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. 2025 Jul 30;10(31):34710–34720. doi: 10.1021/acsomega.5c03594

Comparative Study on Different Clay Minerals for Costabilizing Antibacterial Pickering Emulsions with Sapindus Mukorossi against MRSA and ESBL-E. coli

Aiping Hui 1, Bin Mu 1,*, Yongfeng Zhu 1, Aiqin Wang 1,*
PMCID: PMC12355280  PMID: 40821576

Abstract

Green Pickering emulsions were costabilized by different natural clay minerals and Sapindus mukorossi (S. mukorossi) for the encapsulation of carvacrol in this study, and the effect of the types of clay minerals on the emulsion stability and antibacterial activity of Pickering emulsions was studied, while the stability mechanism and surface/interfacial interaction of Pickering emulsions were revealed. The results suggested that the synergistic effect among the continuous-phase polymer, clay mineral, and organic substances derived from S. mukorossi was the predominant stabilization mechanism of Pickering emulsions, which formed a hybrid rigid shell on the oil-in-water interface. Meanwhile, Pickering emulsions presented excellent antibacterial activities against Staphylococcus aureus (MRSA) and extended-spectrum β-lactamases of Escherichia coli (ESBL- E. coli). By contrast, halloysite and S. mukorossi costabilized Pickering emulsions presented better emulsion stability and antibacterial activities toward MRSA and ESBL- E. coli, and lower visible colonies were observed for MRSA with a minimum inhibitory concentration of 2.5 μL/mL. The enhancement in the antibacterial activity was attributed to the increase in the water solubility of carvacrol and contact chance between the Pickering droplets and bacteria, because of which the structural integrity of drug-resistant bacteria was more easily destroyed. This study provides a feasible approach to engineer efficient antibacterial materials for industrial applications.

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1. Introduction

To date, Pickering emulsions have been widely used to protect, encapsulate, and facilitate the sustained release and transport of core items such as essential oil, drugs, organic substances, bioactive compounds, and antioxidants owing to their multicomponent structure, and they have been applied in food, cosmetic, pharmaceutical, and cell encapsulation. , However, the relevant applications of Pickering emulsions are greatly limited due to the poor oil/water interfacial stability. In order to improve the stability of Pickering emulsions, a large number of amphiphilic solid particles have been developed for the stabilization of oil-in-water emulsions, including synthetic soy protein particles, polysaccharide particles, media-milled agar particles, biopolymeric particles, clay minerals, multiple particles, hybrid particles, complex particles, and colloidal particles. − Compared with traditional surfactants, amphiphilic solid particles effectively reduce the tension of the oil–water interface and possess a large desorption energy to be firmly anchored at the oil–water interface as well. , Thus, much attention has been paid to develop various solid particles for the design of Pickering emulsions, ignoring the cleaner, low-cost, and green preparation process. ,,

Recently, numerous natural polysaccharide and biopolymeric particles were developed for the green fabrication of Pickering emulsions, while the preparation cost is still relatively high owing to the expensive raw materials. Meanwhile, the amphiphilicity of the above particles was only displayed in the appropriate environment; e.g., chitosan could stabilize the emulsion only in the pH range of 3.0–6.5; thus, a more universal approach was developed to prepare green Pickering emulsions using amphipathic composite particles. ,− Among them, natural clay minerals have been recognized as promising candidates due to their unique morphology, large numbers of silica/aluminum hydroxyl groups, and easy modification; especially, two-dimensional lamellar montmorillonite (MMT) was explored to stabilize Pickering emulsions because of its potential conformance control in high-salinity and high-temperature reservoirs, phase inversion, and antibacterial application. Furthermore, the Pickering emulsions were also stabilized using laponite modified with short-chain aliphatic amines in the absence of electrolytes. ,−

Compared with the synthetic surfactants, natural biosurfactants have attracted much attention due to their great advantages as eco-friendly alternatives to synthetic surfactants. Thus, different plant-derived biosurfactants were developed to design eco-friendly Pickering emulsions and aqueous foams with excellent stability by combining them with attapulgite derived from different geological origins in our previous studies. ,,, In fact, the particle shape and size exhibited a dramatic effect on the stability of Pickering emulsions, and it is indispensable to comparatively study the influence of clay minerals with different morphologies and crystal layer structures on the stability of Pickering emulsions. ,

Therefore, different clay minerals, including halloysite (Hal), sepiolite (Sep), montmorillonite (MMT), and kaolinite (Kaol), were employed to fabricate oil-in-water Pickering emulsions combined with S. mukorossi using carvacrol as the active oil phase with antibacterial activity. The effect of the size, morphology, and crystal layer structures of the involved clay minerals on the stability of Pickering emulsions was investigated in detail, while the antibacterial activities of Pickering emulsions against drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum β-lactamases-producing Escherichia coli (ESBL- E. coli) were evaluated. The results of this study are expected to help develop green and stable Pickering emulsions for antibacterial applications based on natural clay minerals, plant-derived biosurfactants, and essential oil.

