Catherine P. Whitby

Some general features of limited coalescence in solid-stabilized emulsions

Abstract.We produce direct and inverse emulsions stabilized by solid mineral particles. If the total amount of particles is initially insufficient to fully cover the oil-water interfaces, the emulsion droplets coalesce such that the total interfacial area between oil and water is progressively reduced. Since it is likely that the particles are irreversibly adsorbed, the degree of surface coverage by them increases until coalescence is halted. We follow the rate of droplet coalescence from the initial fragmented state to the saturated situation. Unlike surfactant-stabilized emulsions, the coalescence frequency depends on time and particle concentration. Both the transient and final droplet size distributions are relatively narrow and we obtain a linear relation between the inverse average droplet diameter and the total amount of solid particles, with a slope that depends on the mixing intensity. The phenomenology is independent of the mixing type and of the droplet volume fraction allowing the fabrication of both direct and inverse emulsion with average droplet sizes ranging from micron to millimetre.

Effect of adding anionic surfactant on the stability of Pickering emulsions.

We investigated the structure and stability of dodecane-in-water emulsions stabilised by partially hydrophobised silica particles after dilution of the emulsions in solutions of sodium dodecyl sulfate and sodium chloride. The emulsions were stable to coalescence on dilution in salt solutions, but did cream over time. The rate and extent of creaming gradually decreased as the salt concentration in the diluted emulsion increased. Dilution in low concentrations of the anionic surfactant did not affect the emulsion stability to coalescence or alter the creaming behaviour of the emulsion. At surfactant concentrations above the critical micelle concentration, however, the rate and extent of creaming and flocculation of the drops were enhanced.

Interfacial displacement of nanoparticles by surfactant molecules in emulsions.

The remarkable stability of nanoparticles attached to oil-water interfaces in macroemulsions hinders controlled detachment of these particles from emulsions. In this work it is shown that adding surfactant molecules which preferentially adsorb at the oil-water interface displaces nanoparticles from the interface. Surfactant adsorption at the oil-water interface is energetically favoured and readily occurs on mixing nanoparticle-stabilised oil-in-water emulsions with surfactant solutions. Depending on the surfactant concentration, there is a significant reduction in the interfacial tension. Hence there is substantial fragmentation of the oil droplets and foaming of the emulsion during mixing. Surfactant concentrations above the critical micelle concentration are required to achieve complete interfacial displacement and hence recovery of the nanoparticles from the emulsions. The effects of surfactant addition have important implications for tailoring the interfacial composition of emulsions.

Effect of oil soluble surfactant in emulsions stabilised by clay particles.

Although surfactants and particles are often mixed together in emulsions, the contribution of each species to the stabilisation of the oil-water interface is poorly understood. We report the results of investigations into the formation of emulsions from solutions of surfactant in oil and aqueous suspensions of laponite. Depending on the salt concentration in the aqueous suspensions, the laponite dispersed as individual disc-shaped particles, 30 nm in diameter, or flocculated into aggregates tens of micrometres in diameter. At the concentrations studied, the flocculated particles alone stabilized oil-in-water emulsions. Synergistic interactions between the particles and octadecylamine at the oil-water interface reduced the average emulsion drop size, while antagonistic interactions with octadecanoic acid enhanced coalescence processes in the emulsions. The state of particle dispersion had dramatic effects on the emulsions formed. Measurements of the oil-water interfacial tension revealed the origins of the interactions between the surfactants and particles.

Shear-induced coalescence of oil-in-water Pickering emulsions.

This work reports on coalescence in oil-in-water Pickering emulsions subjected to simple shear flow. The emulsions were stabilized by silanized fumed silica particles forming layers a few hundred nanometers thick around drops that are tens of micrometers in size. The drop size and particle concentration in the emulsions were fixed, while the salt concentration was varied to adjust the colloidal interactions between the drops and particles. At rest the oil drops do not coalesce. The susceptibility of the drops to orthokinetic coalescence was found to depend on the extent of particle flocculation in the attached particle layer. The evolution of the drop size with time and shear rate was consistent with phenomenological models used to describe the behavior of emulsions under shear.

Oil-in-water Pickering emulsion destabilisation at low particle concentrations.

