous other areas (sensing, molecular recognition, polymerisation reactions, etc.) [12, 13]. Surprisingly, it was remarked that while several papers have been devoted to metalloporphyrin complexes including the metal centers zinc, rhodium, and cobalt [14, 15], little attention has been paid to the use of porphyrins as complexing compounds of aluminum(III), iron(III), and indium(III) (Figure 1) despite their contribution in the interesting fields indicated above. In fact, the adsorption of these three metals on two tested porphyrins (H2TPP and H2TTPP) was examined in the present investigation.Figure 1
Chemical structure of the metalloporphyrin complexes.In the literature, many approaches have been applied for the investigation of the adsorption mechanism of metals such as the sorption/reduction of heavy metal ions in environmental pollution management [16, 17]. Other experimental methods have been applied for the investigation of the adsorption mechanism [18–20], but in this work, the adsorption mechanisms of aluminum chloride, iron chloride, and indium chloride have been investigated by the QCM technique for many reasons. Firstly, this experimental method is a simple mass detector technique which requires the immobilization of porphyrins on the solid support of the quartz crystal. Then, the complexed mass of the metal into the macromolecule cavities can be controlled through the microbalance apparatus. Secondly, the metal-porphyrin complexes do not require an inert environment, and they are stable in the presence of water. Thirdly, the use of the QCM technique enables to plot the adsorption curves which describe the complexed amount of metallic ions on porphyrins at different temperatures. Lastly, the microscopic characteristics of the resulting film can be followed by easily fitting the experimental isotherms with analytical models [21].According to the literature, the physical models were adopted by numerous authors to examine the adsorption problem. For example, the empirical expressions of the Langmuir and Freundlich models [22, 23] have been applied for the description of the isotherm curves, but they did not give a deep physicochemical analysis of the adsorption process. Importantly, the analytical adsorption models of the present investigation were established through the innovative statistical physics theory [24, 25] which provides the new vision about the metalloporphyrin description. Based on this idea, the fundamental aim of the modeling work of adsorption isotherms is to find the sufficient systematic model that can anticipate physical insights of porphyrin adsorption based on the physicochemical parameters of the statistical physics models [26].


## 2. Experimental Adsorption Isotherms

### 2.1. Materials

The aluminum(III) chloride (AlCl3), the iron(III) chloride (FeCl3), and the indium(III) chloride (InCl3) are the tested adsorbates in this paper.The tetrakis(4-tolylphenyl)porphyrin (H2TTPP) and the tetraphenylporphyrin (H2TPP) are the porphyrins used in these experiments. They were synthesized based on a published method (the Adler-Longo strategy) [27].Figure2 shows the synthesis methods of porphyrins H2TPP and H2TTPP.Figure 2
Synthesis methods of the 5,10,15,20-tetraphenylporphyrin (H2TPP) and the 5,10,15,20-tetrakis(4-tolylphenyl)porphyrin (H2TTPP).
(a)(b)H2TPP (Figure 2(a)) was synthesized through the reaction between propanoic acid (2 L) and benzaldehyde (40 mL). The synthesis of H2TTPP (Figure 2(b)) was performed by introducing 4-tolualdehyde (12.5 g) with the propanoic acid (110 mL). Note that the synthesis of the two tested adsorbents has been performed by following the same procedure (reflux for 30 min, allowing for cooling, filtration, and drying under vacuum for 3 hours). The dried solid mass of H2TPP was 9.76 g while the obtained mass of H2TTPP was 3 g. The two solid compounds of porphyrins were dissolved in chloroform giving two solutions with concentration of 2.9 10-2 mol·L-1.


### 2.2. Experimental Measurements

The experimental QCM setup is presented in Figure3.Figure 3
Experimental setup of the quartz crystal microbalance.The QCM measurements were performed based on the piezoelectric quartz crystal [28, 29]. The crystal is a thin disc which is cut from the polished quartz (AT-cut). The fundamental resonant frequency of the quartz is 5 MHz, and the diameter of the disc is 2.54 cm [29]. A cleaning treatment was applied on the crystals by means of a Piranha solution at room temperature followed with rinsing with ethanol and drying with high purity nitrogen.For the adsorption measurement, 60μL of the adsorbents (porphyrins H2TPP and H2TTPP) was doped onto the clean crystal surface by the spin coating technique at 3500 rpm for 30 seconds. The functionalized crystals were dried at 393 K for 2 hours.The functionalized crystal (adsorption cell) was placed in a Teflon probe, which was covered by a protective ring, and it was immersed inVs=100mL of pure water contained in the bain-marie. The stabilization of the resonant frequency in the reactor took about 1 hour, and we noted the crystal frequency F0 after the stabilization. Then, we proceed with the injection of volumes of stock solutions of the adsorbates (AlCl3/FeCl3/InCl3) in the reactor. The added volumes of adsorbates Vad should be calculated according to the next equation:
(1)Vad=cf×Vsc0,whereVs is the initial volume in the reactor, c0 is the concentration of the prepared adsorbate solution, and cf is the final concentration in the reactor.At least 15 injections into the reactor were performed using a micropipette, then the variation of the resonant frequency (corresponding to the new adsorbate concentration in the reactor) was determined as follows:(2)Δf=Fi−F0.withFi as the resonant frequency after each adsorbate injection and F0 characterizing the effect of the hydrostatic pressure on the quartz crystal. It can be understood from this equation that the frequency variation Δf is only the result of the mass variation Δm of the metal on the porphyrin surface and does not consider any effect from the quartz crystal on the adsorption process since the hydrostatic pressure effect on the quartz (frequency F0) is not considered in the measured frequency variation Δf.


### 2.3. Sauerbrey’s Equation

The deposited mass of the metallic ions on the surface after each injection was determined according to the Sauerbrey hypothesis which links the mass variation to the frequency change [30–32]:
(3)Δf=−C×Δm,whereC is the crystal sensitivity factor (Hz·cm2·μg-1). Here, keep in mind that this relation should be applied when the resonant frequency variation is mainly due to the change of the deposited mass on the adsorption surface.In order to justify the application of the Sauerbrey equation in a liquid medium, the total frequency should be quantified according to the crystal properties. This allows writing the total frequency as follows [19, 33, 34]:
(4)Δf=Fi−F0=Δfm+ΔfT+Δfp+Δfr+Δfη,ρ,with,Δfm being the effect of the mass variation, ΔfT being the effect of the temperature variation, Δfp being the effect of the pressure variation, Δfr being the effect of the crystal roughness, and Δfη,ρ being the influences of the liquid properties (viscosity and density of the solution) described by Kanazawa and Gordon [33].Indeed, the influences of the temperature and the roughness on the total frequency should be neglected since the present investigation was carried out using polished crystals at a fixed temperature. The hydrostatic pressure influence was not taken into account according to Equation (2). Furthermore, low effects of the solution properties were noted, so they were not considered. Thus, the impact of the variation of the deposited mass is the dominant factor for the frequency change (Δf=Δfm). This means that the experimental measurements of the frequency variations characterize only the interactions between the metals and the adsorbent sites of porphyrins. Therefore, the physicochemical characterization of the adsorption process should be only focus on the analysis of the interactions between the ions and the porphyrins without taking account of the quartz crystal sensor because the adsorption process is performed without the intervention of the crystal properties.In fact, the adsorbed massesΔm (also noted QA) of the three adsorbates on the surface after each injection was determined from the resonant frequency variation (Equation (3)).


### 2.4. Results of Experimental Measurements

The experimental adsorbed quantities of Al3+, Fe3+, and In3+ on H2TPP and H2TTPP are plotted in Figure 4 at 290-330 K.Figure 4
Experimental isotherms of the six adsorption systems given at five temperatures (290-330 K): system (a) AlCl3-H2TTPP, system (a′) AlCl3-H2TPP, system (b) FeCl3-H2TTPP, system (b′) FeCl3-H2TPP, system (c) InCl3-H2TTPP, and system (c′) InCl3-H2TPP.Observing carefully the experimental data of the six adsorption systems, it is clear that the behavior of the isotherm curve depends on the type of adsorbate. The isotherms of aluminum chloride on H2TTPP and H2TPP (systems (a) and (a′)) show the same behavior (a unique saturation level for all the temperatures). This indicates that the two porphyrin supports adsorbed one layer of aluminum ions without contribution of the chloride ions in the adsorption process. The adsorbed quantities of FeCl3 (systems (b) and (b′)) decrease after the saturation level. In this case, the presence of an irreversible phenomenon is possible at a high iron chloride concentration which reflects weak iron-porphyrin binding. The indium chloride isotherms (systems (c) and (c′)) present two stability states (multilayer ionic adsorption). In this situation, it can be suggested that the two tested porphyrins H2TPP and H2TTPP adsorb many layers based on charge neutralization between particles having opposite charge signs (layer-by-layer (LBL) process) [24, 25, 35, 36].In addition, comparing the performance of the six adsorption systems in terms of quantity, we can note the following order of adsorption performance:QA (system (a)) > QA (system (a′)) > QA (system (b)) > QA (system (b′)) > QA (system (c)) > QA (system (c′)). The adsorbed quantities are the highest for AlCl3-H2TTPP. Then, the adsorption compounds aluminum chloride and porphyrin H2TTPP can be recommended for a real application of the metalloporphyrin complex. In the following section, the microscopic investigation of this experimental result is carried out through the physical modeling of the experimental isotherms.


