Illuminating the Impact of Submicron Particle Size and Surface Chemistry on Interfacial Position and Pickering Emulsion Type

Abstract

Pickering emulsions are increasingly applied in the production of medicines, cosmetics, and in food technology. To apply Pickering emulsions in a rational manner it is insufficient to examine properties solely on a macroscopic scale, as this does not elucidate heterogeneities in contact angles (θ) of individual particles, which may have a profound impact on stability and microstructure. Here, we apply the super-resolution technique iPAINT to elucidate for the first time the microscopic origins of macroscopically observed emulsion phase inversions induced by a variation in particle size and aqueous phase pH. We find θ of single carboxyl polystyrene submicron particles (CPS) significantly decreases due to increasing aqueous phase pH and particle size, respectively. Our findings confirm that θ of submicron particles are both size- and pH-dependent. Interestingly, for CPS stabilized water-octanol emulsions, this enables tuning of emulsion type from water-in-oil to oil-in-water by adjustments in either particle size or pH.

The position of stabilizing submicron particles at the liquid–liquid interface of droplets in a Pickering emulsion determines its type: water-in-oil (w/o) or oil-in-water (o/w).¹ For applications where tailor-made Pickering emulsions are required the formulation of well-defined emulsions at equivalent conditions, but with different microstructures, is optimal. Inverting the type of a Pickering emulsion conventionally requires significant manipulation of the system components. It has been acknowledged that inversion of Pickering emulsion type can be achieved through multiple handles such as altering the oil–water ratio, particle concentration, grafting functionalization, particle roughness, or the addition of a surfactant to modify particle wettability.¹⁻⁸ However, by utilizing less invasive handles we can feasibly create a series of stable Pickering emulsions from the same components but with different microstructures.

The phase inversion of emulsions stabilized with ionizable particles by adjusting the pH or ionic strength of the aqueous phase has been reported.⁹ Carboxyl polystyrene latex (CPS) particles are particularly suited to investigate the potential of these handles on Pickering emulsions. The carboxylic acid groups coating the surface of the particles are un-ionized at low to moderate pH, rendering the particles hydrophobic and as such stabilizing w/o emulsions. Conversely, at high pH the carboxyl groups become increasingly ionized, and as such the particles have the potential to stabilize o/w emulsions. Binks et al. (2005) reported the inversion of hexadecane–water (1:1 volume) emulsions stabilized by 200 nm CPS particles in 1 M NaCl from w/o to o/w by lowering the surface potential and increasing the pH above 10.⁹ More recently it was observed that, for submicron particles, particle size has a non-negligible impact on particle position at fluid interfaces.^10,11 However, inversion of emulsion type merely by changing the particle diameter has not yet been reported. Here, for the first time we showcase a system where the Pickering emulsion type can be inverted merely by changing the size of the stabilizing submicron particles. First, we investigate the macroscopic structure of Pickering emulsions by confocal microscopy. Then, the super-resolution microscopy technique interface Point Accumulation for Imaging in Nanoscale Topography (iPAINT) is applied for the first time to Pickering emulsions with the aim to identify handles for phase inversion. The application of iPAINT requires the use of a (poly(ethylene glycol) (PEG) conjugated) fluorescent probe, which slightly alters the physicochemical properties of the system. However, iPAINT advantageously allows for the direct, simultaneous visualization of submicron particles adsorbed at fluid interfaces in situ at nanometer resolution. Specifically, iPAINT is used to precisely identify the transition in interfacial position of individual submicron particles as a result of varying pH or particle size. By utilizing the impact of both size and ionizability on particle position we have two minimally invasive approaches to selectively influence emulsion microstructure.

To investigate the impact of the submicron particle size and ionizability on emulsion type, a range of Pickering emulsions was created and imaged using confocal microscopy (Figure 1).

Figure 1.

Effect of pH and particle size on the type of Pickering emulsions. Confocal imaging of octanol–water (1:1 volume) Pickering emulsions stabilized by 2 wt % 320 nm (a–c) and 810 nm (d–f) CPS particles at pH 5, 8, and 12.5. Nile red was used to stain the oil phase (in green), while the aqueous phase is nonfluorescent (dark). Scale bars 100 μm. Inversion from w/o to o/w emulsion can be observed upon increasing pH and particle size, respectively.

