Abstract: In recent years, the applications of Pickering emulsions have expanded in cosmetic and biological systems, leading to increased interest in the adsorption behavior of fine particles on oil-water interfaces. Despite this interest, there are still many unanswered questions regarding the connection between particle adsorption and interfacial tension. To address these gaps in knowledge, our study focused on three main areas: (1) the relationship between changes in liquid-liquid interfacial tension and the physical properties of Pickering emulsions, (2) competitive adsorption of fine particles and surfactants at the liquid-liquid interface, and (3) the particle size dependence of nanoparticle adsorption capability based on measurements of surface (interfacial) tension and its thermodynamic analysis. In this short review, we will discuss the findings from our experiments, particularly the possibility of spontaneous demulsification of Pickering emulsions through the control of interfacial tension.

Pickering emulsions, formed from oil and water mixtures, are stabilized by the addition of solid particles which reside at the oil-water (OW) interface^{1) ,2)} . Recently there has been an increased interest in Pickering emulsions because such emulsions are expected to be significantly more stable than corresponding surfactant-stabilized OW emulsions^{3) ,4)} . However, as in the surfactant stabilized emulsions, some applications, such as the enhanced oil recovery^{5) ,6)} , controlled release of drugs^{7) ,8)} , and sewage treatments^{9) ,10)} , the demulsification of Pickering emulsions is also an important issue.

The stability of Pickering emulsion is determined by the reduction in the interfacial energy by the replacement of the OW interface having the interfacial tension, γ_OW, by particles, which is given by^{11) ,12) ,13)}

(1)

where θ is the contact angle that a particle makes with the OW interface and the positive (negative) sign in Eq. (1) is applicable if a particle at the oil-water interface is removed into the oil (water) phase. Thus, it can be realized that the enhanced stability of Pickering emulsions, relative to surfactant emulsions, arises from the large cross-sectional area, Πγ², for a particle of radius γ. The interfacial energy reduction by particle adsorption is much greater than the thermal energy except for very small (0°) or large (180°) contact angles.

The irreversible adsorption of particles also gives rise to slow diffusion-based release of oily ingredients from Pickering droplet through the particle-laden interfaces. To address this issue, many researchers have been actively investigating Pickering emulsions that demulsify in response to external stimuli, such as temperature, pH, electric field strength, or light irradiation. One common approach is the use of chemically modified particles that have a varying affinity (i.e. contact angle) for water or oil, which allows for controlled demulsification^{14) ,15)} .

Although these methods are beneficial for drug delivery systems, they have the drawback of only being applicable to certain chemically modified particles. Consequently, to investigate a more general principle of Pickering emulsion demulsification, we conducted several experiments summarized in this review. Considering Eq. (1) , one possible approach to control the particle adsorption energy is to reduce OW interfacial tension. In general, the interfacial tension between two compatible liquids decreases exponentially as they approach the critical point. In the first topic, we utilized this principle to lower the particle adsorption energy continuously upon cooling.

In the second approach, we prepared Pickering emulsion in the presence of a cationic surfactant, whose adsorbed film shows the first-order surface freezing transition. This surfactant lowered the interfacial tension below the surface freezing temperature and therefore, the interfacial energy of particle laden interface and surfactant adsorbed film can be switched at the surface freezing. In other words, this procedure compares the interfacial energy reduction, ΔE, between the particles and coexisting surfactants in the same system.

Finally, we briefly discuss the change in particle stability at a liquid surface with decreasing particle size. This topic is related to the effect of line tension working at the vapor - liquid - particle three phase contact lines which is not considered in Eq. (1) . Understanding the demulsification conditions for these three unique systems also provides a useful guideline for preparing stable Pickering emulsions.

When a system consisting of 2,6-lutidine and water is emulsified with silica particles, Pickering emulsions spontaneously phase-separated during continuous cooling toward the lower critical solution temperature, T_c, of 34.2°C¹⁶⁾ . As the particle radius increases, the emulsion transitions occur at higher temperatures, resulting in a narrower temperature range for obtaining a stable Pickering emulsion. This trend was also observed in methanol-cyclohexane mixtures with an upper critical point of 46.0°C, which demonstrates the generality of the correlation between particle size and the demulsification transition of Pickering emulsions¹⁷⁾ . The theoretical background of these behaviors is introduced herein.

