# Multiscale Mechanistic Study of the Adsorption of Methyl Orange on the External Surface of Layered Double Hydroxide

Brian Grégoire, Jean-Louis Bantignies, Rozenn Le Parc, Bénédicte Prelot, Jerzy Zajac, Géraldine Layrac, Didier Tichit, Gaëlle Martin-Gassin

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Brian Grégoire, Jean-Louis Bantignies, Rozenn Le Parc, Bénédicte Prelot, Jerzy Zajac, et al. Multiscale Mechanistic Study of the Adsorption of Methyl Orange on the External Surface of Layered Double Hydroxide. Journal of Physical Chemistry C, 2019, 123 (36), pp.22212-22220. 10.1021/acs.jpcc.9b04705 . hal-02302983

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# A Multiscale Mechanistic Study of the Adsorption of Methyl Orange on the External Surface of Layered Double Hydroxide

Brian Gregoire, Jean-Louis Bantignies, Rozenn Le Parc, Benedicte Prelot, Jerzy Zajac, Géraldine Layrac, Didier Tichit, and Gaelle Martin-Gassin

J. Phys. Chem. C, **Just Accepted Manuscript** • DOI: 10.1021/acs.jpcc.9b04705 • Publication Date (Web): 19 Aug 2019

Downloaded from pubs.acs.org on August 24, 2019

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# A Multiscale Mechanistic Study of the Adsorption of Methyl Orange on the External Surface of Layered Double Hydroxide

Brian Grégoire*<sup>a,c</sup>, Jean-Louis Bantignies<sup>b</sup>, Rozenn Le-Parc<sup>b</sup>, Benedicte Prélot<sup>c</sup>,

Jerzy Zajac<sup>c</sup>, Geraldine Layrac<sup>c,d</sup>, Didier Tichit<sup>c</sup>, Gaelle Martin-Gassin<sup>c</sup>.

<footnote><sup>a</sup> Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 CNRS-Université de Poitiers, 5 rue Albert Turpain, TSA 51106, 86073 Poitiers Cedex 9, France.</footnote>

<footnote><sup>b</sup> Laboratoire Charles Coulomb, UMR 5221 CNRS-Université de Montpellier, Montpellier, France</footnote>

<footnote><sup>c</sup> Institut Charles Gerhardt de Montpellier, ICGM - Univ Montpellier - CNRS - ENSCM, Place Eugene Bataillon, Montpellier F-34095 Cedex 5, France</footnote>
d ICPEES, Institut de Chimie et Procédé pour l’Energie l’Environnement et la Santé, CNRS

UMR 7515, ECPM-Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2,
France

* to whom correspondence shall be addressed : brian.gregoire@univ-poitiers.fr
Abstract

The molecular mechanism of Methyl Orange (MO) dye adsorption on the external surface of Mg₂Al-CO₃ layered double hydroxide (LDH) was investigated by combining adsorption isotherms, X-Ray diffraction, and infrared (IR) and Raman spectroscopies coupled to DFT calculations. The vibrational dynamic behavior upon adsorption and the molecular surface organisation were correlated with the adsorption isotherm revealing a four-domain uptake mechanism. The peculiar high adsorption capacity of LDH toward MO was explained by the surface-promoted dye aggregation over the external surface of particles.
## I. Introduction

Dyes are classically used industrially to impart colour to surfaces. They are required to be highly resistant to the action of detergents to ensure the durability of surface treatment. Textile, paper, leather tanning, plastic, cosmetic, and printing industries all use large amounts of dyes ¹⁻³. Due to their chemical resistance, their discharge into wastewater causes them to accumulate there and thus constitute a major pollution source⁴, ⁵. Although the exact quantity of dyes released in the environment is unknown, a rough estimation of their discharge into water streams is evaluated at 100 tons per year ⁶. Among the common methods used to remove dyes from wastewater, adsorption-based processes are probably the most efficient and they are commonly applied to remove hazardous organic pollutants present in the effluents ⁷. Commercial activated carbon benefits from its efficient adsorption capacity but the inherent difficulty to be regenerated and the large cost associated with its use have driven scientists to consider alternative materials⁸.

Due to their structural features, Layered Double Hydroxides (LDHs) have been extensively studied in the past decade as potential adsorbents for environmental
remediation processes ⁹, ¹⁰. The structure of these materials is based on stacking of Mg(OH)₂-like layers, with partial substitution of divalent cations to trivalent ones, thereby leading to a positive layer charge balanced by hydrated interlayer anions.

The structural formula of LDHs is $[M^{II}_{1-x}M^{III}_{x}(OH)_{2}]^{x+}(A^{-}_{x/n}, mH_{2}O)^{x-}$, where MII and MIII are divalent and trivalent cations, respectively, A is the interlayer anion of charge n, and x represents the molar fraction of trivalent cations. The positive charge within the layers is responsible for anionic exchange capacity ¹¹, ¹² generally comprised between 200 and 400 meq / 100 g depending on the cationic composition of the layer ¹³. Various anions can be accommodated within the interlayer domain, but monovalent anions are often preferred due to their easy replacement by almost any organic or inorganic anions ¹⁴.

Numerous papers have been devoted to dye adsorption onto LDHs ¹⁵⁻²³. Using easily exchangeable interlayer anions, such as nitrate or chloride, dyes bearing negative charges can be intercalated in the interlayer domains ²⁴. By analysing the quantity of dyes adsorbed relative to the equilibrium concentration of dyes in solution, the adsorption isotherm can be determined. Adsorption models are then used to link the macroscopic measurements with the microscopic phenomena occurring during
dye uptake. Though the demonstration of the great potential offered by such layered
exchangers for the uptake of Methyl Orange (MO) or inorganic anions has been
recently done, the understanding of the sorption mechanism is still required at the
molecular level <sup>25-27</sup>.