Emulsions of octanol–water (1:1 v/v) were stabilized by 2 wt % CPS particles of 320 and 810 nm diameter with an equivalent surface charge density of 1.33 groups/nm² at pH 5, 8, and 12.5. To stain the oil phase 0.05 wt % of Nile red was incorporated prior to emulsification (Supporting Information, Section 1.2). From previous studies it is expected that, under conventional conditions, that is, 1:1 volume ratios and mild pH, CPS particles preferentially stabilize w/o emulsions.⁹ This is exemplified by the w/o emulsions stabilized by 320 nm CPS particles (Figure 1a–c). Additionally, a pH-induced inversion from w/o to o/w can be observed for Pickering emulsions stabilized by 810 nm CPS particles (Figure 1d–f). This inversion is attributed to acid dissociation of the carboxyl group causing a transition of particle wettability from hydrophobic to hydrophilic. Furthermore, ζ-potential measurements demonstrated a correlation in charge increase as pH increases (Figure S4). However, this hypothesis cannot be confirmed solely by visualization of the bulk emulsion due to concomitant phenomena, which could induce phase inversion of Pickering emulsions, such as particle concentration, mixing conditions, phase viscosity, and water-to-oil ratio.^3,12−14 Additionally, a transition in particle wettability exemplified by a shift in particle position has so far proven difficult to confirm on the single particle level.

Remarkably, a second phase inversion can be observed merely by varying the CPS particle diameter from 320 to 810 nm. At pH 8 (Figure 1b,e) and pH 12 (Figure 1c,f) 320 nm CPS particles stabilize w/o emulsions, whereas o/w emulsions were stabilized by the 810 nm CPS particles. Size-induced inversion presents an important, simple, and noninvasive new handle for the formulation of Pickering emulsions, although the mechanism of this inversion remains unclear on the bulk scale. Therefore, to more closely examine the two inversion phenomena presented here, the interfacial position of the CPS particles as a function of pH and size requires elucidation on the single particle level.

To precisely visualize the position of the colloidal particles and the location of the fluid interface simultaneously, in situ, we employ a recently developed super-resolution (iPAINT).¹⁵⁻¹⁹ For in situ measurement of the position of submicron particles at the octanol–water interface the aqueous particle dispersion is enriched with iPAINT probes, a poly(ethylene glycol) chain carrying a photoactivatable functionality (Supporting Information, Section 1.3).¹⁵ The aqueous phase is then placed on a microscope slide beside an octanol droplet of equal volume and sandwiched with a coverslip to create the fluid interface (Supporting Information, Sections 1.4 and 1.5, Figure S1). A low-power UV light exposure acts as an external trigger to uncage a subset of the photoactivatable probes, ensuring that, stochastically, the distance between two probes in the ON state (fluorescent) is greater than the diffraction limit of light (∼250 nm at λ = 500 nm).²⁰ Given that the iPAINT probes are physically confined in the aqueous phase, all interfaces accessible to water (particles, fluid interface, and microscope coverslip) are coated and fluorescently labeled (green markers in Figure 2), allowing for localization. Following this, the particle position at the interface can be quantified as the contact angle (θ). A disadvantage of the iPAINT technique relates to the influence of PEG–particle and particle–coverslip interactions on the force balance. Although the addition of the iPAINT probe systematically increases the hydrophilicity of the system it concurrently partially shields the particle surface charge, as reflected by ζ-potential measurements (Figure S5). Significantly, the increase in particle charge over corresponding pH remains discernible. Therefore, the systematic impact of significant parameters such as particle size and surface chemistry on particle position can be clearly elucidated by iPAINT.

Figure 2.

Impact of pH on the position of single CPS particles (⌀ = 810 nm) at the water-octanol interface as measured by iPAINT. The pH of the aqueous phase was adjusted to (a) 2, 4, 5, and 6. The red line locates the interface between the aqueous phase (green) and the oil phase (dark). Scale bars 500 nm. (b) Contact angles calculated for n > 30 particles at each pH value.

We begin by examining individual 810 nm CPS particles adsorbed at the octanol–water interface (Figure 2). From the iPAINT visualization, we can retrieve the particle position in relation to that of the fluid interface (Supporting Information, Section 1.6), and hence compute θ, here through the aqueous phase (Equation S1). The transition of the particle position from predominantly oil wetted to water wetted over the pH range from 1.8 to 6 is clearly illuminated in Figure 2a.