The contact angle of particles at the interface depends on the interfacial tensions (energies) through Young equation,

(2)

and, at θ=0° (or 180°) , a wetting transition of particles either by water or oil phase occurs. In the present case, silica particles are desorbed from the interface and dispersed in water at the wetting transition. The abbreviations, S, O, and W represent particle, oil (2,6-lutidine) , and water, respectively.

It is known that both γ_OW and Δγ obey the universal scale law near the critical point:¹⁸⁾

(3)

and

(4)

The scale factors M and B are positive constants determined for individual systems. On the other hand, the critical exponents, μ and β₁ are independent of the system. Although precise values for the critical exponents are still under debate, it is known that the μ takes a value of approximately 1.27¹⁹⁾ . In the original paper written by Cahn¹⁸⁾ , the β₁ was expected to be 0.34 but more larger values were proposed later by other researchers^{20) ,21) ,22)} . However, regardless of which value is adopted as the critical exponents, particles should wet completely by the water at the temperature is reached to

(5)

by applying Eqs. (3) and (4) to Young's criteria for wetting.

Figure 1 displays the phase diagram of 2,6-lutidine and water mixtures. The horizontal lines in Fig. 1 represent the wetting transition temperatures, T_w, which were observed for spherical silica particles with different diameters. When the sample was continuously cooled, the Pickering emulsion was found to be stable at all temperatures above T_w. However, when the temperature reached T_w, the silica particles detached from the interface, and Pickering emulsions were smoothly demulsified, as shown in the pictures in Fig. 1.

Fig.1

Phase diagram of 2,6-lutidine and water mixtures. Horizontal lines represent the wetting transition temperature, T_w, occurred at glass plate (green) , silica particles of diameter 1000 nm (yellow) , 100 nm (blue) , 50 nm (light green) , and 10 nm (red) , respectively. At the wetting transition temperatures, Pickering emulsions stabilized with corresponding particles demulsify spontaneously as shown in the pictures on the right-hand-side. Reproduced with the data originally published in the reference [16] , copyright (2020) American Chemical Society.

From Eq. (1) , it is indicated that Pickering emulsions formed from larger particles would be more stable than Pickering emulsions formed from smaller particles. This is the opposite trend to that observed in Fig. 1, where Pickering emulsions formed from smaller particles are more stable. This discrepancy is caused by the absence of gravitational forces in Eq. (1) , which act on a particle at the lower extremity of a Pickering droplet and destabilize Pickering emulsions prepared with larger particles. The difference in temperature between T_w and T_c is about 10°C for the mixtures of 2,6-lutidine and water, and around 20°C for the mixtures of methanol and cyclohexane, when using 1000 nm silica particles.

In addition, we evaluated the stability of Pickering emulsions containing 100 nm silica particles at various temperatures²³⁾ . Our findings indicate that these emulsions exhibit sufficient stability over time only when the temperature is increased beyond 20°C from the wetting transition temperature (38.0°C) . When water is used as a solvent, the critical temperature may not be observed practically due to bulk freezing or vaporization in most liquid mixtures. Nonetheless, our experiments suggest that the destabilization of Pickering emulsions may be caused for liquid mixtures with considerable compatibility even when the system is away from the critical temperature. The resulting small interfacial tension can occur over a wide temperature range. While small interfacial tensions are in general advantageous for emulsification processes as they reduce energy input, our data suggest that there is an optimal balance between the lowering of the liquid-liquid interfacial tension and the particle adsorption energy in Pickering emulsion systems.

Next example to consider the effect of the interfacial tension on the Pickering emulsion stability is that particles adsorbed at the oil-water interface can be replaced by coexisting surfactants if the surfactant adsorbed film can achieve lower interfacial tension than the particle laden interface²⁴⁾ .

Figure 2 (b) shows the interfacial tension between tetradecane and cetyltrimethylammonium chloride (CTAC) aqueous solutions. Filled squares show the interfacial tension values obtained in the absence of silica particles. The kink on the interfacial tension vs. temperature curve at 10.0°C is corresponding to the phase transition of CTAC adsorbed film from chain-melted liquid like monolayer (surface liquid) to chain-frozen one. The driving force of this surface freezing transition is enhanced lateral van der Waals attraction between CTAC hydrophobic chains and incorporated tetradecane molecules.