Recently, our group has investigated the adsorption of different dyes onto LDHs.
Interestingly, kinetic and thermodynamic measurements have revealed for the first
time that adsorption of MO units is exothermic and exceeds the anionic exchange
capacity, thereby suggesting that both intercalation and adsorption on the external
surface may take place <sup>28</sup>. Such behavior is uncommon with various dyes adsorbed
onto LDHs and further investigation is necessary to shed light on the precise
adsorption mechanism. The molecular mechanism of adsorption on the external
surface of LDH particles has been very scarcely investigated in the literature owing to
the inherent difficulty of separating the contributions due to interlayer anionic
exchange and that related to adsorption on the external surface of LDH layers. The
latter contribution can be probed by preventing the interlayer access when making
use of hardly exchangeable carbonate anions.
The study of the adsorption behaviour based only on a variety of adsorption models to derive the precise retention mechanism at the molecular level is shown to lead to potentially erroneous interpretation <sup>29</sup>. In this context, vibrational spectroscopies create a significant added value since they are highly sensitive to molecular changes and intermolecular interactions, and therefore they allow to gain access to a molecular description of the uptake mechanism.

In this paper, the adsorption of MO on the external surface of LDHs (Figure 1) is examined by referring to an innovative, multi-scaled approach based on the combination of the measurements of adsorption isotherms with X-Ray diffraction, as well as infrared (IR) and Raman spectroscopy studies. Molecular DFT calculations were also performed to precisely assign the vibrational modes of MO. The correlation between macroscopic measurements and the local scale investigations allowed deriving the microscopic uptake mechanism of MO by LDHs.

<figure><img src="image_5.png" /></figure>
S-Ring

<figure><img src="image_6.png" /></figure>

**Figure 1:** Schematic representation of the adsorption system studied: the uptake of anionic MO units (a) is limited only to the (001) external surface of Mg<sub>2</sub>Al-LDH (b) containing interlayer carbonate anions.

## II. Materials and methods

### II.1. Preparation of Mg<sub>2</sub>Al-CO<sub>3</sub> LDH

Mg-Al-CO<sub>3</sub> LDH was prepared by the low supersaturation coprecipitation method. A 0.8 M aqueous solution of Mg(NO<sub>3</sub>)<sub>2</sub>, 6H<sub>2</sub>O and Al(NO<sub>3</sub>)<sub>3</sub>, 9H<sub>2</sub>O with n(Mg)/n(Al) = 2 was slowly (0.3 mL per min) added into a solution containing 0.25 M of Na<sub>2</sub>CO<sub>3</sub>. The pH was maintained at a constant value of 11 by simultaneous addition of a 1M NaOH solution making use of an automatic titrator device (Titrino - Metrohm). After complete addition, the suspension was stirred for 2 additional hours, and then refluxed at 80 °C in an oil bath for 24 h. The slurry was recovered by centrifugation and thoroughly washed three times with deionised water. Finally, the product was dried at 80 °C overnight. The nature of the interlayer anion was controlled by IR spectroscopy which confirmed that only carbonate was present. The layer composition was obtained by
Energy Dispersive X-ray measurement that gave the expected value of n(Mg)/n(Al)

very close to 2.

## II.2. Adsorption isotherm

The uptake property of the as-synthesised Mg-Al-CO₃ LDH was determined by the classical solution depletion method. The MO dye of high purity (dye content > 98%) was purchased from Sigma-Aldrich and used without further purification. Ultrapure water (resistivity of 18.2 MΩ) was used to prepare all aqueous solutions. About 20 mg of LDH was added into each 30 mL Nalgene® tube containing 20 mL of a MO solution with a concentration varying between 0.05 to 10 mM (i.e., mmol L⁻¹). The pH of the solution was recorded before and after reaching of the adsorption equilibrium. The tubes were stirred for 20 h in a thermostatic oven set at 25 °C. The LDH phase was separated from the supernatant liquid by filtration on a polyvinylidene membrane with a pore size of 0.2 µm and then thoroughly washed with deionized water. The supernatant solution was subsequently analysed by UV-visible spectroscopy in the range of 300-800 nm using a V-670 UV-Vis spectrophotometer in order to determine the equilibrium concentration (Cₐₗ). The quantity of MO adsorbed (Qₐₗs) was calculated by means of the following equation:
$$Q_{ads} = \frac{V_0(C_i - C_{eq})}{m_s}$$

where Cᵢ is the initial MO concentration (in mM), V₀ corresponds to the initial volume of solution (in L) and m_s is the mass of the LDH sample (in g). The adsorption isotherms were performed twice to ensure good repeatability (the experimental deviation on the measured dye concentration was lower than 5% between two consecutive experiments).

## II.3. FTIR analysis

Fourier-transformed infrared (FTIR) spectroscopy experiments were performed on a Bruker Tensor 27 spectrometer equipped with a Deuterated Triglycerine Sulphate (DTGS), a Globar source and a potassium bromide (KBr) beamsplitter. LDH samples were finely ground and mixed with dried spectroscopic grade KBr at 0.5 wt% and pressed into pellets. Prior to IR experiments, the pellets were dried overnight at 80 °C to reduce the level of the adsorbed water. The spectra were obtained in transmission mode by averaging 128 scans at a resolution of 4 cm⁻¹. The MO solution (10 mM) spectra were obtained by means of attenuated total reflectance (ATR-FTIR) apparatus equipped with a diamond crystal.

## II.4. FT-Raman analysis
Raman spectra were measured using a Bruker RFS100 FT Raman spectrometer equipped with a nitrogen cooled germanium detector. The excitation source was a continuous Nd:YAG laser emitting at 1064 nm. The power was set at 100 mW and no degradation of the dye was observed during the experiments at such a power. The resolution was about 4 cm⁻¹. The MO solution (10 mM) spectra were recorded by measuring the 180° reflectance.

## II.5. X-ray diffraction

X-ray diffraction patterns of the pristine and reacted Mg-Al-CO₃ LDH were recorded with a X'Pert diffractometer over the 2θ range from 2° to 30° with a Cu Kα radiation (λ = 1.5418 Å) and nickel filter with a scan rate 0.003 deg min⁻¹ at 30 mA, 45 kV, using a 10 mm incident beam mask.