## 2.1. Materials

The aluminum(III) chloride (AlCl3), the iron(III) chloride (FeCl3), and the indium(III) chloride (InCl3) are the tested adsorbates in this paper.The tetrakis(4-tolylphenyl)porphyrin (H2TTPP) and the tetraphenylporphyrin (H2TPP) are the porphyrins used in these experiments. They were synthesized based on a published method (the Adler-Longo strategy) [27].Figure2 shows the synthesis methods of porphyrins H2TPP and H2TTPP.Figure 2
Synthesis methods of the 5,10,15,20-tetraphenylporphyrin (H2TPP) and the 5,10,15,20-tetrakis(4-tolylphenyl)porphyrin (H2TTPP).
(a)(b)H2TPP (Figure 2(a)) was synthesized through the reaction between propanoic acid (2 L) and benzaldehyde (40 mL). The synthesis of H2TTPP (Figure 2(b)) was performed by introducing 4-tolualdehyde (12.5 g) with the propanoic acid (110 mL). Note that the synthesis of the two tested adsorbents has been performed by following the same procedure (reflux for 30 min, allowing for cooling, filtration, and drying under vacuum for 3 hours). The dried solid mass of H2TPP was 9.76 g while the obtained mass of H2TTPP was 3 g. The two solid compounds of porphyrins were dissolved in chloroform giving two solutions with concentration of 2.9 10-2 mol·L-1.


## 2.2. Experimental Measurements

The experimental QCM setup is presented in Figure3.Figure 3
Experimental setup of the quartz crystal microbalance.The QCM measurements were performed based on the piezoelectric quartz crystal [28, 29]. The crystal is a thin disc which is cut from the polished quartz (AT-cut). The fundamental resonant frequency of the quartz is 5 MHz, and the diameter of the disc is 2.54 cm [29]. A cleaning treatment was applied on the crystals by means of a Piranha solution at room temperature followed with rinsing with ethanol and drying with high purity nitrogen.For the adsorption measurement, 60μL of the adsorbents (porphyrins H2TPP and H2TTPP) was doped onto the clean crystal surface by the spin coating technique at 3500 rpm for 30 seconds. The functionalized crystals were dried at 393 K for 2 hours.The functionalized crystal (adsorption cell) was placed in a Teflon probe, which was covered by a protective ring, and it was immersed inVs=100mL of pure water contained in the bain-marie. The stabilization of the resonant frequency in the reactor took about 1 hour, and we noted the crystal frequency F0 after the stabilization. Then, we proceed with the injection of volumes of stock solutions of the adsorbates (AlCl3/FeCl3/InCl3) in the reactor. The added volumes of adsorbates Vad should be calculated according to the next equation:
(1)Vad=cf×Vsc0,whereVs is the initial volume in the reactor, c0 is the concentration of the prepared adsorbate solution, and cf is the final concentration in the reactor.At least 15 injections into the reactor were performed using a micropipette, then the variation of the resonant frequency (corresponding to the new adsorbate concentration in the reactor) was determined as follows:(2)Δf=Fi−F0.withFi as the resonant frequency after each adsorbate injection and F0 characterizing the effect of the hydrostatic pressure on the quartz crystal. It can be understood from this equation that the frequency variation Δf is only the result of the mass variation Δm of the metal on the porphyrin surface and does not consider any effect from the quartz crystal on the adsorption process since the hydrostatic pressure effect on the quartz (frequency F0) is not considered in the measured frequency variation Δf.


## 2.3. Sauerbrey’s Equation

The deposited mass of the metallic ions on the surface after each injection was determined according to the Sauerbrey hypothesis which links the mass variation to the frequency change [30–32]:
(3)Δf=−C×Δm,whereC is the crystal sensitivity factor (Hz·cm2·μg-1). Here, keep in mind that this relation should be applied when the resonant frequency variation is mainly due to the change of the deposited mass on the adsorption surface.In order to justify the application of the Sauerbrey equation in a liquid medium, the total frequency should be quantified according to the crystal properties. This allows writing the total frequency as follows [19, 33, 34]:
(4)Δf=Fi−F0=Δfm+ΔfT+Δfp+Δfr+Δfη,ρ,with,Δfm being the effect of the mass variation, ΔfT being the effect of the temperature variation, Δfp being the effect of the pressure variation, Δfr being the effect of the crystal roughness, and Δfη,ρ being the influences of the liquid properties (viscosity and density of the solution) described by Kanazawa and Gordon [33].Indeed, the influences of the temperature and the roughness on the total frequency should be neglected since the present investigation was carried out using polished crystals at a fixed temperature. The hydrostatic pressure influence was not taken into account according to Equation (2). Furthermore, low effects of the solution properties were noted, so they were not considered. Thus, the impact of the variation of the deposited mass is the dominant factor for the frequency change (Δf=Δfm). This means that the experimental measurements of the frequency variations characterize only the interactions between the metals and the adsorbent sites of porphyrins. Therefore, the physicochemical characterization of the adsorption process should be only focus on the analysis of the interactions between the ions and the porphyrins without taking account of the quartz crystal sensor because the adsorption process is performed without the intervention of the crystal properties.In fact, the adsorbed massesΔm (also noted QA) of the three adsorbates on the surface after each injection was determined from the resonant frequency variation (Equation (3)).


## 2.4. Results of Experimental Measurements

The experimental adsorbed quantities of Al3+, Fe3+, and In3+ on H2TPP and H2TTPP are plotted in Figure 4 at 290-330 K.Figure 4
Experimental isotherms of the six adsorption systems given at five temperatures (290-330 K): system (a) AlCl3-H2TTPP, system (a′) AlCl3-H2TPP, system (b) FeCl3-H2TTPP, system (b′) FeCl3-H2TPP, system (c) InCl3-H2TTPP, and system (c′) InCl3-H2TPP.Observing carefully the experimental data of the six adsorption systems, it is clear that the behavior of the isotherm curve depends on the type of adsorbate. The isotherms of aluminum chloride on H2TTPP and H2TPP (systems (a) and (a′)) show the same behavior (a unique saturation level for all the temperatures). This indicates that the two porphyrin supports adsorbed one layer of aluminum ions without contribution of the chloride ions in the adsorption process. The adsorbed quantities of FeCl3 (systems (b) and (b′)) decrease after the saturation level. In this case, the presence of an irreversible phenomenon is possible at a high iron chloride concentration which reflects weak iron-porphyrin binding. The indium chloride isotherms (systems (c) and (c′)) present two stability states (multilayer ionic adsorption). In this situation, it can be suggested that the two tested porphyrins H2TPP and H2TTPP adsorb many layers based on charge neutralization between particles having opposite charge signs (layer-by-layer (LBL) process) [24, 25, 35, 36].In addition, comparing the performance of the six adsorption systems in terms of quantity, we can note the following order of adsorption performance:QA (system (a)) > QA (system (a′)) > QA (system (b)) > QA (system (b′)) > QA (system (c)) > QA (system (c′)). The adsorbed quantities are the highest for AlCl3-H2TTPP. Then, the adsorption compounds aluminum chloride and porphyrin H2TTPP can be recommended for a real application of the metalloporphyrin complex. In the following section, the microscopic investigation of this experimental result is carried out through the physical modeling of the experimental isotherms.


## 3. Theory/Calculation

### 3.1. Statistical Physics Theory

The estimated values of the physical model parameters represent the key to understand the adsorption mechanism. The progress of this physical treatment is firstly seen against the well-known model of Langmuir (empirical model) [22]. The Langmuir model is developed by considering that one particle can interact with only one adsorbent site without taking account of the external factors. In general, this assumption led to wrong scientific conclusions. On the contrary, our statistical physics models correct this assumption by introducing a parameter defined by the number of adsorbed particles per adsorption site; it is noted in general by “n.” This correction can simply provide useful interpretations regarding the adsorption mechanism.Based on the adsorption isotherm profiles, our statistical physics models can suggest various energies which are linked to different functional groups of the adsorbent surface contrary to the traditional equations of Langmuir [22] or Freundlich [23]. In the same direction, the statistical physics models are able to estimate the total number of the formed layer at all the reaction temperatures [24–26]. Note also that the multilayer adsorption of charged ions can be only carried out via a layer-by-layer (LBL) process [25, 36].A general analysis of the adsorption data led to test two models: the monolayer model for AlCl3 and FeCl3 and the LBL multilayer model for InCl3. The modeling work is then arranged as follows: (a) it is worth mentioning the general methodology which allows developing the physical models expressing the adsorbed amount corresponding to the adsorbate concentration, (b) the statistical physics model can be applied and discussed on the experimental data, and (c) the choice of the adopted model led to describing the adsorption reaction via its parameters.