The open squares in Fig. 2 (b) show the interfacial tension measured in the presence of silica particles. The interfacial tension at temperatures above the surface freezing point decreased by the addition of silica particles, indicating that silica particles adsorbed at the water-tetradecane interface with CTAC adsorbed film in the surface liquid state. However, two interfacial tension curves coincided below 8.0°C: both interfaces are completely covered with CTAC-tetradecane surface frozen film and silica particles are expelled to the aqueous phase. The detachment of particles from the interface can be confirmed by spontaneous Pickering emulsion demulsification under cooling.

Fig.2

Interfacial tension vs. temperature curves measured at tetradecane- CTAC aqueous solutions in the absence (filled squares) and in the presence (open squares) of silica particles of 300 nm diameter. From (a) to (b) , CTAC concentration increased from 0.8 mmol kg^-1 to 1.0 mmol kg^-1 at a given silica concentration (0.0033 wt% in water) . From (b) to (c) , silica concentration increased from 0.0033 wt% to 0.05 wt% at a given CTAC concentration (1.0 mmol kg^-1) . Pickering emulsion prepared at 15°C was stable under cooling to 6°C (red arrows) in situations (a) and (c) , but demulsified in the situation (b) . Reproduced with the data originally published in the reference [24] , copyright (2023) Japan Oil Chemists' Society.

Figures 2 (a) and 2 (b) demonstrate the interfacial tension measurements taken at CTAC concentrations of 0.8 mmol kg^-1 and 1.0 mmol kg^-1. Interestingly, the interfacial tensions differed when silica particles were present or absent even below the surface freezing transition temperature, when the CTAC concentration was decreased to 0.8 mmol kg^-1. In this situation, Pickering emulsions remained stable at the lowest temperature tested. Similar behavior was observed when the silica particle concentration increased at a given CTAC concentration (1.0 mmol kg^-1) as shown in Figs. 2 (b) and 2 (c) . These observations suggest that the Pickering emulsion transition occurs only when the surfactant concentration is sufficient to replace all particles adsorbed at the surface of Pickering droplets.

The surface freezing transition temperature varies depending on the combination of surfactant and n-alkane (see Table 1) . Chain matching between surfactant and alkane is crucial to raise the surface freezing temperature. This is demonstrated by examples 3-5 in Table 1, where alkane mixtures were used as the oil phase, and their surface freezing temperature increased with the mole fraction of longer alkanes in the oil phase. Ellipsometric experiments for these systems revealed that the surface frozen monolayer was composed of CTAC and longer alkanes, while chain-mismatched shorter alkanes were excluded from the OW interface. Saturated alcohols can be used as a substitute for n-alkanes, which significantly increased the surface freezing temperature with their surface activity. In Table 1, we refer to these molecules as cosurfactants that are incorporated into the surfactant adsorbed film.

Table 1

Surface freezing temperature for various combinations of surfactant and cosurfactant.

We recently showed that the Pickering emulsion transition can be achieved at ambient temperatures by using CTAC-hexadecanol surface freezing, as demonstrated in example 7. Similarly, as in example 9, surface freezing also occurs with anionic surfactants, such as sodium hexadecyl sulfate (SHS) . However, determining the surface freezing temperature for nonionic surfactants can be challenging experimentally due to their solubilities in both the oil and water phases significantly changing with temperature.

When three bulk phases intersect at, for example, the solid-liquid-vapor contact line, this contact line can be characterized by an energy per unit length or line tension τ^{30) ,31) ,32)} . The line tension effects can be negligible for large particles where the interfacial energies dominate the line energy; however, this can be crucial for a nanoparticle (NP) at the air-liquid surface. In the last section, we briefly summarize recent knowledge about this issue.