## II.6. Peak fitting procedure

Spectroscopic manipulations, such as baseline subtraction and peak fittings, were performed using the Peak Fitting Module of Origin 7.0 which enabled the function type to be selected and allowed some parameters to be fixed or varied accordingly. Band fittings were performed with pseudo-Voigt functions (η = 0.8) with the minimum number of component bands used for the fitting process. Fittings were obtained with
squared correlation greater than 0.995. Example of band fittings can be found in
Figure S2 in Supporting Information.

II.7. DFT calculations

Since MO contains 35 atoms, it possesses 99 fundamental vibrations. Full
geometry optimisation was performed with the widely used DFT methods
corresponding to the Becke's three parameter exchange functional in combination
with the Lee-Yang-Parr's correlation functional (B3LYP)30-32 using the Gaussian 09
suite of programs33. Calculations were carried out with the all-electrons 6-31G+d
basis sets. The stability of the optimised geometry was confirmed by wavenumber
calculations, which resulted in the absence of imaginary frequency. Infrared and
Raman intensities were calculated in the harmonic approximation, ignoring second
and higher order dipole moment and polarizability derivatives.

The optimised bond lengths and bond angles obtained from DFT calculation are
presented in Table S1 in Supporting Information, along with the experimental values
extracted from the XRD refined structure 34. A very good agreement between these
values can be noticed giving full confidence on the quality of the calculated structure.
A Lorentzian broadening of 4 cm⁻¹ was applied in order to plot the calculated spectrum.
### III. Results and discussion

#### III.1. Spectroscopic studies of solubilised and adsorbed MO units

<figure><img src="image_7.png" /></figure>

**Figure 2:** Experimental and calculated IR (a) and Raman (b) spectra of MO in aqueous solution and adsorbed on LDH surface. The reference sample (in pink), i.e. $Mg_2Al-CO_3$, is also displayed for comparison

The experimental and calculated IR and Raman spectra of MO are presented in Figure 2a and 2b. The mode assignment has been performed based on the comparison with the DFT calculations (see Table 1). Interestingly, the intramolecular dynamic represents a local probe of the interactions between MO and the surface. Sensitivity of head groups of MO are detected through C-N(CH₃)₂ and S=O groups. The C-N(CH₃)₂ IR stretching modes (ν(C-N(CH₃)₂)) are probed at 1519 and 1368 cm⁻¹ coupled with other vibrations (Table 1); the IR antisymmetric and symmetric
stretching modes of N-(CH₃)₂ are revealed at 1255 (νₐs(N-(CH₃)₂) and 1004 cm⁻¹ (νₛ(N-(CH₃)₂). The S=O and -SO₃⁻ groups show bands at 1187 (ν(S=O) IR active) and 1034 cm⁻¹ (ν(SO₃⁻) IR and Raman activity), respectively. The core of the molecules is explored via the stretching (ν(C=C)) and bending (δop(CH)) modes of the sulfonate benzene moiety (ring S) and aminodimethyl benzene group (ring N), respectively, at 848 / 1589 cm⁻¹ and at 818 / 1607 cm⁻¹. In addition, the ν(N=N) mode is observed at 1445 cm⁻¹ coupled to other vibrations in agreement with the literature³⁵, ³⁶. One can notice that the experimental bands at 1662 and 1683 cm⁻¹ in the IR spectra have not been reported in the calculated frequencies. Since these two bands are not observed in the case of salt, we postulate that they arise from specific interactions and will be further discussed in detail.

A comparison between the IR and Raman spectra of MO adsorbed onto Mg₂Al-CO₃ LDH and the pristine MO-free LDH allows the identification of the bands assigned to the pristine LDH and the adsorbed MO molecules (Figure 2a and 2b.). Mg₂Al-CO₃ LDH exhibits very broad IR bands: below 1000 cm⁻¹libration modes of the hydroxyls of the LDH sheets modes are observed, while antisymmetric stretching modes of carbonate are detected around 1360 cm⁻¹ ³⁷, ³⁸. The remaining bands are
attributed to the adsorbed MO molecules. Owing to the absence of any Raman bands of Mg₂Al-CO₃ LDH between 700 and 1800 cm⁻¹, this spectral range originates only from the adsorbed MO units. When comparing the vibrational signatures of the free MO units in aqueous solution and the adsorbed ones, slight frequency shifts are noticed. This indicates that the MO interaction with the surface affects the symmetry and/or the environment of the dye units (Table S2 in Supporting Information).

<table><caption>**Table 1:** Experimental and calculated vibrational frequencies of MO along with the assignment of their vibrational modes. The following notation has been used to identify the two non-equivalent benzene rings: S ring refers to the benzene ring bearing the sulfonate group and the N ring corresponds to the benzene ring bearing the aminodimethyl group (Figure 1)</caption><thead><tr><th>IR</th><th>Raman</th><th>Calculated<br>frequencies<br>(cm<sup>-1</sup>)</th><th>Assignments</th></tr></thead><tbody><tr><td>818</td><td></td><td>835</td><td>δ<sub>op</sub>(CH) ring N</td></tr><tr><td>848</td><td></td><td>862</td><td>δ<sub>op</sub>(CH) ring S</td></tr><tr><td>1004</td><td></td><td>966</td><td>ν<sub>s</sub>(N-(CH<sub>3</sub>)<sub>2</sub>)</td></tr></tbody></table>
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60