Droplet evolution in unstable, dilute oil-in-water Pickering emulsions was characterised using a combination of light scattering, confocal microscopy and rheology. Emulsions were formed at concentrations of silanised fumed silica particles that are not sufficient to prevent destabilisation. The key result is that destabilisation initially occurs via a combination of droplet flocculation and permeation. Close contact between the drops enhances oil transfer from smaller drops to the larger ones. The large drops swell over time until the attached particle density is insufficient to protect the drops against coalescence. Examination of the emulsion microstructure revealed the relationship between drop stability and the structural characteristics of the aggregates formed due to coagulation of the silica particles in the emulsions. The implications of these results for controlling Pickering emulsion stability are discussed.

The Role of Porous Nanostructure in Controlling Lipase-Mediated Digestion of Lipid Loaded into Silica Particles

The rate and extent of lipolysis, the breakdown of fat into molecules that can be absorbed into the bloodstream, depend on the interfacial composition and structure of lipid (fat) particles. A novel method for controlling the interfacial properties is to load the lipid into porous colloidal particles. We report on the role of pore nanostructure and surface coverage in controlling the digestion kinetics of medium-chain and long-chain triglycerides loaded into porous silica powders of different particle size, porosity, and hydrophobicity/hydrophilicity. An in vitro lipolysis model was used to measure digestion kinetics of lipid by pancreatic lipase, a digestive enzyme. The rate and extent of lipid digestion were significantly enhanced when a partial monolayer of lipid was loaded in porous hydrophilic silica particles compared to a submicrometer lipid-in-water emulsion or a coarse emulsion. The inhibitory effect of digestion products was clearly evident for digestion from a submicrometer emulsion and coarse emulsion. This effect was minimal, however, in the two silica-lipid systems. Lipase action was inhibited for lipid loaded in the hydrophobic silica and considered due to the orientation of lipase adsorption on the methylated silica surface. Thus, hydrophilic silica promotes enhanced digestion kinetics, whereas hydrophobic silica exerts an inhibitory effect on hydrolysis. Evaluation of digestion kinetics enabled the mechanism for enhanced rate of lipolysis in silica-lipid systems to be derived and detailed. These investigations provide valuable insights for the optimization of smart food microparticles and lipid-based drug delivery systems based on lipid excipients and porous nanoparticles.

Formation and stability of nanoparticle-stabilised oil-in-water emulsions in a microfluidic chip.

The formation and stability of drops in the presence of nanoparticles was studied in a microfluidic device to directly observe the early stages of Pickering emulsification (low interfacial coverage). We observed several key differences between oil droplet necking and rupture in aqueous phases of nanoparticles (methylated silica) and well-characterised surfactant systems. The presence of particles did not influence drop formation dynamics and thus the size of the drops generated. In addition, observations of in-channel drop stability shortly after formation (several milliseconds) indicated that particles in the aqueous phase slow film thinning processes, but do not prevent coalescence. In contrast, downstream collection and densification (at the microchannel outlet), showed that particle-stabilised drops do not coalesce for several weeks, above a critical particle concentration. The implications of our results for droplet microfluidics and our understanding of conventional emulsification systems are discussed.

Structure of concentrated oil-in-water Pickering emulsions

Following the structural changes in unstable Pickering emulsions is difficult due to the emulsion turbidity. We studied droplet packing in concentrated oil-in-water emulsions stabilized by silanised silica particles using confocal fluorescence microscopy to image thin sections of the emulsions. As the volume fraction of the drops approaches close packing, they deform into polyhedral shapes and flattened areas of contact between droplets appear. We show that the increase in the average number of nearest neighbours of a drop is a power law function of the drop volume fraction, consistent with theoretical predictions. At the volume fraction where the emulsions start to break down, ϕ = 0.88, there is a jump in drop elongation and thus in the number of drops in contact with each other. Drops increase in size, with some forming shapes that resemble an intermediate stage of the coalescence process. A few large drops grow more than most. A key finding is that the rigidity of the droplet surfaces controls the destabilization mechanism.

Structure of oil-in-water emulsions stabilised by silica and hydrophobised titania particles.

We have studied the stability and structure of emulsions formed in the presence of colloidal mixtures of partially hydrophobic titania particles and hydrophilic silica particles. On their own, the titania particles attached strongly to the oil-water interface and stabilised emulsions, while the silica particles did not attach to the interface. Adding silica particles to the titania dispersions enhanced coalescence processes during emulsion formation, except under mixing conditions that favoured particle heteroaggregation. The destabilisation of the emulsions was linked to the presence of silica particles in the particle layers at the interface.