The transition of particle position can be quantitatively described by the contact angle of the particle and indicates the particle wetting. In this case, the inversion of particle wetting from hydrophobic (θ > 90°) to hydrophilic (θ < 90°) as the pH of the aqueous phase increases is apparent. Analyzing the contact angle of each particle over the pH range firmly validates this phenomenon (Figures 2b and S8–S10). A sharp decrease in contact angle is visible, with inversion of particle wettability occurring at pH ≈ 3.5. The dependence of the contact angle on pH for each particle size is provided in the Supporting Information (Figure S10).

As exemplified by the bulk emulsions (Figure 1), phase inversion of water-octanol emulsions stabilized by CPS particles from w/o to o/w results from not only the ionization of surface groups but also by varying the particle size. From previous studies, it is known that, for sub-micrometer colloids, contact angles scale linearly with particle size.^10,11 To evaluate the scope of this dependency, we extend the iPAINT investigation over a large size range. The impact of particle diameter on interfacial position was examined for CPS particles of 320, 450, 510, and 810 nm with an equivalent surface charge density of 1.33 groups/nm² (Figure 3).

Figure 3.

Impact of size on the position of single CPS particles at the water–octanol interface measured by iPAINT. CPS particles with diameters of (a) 320, 450, 510, and 810 nm adsorbed at the water–octanol interface (pH 4). The red line locates the interface between the aqueous phase (green) and the oil phase (dark). Scale bars 500 nm. (b) Dependence of the contact angle of single particles on particle size and pH in the range from 1.8 to 6. θ is calculated for n > 30 particles for each pH. The error bars stem from the standard deviation on θ and ⌀.

Via direct visualization of the CPS particles (pH 4) at the octanol–water interface we observe the transition from predominantly the oil phase (θ > 90°) to the water phase (θ < 90°), as the diameter of the particle increases (Figure 3a). Significantly, a non-negligible dependence of contact angle on particle size can be observed for all conditions examined (Figure 3b). Furthermore, it is apparent that the pH effect holds for all measured particle sizes. These findings imply that the stability of Pickering emulsions significantly depends on particle size dispersity and surface heterogeneity, as these parameters determine whether the particle contact angle is smaller or larger than 90°.

Finally, we extend the investigation to compare the impact of particle size on interfacial position by probing vastly different surface chemistries within a similar size range. Individual silica particles with 80 < ⌀ < 700 nm at pH 5 were examined at the octanol–water interface, and their positions were compared to those of CPS particles over the corresponding size range and pH value (Figure 4, Figure S6, Figure S7). As expected, the contact angles measured for silica particles, an inherently hydrophilic material, are systematically lower than that of the CPS particles (Figure 4). Additionally, we demonstrate that a non-negligible dependence of contact angle on particle size exists regardless of the particle surface chemistry. The difference observed in the slopes of the linear fit for CPS (slope = −0.01) and silica (slope = −0.03) particles is ascribable to the interaction of each material with the iPAINT probe.

Figure 4.

Dependence of the contact angle of single particles on size for CPS particles (△) and silica (□) particles at pH 5 at the water-octanol interface. θ is calculated for n > 30 particles for each surface chemistry. The error bars stem from the standard deviation in θ and ⌀.

The present work illuminates the impact of submicron particle size and surface chemistry on the position of individual particles at oil–water interfaces by super-resolution microscopy. Our results reveal the mechanism of two subtle and independent means to invert Pickering emulsions. As the wettability of ionizable CPS submicron particles can be adjusted significantly from fairly hydrophobic to fairly hydrophilic by both the degree of ionization and particle size, CPS-stabilized Pickering emulsions can be inverted simply by tuning the pH and/or particle size. Moreover, we validate that the inversion of a Pickering emulsion is reliant on the size of stabilizing particles by demonstrating that contact angle is size-dependent, regardless of particle surface chemistry. Exploiting the size effect on the wettability of the particles in combination with the pH effect, we are presented with a unique and noninvasive approach to design custom Pickering emulsions. Future application of iPAINT can aid in the bottom-up design and formulation of Pickering emulsions, as well as being a tool to investigate the effects of parameters such as surface roughness and particle shape on the interfacial position of single particles.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c00709.

• Experimental details relating to emulsion preparation, confocal microscopy, and iPAINT microscopy, as well as additional figures, particle characterization (SEM imaging and ζ-potential measurements), super-resolution microscopy analysis, and iPAINT extended data (PDF)

Author Contributions

^⊥ E.C.G. and A.A. contributed equally. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.