2. Experimental Section

2.1. Preparation of Pickering Emulsions

Typically, 5 g of S. mukorossi was mixed with clay mineral in a mass ratio of 1:1 and ground for 15 min after adding 4 mL of distilled water. Then the mixture was dried at 60 °C and passed through a 200-mesh sieve. Next, 1 g of the above mixture of clay mineral and S. mukorossi was added into 100 mL of 0.25% xanthan gum solution and stirred at 600 rpm for 30 min. After that, 10% carvacrol based on the mass of the aqueous phase was added slowly under continuous stirring for an extra 10 min. Finally, the Pickering emulsions were prepared by high-speed stirring at 6000 rpm for 10 min. Similarly, 1% S. mukorossi as a single stabilizer was used to prepare the control group without clay mineral according to the above process.

3. Results and Discussion

3.1. Preparation of Costabilizers by Combining Clay Minerals with S. mukorossi

Clay minerals have a great potential to stabilize an incompatible interface, but they first need to be modified with organic molecules to adjust the surface amphiphilicity due to their inherent strong hydrophilicity (Figure ). S. mukorossi is rich in saponins, which are a type of nonionic natural surfactants. , Due to the sufficient number of surface silicon hydroxyl groups, the organic molecules leached from S. mukorossi could be adsorbed onto the surface of clay minerals or entered into the interlayer or porous structure of clay minerals via mechanical grinding. In addition, the grinding also contributes to destroying the fiber structure of S. mukorossi and increasing the release efficiency of active substances including saponins, but overgrinding might destroy the morphology and porous structure of clay minerals, which is unfavorable to stabilize Pickering emulsions.

1.

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Schematic illustration of the fabrication of Pickering emulsions by combining clay minerals with S. mukorossi.

Generally, different clay minerals have different morphologies, sizes, and colloidal properties, either one-dimensional or two-dimensional structure. As shown in Figures a and S1, Hal presents a typical one-dimensional tubular morphology with a length of 100–300 nm and good dispersibility, and no clear fractures or small-size fragments were observed. Sep was found to be a one-dimensional nanofiber with different diameters or lengths, ranging up to several micrometers (Figure b). By contrast, a two-dimensional wrinkled sheet-like structure was found for MMT (Figure c), while Kaol exhibited an agglomerated lamellar structure with small size (Figure d).

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TEM images of clay minerals: (a) Hal, (b) Sep, (c) MMT, and (d) Kaol.

In the FTIR spectra of clay minerals (Figure a), the absorption peaks around 3700–3500 and 3400–3300 cm–1 were assigned to the stretching vibrations of hydroxyl groups on the outer surface of the clay minerals. , The peaks at 3440 and 1634 cm–1 were assigned to the stretching vibrations of the hydroxyl groups of water and the bending vibrations of adsorbed water molecules of clay minerals, respectively. , In addition, the absorption bands at 1070, 1034, and 912 cm–1 were attributed to the stretching vibration of Si–O groups and the vibration of the inner Al–OH groups of raw clay minerals, while the bands at 798, 756, and 696 cm–1 and the other three bands around 538, 470, and 452 cm–1 were mainly related to the vibration of O–Al–OH and Si–O–Al, respectively. , In the case of S. mukorossi, the characteristic absorption peaks of 2974, 2928 cm–1 and 1632, 1043 cm–1 corresponded to the stretching vibrations of −CH2– and CC characteristic peaks of the aromatic ring skeleton and aromatic C–O asymmetric stretching, respectively. , After being ground with clay mineral, the absorption peaks of S. mukorossi located at 1738, 1632, and 1043 cm–1 shifted to 1736, 1634, and 1036 or 1040 cm–1, respectively. This phenomenon might be attributed to the weak interaction between S. mukorossi and clay minerals. , Furthermore, it was noted that some bands of S. mukorossi were overlapped with the characteristic bands of clay minerals. Thus, the surface modification of clay minerals might be due to weak interactions such as hydrogen bonding between the organic active substances leached from S. mukorossi and the hydroxyl groups of clay minerals.