### 3.2. Advanced Adsorption Models

To establish the physical model expressions, the equilibrium of the adsorption reaction can be expressed via the next equation [25, 37]:
(5)nA+S⇄An−S,whereA represents the tested adsorbates (AlCl3/FeCl3/InCl3), S is the adsorbent receptor site of porphyrin (H2TPP or H2TTPP), and An−S is the resulting complex. The variable n is a stoichiometric parameter which describes the number of bonded ions per adsorbent site. In general, this parameter can identify the nature of the adsorption process (n≤1: multi-interaction process, n≥1: multi-ionic process) [38, 39].This system is characterized by the chemical potential (μ) and the temperature (T) imposing from the outside towards the considered system (grand canonical situation). In this case, the general expression of the partition function is written as a function of the occupation number (Ni), the adsorption energy (−Ei), and the Boltzmann factor (β) [37]:
(6)zgc=∑Nie−β−Ei−μNi.Concerning the adsorption via one energy level (−E) (one adsorbed layer), it can be expressed by [39]
(7)zgc=1+eβE+μ.For the LBL adsorption process (double-layer or multilayer), two energies can be responsible for this process. Note that the adsorption of the first layer is characterized by the energy (−E1), and the second energy (−E2) is in relationship with the formation of the additional adsorbed layers. The partition function is written as [38] follows:For the double-layer model:(8)zgc=1+eβE1+μ+eβE1+E2+2μ.For the multilayer model:(9)zgc=1+eβE1+μ+∑Ni=2Le−β−E1−Ni−1E2−Niμ,withL as the number of adsorbed layers.Then, the average occupation number ofPM porphyrin sites for the three models should be calculated using the next equation [37, 40]:
(10)No=PMβ∂lnzgc∂μ.In this modeling work, the analytical development can be carried out by introducing the chemical potential of the perfect gas (μp). μp is introduced in the mathematical development of the models through its expression which is written as a function of the partition-function of translation (zTr) and the number of adsorbates (N) [40, 41]:
(11)μp=1βlnNzTr,whereztr can be written as a function of Planck’s constant, the volume V, and the mass m of the adsorbed particle [37]:
(12)zTr=V2πmkBTh23/2.The investigation of the adsorption problem can be also performed using the chemical potential (μr) of the real gas. In this case, the lateral interactions involving the adsorbates at free state are taken into account by means of the parameters a (the pressure of cohesion) and b (the covolume) [24, 25]:
(13)μr=μp+1βln11−bc+1βbc1−bc−2ac.Then, the adsorbed quantityQA (μg/cm2) can be determined by the following equation [37, 40]:
(14)QA=n×N0.Finally, we get the analytical expressions of the six adsorption models:The monolayer model of the ideal gas approach:(15)QA=nPM×c/c1/2n1+c/c1/2n,wherec1/2 involves the molar adsorption energy (−ΔE1/2) (kJ/mol), the adsorption temperature T (K), and the adsorbate solubility S (mol/L). It has the following formula:
(16)c1/2=Se−ΔE1/2/RT.The monolayer model of the real gas approach:(17)QA=nPM1+w1/21−bc/ce2βace−bc/1−bcn,where the energetic coefficientw1/2 is
(18)w1/2=Se−ΔE1/2/RT.The double-layer and multilayer models include two energetic coefficients involving two molar adsorption energies (−ΔE1) and (−ΔE2).The double-layer model of the ideal gas approach:(19)QA=nPMc/c1n+2c/c22n1+c/c1n+c/c22n,where the energetic variablesc1 and c2 are
(20)c1,2=Se−ΔE1,2/RT.The double-layer model of the real gas approach:(21)QA=nPMc/w11−bce2βace−bc/1−bcn+2c/w21−bce2βace−bc/1−bc2n1+c/w11−bce2βace−bc/1−bcn+c/w21−bce2βace−bc/1−bc2n,wherew1 and w2 are
(22)w1,2=Se−ΔE1,2/RT.The multilayer model of the ideal gas approach:(23)QA=nPM×c/c1n+c/c1nc/c2n1−2c/c2nL−Lc/c2nL+1+c/c2n1−c/c2nL/1−c/c2n1−c/c1n1−c/c2n+c/c1nc/c2n1−c/c2nL,wherec1 and c2 are given by Equation (20).The multilayer model of the real gas approach:(24)QA=nPM×c/c1n+c/c1nc/c2n1−2c/c2nL−Lc/c2nL+1+c/c2n1−c/c2nL/1−c/c2n1−c/c1n1−c/c2n+c/c1nc/c2n1−c/c2nL,wherec1 and c2 are written as
(25)c1,2=w1,21−bce2βace−bc/1−bc,withw1 and w2 are previously mentioned (Equation (22)).


### 3.3. Adjustment of Adsorption Models with Experimental Isotherms

The six physical adsorption models were applied on the experimental isotherms by means of a numerical fitting program [42]. The mathematical fitting method is based on the Levenberg-Marquardt iterating algorithm using a multivariable nonlinear regression program [40, 43]. The criteria to adopt a descriptive model are the determination coefficient R2, the RMSE coefficient (residual root mean square error), and the well-known AIC (Akaike Information Criterion) coefficient. The first error coefficient (R2) is a standardized measure of the fit goodness [43, 44]. If the coefficient R2 value is near the unit, we are in the case of the best fitting model. The second error coefficient (RMSE) is a nonstandardized measure of the fit goodness [44–47]. The best fitting model is obtained when the RMSE value is inferior to 2. The third adjustment coefficient (AIC) shows the best fitting with the model having the lowest AIC values [36]. These three adjustment coefficients are interpreted to adopt a physical model.Tables1 and 2 show the numerical values of the error adjustment coefficients.Table 1
Values of the error adjustment coefficientsR2, RMSE, and AIC deduced from fitting the experimental isotherms of AlCl3, FeCl3, and InCl3 on 5,10,15,20-tetrakis(4-methylphenyl) porphyrin H2TTPP (systems (a), (b), and (c)) with the three advanced models.
Adsorption modelMonolayer model (ideal gas)Monolayer model (real gas)Double-layer model (ideal gas)Adjustment coefficientR2RMSEAICR2RMSEAICR2RMSEAICAdsorption system (a): AlCl3-H2TTPP290 K0.981.2314.990.932.6419.880.902.9822.99300 K0.991.2215.020.942.8819.840.912.9923.64310 K0.981.5618.710.952.9720.340.922.9323.84320 K0.991.3417.640.932.5619.650.893.0121.98330 K0.981.8916.760.943.2420.890.914.0222.76Adsorption system (b): FeCl3-H2TTPP290 K0.835.0229.010.981.1320.510.922.8124.72300 K0.845.1228.910.981.1120.30.912.9824.61310 K0.825.4928.230.991.2321.440.933.0725.94320 K0.824.9928.990.971.3220.980.913.5125.16330 K0.825.0128.540.971.2921.520.923.1125.28Adsorption system (c): InCl3-H2TTPP290 K0.627.8936.660.895.7730.760.981.9827.02300 K0.717.9236.70.885.6430.720.992.1127.64310 K0.658.1339.40.876.7832.650.991.9926.91320 K0.648.1338.80.826.3631.470.971.9826.98330 K0.677.6238.40.795.7332.280.972.0226.56Table 2
Values of the error fitting coefficientsR2, RMSE, and AIC deduced from the numerical adjustment of experimental data of AlCl3, FeCl3, and InCl3 on tetraphenylporphyrin H2TPP (systems (a′), (b′), and (c′)) with the three statistical physics models.
Adsorption modelMono-layer model (ideal gas)Mono-layer model (real gas)Double-layer model (ideal gas)Adjustment coefficientR2RMSEAICR2RMSEAICR2RMSEAICAdsorption system (a′): AlCl3-H2TPP290 K0.981.1111.890.923.4415.670.893.8918.22300 K0.971.1212.330.933.1215.800.903.8318.51310 K0.981.3213.560.933.1716.420.893.9919.56320 K0.991.3613.640.943.4317.020.894.1519.20330 K0.981.5914.520.933.5116.450.924.1718.92Adsorption system (b′): FeCl3-H2TPP290 K0.814.0222.310.981.8914.300.903.2418.22300 K0.854.5423.520.971.8214.760.923.5418.97310 K0.834.7223.540.981.7315.640.943.6119.45320 K0.844.9922.920.961.7115.990.924.0418.83330 K0.835.1523.640.981.6615.780.914.2219.34Adsorption system (c′): InCl3-H2TPP290 K0.736.6630.140.864.5426.120.971.2819.04300 K0.736.5131.240.854.8226.450.981.3420.42310 K0.757.2130.740.834.7127.080.991.5220.84320 K0.747.5431.670.805.0227.640.981.4920.94330 K0.716.9431.840.825.6427.540.981.6921.06According to Tables1 and 2, the aluminum chloride isotherms (systems (a) and (a′)) should be analyzed by the monolayer model of ideal gas which shows R2 values near the unit and low values of RMSE and AIC for the five reaction temperatures. However, the adsorption isotherms of FeCl3 (systems (b) and (b′)) show the best coefficients of adjustment with the monolayer model of real gas. This explains that the decline of the FeCl3 isotherms at high equilibrium concentration is fundamentally due to the lateral interaction impacts and confirms that the aluminum ions are the best compounds for porphyrin complexation in terms of stability. On the other hand, in light of the selection criteria, the indium chloride adsorption (systems (c) and (c′)) should be described via the double-layer model of ideal gas. In this case, two adsorbed layers are formed based on charge neutralization between cations (In3+) and anions (Cl-).Figure5 illustrates the monolayer adsorptions of aluminum chloride and iron chloride and the double-layer adsorption of indium chloride on H2TTP and H2TPP.Figure 5
Illustration of the monolayer adsorption of AlCl3 and FeCl3 and the LBL double-layer adsorption of InCl3 on the two porphyrins H2TPP and H2TTPP.
(a)(b)