<table><tr><td>1034</td><td>1034</td><td>974</td><td>v(SO<sub>3</sub>)</td></tr><tr><td>1068</td><td></td><td>1086</td><td>δ(CH<sub>3</sub>)</td></tr><tr><td>1098</td><td>1098</td><td>1109</td><td>v(C-SO<sub>3</sub>), δ(CH) ring S</td></tr><tr><td>1118</td><td>1119</td><td>1122</td><td>δ<sub>ip</sub>(CH) ring S</td></tr><tr><td></td><td>1142</td><td>1148</td><td>δ<sub>ip</sub>(CH) ring N</td></tr><tr><td>1152</td><td>1152</td><td>1153</td><td>δ(CH<sub>3</sub>)</td></tr><tr><td>1167</td><td></td><td>1169</td><td>v(SO<sub>2</sub>), δ<sub>ip</sub>(CH) ring N, δ<sub>ip</sub>(CH) ring S</td></tr><tr><td>1187</td><td></td><td>1182</td><td>v(S=O) + δ<sub>ip</sub>(CH) ring S</td></tr><tr><td>1198</td><td>1200</td><td>1186</td><td></td></tr><tr><td>1232</td><td>1234</td><td>1231</td><td>δ<sub>ip</sub>(CH) ring N, δ<sub>ip</sub>(CH) ring S</td></tr><tr><td>1255</td><td></td><td>1269</td><td>v<sub>as</sub>(N-(CH<sub>3</sub>)<sub>2</sub>) + δ<sub>op</sub>(CH) ring N</td></tr><tr><td>1270</td><td></td><td>1281</td><td>v(C-N<sub>2</sub>), v(C=C) ring N et S</td></tr><tr><td>1315</td><td>1317</td><td>1323</td><td>δ<sub>ip</sub>(CH) ring S</td></tr><tr><td></td><td>1340</td><td>1347</td><td>δ<sub>ip</sub>(CH) ring N</td></tr><tr><td>1360</td><td>1367</td><td>1367</td><td>v(C=C) ring N, v(C=C) ring S</td></tr><tr><td>1368</td><td></td><td>1376</td><td>v(C-N(CH<sub>3</sub>)<sub>2</sub>) + δ<sub>ip</sub>(CH) ring N</td></tr><tr><td>1389</td><td>1393</td><td>1435</td><td>δ<sub>ip</sub>(CH) ring S, v(N=N)</td></tr><tr><td></td><td>1410</td><td>1458</td><td>δ(CH<sub>3</sub>), δ<sub>ip</sub>(CH) ring N</td></tr><tr><td>1415</td><td></td><td>1473</td><td>δ(CH<sub>3</sub>), v(C=C) ring N</td></tr><tr><td>1421</td><td>1421</td><td>1499</td><td>v(N=N), δ(CH<sub>3</sub>), v(C=C) ring N</td></tr><tr><td>1445</td><td>1445</td><td>1528</td><td>v(N=N), v(C=C) ring S, v(C=C) ring N, δ(CH<sub>3</sub>)</td></tr><tr><td>1519</td><td></td><td>1558</td><td>v(C-N(CH<sub>3</sub>)<sub>2</sub>) + v(C=C) ring N</td></tr><tr><td>1566</td><td></td><td>1604</td><td>v(C=C) ring N + v(C=C) ring S</td></tr></table>
<table><tr><td>1589</td><td>1592</td><td>1637</td><td>v(C=C) ring S</td></tr><tr><td>1607</td><td>1606</td><td>1658</td><td>v(C=C) ring N</td></tr></table>

Note : v : stretching; δ : bending; op : out of plane; ip : in plane.

### III.2 Dye adsorption onto Mg<sub>2</sub>Al-CO<sub>3</sub> LDH

<figure><img src="image_8.png" /><figcaption>**Figure 3:** The log-log plot of adsorption isotherm for MO uptake by Mg<sub>2</sub>Al-CO<sub>3</sub> sample (panel a) and XRD patterns for selected points (pointed by four arrows) compared to those obtained on the reference sample Mg<sub>2</sub>Al-CO<sub>3</sub> LDH (panel b).</figcaption></figure>

The isotherm of MO adsorption onto Mg<sub>2</sub>Al-CO<sub>3</sub> is plotted on a log-log scale (Figure 3a) due to some similarity with the adsorption of ionic surfactants, conventionally represented in this manner<sup>39-43</sup>. It is worth noting here that Böhmer and Koopal have theoretically predicted a model four-region isotherm for anionic surfactant adsorption on solid surfaces characterised by a constant surface potential
(e.g., metal oxides at a fixed pH). Since the MO units contain a polar sulfonate headgroup and a hydrophobic moiety, they can to a certain extent be considered as having an amphiphilic structure. In the case of anionic surfactants adsorbed on a surface of opposite charge, the four-region pattern is classically attributed to the initial individual adsorption of monomer, formation of primary aggregates due to strong lateral attraction between the surfactant tails, and growth of these aggregates into some extended self-assembly structures<sup>41-43</sup>. For the adsorption of MO dye on LDH surface, a four-region isotherm is also observed. However, the profile of the isotherm is markedly different from that obtained classically with surfactant adsorption, suggesting different adsorption mechanisms. It is noteworthy that unlike ionic surfactants, and due to their disc-shaped form, the MO units aggregate by pilling up on the top of each other instead of forming spherical or ellipsoidal micelles like surfactants do<sup>44, 45</sup>. While the growth limitation of micelle originates from the balance between the hydrophobic chain and size of the polar head, dye aggregation occurs from successive assembly of monomers to oligomers. As a consequence, there is no evidence that a critical micelle concentration may exist in the case of dye molecules. A detailed description of the isotherm profile indicates that domain I is characterised
by a slope value of 1.8 which is greater than the value of 1 obtained usually with
surfactant adsorption adsorbing randomly on the surface of mineral oxides at a fixed
pH. Such a value may indicate some cooperative adsorption of the monomer at the
solid-solution interface. The increase of the adsorption in this range is interpreted as
due to the electrostatic interaction between the negatively charged sulfonate group
and the positive surface charge, hydrogen bonding between the surface hydrogen
groups and the proton-acceptor sulfonate head group of MO monomers, as well as
lateral interactions between the benzene rings. The slope is much lower in the
second domain with a value of 0.25, thereby suggesting a decrease in the attractive
forces between the dye units and the surface. In domain III, the slope is twice higher
than in domain II, with a value of 0.5. Then the plateau is obtained in domain IV,
where the dye adsorption levels off. At the maximum equilibrium concentration, the
MO uptake reaches a value of 1.6 mmol g⁻¹ (c.f., Figure S1 in Supporting
Information).