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FITR spectra of clay minerals (a) before and (b) after being ground with S. mukorossi.

Clay minerals have been used as carriers to build functional materials owing to their good hydrophilic properties, which are suitable to serve as a fixed platform of biopolymers or surfactants. The introduction of surfactants can adjust the surface activities and hydrophilic or hydrophobic features of clay minerals, so small surfactant molecules are prone to interact with clay minerals after the semidrying process. The contact angles of raw Hal, Sep, MMT, and Kaol were 27.6, 22.5, 17.2, and 15.1°, respectively (Figure a). After the grinding process, the corresponding contact angle values increased to 55.1, 44.5, 40.5, and 59.2°, respectively (Figure b), revealing that the hydrophobicity of clay minerals increased after the semidry grinding process with S. mukorossi. In fact, the compositions of S. mukorossi were composed of saponins, fat oil, protein, and plant fiber particles, and the optical microscopic images of the ground S. mukorossi in water illustrated the abundance morphology including microtubes, sheets, short rods, irregular spherical particles, and vesicles of different sizes (Figure S2), which might be attributed to the complex compositions derived from S. mukorossi. Thus, clay minerals were considered to combine with active substances leached from S. mukorossi to enhance their hrydrophobicity based on the interaction between S. mukorossi and clay minerals (Figure c).

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Contact angles of clay minerals (a) before and (b) after being ground with S. mukorossi; (c) schematic illustration of a clay mineral hydrophobic tablet.

3.2. Preparation of Pickering Emulsions

Figure depicts the optical microscopic and droplet size distribution images of Pickering emulsions costabilized by clay minerals and S. mukorossi. Clearly, the droplet diameter of the Pickering emulsions was not significantly affected by the different costabilizers composed of clay minerals and S. mukorossi, which was 4.6, 4.4, 4.5, and 5.4 μm for the Pickering emulsions prepared with Hal, Sep, MMT, and Kaol, respectively. The droplet size was equal to that of the control group stabilized alone with S. mukorossi. Moreover, the Pickering emulsions costabilized by clay minerals and S. mukorossi exhibited excellent stability without clear creaming or demulsification after being stored at room temperature for 60 days (Figure ), but poor stability was observed for the control group, and the flocculation phenomenon occurred after 60 days. With the increase in storage time from 60 to 180 days, the Pickering emulsions prepared with clay minerals and S. mukorossi presented different stabilities. Obviously, the droplet diameters of all Pickering emulsions had an increasing tendency due to the coalescence of small droplets, which resulted in poor stability of the system; the appearance of flocculation and demulsification is indicated by the green arrows in Figure .

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Optical microscopic images, droplet size distribution, and digital images of the Pickering emulsion costabilized by clay minerals and S. mukorossi.

As a comparative study, the Pickering emulsions costabilized by Hal and S. mukorossi exhibited the best stability after being stored at room temperature for 180 days, which might be attributed to the differences in the morphology and surface properties of clay minerals derived from the interaction between clay minerals and active substances leached from S. mukorossi during grinding. To further correlate the relationship between the hydrated particle size of clay minerals and the droplet size of the emulsions, it could be observed that the hydrated size of clay minerals was obviously increased in water, and the D 50 value of clay minerals was 15.7, 30.8, 14.4, and 10.3 μm, respectively (Figure S3). The presence of multiple peaks indicated that the particle size of clay minerals was not uniform. However, the droplet sizes of the Pickering emulsions had no significant differences, which was consistent with the optical microscope results, except that the tested droplet size was larger in the aqueous system (Figure S4).

Interestingly, the Pickering emulsions prepared using Sep and S. mukorossi exhibited better stability in the tube after being stored for 180 days followed by shaking with hand and standing for 2 weeks (Figure a). On the contrary, a clear demulsification and stratification occurred for the emulsion systems costabilized with other clay minerals and S. mukorossi. Combined with the effect of morphology and size on the stability of the emulsions, a probable mechanism was proposed that the size of Sep matched with the interface necessary for the protective layer anchored on the oil-in-water interface of larger droplets (Figure b,c). Meanwhile, this phenomenon also suggested that the Pickering emulsions costabilized by clay minerals and S. mukorossi could be re-emulsified to obtain the stable emulsion by simple shaking before use, even if there was sedimentation and delamination.

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(a) Digital images of Pickering emulsions costabilized by clay minerals and S. mukorossi after being stored for 180 days followed by shaking with hand and standing for 2 weeks; (b) optical microscopic images of Pickering emulsions-stabilized Sep and S. mukorossi after being stored for 180 days followed by shaking with hand and standing for 2 weeks; (c) FESEM image of Sep confined at the oil–water interface of Pickering emulsions.