### 3.4. Analysis of Physicochemical Parameters

The mon-layer model (ideal gas), devoted for the description of the AlCl3 isotherms, was governed by three parameters (the number of aluminum ions per receptor site n, the number of porphyrin sites PM, and the energetic parameter c1/2 (Equation (16))). For FeCl3, the monolayer model (real gas) presents the parameter a (cohesion pressure) and the parameter b (covolume) in addition to the steric variables n and Pm and the energetic parameter w1/2. The LBL double-layer adsorption of InCl3 can be interpreted via four physicochemical variables (n and PM (steric variables) and c1 and c2 (Equation (20))).All fitting parameters values are given in Tables3 and 4. The study of the adsorption model parameters attributes interesting microscopic interpretations of the porphyrin complexation.Table 3
Values of the physicochemical parameters (n, PM, a, b, c1/2, w1/2, c1, and c2) affecting the adsorption of AlCl3, FeCl3, and InCl3 on 5,10,15,20-tetrakis(4-methylphenyl) porphyrin H2TTPP (systems (a), (b), and (c)) at five temperatures.
SystemParameters290 K300 K310 K320 K330 KSystem (a): AlCl3-H2TTPPn0.80 (±0.02)0.81 (±0.015)0.89 (±0.019)0.99 (±0.024)1.01 (±0.019)Pm281.7 (±8.2)320.6 (±9.4)366.1 (±8.9)401.9 (±10.9)450.7 (±11.7)c1/20.015 (±0.005)0.015 (±0.006)0.016 (±0.005)0.016 (±0.005)0.017 (±0.006)System (b): FeCl3-H2TTPPn0.71 (±0.025)0.73 (±0.015)0.79 (±0.022)0.84 (±0.019)0.89 (±0.02)Pm228.4 (±12.1)262.6 (±10.9)298.7 (±9.99)333.6 (±8.79)389.8 (±9.78)w1/20.011 (±0.004)0.011 (±0.005)0.012 (±0.006)0.011 (±0.006)0.012 (±0.007)a (×10−9)8.3 (±0.2)7.2 (±0.3)6.1 (±0.4)5.8 (±0.3)5.1 (±0.5)b (×10−12)2.6 (±0.1)3.2 (±0.2)3.9 (±0.2)4.8 (±0.4)5.7 (±0.4)System (c): InCl3-H2TTPPn0.63 (±0.018)0.66 (±0.023)0.69 (±0.016)0.74 (±0.022)0.82 (±0.016)Pm142.3 (±9.52)189.4 (±10.7)233.8 (±12.8)289.4 (±11.2)320.5 (±10.9)c10.0033 (±0.0007)0.003 (±0.008)0.0035 (±0.0003)0.0032 (±0.0009)0.0036 (±0.0004)c20.026 (±0.007)0.025 (±0.009)0.026 (±0.008)0.025 (±0.004)0.027 (±0.007)Table 4
Values of the physicochemical variables (n, PM, a, b, c1/2, w1/2, c1, and c2) deduced from the fitting of experimental data of AlCl3, FeCl3, and InCl3 on tetraphenylporphyrin H2TPP (systems (a′), (b′), and (c′)) with the three adopted models.
SystemParameters290 K300 K310 K320 K330 KSystem (a′): AlCl3-H2TPPn0.75 (±0.021)0.77 (±0.026)0.83 (±0.019)0.91 (±0.023)0.95 (±0.017)Pm258.2 (±12.2)301.2 (±9.77)332.7 (±8.97)368.2 (±9.65)421.4 (±10.7)c1/20.009 (±0.0007)0.011 (±0.006)0.009 (±0.0008)0.01 (±0.003)0.008 (±0.0009)System (b′): FeCl3-H2TPPn0.67 (±0.023)0.70 (±0.019)0.74 (±0.02)0.79 (±0.022)0.85 (±0.016)Pm184.5 (±9.71)231.4 (±8.69)268.2 (±9.43)312.2 (±10.3)355.2 (±8.46)w1/20.007 (±0.0002)0.008 (±0.0001)0.008 (±0.0003)0.0075 (±0.0001)0.0069 (±0.0006)a (×10−9)10.5 (±0.8)9.4 (±0.9)8.4 (±0.6)7.9 (±0.5)7.12 (±0.5)b (×10−12)1.2 (±0.2)1.7 (±0.1)2.4 (±0.1)3.3 (±0.2)4.1 (±0.3)System (c′): InCl3-H2TPPn0.50 (±0.015)0.56 (±0.016)0.61 (±0.015)0.70 (±0.019)0.79 (±0.02)Pm105.4 (±7.89)147.3 (±8.45)194.3 (±10.5)238.4 (±9.34)279.4 (±8.64)c10.002 (±0.0007)0.003 (±0.006)0.0031 (±0.0009)0.0029 (±0.008)0.0032 (±0.0005)c20.023 (±0.005)0.024 (±0.007)0.025 (±0.003)0.025 (±0.008)0.026 (±0.002)


#### 3.4.1. Steric Interpretation

Basically, the parametersn and PM are typified by a steric aspect. The product of these parameters is the result of the maximum adsorption capacity [48].The evolution of the two steric parameters versus temperature is reported in Figure6.Figure 6
Evolutions of the two steric parameters as a function of temperature: (a) evolution of the number of bonded ions per siten and (b) variation of the density of receptor porphyrin sites PM.
(a)(b)From Figure6(a), the n values are found inferior to 1 for the six adsorption systems. For example, regarding the values of this parameter that are inferior to 0.5, it is clear that the complexation mechanism is a multi-interaction process (the ions can be interacted with two adsorption sites) [39].The fitted values ofPM describe the number of porphyrin sites accessible to the adsorbate ions at each temperature. Figure 6(b) demonstrates that the adsorptions of AlCl3 and FeCl3 present the highest values of PM at all the temperatures: PM (AlCl3) > PM (FeCl3) > PM (InCl3). In fact, the anions are not involved in the complexation processes of AlCl3 and FeCl3, so there is a fast insertion of the metallic ions in the porphyrin cavities. However, the contribution of the anionic particles in InCl3 adsorption prevents the complexation of porphyrin by the indium ions because of the interaction between the two adsorbed layers.Furthermore, it is noted from Figure4 that the temperature exerts exactly the same influence on the six complexation systems: once the temperature increases, the adsorption capacities increase. This is often explained from Figure 6 which shows that the values of the coefficients n (Figure 6(a)) and PM (Figure 6(b)) rise with the temperature. It can be concluded that the thermal agitation effect favors the adsorption dynamics: the rise of temperature activates other receptor sites to contribute in the complexation process.


#### 3.4.2. Van der Waals Parameter Behaviors

It can be noticed that despite the AlCl3 and FeCl3 adsorptions being both monolayer adsorption processes, where there is no contribution of the chloride ions at the layer formation, the fitted values of PM are the lowest for FeCl3. Thus, the physical model that describes the FeCl3 adsorption includes the lateral interaction effect by the intermediate of the parameter a (cohesion pressure) and the parameter b (covolume) [49].It can be concluded that the decrease of FeCl3 isotherms (Figures 4(b) and 4(b′)) can be the result of the high adsorbate-adsorbate interaction which reflects a weak binding Fe3+-porphyrin compared to the Al3+-porphyrin binding.Overall, it can be concluded that the use of aluminum chloride guarantees more stability during the metalloporphyrin formation.The values of these two parameters for H2TPP and H2TTPP are summarized in Figure 7.Figure 7
Variations of the adjusted values of the cohesion pressurea and the covolume b as a function of temperature (290-330 K) for the indium chloride adsorption.Generally, the interactions of the particles (lateral interactions) are characterized by the cohesion pressurea. One can see from Figure 7 that aFeCl3‐H2TTPP<aFeCl3‐H2TPP. Therefore, the H2TTPP is the best complexing porphyrin of the iron ions because it presents the lowest impact of the lateral interactions on the adsorption process. On the other hand, it should be noted, from Figure 7, that the H2TTPP presents the highest values of the covolume b: bFeCl3‐H2TTPP>bFeCl3‐H2TPP. In this case, the distance between the adsorbates is high which indicates that the iron ions can be easily adsorbed by the surface and causes an expansion in the adsorbed amount. This result confirms the suggestion of the porphyrin H2TTPP as a suitable material for the metalloporphyrin adsorption.Furthermore, it is seen from Figure7 that the parameter a declines with the expansion of the temperature for the FeCl3 adsorption while the covolume b increases. The decrease of the cohesion pressure indicates that the lateral interaction effect is low at high temperature. The increase of the parameter b reflects a strong distance between the adsorbates. The lateral interaction description explains the highest performance of the iron adsorption at 330 K and shows the endothermic nature of the studied process.