In our samples, non-exchangeable carbonates are present in the interlayer space
thus preventing potential MO intercalation⁴⁶. Samples taken in the different
aforementioned domains were analysed by XRD together with the reference Mg₂Al-
CO₃ LDH one (Figure 3b). The 2θ range was limited to 2-30° in which only the basal (00l) reflections are observed. Diffraction peaks located at 11.8° and 23.4° assigned to the (003) and (006) reflections of Mg₂Al-CO₃ LDH phases appear insensitive to the adsorption process. The calculated c parameter is 22.6 Å corresponding well to the reported value for the pristine Mg₂Al-CO₃ phases which points out that the adsorption of MO units is restricted to the external surface as expected. The permanent positive charge of the brucite-like layers due to the presence of trivalent cations is presumably balanced by the interlayer carbonate anions, external surface charges and hydrogen bonding are therefore responsible for the MO uptake on the surface. The point of zero charge for LDH is around 10.2 <sup>47-49</sup>. As our MO sorption measurements have been performed at a pH between 9 and 9.5, the surface bears a global positive charge originated from the partial protonation of some OH groups preferentially located on the external surface (basal face and edges to a lower extent) <sup>48-50</sup>. Interestingly, significant modifications of the XRD data in domain III cannot be attributed to the LDH phases. The new XRD patterns neither correspond to the pure MO salt ruling out the hypothesis of surface precipitation of MO (c.f., Figure S3 in
Supporting Information). The origin of this behaviour in domain III will be discussed in more detail in the following section.

### IIII.3. Adsorption mechanism probed through vibrational spectroscopies

To achieve a molecular description of the dye organisation at the interface, we used IR and Raman active modes as local probes to shed light on interactions between MO units and LDH surface. We compare below the behaviour of the vibrational dynamic (Figures 4 and 5) and that of isotherm adsorption to get complementary insight about the MO uptake process. Despite that edge sites are among the most reactive sites, they represent only a small fraction of all adsorption sites of LDH layers <sup>51</sup>. Thus, due to the very low aspect ratio of LDH particles (particle thickness to length ratio), only adsorption on (001) surface sites is believed to mainly contribute to the vibrational signature of adsorbed MO molecules so that the potential contribution of edges sites will be ignored.

In domain I, the v(SO<sub>3</sub>) vibrations at 1034 cm<sup>-1</sup> (specific of the MO extremity bearing negatively charged sulfonate groups) are impacted by the adsorption where slight downshifts are observed (Figures 5a). As expected, the aminodimethyl group
(specific of the other extremity of MO $v(N-(CH_3)_2$ group) does not exhibit any sensitivity of $v_s(N-(CH_3)_2$) at 1004 cm⁻¹ in terms of frequencies or band profiles (Figures 5b).

Therefore, it can be concluded that domain I in the isotherm profile corresponds to the adsorption of the first molecule of MO on the surface with the negatively charged sulfonate groups interacting with the protonated surface hydroxyl groups presumably by electrostatic interaction. The cooperative adsorption revealed by the adsorption isotherm previously discussed in III.2 suggests the presence of aggregated structures on the hydroxyl surface, stabilised by lateral interactions. From the spectroscopic analysis of samples in this domain, this cooperative adsorption is not clearly observed, owing to the very low intensity of the MO vibrational bands resulting from the very low uptake in this domain (figure 4). Looking more precisely in the three points of domain I, one can notice a very slight upshift of the $δ_{ip}(CH)$ of ring S from 1018.5 to 1019.5 cm⁻¹ (Figure 5c). These vibrational changes may account for a change in the orientation of the adsorbed dye molecules. This suggests that at very low coverage, namely in the beginning of domain I, the molecules are tilted with respect to the normal to the surface. As the surface coverage increases, the molecules tend to straighten up which results in changes in the frequency of the
sulfonate moieties and the ring S is no more interacting with the surface (Figure 6a).

Since the pH of adsorption is not that far from the PZC value, the end of domain I may correspond to a compensation of all positive surface charges but may not indicate a complete surface coverage. As revealed by the adsorption isotherm, a different mechanism seems to take place in domain II, with a much lower slope. The most obvious changes from Figure 5 concern the lower part of the molecule: an additional higher frequency v(SO₃) band appears, thus suggesting the presence of two distinct dye molecules over the surface. Both v(SO₃) vibrational bands increase in frequency as the dye uptake increases. Besides, the δop(CH) of ring S is also affected with a slight frequency increase from 847.6 to 849 cm⁻¹. As a result, due to the aggregation process, the S rings of two neighbouring molecules may interact with each other, presumably by aromatic-aromatic (π-stacking) interaction (Figure 6b). As a consequence, the surface acquires an enhanced structural order and may explain the presence of two distinct molecules vibrations over the surface, as indicated by the two v(SO₃) bands. However, the upper part, including the amino-dimethyl group and the ring N, does not present any frequency evolution and may be in an environment very similar to that of domain I. This may be explained by the steric hindrance
induced by the voluminous amino-dimethyl group, preventing pi-interactions between two N rings. This implies that the upper part of molecules are not facing to each other while the lower part must be in a closed contact to enable π-π interaction of S rings. This spatial organisation leads to an increase of the surface coverage with a maximum molecule density reached at the end of domain II

In order to gain more accessibility and to increase the stability of the adsorbed molecular layer, a structural reorganisation may account for the slope increase of the adsorption isotherm in domain III (Figure 3a). As far as the vibrational spectra are concerned, drastic frequency changes in almost all functional groups under investigations are observed, reflecting an important structural reorganisation. As stated before, the LDH external surface is saturated with no more accessibility for incoming molecules in complete agreement with the constant frequency of δ(CH) ring S. However, the tilted orientation adopted by successive molecules adsorbed on the layer at the end of domain II generates a void that can be regarded as additional adsorption space. The vibrational frequency of functional groups in the upper part of the molecules undergoes downshift that may result from interactions between two N rings of neighbouring molecules. Therefore, based on these observations, a flip-flop
orientation is proposed, with the upper part of the newly adsorbed molecules fitting
the available void and interacting with the N ring of already adsorbed units (Figure
6c). Therefore, the sulfonate group pointing toward the solution may be compensated
by sodium ions as observed by Darmograi *et al.*²⁸. In consequence, its environment
is almost similar to the sulfonate group interacting with the surface which may explain
its constant frequency ²⁸. Note that the δ<sub>ip</sub> (CH) ring S is also affected presumably
due to the proximity of the ring S of one molecule with the amino-dimethyl group of a
second molecule and vice versa. The proposed flip/flop orientation is consistent with
the sur-structure observed on the XRD pattern in domain III. Indeed, assuming that
the rational peaks located at 4° 2θ giving a distance of 19.6 Å corresponds to the
thickness of the molecular layer, one can note that this distance is well above the
dimension of one layer of molecules standing upright at the surface, which would give
a thickness of 15 Å, as well as below the formation of a second layer on the top of
the first one. This again points toward the formation of a flip/flop structure on the
surface. This particular arrangement may be responsible for the additional vibrational
band at 1663 and 1687 cm⁻¹ (figure 4b) that is also observed in ATR spectra of
solution whose concentration is above the oligomerization concentration (i.e 10 mM).
Compared to solubilised oligomer, the surface adsorbed oligomer is upshifted of 11 and 7 cm⁻¹, respectively, possibly reflecting a denser aggregation.