CLSM observation was performed as a direct method to visualize the formation of Pickering emulsions costabilized by clay minerals and S. mukorossi (Figure ). The oil phase was labeled by Nile red dye, and Hal was stained with Safranine T. It was clearly observed that the red fluorescence was located inside the droplets (Figure a,c) and green fluorescence occurred in the aqueous phase (Figure b), suggesting that the typical oil-in-water emulsion was formed. Meanwhile, a sparse red fluorescence appeared on the oil–water interface (Figure d,f), while green fluorescence appeared in aqueous phase without the formation of a green circle at the interface (Figure e), indicating that clay minerals (e.g., Hal) were located around the droplets, and S. mukorossi was still dispersed in the solution and interface. When Hal and S. mukorossi were added into the system at the same time, an assembled behavior occurred at the oil/water interface to form a rigid interfacial shell to costabilize the Pickering emulsions to effectively prevent the aggregation and condensation of oil droplets (Figure g), which was almost consistent with the reported stabilization mechanism of Pickering emulsions combining with clay minerals and plant-derived biosurfactants. ,,,,, Therefore, organic active substances released from S. mukorossi played key roles in synergistically stabilizing the oil–water interface to form Pickering emulsions.

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CLSM images and schematic illustration of Pickering emulsions costabilized by Hal and S. mukorossi, in which the oil phase was labeled by Nile red (a, b, c), and Hal stained with Safranine T (d, e, f); (g) costabilization mechanism of Pickering emulsions using clay minerals and S. mukorossi (scale length = 100 μm).

3.3. Rheological Properties of Pickering Emulsions

Rheological properties of clay minerals and Pickering emulsions are vital characteristics and closely linked to their storage stability. As shown in Figure a,b, all of clay minerals and Pickering emulsions stabilized by clay minerals and S. mukorossi presented a typical shear thinning behavior (a non-Newtonian fluid behavior). , The shear viscosity of clay minerals and Pickering emulsions decreased gradually and then linearly increased with the increase in the shear rate, indicating a weak interaction among the droplets and a weak elastic gel network in the emulsions. Clearly, clay minerals have a great difference in viscosity, and the viscosity also played an important role in the stabilization of Pickering emulsions. By contrast, the viscosity of the emulsions was impressively enhanced by adding the clay minerals in system, original Hal and Hal combined with S. mukorossi costabilized Pickering emulsion exhibited a higher viscosity at a low shear rate.This phenomenon was associated with the size of clay minerals, and small size of Hal was more conducive to enhancing the viscosity to form more stable Pickering emulsions, which was consistent with the storage stability of Pickering emulsions (Figure b).

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Rheological characterization of (a) clay minerals and (b) Pickering emulsions costabilized by clay minerals and S. mukorossi, (c) the elastic or storage modulus (G′) and the viscous or loss modulus (G″) of Pickering emulsions, G’ is represented by square point, G″ is represented by spherical point, (d) Zeta potential of clay minerals and Pickering emulsions at a pH value of 6.05.

Furthermore, the G′ value of Pickering emulsions was always greater than that of G″ from 0.1 to 100 rad s–1 (Figure c), indicating that the elastic properties of the emulsions were dominant, and there was elastic gel-like structure in Pickering emulsions. At high frequencies, the G″ value was clearly above that of G′, and the crossover at around 60 Hz occurred, implying liquid-like behavior of Pickering emulsions. Meanwhile, the exception was observed for the S. mukorossi-stabilized emulsions without clay minerals, while the modulus in entire frequencies range were not crossover point than Pickering emulsions costabilized by different clay minerals and S. mukorossi. It indicated that clay minerals had superiority to form the network structure at the interface of emulsion droplets. Moreover, the viscosity and modulus of Pickering emulsions costabilized by Hal and S. mukorossi was high compared with other clay minerals and S. mukorossi system in the entire frequency range, which might be due to the semisolid-like behavior and the existence of strong gel structure. The ζ-Potential of Pickering emulsions increased compared with the control except for Sep group (Figure d), thus the increase in the negative charges of Pickering emulsions could be ascribed to the adsorption of negatively charged xanthan gum on the oil–water interface simultaneously, implying a costabilization interaction of continuous-phase polymer with clay minerals and S. mukorossi (Figure S5).