#### 3.4.3. Energetic Calculation

According to Equations (16), (18), and (20), the molar energies of the six adsorption mechanisms can be calculated by means of the energetic coefficients, c1/2 for AlCl3, w1/2 for FeCl3, and c1 and c2 for InCl3, which are calculated from the numerical simulation of the three adopted models [36, 48].For aluminum chloride adsorption,(26)−ΔE1/2=RT×lnc1/2SAlCl3.For iron chloride adsorption,(27)−ΔE1/2=RT×lnw1/2SFeCl3.For indium chloride adsorption,(28)−ΔE1=RT×lnc1SInCl3,(29)−ΔE2=RT×lnc2SInCl3.The values of the molar energies and their variations via the temperature are reported in Table5 and Figure 8.Table 5
Values of the molar adsorption energies∣−ΔE1/2∣ for AlCl3 and FeCl3 adsorptions and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3 adsorption given in modulus values at 290, 300, 310, 320, and 330 K.
Adsorption systemAdsorption energy290 K300 K310 K320 K330 KSystem (a): AlCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)61.4 (±2.1)69.5 (±2.3)70.6 (±3.1)75.4 (±2.9)80.3 (±3.9)System (a′): AlCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)48.4 (±2.1)57.2 (±2.4)60.2 (±1.8)63.4 (±3.1)67.4 (±2.9)System (b): FeCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)32.9 (±1.8)36.4 (±1.7)38.7 (±2.5)40.8 (±2.2)43.1 (±2.3)System (b′): FeCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)30.2 (±1.9)33.6 (±1.8)36.7 (±1.8)38.4 (±2.1)40.9 (±2.2)System (c): InCl3-H2TTPP∣−ΔE1∣ (kJ/mol)26.4 (±2.3)30.4 (±1.6)33.5 (±1.5)36.8 (±1.9)38.7 (±1.7)∣−ΔE2∣ (kJ/mol)17.2 (±1.2)20.4 (±1.5)23.3 (±1.4)25.7 (±1.1)28.8 (±1.6)System (c′): InCl3-H2TPP∣−ΔE1∣ (kJ/mol)22.3 (±2.7)26.1 (±2.4)29.4 (±2.1)33.1 (±1.9)35.6 (±1.8)∣−ΔE2∣ (kJ/mol)12.7 (±1.5)16.7 (±1.6)18.9 (±2.0)20.4 (±2.1)21.1 (±1.8)Figure 8
Evolutions of the molar adsorption energies of the six adsorption systems as a function of temperature:∣−ΔE1/2∣ for AlCl3 and FeCl3, and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3.According to Table5, for the indium chloride adsorption, it is obviously remarked that the calculated values of ∣−ΔE1∣ which characterizes the indium-porphyrin interaction are greater than those of ∣−ΔE2∣ (interaction between the two adsorbed layers [36]). Therefore, we can conclude that the interaction ∣−ΔE1∣ should be compared to the other adsorption system energies (∣−ΔE1/2∣) to evaluate the stability of the formed metalloporphyrin complexes.Comparing the∣−ΔE1/2∣ and the ∣−ΔE1∣ values of the six complexation systems (Table 5 and Figure 8), we observe that ∣−ΔE1/2∣ (AlCl3-H2TTPP) > ∣−ΔE1/2∣ (AlCl3-H2TPP) > ∣−ΔE1∣ (FeCl3-H2TTPP) > ∣−ΔE1∣ (FeCl3-H2TPP) > ∣−ΔE1/2∣ (InCl3-H2TTPP) > ∣−ΔE1/2∣ (InCl3-H2TPP). It can be noticed that the affinity of the porphyrin H2TTPP cavities to the metallic ions is higher than the tetraphenylporphyrin sites H2TPP for the three ions and confirms that the aluminum chloride-porphyrin (H2TTPP) is the best adsorption system suited for metalloporphyrin application because it presents the highest adsorption energies values for the five reaction temperatures.It can be also observed that the molar energies rise with the expansion of the temperature (Figure8). This can be interpreted by the endothermic behavior of the six studied processes.Furthermore, the energies∣−ΔE1/2∣ of AlCl3 are superior to 40 kJ/mol for all the temperatures. For this system, the adsorption is carried out via a chemical process involving covalent bonds (irreversible phenomenon) [39, 50–52]. In this case, the adsorption is a dissociative mechanism (there is no possibility of a desorption process) because there is a change in the electron density between the adsorbent and the adsorbed particle. Moreover, a multilayer adsorption is impossible due to the high interaction between the adsorbed layer and the adsorbent surface (e.g., AlCl3 adsorption).In contrast, all the other adsorption mechanisms (FeCl3 and InCl3) take place via a physical process since they present adsorption energy values lower than 40 kJ/mol [42, 50, 51]. During physisorption, the adsorbed particles retain their individual properties and there is no change in the structure of the components of the adsorption system. The system is weakly energetic (low adsorbate-adsorbent interaction), so the presence of a desorption phenomenon is possible (e.g., FeCl3 adsorption). Moreover, the adsorbed layer can interact with other adsorbed layers and can act as a new template for the adsorption of the following layer (multilayer adsorption mechanism) such as the case of InCl3 in the present investigation.


## 3.1. Statistical Physics Theory

The estimated values of the physical model parameters represent the key to understand the adsorption mechanism. The progress of this physical treatment is firstly seen against the well-known model of Langmuir (empirical model) [22]. The Langmuir model is developed by considering that one particle can interact with only one adsorbent site without taking account of the external factors. In general, this assumption led to wrong scientific conclusions. On the contrary, our statistical physics models correct this assumption by introducing a parameter defined by the number of adsorbed particles per adsorption site; it is noted in general by “n.” This correction can simply provide useful interpretations regarding the adsorption mechanism.Based on the adsorption isotherm profiles, our statistical physics models can suggest various energies which are linked to different functional groups of the adsorbent surface contrary to the traditional equations of Langmuir [22] or Freundlich [23]. In the same direction, the statistical physics models are able to estimate the total number of the formed layer at all the reaction temperatures [24–26]. Note also that the multilayer adsorption of charged ions can be only carried out via a layer-by-layer (LBL) process [25, 36].A general analysis of the adsorption data led to test two models: the monolayer model for AlCl3 and FeCl3 and the LBL multilayer model for InCl3. The modeling work is then arranged as follows: (a) it is worth mentioning the general methodology which allows developing the physical models expressing the adsorbed amount corresponding to the adsorbate concentration, (b) the statistical physics model can be applied and discussed on the experimental data, and (c) the choice of the adopted model led to describing the adsorption reaction via its parameters.


## 3.2. Advanced Adsorption Models

To establish the physical model expressions, the equilibrium of the adsorption reaction can be expressed via the next equation [25, 37]:
(5)nA+S⇄An−S,whereA represents the tested adsorbates (AlCl3/FeCl3/InCl3), S is the adsorbent receptor site of porphyrin (H2TPP or H2TTPP), and An−S is the resulting complex. The variable n is a stoichiometric parameter which describes the number of bonded ions per adsorbent site. In general, this parameter can identify the nature of the adsorption process (n≤1: multi-interaction process, n≥1: multi-ionic process) [38, 39].This system is characterized by the chemical potential (μ) and the temperature (T) imposing from the outside towards the considered system (grand canonical situation). In this case, the general expression of the partition function is written as a function of the occupation number (Ni), the adsorption energy (−Ei), and the Boltzmann factor (β) [37]:
(6)zgc=∑Nie−β−Ei−μNi.Concerning the adsorption via one energy level (−E) (one adsorbed layer), it can be expressed by [39]
(7)zgc=1+eβE+μ.For the LBL adsorption process (double-layer or multilayer), two energies can be responsible for this process. Note that the adsorption of the first layer is characterized by the energy (−E1), and the second energy (−E2) is in relationship with the formation of the additional adsorbed layers. The partition function is written as [38] follows:For the double-layer model:(8)zgc=1+eβE1+μ+eβE1+E2+2μ.For the multilayer model:(9)zgc=1+eβE1+μ+∑Ni=2Le−β−E1−Ni−1E2−Niμ,withL as the number of adsorbed layers.Then, the average occupation number ofPM porphyrin sites for the three models should be calculated using the next equation [37, 40]:
(10)No=PMβ∂lnzgc∂μ.In this modeling work, the analytical development can be carried out by introducing the chemical potential of the perfect gas (μp). μp is introduced in the mathematical development of the models through its expression which is written as a function of the partition-function of translation (zTr) and the number of adsorbates (N) [40, 41]:
(11)μp=1βlnNzTr,whereztr can be written as a function of Planck’s constant, the volume V, and the mass m of the adsorbed particle [37]:
(12)zTr=V2πmkBTh23/2.The investigation of the adsorption problem can be also performed using the chemical potential (μr) of the real gas. In this case, the lateral interactions involving the adsorbates at free state are taken into account by means of the parameters a (the pressure of cohesion) and b (the covolume) [24, 25]:
(13)μr=μp+1βln11−bc+1βbc1−bc−2ac.Then, the adsorbed quantityQA (μg/cm2) can be determined by the following equation [37, 40]:
(14)QA=n×N0.Finally, we get the analytical expressions of the six adsorption models:The monolayer model of the ideal gas approach:(15)QA=nPM×c/c1/2n1+c/c1/2n,wherec1/2 involves the molar adsorption energy (−ΔE1/2) (kJ/mol), the adsorption temperature T (K), and the adsorbate solubility S (mol/L). It has the following formula:
(16)c1/2=Se−ΔE1/2/RT.The monolayer model of the real gas approach:(17)QA=nPM1+w1/21−bc/ce2βace−bc/1−bcn,where the energetic coefficientw1/2 is
(18)w1/2=Se−ΔE1/2/RT.The double-layer and multilayer models include two energetic coefficients involving two molar adsorption energies (−ΔE1) and (−ΔE2).The double-layer model of the ideal gas approach:(19)QA=nPMc/c1n+2c/c22n1+c/c1n+c/c22n,where the energetic variablesc1 and c2 are
(20)c1,2=Se−ΔE1,2/RT.The double-layer model of the real gas approach:(21)QA=nPMc/w11−bce2βace−bc/1−bcn+2c/w21−bce2βace−bc/1−bc2n1+c/w11−bce2βace−bc/1−bcn+c/w21−bce2βace−bc/1−bc2n,wherew1 and w2 are
(22)w1,2=Se−ΔE1,2/RT.The multilayer model of the ideal gas approach:(23)QA=nPM×c/c1n+c/c1nc/c2n1−2c/c2nL−Lc/c2nL+1+c/c2n1−c/c2nL/1−c/c2n1−c/c1n1−c/c2n+c/c1nc/c2n1−c/c2nL,wherec1 and c2 are given by Equation (20).The multilayer model of the real gas approach:(24)QA=nPM×c/c1n+c/c1nc/c2n1−2c/c2nL−Lc/c2nL+1+c/c2n1−c/c2nL/1−c/c2n1−c/c1n1−c/c2n+c/c1nc/c2n1−c/c2nL,wherec1 and c2 are written as
(25)c1,2=w1,21−bce2βace−bc/1−bc,withw1 and w2 are previously mentioned (Equation (22)).


