# Preparation of Pickering Emulsions Stabilized by Modified Silica Nanoparticles via the Taguchi Approach

Fatemeh Heidari, Seid Mahdi Jafari, Aman Mohammad Ziaiifar, Nicolas

Anton

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Fatemeh Heidari, Seid Mahdi Jafari, Aman Mohammad Ziaiifar, Nicolas Anton. Preparation of Picker- ing Emulsions Stabilized by Modified Silica Nanoparticles via the Taguchi Approach. Pharmaceutics, 2022, 14 (8), pp.1561. 10.3390/pharmaceutics14081561. hal-05502110

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Article

# Preparation of Pickering Emulsions Stabilized by Modified Silica Nanoparticles via the Taguchi Approach

Fatemeh Heidari<fnref n="1" />, Seid Mahdi Jafari <fnref n="1" />,* , Aman Mohammad Ziaiifar<fnref n="1" /> and Nicolas Anton<fnref n="2" />

**Abstract:** In this study, oil-in-water Pickering emulsions (PEs) were prepared by modified silica nanoparticles (MSNs) with cetyltrimethylammonium bromide (CTAB) using the Taguchi approach. The surface modification of SiO₂ nanoparticles (NPs) was performed in different conditions, temperatures, pH levels, and amounts of CTAB as a coating agent, followed by an evaluation of their physicochemical properties. After treatment of the SiO₂ NPs, the relationship of the MSNs' surface properties and their efficiency in stabilizing Pickering emulsions was investigated by considering the zeta potential (ZP) and emulsion physical stability as main responses, respectively. Results disclosed were then supported by additional characterization, such as thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, contact angle (CA), and scanning electron microscopy. Results demonstrated that temperature has the most important role in the treatment of SiO₂ nanoparticles, and allows for the identification of the best experimental conditions, i.e., range of zeta potential of MSNs to produce more efficient NPs, as well as the best stabilization of PEs.

**Keywords:** silica nanoparticles; surface modification; Pickering emulsion; optimization; physical stability

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**Citation:** Heidari, F.; Jafari, S.M.; Ziaiifar, A.M.; Anton, N. Preparation of Pickering Emulsions Stabilized by Modified Silica Nanoparticles via the Taguchi Approach. *Pharmaceutics* **2022**, *14*, 1561. https://doi.org/10.3390/pharmaceutics14081561

Academic Editor:
Korbinian Löbmann

Received: 27 June 2022

Accepted: 24 July 2022

Published: 28 July 2022

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**Copyright:** © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

In Pickering emulsions (PEs), surfactants are replaced by colloidal particles to stabilize the interface between the oil and water phases [1,2]. These systems display greater physical/chemical stabilities, such as long-term stability and high resistance to coalescence compared with surfactant-stabilized emulsions [3]. This arises from the fact that more energy is required to remove the particles from the liquid-liquid interface relative to desorbing the surfactants [4]. These systems are therefore widely used in various food, pharmaceutical, and cosmetic industries [5]. Different inorganic and organic particles can be employed as stabilizers for PEs, including SiO₂, CaCO₃, BaSO₄, clays, carbon black, carbon nanotubes, cellulose, starch, and chitosan [6,7].

SiO₂ nanoparticles (NPs), also called nano-silica, are extensively applied due to their cost-effectiveness, biocompatibility, low in vivo toxicity, and antimicrobial properties. However, they cannot be directly used to stabilize PEs due to the presence of plentiful hydroxyl groups (–OH) on their surface, which cause the NPs to have a strong degree of hydrophilicity and low surface activity [8–10]. Accordingly, the surface of SiO₂ NPs should be modified to achieve a proper degree of hydrophobicity and optimum wettability at the oil-water (O/W) interface [2,5]. Properties of SiO₂ NPs can be altered by chemical and/or physical methods [11]. Physical modification is more noticeable since it is more straightforward and has a lower experimental cost in comparison to chemical modification [12]. Physical modification is induced from physical interactions between surfactants (or large molecules) and NPs, resulting in the modification of the surface properties of NPs, which can lead to greater adsorption of NPs at the O/W interface, as well as high stability [9,13,14].

<footnote n="1">¹ Faculty of Food Science and Technology, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan 4913815739, Iran; fatemeh_heidari87@yahoo.com (F.H.); amziaiifar@gau.ac.ir (A.M.Z.)</footnote>

<footnote n="2">² INSERM (French National Institute of Health and Medical Research), UMR 1260, Regenerative Nanomedicine (RNM), FMTS, University of Strasbourg, F-67000 Strasbourg, France; nanton@unistra.fr</footnote>

<footnote>* Correspondence: smjafari@gau.ac.ir</footnote>
Numerous studies have previously focused on particle surface modification through the adsorption of surface-active agents (i.e., surfactants, proteins, and polymers), e.g., chitosan, alginate, and cetyltrimethylammonium bromide (CTAB) for polystyrene, silica, and bentonite, respectively [15–17]. CTAB is a cationic surfactant widely employed in the pharma industry. When CTAB is mixed with anionic stabilizers such as silica and bentonite nanoparticles, electrostatic interaction between their surface charges increases the adsorption of CTAB molecules on the surface of nanoparticles. As a result, this can lead to more surface activity and strongly affect their hydrophilic/hydrophobic properties [18]. Some systematic studies on the surface modification of SiO₂ NPs with CTAB at water-air [19], water-hexane [20], water-n heptane [21], and wax-water [22] interfaces are available. These authors obtained an increase in the NPs’ hydrophobicity, attributed to the electrostatic interactions between the negatively charged surface of SiO₂ NPs and the positively charged surface of CTAB [19,21]. As a result, NPs are coated with hydrophilic moieties of CTAB. On the other hand, researchers have also demonstrated that surface modification of NPs directly depends on several factors, such as the properties of NPs and surfactants, the ratio of NPs/surfactant, temperature, and pH [22,23].