3.4. Antibacterial Test

The antibacterial activities of Pickering emulsions against MRSA and ESBL-E. coli were investigated via a standard plate counting method. It was found that both the blank group and positive control groups were within the normal range (Figure a–c), and the growth of MRSA was almost completely suppressed after incorporation of the prepared Pickering emulsions with a concentration of 2.5 μL/mL, while the plates presented visible colonies (Figure d–h). Compared with the antibacterial performance of emulsions against MRSA, the emulsion system containing Hal had a superior effect. However, Pickering emulsions costabilized by Hal, Sep, and MMT combined with S. mukorossi exhibited better antibacterial effect toward ESBL-E. coli with a minimum inhibitory concentration of 2.5 μL/mL, but a high concentration of 5 μL/mL was observed for the Kaol system (Figure i–m). When carvacrol was encapsulated into Pickering emulsions stabilized by clay minerals and S. mukorossi, there was an obvious difference in the antibacterial properties (Figure n), and the bacteriostatic concentrations of carvacrol for MRSA and ESBL-E. coli were 1 and 2.5 μL/mL, respectively (Figure o). Due to the low content of carvacrol in the emulsions (only 10%), the antibacterial properties of Pickering emulsions toward drug-resistant bacteria were superior to those with the direct use of carvacrol, while the Pickering emulsions presented a clearly enhanced antibacterial performance against ESBL-E. coli.

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Photographs of (a) blank, (b) control of MRSA, (c) control of ESBL-E. coli, (d–h) MRSA, and (i–m) ESBL-E. coli cultured after treatment using Pickering emulsions costabilized by clay minerals and S. mukorossi with a content of 2.5 μL/mL; (n) inhibition ratio of bacteria treated with Pickering emulsions; (o) bacteria cultured after treatment with carvacrol.

Taking into account the component and structure of the designed Pickering emulsions, the enhancement in the antibacterial properties of Pickering emulsions might be due to the following reasons (Figure ): First, the emulsion droplets possessed better compatibility with the bacteria and the essential oil molecules might have been able to more easily contact with the bacteria. , Second, there was a rapid spreading effect after the hydrophobic essential oil was transformed to hydrophilic oil-in-water emulsion droplets. In a word, the difference in the antibacterial properties of clay minerals-costabilized Pickering emulsions was mainly related to the emulsion stability, emulsifying efficiency of the oil phase, and utilization ratio of active essential oil molecules.

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Schematic illustration of the enhanced antibacterial properties of Pickering emulsions.

4. Conclusions

In this study, different clay minerals including Hal, Sep, MMT, and Kaol were employed to fabricate Pickering emulsions by combining with natural S. mukorossi. The results indicated that continuous-phase polymer with clay minerals and S. mukorossi were assembled at the oil–water interface to form a rigid interfacial layer to stabilize the Pickering emulsions, in which clay minerals interacted with the organic active substances leached from S. mukorossi. The storage stability of the prepared Pickering emulsions was mainly determined by the size, morphology, hydratability, and surface properties. Interestingly, the Pickering emulsions exhibited strong antibacterial activities, and the inhibition ratio against MRSA and ESBL-E. coli reached 100% with a low concentration of 5 and 2.5 μL/mL, respectively. Compared with pure carvacrol, Pickering emulsions exhibited low visible colonies for MRSA with a minimum inhibitory concentration of 2.5 μL/mL, which completely prevented the growth of bacteria. The main reason for the enhancement of antibacterial activities was attributed to the increase in compatibility and the contact opportunity between the essential oil and bacteria.

Supplementary Material

ao5c03594_si_001.pdf (565.7KB, pdf)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (22105212), Basic Research Creative Groups Project of the Science and Technology Plan of Gansu Province (23JRRA568), and Major Scientific and Technological Project of National Building Materials Industry of China (202201JBGS20-01).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c03594.

  • Additional experimental section including materials, antibacterial test details, and characterization techniques. FESEM images of the involved clay minerals, optical microscopic images of S. mukorossi aqueous solution, hydrated particle size of clay minerals, droplets size of Pickering emulsions, ζ-Potential of the supernatant of S. mukorossi aqueous solution and corresponding Hal-based Pickering emulsions containing different contents of saponins (PDF)

A.H.: Methodology, visualization, data curation, formal analysis, funding acquisition, writingoriginal draft. B.M.: Visualization, supervision, project administration, funding acquisition, writingreview and editing. Y.Z.: Data curation, formal analysis. A.W.: Conceptualization, supervision, project administration, funding acquisition, writingreview and editing.

The authors declare no competing financial interest.

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