## 3.3. Adjustment of Adsorption Models with Experimental Isotherms

The six physical adsorption models were applied on the experimental isotherms by means of a numerical fitting program [42]. The mathematical fitting method is based on the Levenberg-Marquardt iterating algorithm using a multivariable nonlinear regression program [40, 43]. The criteria to adopt a descriptive model are the determination coefficient R2, the RMSE coefficient (residual root mean square error), and the well-known AIC (Akaike Information Criterion) coefficient. The first error coefficient (R2) is a standardized measure of the fit goodness [43, 44]. If the coefficient R2 value is near the unit, we are in the case of the best fitting model. The second error coefficient (RMSE) is a nonstandardized measure of the fit goodness [44–47]. The best fitting model is obtained when the RMSE value is inferior to 2. The third adjustment coefficient (AIC) shows the best fitting with the model having the lowest AIC values [36]. These three adjustment coefficients are interpreted to adopt a physical model.Tables1 and 2 show the numerical values of the error adjustment coefficients.Table 1
Values of the error adjustment coefficientsR2, RMSE, and AIC deduced from fitting the experimental isotherms of AlCl3, FeCl3, and InCl3 on 5,10,15,20-tetrakis(4-methylphenyl) porphyrin H2TTPP (systems (a), (b), and (c)) with the three advanced models.
Adsorption modelMonolayer model (ideal gas)Monolayer model (real gas)Double-layer model (ideal gas)Adjustment coefficientR2RMSEAICR2RMSEAICR2RMSEAICAdsorption system (a): AlCl3-H2TTPP290 K0.981.2314.990.932.6419.880.902.9822.99300 K0.991.2215.020.942.8819.840.912.9923.64310 K0.981.5618.710.952.9720.340.922.9323.84320 K0.991.3417.640.932.5619.650.893.0121.98330 K0.981.8916.760.943.2420.890.914.0222.76Adsorption system (b): FeCl3-H2TTPP290 K0.835.0229.010.981.1320.510.922.8124.72300 K0.845.1228.910.981.1120.30.912.9824.61310 K0.825.4928.230.991.2321.440.933.0725.94320 K0.824.9928.990.971.3220.980.913.5125.16330 K0.825.0128.540.971.2921.520.923.1125.28Adsorption system (c): InCl3-H2TTPP290 K0.627.8936.660.895.7730.760.981.9827.02300 K0.717.9236.70.885.6430.720.992.1127.64310 K0.658.1339.40.876.7832.650.991.9926.91320 K0.648.1338.80.826.3631.470.971.9826.98330 K0.677.6238.40.795.7332.280.972.0226.56Table 2
Values of the error fitting coefficientsR2, RMSE, and AIC deduced from the numerical adjustment of experimental data of AlCl3, FeCl3, and InCl3 on tetraphenylporphyrin H2TPP (systems (a′), (b′), and (c′)) with the three statistical physics models.
Adsorption modelMono-layer model (ideal gas)Mono-layer model (real gas)Double-layer model (ideal gas)Adjustment coefficientR2RMSEAICR2RMSEAICR2RMSEAICAdsorption system (a′): AlCl3-H2TPP290 K0.981.1111.890.923.4415.670.893.8918.22300 K0.971.1212.330.933.1215.800.903.8318.51310 K0.981.3213.560.933.1716.420.893.9919.56320 K0.991.3613.640.943.4317.020.894.1519.20330 K0.981.5914.520.933.5116.450.924.1718.92Adsorption system (b′): FeCl3-H2TPP290 K0.814.0222.310.981.8914.300.903.2418.22300 K0.854.5423.520.971.8214.760.923.5418.97310 K0.834.7223.540.981.7315.640.943.6119.45320 K0.844.9922.920.961.7115.990.924.0418.83330 K0.835.1523.640.981.6615.780.914.2219.34Adsorption system (c′): InCl3-H2TPP290 K0.736.6630.140.864.5426.120.971.2819.04300 K0.736.5131.240.854.8226.450.981.3420.42310 K0.757.2130.740.834.7127.080.991.5220.84320 K0.747.5431.670.805.0227.640.981.4920.94330 K0.716.9431.840.825.6427.540.981.6921.06According to Tables1 and 2, the aluminum chloride isotherms (systems (a) and (a′)) should be analyzed by the monolayer model of ideal gas which shows R2 values near the unit and low values of RMSE and AIC for the five reaction temperatures. However, the adsorption isotherms of FeCl3 (systems (b) and (b′)) show the best coefficients of adjustment with the monolayer model of real gas. This explains that the decline of the FeCl3 isotherms at high equilibrium concentration is fundamentally due to the lateral interaction impacts and confirms that the aluminum ions are the best compounds for porphyrin complexation in terms of stability. On the other hand, in light of the selection criteria, the indium chloride adsorption (systems (c) and (c′)) should be described via the double-layer model of ideal gas. In this case, two adsorbed layers are formed based on charge neutralization between cations (In3+) and anions (Cl-).Figure5 illustrates the monolayer adsorptions of aluminum chloride and iron chloride and the double-layer adsorption of indium chloride on H2TTP and H2TPP.Figure 5
Illustration of the monolayer adsorption of AlCl3 and FeCl3 and the LBL double-layer adsorption of InCl3 on the two porphyrins H2TPP and H2TTPP.
(a)(b)


## 3.4. Analysis of Physicochemical Parameters

The mon-layer model (ideal gas), devoted for the description of the AlCl3 isotherms, was governed by three parameters (the number of aluminum ions per receptor site n, the number of porphyrin sites PM, and the energetic parameter c1/2 (Equation (16))). For FeCl3, the monolayer model (real gas) presents the parameter a (cohesion pressure) and the parameter b (covolume) in addition to the steric variables n and Pm and the energetic parameter w1/2. The LBL double-layer adsorption of InCl3 can be interpreted via four physicochemical variables (n and PM (steric variables) and c1 and c2 (Equation (20))).All fitting parameters values are given in Tables3 and 4. The study of the adsorption model parameters attributes interesting microscopic interpretations of the porphyrin complexation.Table 3
Values of the physicochemical parameters (n, PM, a, b, c1/2, w1/2, c1, and c2) affecting the adsorption of AlCl3, FeCl3, and InCl3 on 5,10,15,20-tetrakis(4-methylphenyl) porphyrin H2TTPP (systems (a), (b), and (c)) at five temperatures.
SystemParameters290 K300 K310 K320 K330 KSystem (a): AlCl3-H2TTPPn0.80 (±0.02)0.81 (±0.015)0.89 (±0.019)0.99 (±0.024)1.01 (±0.019)Pm281.7 (±8.2)320.6 (±9.4)366.1 (±8.9)401.9 (±10.9)450.7 (±11.7)c1/20.015 (±0.005)0.015 (±0.006)0.016 (±0.005)0.016 (±0.005)0.017 (±0.006)System (b): FeCl3-H2TTPPn0.71 (±0.025)0.73 (±0.015)0.79 (±0.022)0.84 (±0.019)0.89 (±0.02)Pm228.4 (±12.1)262.6 (±10.9)298.7 (±9.99)333.6 (±8.79)389.8 (±9.78)w1/20.011 (±0.004)0.011 (±0.005)0.012 (±0.006)0.011 (±0.006)0.012 (±0.007)a (×10−9)8.3 (±0.2)7.2 (±0.3)6.1 (±0.4)5.8 (±0.3)5.1 (±0.5)b (×10−12)2.6 (±0.1)3.2 (±0.2)3.9 (±0.2)4.8 (±0.4)5.7 (±0.4)System (c): InCl3-H2TTPPn0.63 (±0.018)0.66 (±0.023)0.69 (±0.016)0.74 (±0.022)0.82 (±0.016)Pm142.3 (±9.52)189.4 (±10.7)233.8 (±12.8)289.4 (±11.2)320.5 (±10.9)c10.0033 (±0.0007)0.003 (±0.008)0.0035 (±0.0003)0.0032 (±0.0009)0.0036 (±0.0004)c20.026 (±0.007)0.025 (±0.009)0.026 (±0.008)0.025 (±0.004)0.027 (±0.007)Table 4
Values of the physicochemical variables (n, PM, a, b, c1/2, w1/2, c1, and c2) deduced from the fitting of experimental data of AlCl3, FeCl3, and InCl3 on tetraphenylporphyrin H2TPP (systems (a′), (b′), and (c′)) with the three adopted models.
SystemParameters290 K300 K310 K320 K330 KSystem (a′): AlCl3-H2TPPn0.75 (±0.021)0.77 (±0.026)0.83 (±0.019)0.91 (±0.023)0.95 (±0.017)Pm258.2 (±12.2)301.2 (±9.77)332.7 (±8.97)368.2 (±9.65)421.4 (±10.7)c1/20.009 (±0.0007)0.011 (±0.006)0.009 (±0.0008)0.01 (±0.003)0.008 (±0.0009)System (b′): FeCl3-H2TPPn0.67 (±0.023)0.70 (±0.019)0.74 (±0.02)0.79 (±0.022)0.85 (±0.016)Pm184.5 (±9.71)231.4 (±8.69)268.2 (±9.43)312.2 (±10.3)355.2 (±8.46)w1/20.007 (±0.0002)0.008 (±0.0001)0.008 (±0.0003)0.0075 (±0.0001)0.0069 (±0.0006)a (×10−9)10.5 (±0.8)9.4 (±0.9)8.4 (±0.6)7.9 (±0.5)7.12 (±0.5)b (×10−12)1.2 (±0.2)1.7 (±0.1)2.4 (±0.1)3.3 (±0.2)4.1 (±0.3)System (c′): InCl3-H2TPPn0.50 (±0.015)0.56 (±0.016)0.61 (±0.015)0.70 (±0.019)0.79 (±0.02)Pm105.4 (±7.89)147.3 (±8.45)194.3 (±10.5)238.4 (±9.34)279.4 (±8.64)c10.002 (±0.0007)0.003 (±0.006)0.0031 (±0.0009)0.0029 (±0.008)0.0032 (±0.0005)c20.023 (±0.005)0.024 (±0.007)0.025 (±0.003)0.025 (±0.008)0.026 (±0.002)