The physicochemical parameters of NPs such as size, shape, wettability, and surface charge play a key role in the stability of PEs [24]. The particle surface wettability is the most important factor in determining the type of emulsions related to the hydrophilicity or hydrophobicity of the particles, which is evaluated by measuring the contact angle [25]. Particle size affects the particle mobility in the continuous phase and the droplet size of the emulsion [26]. There are colloidal particles with different shapes such as spherical, ellipsoidal, and rod-like particles; among these, spherical particles are the most widely used particles [27]. The desorption energy of particles with various shapes depends on different parameters. For example, in spherical particles, it depends on the interfacial tension, radius of NPs and contact angle, while in rod particles, it depends on the length, diameter, rigidity, and amphiphilicity [28,29].

Emulsions prepared by NPs with a high or low surface charge are not stable. Therefore, this study aims to first modify the surface of SiO₂ NPs, and then investigate their application as a stabilizer in the formulation of PEs as a potential delivery system for bioactive compounds. To this end, the optimization of the surface properties of SiO₂ NPs is performed according to the Taguchi method, investigating the effects of SiO₂ NPs/CTAB ratios, temperature, and pH. As a result, the main outputs were evaluations of the surface charge, chemical structure, wettability, and performance of MSNs as PE stabilizers. One unique aspect of this study lies in how the experimental approach was carried out, using and optimizing SiO₂ nanoparticles as an efficient solution to stabilize Pickering emulsions. The multistep process, from the surface modification of SiO₂ NPs, the formulation of Pickering NEs, and up to the optimization of their stability according to the Taguchi method, also constitutes new insight into the field of pharmaceutical formulation.

## 2. Materials and Methods

### 2.1. Materials

Rice husks and glycerol used for the synthesis of SiO₂ NPs were purchased from a local rice-processing complex in Gorgan (Golestan, Iran) and the Kian Kaveh Amaze Company (Tehran, Iran), respectively. CTAB, acetic acid, and sodium hydroxide were obtained from Merck, Germany. In addition, the applied sunflower oil for the preparation of PEs was provided from a local market in Gorgan (Golestan, Iran). All other chemicals used in this study were of analytical grade. Distilled and deionized water were employed in experiments.

### 2.2. Preparation of SiO₂ NPs

SiO₂ NPs were synthesized from rice husks according to the modified method of Suttiruengwong et al. [30]. Rice husk ash (RHA) was first obtained through calcining the rice husks at 500 °C for 24 h in a furnace (LE 14/11/B150, Nabertherm, Lilienthal,
Germany). RHA was then mixed with glycerol as a depolymerizing agent at a ratio of 1:10 *w/v* at 200 °C using a magnetic hotplate stirrer for 2 h. The obtained mixture was stored at an ambient temperature for 24 h. Excess glycerol was then evacuated, deionized water was added, and the mixture was stored for 24 h. The obtained gel was washed with distilled water three times and dried at 105 °C for 24 h in an oven (800, Memert, Schwabach, Germany). The NPs were finally formed by calcinating the dried gel at 500 °C for 24 h.

## 2.3. *Surface Modification of SiO₂ NPs*

The surface modification of SiO₂ NPs was performed according to a protocol previously reported [1,20]: 1 g of SiO₂ NPs was dispersed in 100 mL of deionized water, and the pH of the solution was adjusted at different values by adding acetic acid (1% *v/v*) and NaOH (0.5 M). The different treatments are reported in Table 1, according to the Taguchi method; different values of CTAB (powder) were slowly added to suspensions of SiO₂ NPs at different weight ratios and for different pHs and temperatures under constant stirring. These mixtures were then stirred with a magnetic stirrer for a minimum of 6 h. The suspensions were then centrifuged at 4500 rpm for 10 min, washed with ultrapure water several times, and dried at 70 °C for 24 h in an oven.

