Influence of Surfactant Concentration on Spontaneous Emulsification Kinetics

Self-stabilising water-in-oil droplets through controlled spontaneous emulsification

HYPOTHESIS

The application of emulsion droplets as vessels for confined reactions, encapsulation, and material assembly is limited by the difficulty in achieving strong stabilisation against coalescence, particularly for water-in-oil (W/O) emulsions. Here, we investigate the time-dependent influence of spontaneous emulsification on macroemulsion stability, which remains poorly understood. We hypothesise that this non-equilibrium process can be harnessed as a practical stability-enhancing strategy and characterise the factors influencing this process in a variety of systems.

EXPERIMENTS

Daughter droplet behaviour in W/O macroemulsions predominantly containing the non-ionic surfactant Span 80 was studied through time-resolved optical microscopy, dynamic light scattering, and rheology. The effects of surfactant composition and viscosity on spontaneous emulsification behaviour and stability were investigated in mixed oil phases of isoparaffin and either oleic acid or mineral oil.

FINDINGS

Our study provides mechanistic insights and design guidelines for formulating emulsions which self-stabilise through daughter droplets assembling into elastic interfacial structures. This strategy does not require expensive components and is not specific to emulsion composition. Importantly, we also show that spontaneous emulsification does not always enhance stability. Effective stabilisation requires robust interfacial films between daughter and parent droplets and controlled continuous phase viscosity: if the viscosity is low, interfacial structures do not assemble, whereas a very high viscosity leads to buckling and deformation.

Factors, Mechanisms, and Kinetics of Spontaneous Emulsification for Heavy Oil-in-Water Emulsions

In challenging reservoirs where thermal recovery falls short, cold or chemical oil recovery methods are crucial. Spontaneous emulsification (SE), triggered by gentle disturbance, significantly enhances oil recovery. In elucidating SE mechanisms and kinetics, SE processes via direct contact between oil and aqueous phases without stirring were conducted. The effects of temperature, emulsifier concentration, pH, NaCl concentration, and the oil-to-water ratio on SE were investigated through droplet size analysis and turbidity measurements. Furthermore, the emulsification mechanism and derived emulsification kinetics based on turbidity data were obtained. The results underscore the feasibility of SE for oil-water systems, reducing viscous and capillary resistances without agitation. The emulsified oil mass increased with the temperature, pH, and aqueous-to-oil phase volume ratio while decreasing with the NaCl concentration. In this study, for GD-2 crude oil, the optimal emulsified oil amount occurred at a betaine surfactant (BetS-2) emulsifier concentration of 0.45%. Microscopic photo analysis indicated narrow particle size distributions and small droplets, which remained stable over time under various experimental conditions. A combined SE mechanism involving ultralow interfacial tension, interfacial turbulence due to Marangoni effects, and "diffusion and stranding" due to in situ emulsifier hydrophilicity, was speculated. Additionally, an analogous second-order kinetic equation for SE was proposed, indicating exceptional correlation with calculated and experimentally measured values. This study offers theoretical insight for enhancing oil recovery in chemical and cold production of heavy oil in oilfields.

Dynamic Oscillation and Motion of Oil-in-Water Emulsion Droplets

The interplay of interfacial tensions on droplets results in a range of self-powered motions that mimic those of living systems and serve as a tunable model to understand their complex non-equilibrium behavior. Spontaneous shape deformations and oscillations are crucial features observed in nature but difficult to incorporate in synthetic artificial systems. Here, we report sessile oil-in-water emulsions that exhibit rapid oscillating behavior. The oscillations depend on the nature and concentration of the surfactant, the chemical composition of the oil, and the wettability of the solid substrate. The rapid changes in the contact angle per oscillation are observed using side-view optical microscopy. We propose that the changes in the interfacial tension of the oil droplets is due to the partitioning of the surfactant into the oil phase and the movement of self-emulsified oil out of the parent droplets giving rise to the rhythmic variation in droplet contact-line. The ability to control and understand droplet oscillation can help model similar oscillations in out-of-equilibrium systems in nature and reproduce biomimetic behavior in artificial systems for various applications, such as microfluidic lab-on-a-chip and adaptive materials.