### 3.4.1. Steric Interpretation

Basically, the parametersn and PM are typified by a steric aspect. The product of these parameters is the result of the maximum adsorption capacity [48].The evolution of the two steric parameters versus temperature is reported in Figure6.Figure 6
Evolutions of the two steric parameters as a function of temperature: (a) evolution of the number of bonded ions per siten and (b) variation of the density of receptor porphyrin sites PM.
(a)(b)From Figure6(a), the n values are found inferior to 1 for the six adsorption systems. For example, regarding the values of this parameter that are inferior to 0.5, it is clear that the complexation mechanism is a multi-interaction process (the ions can be interacted with two adsorption sites) [39].The fitted values ofPM describe the number of porphyrin sites accessible to the adsorbate ions at each temperature. Figure 6(b) demonstrates that the adsorptions of AlCl3 and FeCl3 present the highest values of PM at all the temperatures: PM (AlCl3) > PM (FeCl3) > PM (InCl3). In fact, the anions are not involved in the complexation processes of AlCl3 and FeCl3, so there is a fast insertion of the metallic ions in the porphyrin cavities. However, the contribution of the anionic particles in InCl3 adsorption prevents the complexation of porphyrin by the indium ions because of the interaction between the two adsorbed layers.Furthermore, it is noted from Figure4 that the temperature exerts exactly the same influence on the six complexation systems: once the temperature increases, the adsorption capacities increase. This is often explained from Figure 6 which shows that the values of the coefficients n (Figure 6(a)) and PM (Figure 6(b)) rise with the temperature. It can be concluded that the thermal agitation effect favors the adsorption dynamics: the rise of temperature activates other receptor sites to contribute in the complexation process.


### 3.4.2. Van der Waals Parameter Behaviors

It can be noticed that despite the AlCl3 and FeCl3 adsorptions being both monolayer adsorption processes, where there is no contribution of the chloride ions at the layer formation, the fitted values of PM are the lowest for FeCl3. Thus, the physical model that describes the FeCl3 adsorption includes the lateral interaction effect by the intermediate of the parameter a (cohesion pressure) and the parameter b (covolume) [49].It can be concluded that the decrease of FeCl3 isotherms (Figures 4(b) and 4(b′)) can be the result of the high adsorbate-adsorbate interaction which reflects a weak binding Fe3+-porphyrin compared to the Al3+-porphyrin binding.Overall, it can be concluded that the use of aluminum chloride guarantees more stability during the metalloporphyrin formation.The values of these two parameters for H2TPP and H2TTPP are summarized in Figure 7.Figure 7
Variations of the adjusted values of the cohesion pressurea and the covolume b as a function of temperature (290-330 K) for the indium chloride adsorption.Generally, the interactions of the particles (lateral interactions) are characterized by the cohesion pressurea. One can see from Figure 7 that aFeCl3‐H2TTPP<aFeCl3‐H2TPP. Therefore, the H2TTPP is the best complexing porphyrin of the iron ions because it presents the lowest impact of the lateral interactions on the adsorption process. On the other hand, it should be noted, from Figure 7, that the H2TTPP presents the highest values of the covolume b: bFeCl3‐H2TTPP>bFeCl3‐H2TPP. In this case, the distance between the adsorbates is high which indicates that the iron ions can be easily adsorbed by the surface and causes an expansion in the adsorbed amount. This result confirms the suggestion of the porphyrin H2TTPP as a suitable material for the metalloporphyrin adsorption.Furthermore, it is seen from Figure7 that the parameter a declines with the expansion of the temperature for the FeCl3 adsorption while the covolume b increases. The decrease of the cohesion pressure indicates that the lateral interaction effect is low at high temperature. The increase of the parameter b reflects a strong distance between the adsorbates. The lateral interaction description explains the highest performance of the iron adsorption at 330 K and shows the endothermic nature of the studied process.


### 3.4.3. Energetic Calculation

According to Equations (16), (18), and (20), the molar energies of the six adsorption mechanisms can be calculated by means of the energetic coefficients, c1/2 for AlCl3, w1/2 for FeCl3, and c1 and c2 for InCl3, which are calculated from the numerical simulation of the three adopted models [36, 48].For aluminum chloride adsorption,(26)−ΔE1/2=RT×lnc1/2SAlCl3.For iron chloride adsorption,(27)−ΔE1/2=RT×lnw1/2SFeCl3.For indium chloride adsorption,(28)−ΔE1=RT×lnc1SInCl3,(29)−ΔE2=RT×lnc2SInCl3.The values of the molar energies and their variations via the temperature are reported in Table5 and Figure 8.Table 5
Values of the molar adsorption energies∣−ΔE1/2∣ for AlCl3 and FeCl3 adsorptions and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3 adsorption given in modulus values at 290, 300, 310, 320, and 330 K.
Adsorption systemAdsorption energy290 K300 K310 K320 K330 KSystem (a): AlCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)61.4 (±2.1)69.5 (±2.3)70.6 (±3.1)75.4 (±2.9)80.3 (±3.9)System (a′): AlCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)48.4 (±2.1)57.2 (±2.4)60.2 (±1.8)63.4 (±3.1)67.4 (±2.9)System (b): FeCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)32.9 (±1.8)36.4 (±1.7)38.7 (±2.5)40.8 (±2.2)43.1 (±2.3)System (b′): FeCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)30.2 (±1.9)33.6 (±1.8)36.7 (±1.8)38.4 (±2.1)40.9 (±2.2)System (c): InCl3-H2TTPP∣−ΔE1∣ (kJ/mol)26.4 (±2.3)30.4 (±1.6)33.5 (±1.5)36.8 (±1.9)38.7 (±1.7)∣−ΔE2∣ (kJ/mol)17.2 (±1.2)20.4 (±1.5)23.3 (±1.4)25.7 (±1.1)28.8 (±1.6)System (c′): InCl3-H2TPP∣−ΔE1∣ (kJ/mol)22.3 (±2.7)26.1 (±2.4)29.4 (±2.1)33.1 (±1.9)35.6 (±1.8)∣−ΔE2∣ (kJ/mol)12.7 (±1.5)16.7 (±1.6)18.9 (±2.0)20.4 (±2.1)21.1 (±1.8)Figure 8
Evolutions of the molar adsorption energies of the six adsorption systems as a function of temperature:∣−ΔE1/2∣ for AlCl3 and FeCl3, and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3.According to Table5, for the indium chloride adsorption, it is obviously remarked that the calculated values of ∣−ΔE1∣ which characterizes the indium-porphyrin interaction are greater than those of ∣−ΔE2∣ (interaction between the two adsorbed layers [36]). Therefore, we can conclude that the interaction ∣−ΔE1∣ should be compared to the other adsorption system energies (∣−ΔE1/2∣) to evaluate the stability of the formed metalloporphyrin complexes.Comparing the∣−ΔE1/2∣ and the ∣−ΔE1∣ values of the six complexation systems (Table 5 and Figure 8), we observe that ∣−ΔE1/2∣ (AlCl3-H2TTPP) > ∣−ΔE1/2∣ (AlCl3-H2TPP) > ∣−ΔE1∣ (FeCl3-H2TTPP) > ∣−ΔE1∣ (FeCl3-H2TPP) > ∣−ΔE1/2∣ (InCl3-H2TTPP) > ∣−ΔE1/2∣ (InCl3-H2TPP). It can be noticed that the affinity of the porphyrin H2TTPP cavities to the metallic ions is higher than the tetraphenylporphyrin sites H2TPP for the three ions and confirms that the aluminum chloride-porphyrin (H2TTPP) is the best adsorption system suited for metalloporphyrin application because it presents the highest adsorption energies values for the five reaction temperatures.It can be also observed that the molar energies rise with the expansion of the temperature (Figure8). This can be interpreted by the endothermic behavior of the six studied processes.Furthermore, the energies∣−ΔE1/2∣ of AlCl3 are superior to 40 kJ/mol for all the temperatures. For this system, the adsorption is carried out via a chemical process involving covalent bonds (irreversible phenomenon) [39, 50–52]. In this case, the adsorption is a dissociative mechanism (there is no possibility of a desorption process) because there is a change in the electron density between the adsorbent and the adsorbed particle. Moreover, a multilayer adsorption is impossible due to the high interaction between the adsorbed layer and the adsorbent surface (e.g., AlCl3 adsorption).In contrast, all the other adsorption mechanisms (FeCl3 and InCl3) take place via a physical process since they present adsorption energy values lower than 40 kJ/mol [42, 50, 51]. During physisorption, the adsorbed particles retain their individual properties and there is no change in the structure of the components of the adsorption system. The system is weakly energetic (low adsorbate-adsorbent interaction), so the presence of a desorption phenomenon is possible (e.g., FeCl3 adsorption). Moreover, the adsorbed layer can interact with other adsorbed layers and can act as a new template for the adsorption of the following layer (multilayer adsorption mechanism) such as the case of InCl3 in the present investigation.