<table><caption>**Table 1.** Experimental design according to the Taguchi method.</caption><thead><tr><td>Treatment No</td><td>pH</td><td>Surfactant/SiO<sub>2</sub> NPs Ratio</td><td>Temperature (°C)</td></tr></thead><tbody><tr><td>1</td><td>5.5</td><td>1:8</td><td>30</td></tr><tr><td>2</td><td>5.5</td><td>2:8</td><td>50</td></tr><tr><td>3</td><td>5.5</td><td>3:8</td><td>70</td></tr><tr><td>4</td><td>7.5</td><td>1:8</td><td>50</td></tr><tr><td>5</td><td>7.5</td><td>2:8</td><td>70</td></tr><tr><td>6</td><td>7.5</td><td>3:8</td><td>30</td></tr><tr><td>7</td><td>9.5</td><td>1:8</td><td>70</td></tr><tr><td>8</td><td>9.5</td><td>2:8</td><td>30</td></tr><tr><td>9</td><td>9.5</td><td>3:8</td><td>50</td></tr></tbody></table>

## 2.4. *Preparation of PEs*

To prepare the oil-in-water PEs, 2 mL of sunflower oil was slowly added to 8 mL of MSN suspensions (1%) and then homogenized by a rotor stator homogenizer (SilentCrusher M, Heidolph, Schwabach, Germany) at 14,000 rpm for 6 min.

## 2.5. *Determination of Particle Size and Zeta Potential (ZP)*

The average diameter, polydispersity index (PDI), and ZP values of SiO₂ NPs and MSNs were measured using the Zetasizer instrument (Malvern, UK) at 25 °C [22].

## 2.6. *Fourier Transform Infrared (FTIR) Spectroscopy*

To characterize the chemical interactions, FTIR spectra of SiO₂ NPs, CTAB, and MSNs were obtained by an FTIR spectrometer (Tensor II, Bruker Spectrometer, Billerica, MA, USA). The samples were mixed with KBr in a ratio of 1:150, and then pressed under a pressure of 4 t, and FTIR spectra were recorded on pressed pellets in the wavenumber range of 500–4000 cm⁻¹ [23]. The interactions were analyzed using the Omnic 3.2 software.

## 2.7. *Droplet Size Measurement of PEs*

The size of droplets in PEs was measured using a dynamic light scattering (DLS) method (Zetasizer, Malvern Instrument, Malvern, UK). The refractive indices of the sunflower oil and deionized water were set at 1.472 and 1.33, respectively [29]. The droplet size of some samples was measured through microscope pictures taken from an optical microscope (Zeiss, Oberkochen, Germany) and analyzed by the Image J software.
## 2.8. Physical Stability of PEs

The storage stability of PEs against separation and coalescence was determined by measuring the emulsion creaming index (CI) (Equation (1)) and droplet size, respectively. The samples were sealed into plastic tubes and stored at 25 °C for 30 days.

$$CI(\%) = \frac{H_s}{H_t} \qquad (1)$$

Here, $H_s$ and $H_t$ are the height of the serum layer (mm) and the total emulsion (mm), respectively, as measured with a digital caliper [31].

## 2.9. Thermogravimetric Analysis (TGA)

The weight loss in SiO₂ NPs and MSNs was determined using a thermogravimetric analyzer (Q600, TA Instruments, New Castle, DE, USA) by heating the samples from 25 to 800 °C at a heating rate of 10 °C/min in the argon atmosphere [20]. Before the test, NPs were dried in the oven at 100 °C for 24 h to remove the excess water on their surface.

## 2.10. Wettability Measurement

An optical contact angle measuring device (OCA15 Plus, Dataphysics, Filderstadt, Germany) was used to characterize the hydrophilic/hydrophobic properties of SiO₂ NPs and MSNs [22].

## 2.11. Morphological Analysis

Field emission scanning electron microscopy (FESEM) (MIRA III, TESCAN, Ostrau, Czech Republic) was used to determine the microstructure of SiO₂ NPs; scanning electron microscopy (SEM) (SU3500, Hitachi, Tokyo, Japan) was applied to determine the arrangement of MSNs at the O/W interface (the samples were diluted 20 times, then 1 mL of the obtained solution was dried on aluminum foil) [22].

## 2.12. Statistical Analysis

Statistical analyses were performed by analysis of variance (ANOVA) and significant difference analysis ($p < 0.05$) using the Qualitek software (version 4, Nutek, Albany, NY, USA).

# 3. Results and Discussion

## 3.1. Characteristics of SiO₂ NPs

The main results on native SiO₂ NPs are reported in Figure 1. It was revealed that SiO₂ NPs prepared from rice husks were predominantly spherical, with an average size of 42 ± 0.7 nm and PDI = 0.627 (Figure 1b). ZP measurements of SiO₂ NPs (Figure 1c) revealed a significantly negative surface charge over a wide pH range (−56.6, −62.2 and −75.7 mV for pH= 5.5, 7.5, and 9.5, respectively) due to abundant hydroxyl groups on their surface. Finally, the wettability of NPs (obtained through the determination of a three-phase CA of water onto a surface coated with NPs; see details in Section 3.6 below) disclosed the total wettability ($\theta = 0°$) of the NPs. This confirmed that the surface of the NPs was completely hydrophilic. The set of results emphasized in Figure 1 appears in line with those reported in the literature [22,32,33]. We can conclude that in native form, these SiO₂ NPs present an important hydrophilicity and affinity to the water phase, mainly due to the high negative charge of their surface, resulting in total wettability with water. The solution that appears to adapt their use as a potential stabilizer for PEs, as presented in the Introduction, plays on the surface charge and composition, an objective that can be achieved through interaction with CTAB.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Characterization of native SiO₂ NPs. (a) SEM picture with (b) the corresponding size distribution obtained from DLS, and (c) measurement of zeta potentials at different pHs. (d) Contact angle of native SiO₂ NPs.</figcaption></figure>