Once the available space is filled with molecules at the end of domain III, no more changes are detected on the vibrational spectra and no more molecules uptake is recorded in domain IV.

To investigate the thermodynamic of this system in full, DFT calculation will have to be brought into this methodology to derive the energetic of adsorption <sup>52</sup>. The calculated optimum geometry could be further validated by comparing experimental and calculated vibrational spectrum. This would be an interesting topic for future work but is out of the scope of the present study.
<figure><img src="image_9.png" /><figcaption>**Figure 4:** Changes in the vibrational spectra for selected modes as a function of the MO uptake. Line colours refer to the different domains of the adsorption isotherm as specified in Fig. 3a. The related quantities of adsorption are as follows: 0.05, 0.15,</figcaption></figure>
0.31 mmol g⁻¹ (black lines); 0.41, 0.48, 0.54, 0.59 mmol g⁻¹ (blue lines); 0.80, 1.39 mmol g⁻¹ (red lines); 1.65 mmol g⁻¹ (green lines)

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>
**Figure 5:** Frequency evolution for selected vibrational modes. The standard deviation on the band's positions has been calculated during the fitting process. Numbers refer to the amount of MO adsorbed onto LDH (mmol g⁻¹)

I
a) Domain I

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

b) Domain II

<figure><img src="image_18.png" /><figcaption>Side View</figcaption></figure>

<figure><img src="image_19.png" /></figure>

c) Domain III

<figure><img src="image_20.png" /><figcaption>Side View</figcaption></figure>

<figure><img src="image_21.png" /></figure>
<caption>**Figure 6:** Scheme of the proposed mechanism of MO adsorption on the external surface of Mg<sub>2</sub>Al-CO<sub>3</sub> LDH</caption>

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## Conclusion

Using carbonate anions as interlayer counter-ions to hinder the access to the interlayer space of LDHs, it was possible to elucidate the mechanism of Methyl Orange adsorption on the external solid surface from aqueous solution at the molecular level. The methodology applied in the present study included a combination of such experimental techniques as the measurements of adsorption isotherms, vibrational (IR and Raman) spectroscopies, and X-Ray diffraction. The adsorption isotherm presented a four-region pattern well correlated with the changes in the vibrational spectra of the adsorbed dye upon various adsorption stages. For the first time, the formation of aggregates on the external LDH surface, has been evidenced by X-Ray Diffraction and vibrational spectroscopies. This particular adsorption pathway was postulated to be responsible for the oligomerisation of dye units at a concentration lower than that in solution as well as for the unexpectedly high MO uptake on the external surface of the LDH sample. The adequate mechanism of the dye uptake by LDH was proposed

## Supporting information
Bond length and bond angle of the MO optimized geometry, IR and Raman band frequencies of aqueous and adsorbed MO, adsorption isotherm of MO uptake on Mg₂Al LDH surface, vibrational band fittings, XRD pattern of MO@LDH and MO salt.

## Acknowledgements

BG acknowledges the Labex ChemiSyst for supporting his post-doctoral position.

GG and BG are grateful to LabEx Chemistry of Molecular and Interfacial Systems (LabEx CheMISyst) (ANR-10-LABX-05-01) for funding. The authors thank the IRAMAN platform of the University of Montpellier for providing access to IR and Raman facilities.
References

(1) Robinson, T.; McMullan, G.; Marchant, R.; Nigam, P., Remediation of Dyes in Textile Effluent: A Critical Review on Current Treatment Technologies with a Proposed Alternative. *Bioresour. Technol.* **2001**, *77* (3), 247-255.

(2) Srinivasan, S. V.; Rema, T.; Chitra, K.; Sri Balakameswari, K.; Suthanthararajan, R.; Uma Maheswari, B.; Ravindranath, E.; Rajamani, S., Decolourisation of Leather Dye by Ozonation. *Desalination* **2009**, *235* (1), 88-92.

(3) Yagub, M. T.; Sen, T. K.; Afroze, S.; Ang, H. M., Dye and Its Removal from Aqueous Solution by Adsorption: A Review. *Adv. Colloid Interface Sci.* **2014**, *209*, 172-184.

(4) Moran, C.; Hall, M. E.; Howell, R., Effects of Sewage Treatment on Textile Effluent. *J. Soc. Dyers Colour.* **1997**, *113* (10), 272-274.

(5) Nigam, P.; Armour, G.; Banat, I. M.; Singh, D.; Marchant, R., Physical Removal of Textile Dyes from Effluents and Solid-State Fermentation of Dye-Adsorbed Agricultural Residues. *Bioresour. Technol.* **2000**, *72* (3), 219-226.

(6) Yagub, M. T.; Sen, T. K.; Ang, H. M., Equilibrium, Kinetics, and Thermodynamics of Methylene Blue Adsorption by Pine Tree Leaves. *Water, Air, Soil Pollut.* **2012**, *223* (8), 5267-5282.

(7) Rafatullah, M.; Sulaiman, O.; Hashim, R.; Ahmad, A., Adsorption of Methylene Blue on Low-Cost Adsorbents: A Review. *J. Hazard. Mater.* **2010**, *177* (1-3), 70-80.