McBride and Law measured the contact angle of dodecyltrichlorosilane coated silica spheres at the liquid polystyrene (PS) - air interface using atomic force microscopy (AFM) at the PS glass transition temperature (Fig. 3 (a) ) ³³⁾ . They found that the contact angle of silica decreased from 64.2° to 38.8° with decreasing particle radius from 498 nm to 88 nm. Using a modified Young equation,

(6)

the deviation of contact angle from its macroscopic value, θ_∞=64.2°, can be understood because of the line tension associated with the three-phase contact line of length 2Πb becomes comparable to the liquid PS surface tension, γ_LV, for small particles. The estimated line tension for this system was 0.93 nN. Substituting τ and measured γ_LV=39.9 mN m^-1 into Eq. (6) , the estimated contact angle for b=100 nm is approximately 57.5°.

They also claimed that there is a minimum angle, θ_min, below which single isolated particle can no longer exist at the liquid surface where

(7)

The obtained minimum contact angle, 41.2°, and corresponding minimum particle radius, 81.6 nm, are well coincided to those of the smallest silica spheres existed at the PS surface. The magnitude of the line tension is determined by the similarity in chemical structure of the ligand and that of the solvent. For example, the dodecanthioil ligated gold NPs showed much smaller line tension (～1 pN) at the surface of n-alkanes³⁴⁾ . In such a case, the line tension effects appear for much smaller particle radius and hence, the particles can be more stably exist at the liquid surface.

Evaluating the contact angle of particles adsorbed at an interface can be a challenging task. However, when the particles are sufficiently small, such as nanoparticles (NPs) , and an adsorption equilibrium exists between the particle dispersion and the interface, the contact angle can be determined by measuring the surface tension of the particle dispersion. We applied this theory to octadecanethiol ligated gold NPs (radius less than 12 nm) adsorbed at various n-alkanes³⁵⁾ . The line tension measured was in the order of piconewtons (pN) and even became negative for single nanometer radius gold NPs as shown in Fig. 3 (b) . The minimum contact angle theoretically disappeared for negative line tension systems, and the contact angle increased with decreasing NP radius to increase the length of energetically favored three-phase contact.

Fig.3

(a) Contact angles of silica nano particles with different radius at the polystyrene - air interface. Reprinted (adapted) with permission from [33] , copyright (2012) American Physical Society. (b) Contact angle and line tension measured for gold nanoparticles at the air-octadecane interface. The vertical axis on the right-hand-side of Fig. 3 (b) shows the line tension of gold nanoparticles at the air-octadecane interface. Reproduced from [35] , copyright (2018) American Chemical Society.

We have not conducted the line tension measurement for NPs at the OW interface. However, it is evident that the chemical structures of NPs must differ with either water or oil when adsorbed at the OW interface. Therefore, it is expected that the line instability of NP adsorption due to positive line tension is more commonly observed phenomenon at the oil-water interface. To minimize this effect, it would be effective to use particles such as Janus particles that have a high affinity for both water and oil.

Experimental validation of the aforementioned findings has not been conducted in Pickering emulsion systems. However, the instability of particle adsorption at the liquid-liquid interface due to line tension effects could provide a significant physicochemical insight responsible for the destabilization of Pickering emulsions when utilizing small particles.

This review provides an overview of recent insights into the stability of particles adsorbed at the oil-water (partly air-oil) interface. Key findings include:

(1) When emulsification fails due to low interfacial tension in compatible liquid mixtures, it is essential to consider the wetting transition and gravity effect. In this scenario, using particles with a smaller size can improve the situation.

(2) When a surfactant and n-alkanes (or n-alcohols) with similar chain lengths coexist in the system, the particles at the OW interface can be replaced by the surface frozen film of surfactants and n-alkanes (n-alcohols) .

(3) When the contact angle of particles at the interface varies with radius, the line tension effect must be taken into account. Line tension can be controlled by particle chemical coating, but it is practically challenging to modify the particle surface to have affinities for both water and oil at the OW interface.

Confirmation of the generality of spontaneous demulsification of Pickering emulsions by surface freezing and line tension measurements for particles at the OW interface will be a straightforward extension of our works.

This work was supported by JSPS KAKENHI (22K03551) , KOSÈ Cosmetology Research Foundation (2022) , Hosokawa Powder Technology Foundation (HPTF221166) , Iketani Science and Technology Foundation (0351182-A) , and Amano Institute of Technology (2023-2024) . The author appreciates Prof. B.M. Law at Kansas University for the collaboration of the original papers.

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