## 3.2. Zeta Potential of MSNs

ZP appears to be the main parameter determining the surface properties of NPs. Numerous factors can influence this parameter, investigated in preliminary experiments to select the most important factors, and disclosing their working ranges. The experimental ranges of pH, CTAB/SiO₂ NPs ratios, and temperature were found to be 5.5 to 9.5, 1:8 to 3:8 *w/w*, and 30 to 70 °C, respectively (Table 1). The results presented in Figure 2a show the raw ZP values of MSNs, which can vary from -52 to -17 mV. This indicates the effectiveness of the CTAB coating method, and the need to rationalize the selection of process conditions. Treatment No. 7, with CTAB/SiO₂ NPs ratios of 1:8, pH = 9.5, and T = 70 °C, had the highest ZP (-52 mV), whereas treatment No. 9, with CTAB/SiO₂ NPs ratios of 3:8, pH = 9.5, and T = 50 °C, led to the lowest ZP (-17 mV).

Based on the obtained data, all independent parameters were effective on the final ZP of SiO₂ NPs, reported in Figure 2b, illustrating the individual effects of independent factors on the values of ZP. ANOVA results (Figure 2c) showed that pH and temperature were the least (0.782%) and most effective factors (68.688%) on ZP, respectively. As shown in Figure 2a, increasing the modification temperature from 25 to 50 °C decreases the ZP of SiO₂ NPs, probably due to higher electrostatic interactions between the NPs with surfactant (at the higher temperature) and the increase in the amount of bonded CTAB on the SiO₂ NPs' surface. However, a further increase in temperature to 70 °C during the surface modification process resulted in the highest ZP. An explanation for this result is that by exceeding the temperature > 50 °C, a higher amount of CTAB molecules is converted to micelles in water due to the dependence of critical micelle concentration on temperature.
The impact of the amount of CTAB on ZP appeared to be rather logical, decreasing the ZP with the increase in surfactants that interact with hydroxyl groups present on the SiO₂ NPs' surface. This means that more CTAB has been absorbed on the surface of SiO₂ NPs.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Characterization of the surface modification of SiO₂ nanoparticles. (a) zeta potential of modified NPs according to different treatments detailed in Table 1. (b) Mean values of zeta potentials of individual factors, completed with the values of (c) variance and (d) severity index.</figcaption></figure>

Our findings were consistent with those of Ma et al. [23] on the surface modification of SiO₂ NPs by a cationic surfactant, as well as those of Kang et al. [34], reporting similar behavior for the surface modification of Sb₂O₃ NPs with CTAB, where the ZP of Sb₂O₃ NPs increased significantly at higher CTAB levels. Similarly, although weak, pH induces
a decrease in ZP values due to the ionization of -OH groups. However, compared to the effect of temperature, the effect of pH is rather negligible. Thus, the electrostatic interaction between surfactants and SiO₂ NPs becomes stronger, resulting in lower ZP values of SiO₂ NPs [1]. These data are completed with the comparison of the variance between the different parameters (Figure 2c) and severity index (Figure 2d). The highest interaction between two independent variables was for the CTAB/SiO₂ NPs ratio and pH with a severity index of about 38.6%, and the lowest for the CTAB/SiO₂ NPs ratio and a temperature with an index < 7.7%.

### 3.3. Contact Angle of NPS

In line with the former section on the characterization and optimization of the surface modification of SiO₂ NPs, another important characterization is the impact of treatments on the wettability of water on a surface composed of such NPs as shown in Figure 1d for native NPs. This was evaluated through the water CA formed over NPs pressed into a tablet form, reported in Figure 3a,b after treatment No. 7 and No. 2, respectively. Many studies have investigated the behavior of various particles at the O/W interface in emulsions by measuring the CA [35–37]. Generally, the wetting tendency increases by decreasing the CA < 90°, while it decreases by increasing the CA > 90° [9].

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Contact angle θ for water on a surface coated with SiO₂ NPs after (a) treatment No. 7, and (b) treatment No. 2.</figcaption></figure>

As shown in Figure 3, compared to Figure 1d, the CA changed with the surface modification of SiO₂ NPs from 0° for native SiO₂ NPs (indicating that the produced NPs exhibited an absolute hydrophilic behavior) to CA = 85° after treatment No. 2. This confirms the modification of the nature of the SiO₂ NPs' surface by CTAB. In addition, θ was 13° after treatment No. 7, demonstrating that SiO₂ NPs were slightly modified and almost completely hydrophilic. Wei et al. [1] examined the effect of different surfactants on SiO₂ NPs and found a significant effect of surfactant concentrations on surface wettability.