(8) Babel, S.; Kurniawan, T. A., Low-Cost Adsorbents for Heavy Metals Uptake from Contaminated Water: A Review. *J. Hazard. Mater.* **2003**, *97* (1), 219-243.

(9) Goh, K.-H.; Lim, T.-T.; Dong, Z., Application of Layered Double Hydroxides for Removal of Oxyanions: A Review. *Water Res.* **2008**, *42* (6-7), 1343-1368.

(10) Chuang, Y. H.; Tzou, Y. M.; Wang, M. K.; Liu, C. H.; Chiang, P. N., Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (Ldh) and Modified Ldh. *Ind. Eng. Chem. Res.* **2008**, *47* (11), 3813-3819.

(11) Newman, S. P.; Jones, W., Comparative Study of Some Layered Hydroxide Salts Containing Exchangeable Interlayer Anions. *J. Solid State Chem.* **1999**, *148* (1), 26-40.

(12) Châtelet, L.; Bottero, J. Y.; Yvon, J.; Bouchelaghem, A., Competition between Monovalent and Divalent Anions for Calcined and Uncalcined Hydrotalcite: Anion Exchange and Adsorption Sites. *Colloids Surf.*, **A** **1996**, *111* (3), 167-175.

(13) Duan, X.; Evans, D. G., Layered Double Hydroxides. Springer: 2006.

(14) Meyn, M.; Beneke, K.; Lagaly, G., Anion-Exchange Reactions of Layered Double Hydroxides. *Inorg. Chem.* **1990**, *29* (26), 5201-5207.

(15) Mustapha Bouhent, M.; Derriche, Z.; Denoyel, R.; Prevot, V.; Forano, C., Thermodynamical and Structural Insights of Orange Ii Adsorption by Mgralno3 Layered Double Hydroxides. *J. Solid State Chem.* **2011**, *184* (5), 1016-1024.

(16) Benselka-Hadj Abdelkader, N.; Bentouami, A.; Derriche, Z.; Bettahar, N.; de Ménorval, L. C., Synthesis and Characterization of Mg-Fe Layered Double Hydroxides and Its Application on Adsorption of Orange G from Aqueous Solution. *Chem. Eng. J.* **2011**, *169* (1-3), 231-238.

(17) Drici Setti, N.; Jouini, N.; Derriche, Z., Sorption Study of an Anionic Dye – Benzopurpurine 4b – on Calcined and Uncalcined Mg-Al Layered Double Hydroxides. *J. Phys. Chem. Solids* **2010**, *71* (4), 556-559.

(18) Bouraada, M.; Belhalfaoui, F.; Ouali, M. S.; de Ménorval, L.-C., Sorption Study of an Acid Dye from an Aqueous Solution on Modified Mg-Al Layered Double Hydroxides. *J. Hazard. Mater.* **2009**, *163* (1), 463-467.

(19) El Gaini, L.; Lakraimi, M.; Sebbar, E.; Meghea, A.; Bakasse, M., Removal of Indigo Carmine Dye from Water to Mg-Al-Co3-Calcined Layered Double Hydroxides. *J. Hazard. Mater.* **2009**, *161* (2-3), 627-632.

(20) Laguna, H.; Loera, S.; Ibarra, I. A.; Lima, E.; Vera, M. A.; Lara, V., Azoic Dyes Hosted on Hydrotalcite-Like Compounds: Non-Toxic Hybrid Pigments. *Microporous Mesoporous Mater.* **2007**, *98* (1-3), 234-241.
(21) Zhu, M.-X.; Li, Y.-P.; Xie, M.; Xin, H.-Z., Sorption of an Anionic Dye by Uncalcined and Calcined Layered Double Hydroxides: A Case Study. *J. Hazard. Mater.* **2005**, *120* (1-3), 163-171.

(22) bin Hussein, M. Z.; Zainal, Z.; Yahaya, A. H.; Aziz, A. b. A., Synthesis of Layered Organic–Inorganic Nanohybrid Material: An Organic Dye, Naphthol Blue Black in Magnesium–Aluminum Layered Double Hydroxide Inorganic Lamella. *Mater. Sci. Eng., B* **2002**, *88* (1), 98-102.

(23) Gassin, P.-M.; Prelot, B.; Grégoire, B.; Martin-Gassin, G., Second-Harmonic Scattering in Layered Double Hydroxide Colloids: A Microscopic View of Adsorption and Intercalation. *Langmuir* **2018**, *34* (40), 12206-12213.

(24) Mandal, S.; Tichit, D.; Lerner, D. A.; Marcotte, N., Azoic Dye Hosted in Layered Double Hydroxide: Physicochemical Characterization of the Intercalated Materials. *Langmuir* **2009**, *25* (18), 10980-10986.

(25) Lu, Y.; Jiang, B.; Fang, L.; Ling, F.; Gao, J.; Wu, F.; Zhang, X., High Performance Nife Layered Double Hydroxide for Methyl Orange Dye and Cr(Vi) Adsorption. *Chemosphere* **2016**, *152*, 415-422.

(26) Ling, F.; Fang, L.; Lu, Y.; Gao, J.; Wu, F.; Zhou, M.; Hu, B., A Novel Cofe Layered Double Hydroxides Adsorbent: High Adsorption Amount for Methyl Orange Dye and Fast Removal of Cr(Vi). *Microporous Mesoporous Mater.* **2016**, *234*, 230-238.

(27) Darmograi, G.; Prelot, B.; Geneste, A.; Martin-Gassin, G.; Salles, F.; Zajac, J., How Does Competition between Anionic Pollutants Affect Adsorption onto Mg–Al Layered Double Hydroxide? Three Competition Schemes. *J. Phys. Chem. C* **2016**, *120* (19), 10410-10418.

(28) Darmograi, G.; Prelot, B.; Layrac, G.; Tichit, D.; Martin-Gassin, G.; Salles, F.; Zajac, J., Study of Adsorption and Intercalation of Orange-Type Dyes into Mg-Al Layered Double Hydroxide. *J. Phys. Chem. C* **2015**, *119* (41), 23388-23397.