## 3.4.1. Steric Interpretation

Basically, the parametersn and PM are typified by a steric aspect. The product of these parameters is the result of the maximum adsorption capacity [48].The evolution of the two steric parameters versus temperature is reported in Figure6.Figure 6
Evolutions of the two steric parameters as a function of temperature: (a) evolution of the number of bonded ions per siten and (b) variation of the density of receptor porphyrin sites PM.
(a)(b)From Figure6(a), the n values are found inferior to 1 for the six adsorption systems. For example, regarding the values of this parameter that are inferior to 0.5, it is clear that the complexation mechanism is a multi-interaction process (the ions can be interacted with two adsorption sites) [39].The fitted values ofPM describe the number of porphyrin sites accessible to the adsorbate ions at each temperature. Figure 6(b) demonstrates that the adsorptions of AlCl3 and FeCl3 present the highest values of PM at all the temperatures: PM (AlCl3) > PM (FeCl3) > PM (InCl3). In fact, the anions are not involved in the complexation processes of AlCl3 and FeCl3, so there is a fast insertion of the metallic ions in the porphyrin cavities. However, the contribution of the anionic particles in InCl3 adsorption prevents the complexation of porphyrin by the indium ions because of the interaction between the two adsorbed layers.Furthermore, it is noted from Figure4 that the temperature exerts exactly the same influence on the six complexation systems: once the temperature increases, the adsorption capacities increase. This is often explained from Figure 6 which shows that the values of the coefficients n (Figure 6(a)) and PM (Figure 6(b)) rise with the temperature. It can be concluded that the thermal agitation effect favors the adsorption dynamics: the rise of temperature activates other receptor sites to contribute in the complexation process.


## 3.4.2. Van der Waals Parameter Behaviors

It can be noticed that despite the AlCl3 and FeCl3 adsorptions being both monolayer adsorption processes, where there is no contribution of the chloride ions at the layer formation, the fitted values of PM are the lowest for FeCl3. Thus, the physical model that describes the FeCl3 adsorption includes the lateral interaction effect by the intermediate of the parameter a (cohesion pressure) and the parameter b (covolume) [49].It can be concluded that the decrease of FeCl3 isotherms (Figures 4(b) and 4(b′)) can be the result of the high adsorbate-adsorbate interaction which reflects a weak binding Fe3+-porphyrin compared to the Al3+-porphyrin binding.Overall, it can be concluded that the use of aluminum chloride guarantees more stability during the metalloporphyrin formation.The values of these two parameters for H2TPP and H2TTPP are summarized in Figure 7.Figure 7
Variations of the adjusted values of the cohesion pressurea and the covolume b as a function of temperature (290-330 K) for the indium chloride adsorption.Generally, the interactions of the particles (lateral interactions) are characterized by the cohesion pressurea. One can see from Figure 7 that aFeCl3‐H2TTPP<aFeCl3‐H2TPP. Therefore, the H2TTPP is the best complexing porphyrin of the iron ions because it presents the lowest impact of the lateral interactions on the adsorption process. On the other hand, it should be noted, from Figure 7, that the H2TTPP presents the highest values of the covolume b: bFeCl3‐H2TTPP>bFeCl3‐H2TPP. In this case, the distance between the adsorbates is high which indicates that the iron ions can be easily adsorbed by the surface and causes an expansion in the adsorbed amount. This result confirms the suggestion of the porphyrin H2TTPP as a suitable material for the metalloporphyrin adsorption.Furthermore, it is seen from Figure7 that the parameter a declines with the expansion of the temperature for the FeCl3 adsorption while the covolume b increases. The decrease of the cohesion pressure indicates that the lateral interaction effect is low at high temperature. The increase of the parameter b reflects a strong distance between the adsorbates. The lateral interaction description explains the highest performance of the iron adsorption at 330 K and shows the endothermic nature of the studied process.


## 3.4.3. Energetic Calculation

According to Equations (16), (18), and (20), the molar energies of the six adsorption mechanisms can be calculated by means of the energetic coefficients, c1/2 for AlCl3, w1/2 for FeCl3, and c1 and c2 for InCl3, which are calculated from the numerical simulation of the three adopted models [36, 48].For aluminum chloride adsorption,(26)−ΔE1/2=RT×lnc1/2SAlCl3.For iron chloride adsorption,(27)−ΔE1/2=RT×lnw1/2SFeCl3.For indium chloride adsorption,(28)−ΔE1=RT×lnc1SInCl3,(29)−ΔE2=RT×lnc2SInCl3.The values of the molar energies and their variations via the temperature are reported in Table5 and Figure 8.Table 5
Values of the molar adsorption energies∣−ΔE1/2∣ for AlCl3 and FeCl3 adsorptions and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3 adsorption given in modulus values at 290, 300, 310, 320, and 330 K.
Adsorption systemAdsorption energy290 K300 K310 K320 K330 KSystem (a): AlCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)61.4 (±2.1)69.5 (±2.3)70.6 (±3.1)75.4 (±2.9)80.3 (±3.9)System (a′): AlCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)48.4 (±2.1)57.2 (±2.4)60.2 (±1.8)63.4 (±3.1)67.4 (±2.9)System (b): FeCl3-H2TTPP∣−ΔE1/2∣ (kJ/mol)32.9 (±1.8)36.4 (±1.7)38.7 (±2.5)40.8 (±2.2)43.1 (±2.3)System (b′): FeCl3-H2TPP∣−ΔE1/2∣ (kJ/mol)30.2 (±1.9)33.6 (±1.8)36.7 (±1.8)38.4 (±2.1)40.9 (±2.2)System (c): InCl3-H2TTPP∣−ΔE1∣ (kJ/mol)26.4 (±2.3)30.4 (±1.6)33.5 (±1.5)36.8 (±1.9)38.7 (±1.7)∣−ΔE2∣ (kJ/mol)17.2 (±1.2)20.4 (±1.5)23.3 (±1.4)25.7 (±1.1)28.8 (±1.6)System (c′): InCl3-H2TPP∣−ΔE1∣ (kJ/mol)22.3 (±2.7)26.1 (±2.4)29.4 (±2.1)33.1 (±1.9)35.6 (±1.8)∣−ΔE2∣ (kJ/mol)12.7 (±1.5)16.7 (±1.6)18.9 (±2.0)20.4 (±2.1)21.1 (±1.8)Figure 8
Evolutions of the molar adsorption energies of the six adsorption systems as a function of temperature:∣−ΔE1/2∣ for AlCl3 and FeCl3, and ∣−ΔE1∣ and ∣−ΔE2∣ for InCl3.According to Table5, for the indium chloride adsorption, it is obviously remarked that the calculated values of ∣−ΔE1∣ which characterizes the indium-porphyrin interaction are greater than those of ∣−ΔE2∣ (interaction between the two adsorbed layers [36]). Therefore, we can conclude that the interaction ∣−ΔE1∣ should be compared to the other adsorption system energies (∣−ΔE1/2∣) to evaluate the stability of the formed metalloporphyrin complexes.Comparing the∣−ΔE1/2∣ and the ∣−ΔE1∣ values of the six complexation systems (Table 5 and Figure 8), we observe that ∣−ΔE1/2∣ (AlCl3-H2TTPP) > ∣−ΔE1/2∣ (AlCl3-H2TPP) > ∣−ΔE1∣ (FeCl3-H2TTPP) > ∣−ΔE1∣ (FeCl3-H2TPP) > ∣−ΔE1/2∣ (InCl3-H2TTPP) > ∣−ΔE1/2∣ (InCl3-H2TPP). It can be noticed that the affinity of the porphyrin H2TTPP cavities to the metallic ions is higher than the tetraphenylporphyrin sites H2TPP for the three ions and confirms that the aluminum chloride-porphyrin (H2TTPP) is the best adsorption system suited for metalloporphyrin application because it presents the highest adsorption energies values for the five reaction temperatures.It can be also observed that the molar energies rise with the expansion of the temperature (Figure8). This can be interpreted by the endothermic behavior of the six studied processes.Furthermore, the energies∣−ΔE1/2∣ of AlCl3 are superior to 40 kJ/mol for all the temperatures. For this system, the adsorption is carried out via a chemical process involving covalent bonds (irreversible phenomenon) [39, 50–52]. In this case, the adsorption is a dissociative mechanism (there is no possibility of a desorption process) because there is a change in the electron density between the adsorbent and the adsorbed particle. Moreover, a multilayer adsorption is impossible due to the high interaction between the adsorbed layer and the adsorbent surface (e.g., AlCl3 adsorption).In contrast, all the other adsorption mechanisms (FeCl3 and InCl3) take place via a physical process since they present adsorption energy values lower than 40 kJ/mol [42, 50, 51]. During physisorption, the adsorbed particles retain their individual properties and there is no change in the structure of the components of the adsorption system. The system is weakly energetic (low adsorbate-adsorbent interaction), so the presence of a desorption phenomenon is possible (e.g., FeCl3 adsorption). Moreover, the adsorbed layer can interact with other adsorbed layers and can act as a new template for the adsorption of the following layer (multilayer adsorption mechanism) such as the case of InCl3 in the present investigation.


## 4. Conclusion

In this work, the experimental QCM tool is used to control the adsorption of aluminum chloride, iron chloride, and indium chloride on porphyrins H2TTPP and H2TPP. By analyzing the experimental isotherms, the AlCl3-H2TTPP complex was found as the best adsorption system in terms of reproducibility since it showed the highest adsorbed quantities. Theoretically, statistical physical models were suggested for the analysis of the experimental results. The numerical investigation confirmed that the chloride ions contributed only in the adsorption of InCl3 (double-layer adsorption). The iron chloride adsorption was described by a monolayer model that considers the lateral interactions of the ions indicating the lowest stability of the formed iron-porphyrin complex. The physicochemical parameters of these models gave interesting microscopic insights about the complexation of the two porphyrins. Indeed, it was found that the six adsorption processes were governed by a multi-interaction process (n is inferior to 1). The thermal agitation caused an increase of the number of occupied sites PM which indicated that some porphyrin sites contributed to the adsorption only at high temperature (endothermic process). The energetic study, through adsorption energy calculation, demonstrated that the adsorption of AlCl3 on H2TTPP is a chemisorption process involving covalent bonds.



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*Source: 5540517-2021-04-21.xml*