### 3.4. Physical Stability of Pickering Emulsions

After the formulation of PEs, stability study is crucial and confirms the performance of MSNs as a stabilizer [24]. This parameter can be evaluated based on the rate of droplet coalescence, creaming, or emulsion flocculation [29,38]. Considering that the creaming is mostly a precursor to coalescence, the emulsion stability was primarily evaluated visually by measuring the creaming index (CI) as a function of the storage time to acquire the most
stable PEs against creaming. Results are reported in Figure 4a, showing the impact of the ZP of MSNs on the CI.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Characterization of PEs stabilized by modified SiO₂ NPs. (a) Creaming index with corresponding images at 30 days and (b) coalescence of PEs, both as a function of the zeta potential of MSNs.</figcaption></figure>

These results showed that freshly prepared PEs were stable and homogeneous, but after 3 days of storage, the lipid-droplet-enriched cream layer was gradually observed for more than half of PEs, and the CI for emulsions stabilized by MSNs with a ZP > -34 mV exceeded 9%. This disclosed a relationship between the surface activity of MSNs and emulsion stability, and subsequently the droplet flocculation and their coalescence. Furthermore, PEs stabilized by MSNs with a ZP = -52 mV had clear phase separation after one day of storage. Their CI quickly increased to 50% after 5 days of storage. In addition, on the 8th day, PEs failed to form, and their structure completely disappeared. This behavior can be attributed to the rapid reduction of adsorbed NPs on the O/W interface during storage and the subsequent formation of dispersed phase droplets with larger sizes and instability of PEs. In contrast, in the sample stabilized by MSNs with a ZP = -26 mV, no phase separation was observed even after one month of storage, and the measured CI
values remained close to zero. This result suggests that when MSNs have a high stabilizer efficiency, oil droplet aggregation and creaming is much more inhibited.

As illustrated in Figure 4a, due to the intense coalescence of oil droplets in PEs stabilized by MSNs with a ZP < -34 mV, free oil was observed after 30 days of storage. In contrast, the coalescence in PEs stabilized by MSNs with a ZP > -34 mV appeared negligible and seemed to increase again for the higher ZP (MSN with a ZP = -17 mV). These results brought a range of optimized MSNs for stabilizing PEs. The stability comparison between fresh PEs and those PEs stored for 30 days, reported in Figure 4b, confirmed this optimized formulation range. PEs stabilized by MSN with a ZP = -49 mV displayed significant changes in mean droplet size after 30 days of storage compared to other PEs. Our results indicated the highly charged MSNs contributed to a great degree of instability in PEs, in line with previous studies, e.g., those of de Folter et al. [39] and Hu et al. [29], which similarly reported low stability of PEs prepared by zein and gliadin NPs with an approximate ZP of 60 mV and 25 mV, respectively. NPs with high ZP values produce a repulsive energy barrier when they come close to an O/W interface [40,41]. This can be attributed to the repulsive forces created between NPs, which overcome the convective forces (responsible for transporting them to the O/W interface during emulsification); their adsorption onto the interface is consequently limited, such that unstable emulsions are formed [29].

The CI and coalescence in treatment No. 9 (with the highest ZP) were higher than other PEs stabilized by NPs with a ZP > -34 mV. This phenomenon can be attributed to SiO₂ NPs modified with more hydrophobicity than other MSNs [42]. In this case, a large surface area of the SiO₂ NP is modified due to the greater interaction between the hydroxyl groups and the CTAB, leading to the modification of hydrophobicity and hydrophilicity, impacting their PEs' stabilizer properties. In conclusion, analysis of the results revealed that the optimum conditions to prepare an MSN with suitable wettability, which leads to the formation of a PE with the highest stability, are pH = 5.5, a CTAB/SiO₂ NPs ratio of 2:8, and a temperature of 50 °C (treatment No. 2).

A complementary characterization was performed (Figure 5), for which SEM images of PEs clearly show the presence of SiO₂ NPs as a PE stabilizer in the O/W interface. It seems that the NPs' coverage was very homogeneous and very stable against vacuum conditions undergone during SEM experiments.

### 3.5. *Chemical Interactions in NPs*

FTIR spectra were studied to evaluate the interactions between SiO₂ NPs and CTAB (Figure 6), which can be confirmed by the shifts in characteristic peaks and the appearance of new peaks in the MSNs. According to Figure 6, the characteristic absorption peaks at 2849 and 2915 cm⁻¹ indicate the typical stretching vibrations of the C–H resulting from the –CH2 and –CH3 groups in the pure CTAB spectrum, respectively [43]. The characteristic peaks in the SiO₂ NP at 1094, 803, and 3448 cm⁻¹ are associated with the stretching vibrations of Si–O, Si–O–Si, and –OH groups, respectively. The adsorption bands for SiO₂ NPs are consistent with those reported in previous studies [30,43]. The decreased strength of peak 3448 cm⁻¹ and the formation of new peaks at 2923 and 2854 cm⁻¹ on the surface of MSNs confirm the interaction between SiO₂ NPs and CTAB.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** SiO₂ NPs' surface modification process and formation of Pickering emulsion stabilized by MSNs (a). SEM characterization of Pickering emulsions stabilized with SiO₂ NPs (b).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6.** FTIR spectra of (a) SiO₂ NPs, (b) CTAB, and (c) MSNs.</figcaption></figure>