(29) Limousin, G.; Gaudet, J. P.; Charlet, L.; Szenknect, S.; Barthès, V.; Krimissa, M., Sorption Isotherms: A Review on Physical Bases, Modeling and Measurement. *Appl. Geochem.* **2007**, *22* (2), 249-275.

(30) Becke, A. D., Density-Functional Thermochemistry. lii. The Role of Exact Exchange. *J. Chem. Phys.* **1993**, *98* (7), 5648-5652.

(31) Lee, C.; Yang, W.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. **1988**, *37* (2), 785-789.

(32) Zvereva, E. E.; Shagidullin, A. R.; Katsyuba, S. A., Ab Initio and Dft Predictions of Infrared Intensities and Raman Activities. *J. Phys. Chem. A* **2011**, *115* (1), 63-69.

(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al., Gaussian 09, Revision B.01. Wallingford CT, 2009.

(34) Hanson, A., The Crystal Structure of Methyl Orange Monohydrate Monoethanolate. *Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater* **1973**, *29* (3), 454-460.

(35) Almeida, M. R.; Stephani, R.; Dos Santos, H. F.; Oliveira, L. F. C. d., Spectroscopic and Theoretical Study of the “Azo”-Dye E124 in Condensate Phase: Evidence of a Dominant Hydrazo Form. *J. Phys. Chem. A* **2010**, *114* (1), 526-534.

(36) Biswas, N.; Umapathy, S., Structures, Vibrational Frequencies, and Normal Modes of Substituted Azo Dyes: Infrared, Raman, and Density Functional Calculations. *J. Phys. Chem. A* **2000**, *104* (12), 2734-2745.

(37) Palmer, S. J.; Frost, R. L.; Spratt, H. J., Synthesis and Raman Spectroscopic Study of Mg/Al,Fe Hydrotalcites with Variable Cationic Ratios. *J. Raman Spectrosc.* **2009**, *40* (9), 1138-1143.

(38) Kagunya, W.; Baddour-Hadjean, R.; Kooli, F.; Jones, W., Vibrational Modes in Layered Double Hydroxides and Their Calcined Derivatives. *Chem. Phys.* **1998**, *236* (1-3), 225-234.

(39) Somasundaran, P.; Fuerstenau, D. W., Mechanisms of Alkyl Sulfonate Adsorption at the Alumina-Water Interface1. *J. Phys. Chem.* **1966**, *70* (1), 90-96.

(40) Paria, S.; Khilar, K. C., A Review on Experimental Studies of Surfactant Adsorption at the Hydrophilic Solid-Water Interface. *Adv. Colloid Interface Sci.* **2004**, *110* (3), 75-95.

(41) Bohmer, M. R.; Koopal, L. K., Adsorption of Ionic Surfactants on Constant Charge Surfaces. Analysis Based on a Self-Consistent Field Lattice Model. *Langmuir* **1992**, *8* (6), 1594-1602.
(42) Bohmer, M. R.; Koopal, L. K., Adsorption of Ionic Surfactants on Variable-Charge Surfaces. 1. Charge Effects and Structure of the Adsorbed Layer. *Langmuir* **1992**, *8* (11), 2649-2659.

(43) Bohmer, M. R.; Koopal, L. K., Adsorption of Ionic Surfactants on Variable-Charge Surfaces. 2. Molecular Architecture and Structure of the Adsorbed Layer. *Langmuir* **1992**, *8* (11), 2660-2665.

(44) Kendrick, K. L.; Gilkerson, W. R., The State of Aggregation of Methyl Orange in Water. *J. Solution Chem.* **1987**, *16* (4), 257-267.

(45) de Oliveira, H. P.; Oliveira, E. G. L.; de Melo, C. P., Aggregation of Methyl Orange Probed by Electrical Impedance Spectroscopy. *J. Colloid Interface Sci.* **2006**, *303* (2), 444-449.

(46) Gregoire, B.; Andre, E.; Ruby, C.; Carteret, C., Tuning and Investigating the Structure of $M^{II}-Fe^{III}$ Layered Double Hydroxides ($M^{II}= Ni^{II}$, $Co^{II}$ and $Mg^{II}$) in Relation to Their Composition: From Synthesis to Anionic Exchange Properties. *Curr. Inorg. Chem.* **2015**, *5* (3), 169-183.

(47) Rojas Delgado, R.; De Pauli, C. P.; Carrasco, C. B.; Avena, M. J., Influence of $M^{II}/M^{III}$ Ratio in Surface-Charging Behavior of Zn–Al Layered Double Hydroxides. *Appl. Clay Sci.* **2008**, *40* (1-4), 27-37.

(48) Rojas, R.; Barriga, C.; De Pauli, C. P.; Avena, M. J., Influence of Carbonate Intercalation in the Surface-Charging Behavior of Zn-Cr Layered Double Hydroxides. *Mater. Chem. Phys.* **2010**, *119* (1-2), 303-308.

(49) Xu, Z. P.; Jin, Y.; Liu, S.; Hao, Z. P.; Lu, G. Q., Surface Charging of Layered Double Hydroxides During Dynamic Interactions of Anions at the Interfaces. *J. Colloid Interface Sci.* **2008**, *326* (2), 522-529.

(50) Hou, W.-G.; Su, Y.-L.; Sun, D.-J.; Zhang, C.-G., Studies on Zero Point of Charge and Permanent Charge Density of Mg-Fe Hydrotalcite-Like Compounds *Langmuir* **2001**, *17* (6), 1885-1888.

(51) Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J., Spatially Resolved Observation of Crystal-Face-Dependent Catalysis by Single Turnover Counting. *Nature* **2006**, *439* (7076), 572-575.

(52) Cuautli, C.; Valente, J. S.; Conesa, J. C.; Ganduglia-Pirovano, M. V.; Ireta, J., Theoretical Study of the Catalytic Performance of Activated Layered Double Hydroxides in the Cyanoethylation of Alcohols. *J. Phys. Chem. C* **2019**, *123* (14), 8777-8784.
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<figure><img src="image_22.png" /></figure>