### 3.6. *Thermal Properties of NPs*

TGA thermograms of SiO₂ NPs and MSNs are reported in Figure 7. Although the NPs were dried in the oven, further significant weight loss was observed in the samples. The weight loss at temperatures < 200 °C demonstrated the evaporation of the absorbed water on the surface of NPs. These results showed that SiO₂ NPs and MSNs had about 5 and 12% weight loss when the temperature was raised > 200 °C. This weight loss can be explained by the dehydration condensation of Si-OH in both samples and the thermal
decomposition of CTAB in the MSNs [23]. According to TGA plots, MSNs showed less weight loss compared to SiO₂ NPs up to 238 °C. This phenomenon could be associated with less water absorption on the surface of MSNs. Accordingly, the TGA results are in good agreement with those obtained from the FTIR.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** TGA thermograms of native (SiO₂ NPs) and modified NPs (MSNs).</figcaption></figure>

**4. Conclusions**

In this study, the stabilization of PEs by SiO₂ NPs was investigated as a function of the surface modification of silica NPs. This modification was undertaken under the Taguchi approach, for which surface modification of SiO₂ NPs was performed using a coating with CTAB surfactant. Then, such modified MSNs were used to prepare PEs. The independent variables selected in the Taguchi approach were temperature, amount of CTAB respective to that of SiO₂ NPs, and pH. The ANOVA results revealed that temperature and pH were the most and least effective parameters on the ZP of SiO₂ NPs, respectively, and that a strong interaction between the CTAB/SiO₂ NPs ratio and pH were found. According to the findings, increasing the modification temperature from 30 to 50 °C decreased the ZP of SiO₂ NPs, which could be due to higher electrostatic interactions and the increase in bonded CTAB on the surface of SiO₂ NPs. The results of FTIR, TGA, and CA confirmed the surface modifications of SiO₂ NPs. Furthermore, the study of PEs stabilized by MSNs with different ZP values revealed that using MSNs with a ZP = -26 mV was the optimal condition for the best physical stability, obtained with the following treatment: pH = 5.5, the CTAB/SiO₂ NPs ratio = 2:8, and T = 50 °C. In general, the optimal surface modification conditions of SiO₂ NPs reported in this work might be expanded to be used as a PE stabilizer in the food and pharmaceutical industries and related fields.

**Author Contributions:** Data curation: F.H.; investigations: F.H. and N.A.; methodology: F.H. and N.A.; conceptualization: S.M.J. and A.M.Z.; project administration: S.M.J. and A.M.Z.; resources: S.M.J. and A.M.Z.; validation: S.M.J. and A.M.Z.; visualization: S.M.J. and A.M.Z.; writing—original draft: A.M.Z., S.M.J. and F.H.; writing—review and editing: S.M.J., F.H. and N.A.; formal analysis: N.A, S.M.J. and F.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Iranian National Science Foundation (INFS), Grant number 4001004.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author, upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
## References

1. Wei, X.Q.; Zhang, W.J.; Lai, L.; Mei, P.; Wu, L.M.; Wang, Y.Q. Different cationic surfactants-modified silica nanoparticles for Pickering emulsions. *J. Mol. Liq.* **2019**, *291*, 111341. [CrossRef]

2. Chevalier, Y.; Bolzinger, M.-A. Emulsions stabilized with solid nanoparticles: Pickering emulsions. *Colloids Surf. A Physicochem. Eng. Asp.* **2013**, *439*, 23–34. [CrossRef]

3. Zou, Y.; Guo, J.; Yin, S.W.; Wang, J.M.; Yang, X.Q. Pickering emulsion gels prepared by hydrogen-bonded zein/tannic acid complex colloidal particles. *J. Agric. Food Chem.* **2015**, *63*, 7405–7414. [CrossRef] [PubMed]

4. Leal-Calderon, F.; Schmitt, V. Solid-stabilized emulsions. *Curr. Opin. Colloid Interface Sci.* **2008**, *13*, 217–227. [CrossRef]

5. Yang, Y.; Fang, Z.; Chen, X.; Zhang, W.; Xie, Y.; Chen, Y.; Liu, Z.; Yuan, W. An overview of Pickering emulsions: Solid-particle materials, classification, morphology, and applications. *Front. Pharmacol.* **2017**, *8*, 287. [CrossRef] [PubMed]

6. Ye, F.; Miao, M.; Jiang, B.; Campanella, O.H.; Jin, Z.; Zhang, T. Elucidation of stabilizing oil-in-water Pickering emulsion with different modified maize starch-based nanoparticles. *Food Chem.* **2017**, *229*, 152–158. [CrossRef] [PubMed]

7. Alehosseini, E.; Jafari, S.M.; Tabarestani, H.S. Production of d-limonene-loaded Pickering emulsions stabilized by chitosan nanoparticles. *Food Chem.* **2021**, *354*, 129591. [CrossRef] [PubMed]

8. Croissant, J.G.; Fatieiev, Y.; Almalik, A.; Khashab, N.M. Mesoporous silica and organosilica nanoparticles: Physical chemistry, biosafety, delivery strategies, and biomedical applications. *Adv. Healthc. Mater.* **2018**, *7*, 1700831. [CrossRef] [PubMed]

9. Binks, B.P. Particles as surfactants—similarities and differences. *Curr. Opin. Colloid Interface Sci.* **2002**, *7*, 21–41. [CrossRef]

10. Miyazawa, T.; Itaya, M.; Burdeos, G.C.; Nakagawa, K.; Miyazawa, T. A critical review of the use of surfactant-Coated nanoparticles in nanomedicine. *Int. J. Nanomed.* **2021**, *16*, 3937–3999. [CrossRef]

11. Fuji, M.; Takei, T.; Watanabe, T.; Chikazawa, M. Wettability of fine silica powder surfaces modified with several normal alcohols. *Colloids Surf. A Physicochem. Eng. Asp.* **1999**, *154*, 13–24. [CrossRef]

12. Binks, B.P.; Rodrigues, J.A.; Frith, W.J. Synergistic interaction in emulsions stabilized by a mixture of silica nanoparticles and cationic surfactant. *Langmuir* **2007**, *23*, 3626–3636. [CrossRef] [PubMed]

13. Wu, T.M.; Chu, M.S. Preparation and characterization of thermoplastic vulcanizate/silica nanocomposites. *J. Appl. Polym. Sci.* **2005**, *98*, 2058–2063. [CrossRef]

14. Ahn, S.H.; Kim, S.H.; Lee, S.G. Surface-modified silica nanoparticle-reinforced poly (ethylene 2, 6-naphthalate). *J. Appl. Polym. Sci.* **2004**, *94*, 812–818. [CrossRef]

15. Zhao, X.; Yu, G.; Li, J.; Feng, Y.; Zhang, L.; Peng, Y.; Tang, Y.; Wang, L. Eco-friendly pickering emulsion stabilized by silica nanoparticles dispersed with high-molecular-weight amphiphilic alginate derivatives. *ACS Sustain. Chem. Eng.* **2018**, *6*, 4105–4114. [CrossRef]

16. Zhang, S.; Zhou, Y.; Yang, C. Pickering emulsions stabilized by the complex of polystyrene particles and chitosan. *Colloids Surf. A Physicochem. Eng. Asp.* **2015**, *482*, 338–344. [CrossRef]

17. Torres, L.G.; Iturbe, R.; Snowden, M.J.; Chowdhry, B.Z.; Leharne, S.A. Preparation of o/w emulsions stabilized by solid particles and their characterization by oscillatory rheology. *Colloids Surf. A Physicochem. Eng. Asp.* **2007**, *302*, 439–448. [CrossRef]

18. Guivar, J.A.R.; Sanches, E.A.; Magon, C.J.; Fernandes, E.G.R. Preparation and characterization of cetyltrimethylammonium bromide (CTAB)-stabilized Fe3O4 nanoparticles for electrochemistry detection of citric acid. *J. Electroanal. Chem.* **2015**, *755*, 158–166. [CrossRef]

19. Maestro, A.; Guzmán, E.; Santini, E.; Ravera, F.; Liggieri, L.; Ortega, F.; Rubio, R.G. Wettability of silica nanoparticle-surfactant nanocomposite interfacial layers. *Soft Matter* **2012**, *8*, 837–843. [CrossRef]

20. Lan, Q.; Yang, F.; Zhang, S.; Liu, S.; Xu, J.; Sun, D. Synergistic effect of silica nanoparticle and cetyltrimethyl ammonium bromide on the stabilization of O/W emulsions. *Colloids Surf. A Physicochem. Eng. Asp.* **2007**, *302*, 126–135. [CrossRef]

21. Vatanparast, H.; Javadi, A.; Bahramian, A. Silica nanoparticles cationic surfactants interaction in water-oil system. *Colloids Surf. A Physicochem. Eng. Asp.* **2017**, *521*, 221–230. [CrossRef]

22. Lebdioua, K.; Aimable, A.; Cerbelaud, M.; Videcoq, A.; Peyratout, C. Influence of different surfactants on Pickering emulsions stabilized by submicronic silica particles. *J. Colloid Interface Sci.* **2018**, *520*, 127–133. [CrossRef]

23. Ma, X.K.; Lee, N.H.; Oh, H.J.; Kim, J.W.; Rhee, C.K.; Park, K.S.; Kim, S.J. Surface modification and characterization of highly dispersed silica nanoparticles by a cationic surfactant. *Colloids Surf. A Physicochem. Eng. Asp.* **2010**, *358*, 172–176. [CrossRef]

24. Heidari, F.; Jafari, S.M.; Ziaiifar, A.M.; Malekjani, N. Stability and release mechanisms of double emulsions loaded with bioactive compounds; a critical review. *Adv. Colloid Interface Sci.* **2022**, *299*, 102567. [CrossRef] [PubMed]

25. Tavasoli, S.; Liu, Q.; Jafari, S.M. Development of Pickering emulsions stabilized by hybrid biopolymeric particles/nanoparticles for nutraceutical delivery. *Food Hydrocoll.* **2022**, *124*, 107280. [CrossRef]

26. Sun, Z.; Yan, X.; Xiao, Y.; Hu, L.; Eggersdorfer, M.; Chen, D.; Yang, Z.; Weitz, D.A. Pickering emulsions stabilized by colloidal surfactants: Role of solid particles. *Particuology* **2021**, *64*, 153–163. [CrossRef]

27. Ortiz, D.G.; Pochat-Bohatier, C.; Cambedouzou, J.; Bechelany, M.; Miele, P. Current trends in Pickering emulsions: Particle morphology and applications. *Engineering* **2020**, *6*, 468–482. [CrossRef]

28. Yan, H.; Zhao, B.; Long, Y.; Zheng, L.; Tung, C.H.; Song, K. New pickering emulsions stabilized by silica nanowires. *Colloids Surf. A Physicochem. Eng. Asp.* **2015**, *482*, 639–646. [CrossRef]
29. Hu, Y.Q.; Yin, S.W.; Zhu, J.H.; Qi, J.R.; Guo, J.; Wu, L.Y.; Tang, C.H.; Yang, X.Q. Fabrication and characterization of novel Pickering emulsions and Pickering high internal emulsions stabilized by gliadin colloidal particles. *Food Hydrocoll.* **2016**, *61*, 300–310.
[CrossRef]

30. Suttiruengwong, S.; Puathawee, P.; Chareonpanich, M. Preparation of mesoporous silica form rice husk ash: Effect of depolymerizing agent on physico-chemical properties. *Adv. Mater. Res.* **2010**, *93–94*, 664–667. [CrossRef]

31. Ge, S.; Xiong, L.; Li, M.; Liu, J.; Yang, J.; Chang, R.; Liang, C.; Sun, Q. Characterizations of Pickering emulsions stabilized by starch nanoparticles: Influence of starch variety and particle size. *Food Chem.* **2017**, *234*, 339–347. [CrossRef] [PubMed]

32. Sun, D.; Kang, S.; Liu, C.; Lu, Q.; Cui, L.; Hu, B. Effect of zeta potential and particle size on the stability of SiO2 nanospheres as carrier for ultrasound imaging contrast agents. *Int. J. Electrochem. Sci.* **2016**, *11*, 8520–8529. [CrossRef]

33. Skelhon, T.S.; Grossiord, N.; Morgan, A.R.; Bon, S.A. Quiescent water-in-oil Pickering emulsions as a route toward healthier fruit juice infused chocolate confectionary. *J. Mater. Chem.* **2012**, *22*, 19289–19295. [CrossRef]

34. Kang, C.; Xu, J.; Niu, L.; Wang, T.; An, J. A novel surface modification of Sb2O3 nanoparticles with a combination of cationic surfactant and silane coupling agent. *J. Dispers. Sci. Technol.* **2021**, *42*, 2116–2132. [CrossRef]

35. Ribeiro, E.F.; de Barros-Alexandrino, T.T.; Assis, O.B.G.; Junior, A.C.; Quiles, A.; Hernando, I.; Nicoletti, V.R. Chitosan and crosslinked chitosan nanoparticles: Synthesis, characterization and their role as Pickering emulsifiers. *Carbohydr. Polym.* **2020**, *250*, 116878. [CrossRef] [PubMed]

36. Linke, C.; Drusch, S. Pickering emulsions in foods-opportunities and limitations. *Crit. Rev. Food Sci. Nutr.* **2018**, *58*, 1971–1985.
[CrossRef]

37. Ogunlaja, S.B.; Pal, R.; Sarikhani, K. Effects of starch nanoparticles on phase inversion of Pickering emulsions. *Can. J. Chem. Eng.* **2018**, *96*, 1089–1097. [CrossRef]

38. Tian, H.; Lu, Z.; Yu, H.; Chen, C.; Hu, J. Fabrication and characterization of citral-loaded oil-in-water Pickering emulsions stabilized by chitosan-tripolyphosphate particles. *Food Funct.* **2019**, *10*, 2595–2604. [CrossRef]

39. de Folter, J.W.; van Ruijven, M.W.; Velikov, K.P. Oil-in-water Pickering emulsions stabilized by colloidal particles from the water-insoluble protein zein. *Soft Matter* **2012**, *8*, 6807–6815. [CrossRef]

40. Wang, H.; Singh, V.; Behrens, S.H. Image charge effects on the formation of pickering emulsions. *J. Phys. Chem. Lett.* **2012**, *3*, 2986–2990. [CrossRef]

41. Danov, K.D.; Kralchevsky, P.A.; Ananthapadmanabhan, K.P.; Lips, A. Particle– Interface Interaction across a Nonpolar Medium in Relation to the Production of Particle-Stabilized Emulsions. *Langmuir* **2006**, *22*, 106–115. [CrossRef] [PubMed]

42. Lu, S.; Kunjappu, J.T.; Somasundaran, P.; Zhang, L. Adsorption of a double-chain surfactant on an oxide. *Colloids Surf. A Physicochem. Eng. Asp.* **2008**, *324*, 65–70. [CrossRef]

43. Lin, C.; Fan, B.; Zhang, J.X.; Yang, X.; Zhang, H. Study on lead ion wastewater treatment of self-assembled film. *Desalination Water Treat.* **2016**, *57*, 21627–21633. [CrossRef]