Predicting Spontaneous Emulsification in Saltwater Environments Using the HLD Model

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.

Nanoemulsions Stable against Ostwald Ripening

Ostwald ripening, the dominant mechanism of droplet size growth for an O/W nanoemulsion at high surfactant concentrations, depends on micelles in the water phase and high aqueous solubility of oil, especially for spontaneously formed nanoemulsions. In our study, O/W nanoemulsions were formed spontaneously by mixing a water phase with an oil phase containing fatty alcohol polyoxypropylene polyoxyethylene ether (APE). By monitoring periodically the droplet size of the nanoemulsions via dynamic light scattering, we demonstrated that the formed O/W nanoemulsions are stable against Ostwald ripening, i.e., droplet growth. In contrast, the nanoemulsion droplets grew with the addition of micelles, demonstrating the pivotal role of the presence of micelles in the water phase in the occurrence of Ostwald ripening. The influence of the initial phase of APE, the oil or water phase in which APE is present, on the micelle formation is discussed by the partition coefficient and interfacial adsorption of APE between the oil and water phase using a surface and interfacial tensiometer. In addition, the spontaneously formed O/W nanoemulsion, which is stable against Ostwald ripening, can be used as a nanocarrier for the delivery of water-insoluble pesticides. These results provide a novel approach for the preparation of stable nanoemulsions and contribute to elucidating the mechanism of instability of nanoemulsions.

Quantifying the Influence of Electrolytes on the Kinetics of Spontaneous Emulsification

This study quantifies the influence of electrolytes on the kinetics of the spontaneous emulsification phenomenon (SEP) of heavy hydrocarbons in a nonionic surfactant solution. The rate of emulsifying hexadecane in Triton X-100, with the presence of sodium chloride and potassium chloride, has been measured using a technique of monitoring single oil droplet photography. The emulsion droplet size produced in the process was measured under the same conditions by using dynamic light scattering. The data obtained from the two experiments were employed to investigate the mass transfer coefficient of the surfactant molecules through the intermediate layer formed between hexadecane and the surfactant solution. It was found that the electrolytes in an aqueous solution increase the surfactant diffusion rate through the intermediate layer and reduce the emulsion droplet size. As a result, both electrolytes reduce the rate of spontaneous emulsification, with potassium chloride having a more substantial reduction. A model was developed to quantify the influence of electrolytes on the kinetics of the SEP. The data and modeling results verify the influence of ions on the kinetics of spontaneous emulsification. The results provide a significant foundation for predicting the solubilization of heavy hydrocarbons in an electrolyte solution.

Spontaneous Emulsification: Elucidation of the Local Processes

While various attempts have been made to understand the mechanism of spontaneous emulsification within the scopes of equilibrium and nonequilibrium thermodynamics, the local processes underlying spontaneous emulsification still remain elusive. In this research, we investigate the local processes that involve the transfer of surfactants as well as water from an aqueous phase to oil, which results in the formation of a water-in-oil emulsion in the organic phase. Subsequently, these emulsions invert into the oil-in-water emulsion rather abruptly as they cross the phase boundary. Studies based on ultraviolet (UV) spectroscopy and nonequilibrium molecular dynamics simulations indicate that the crossing of the phase boundary may involve catastrophic explosions and subsequent assembly of the resulting fragments to other organized structures. These processes lead to the fluctuation of the component concentrations below the interface that also becomes evident in the fast (athermal) diffusion of the emulsion droplets from the interfacial region farther into bulk water. Spontaneous emulsification is found to be amplified in mixed solvents, but it can be arrested with additives that reduce solubility or inhibit the transfer of water and surfactants in the organic phase.

Spontaneous Emulsions: Adjusting Spontaneity and Phase Behavior by Hydrophilic-Lipophilic Difference-Guided Surfactant, Salt, and Oil Selection

Spontaneous emulsion behavior has been difficult to predict and could be influenced by many variables including salinity, temperature, and chemical composition of the oil and surfactant. In this work, the hydrophilic-lipophilic difference (HLD) framework was used to predict the formation of spontaneous emulsions using a mixture of Span-80 and SLES surfactants. The spontaneity and emulsion behavior of different systems were modeled by estimating the HLD_mix. The influence of surfactant ratio, salinity, and oil type was investigated. Spontaneous emulsification could only be observed when the HLD_mix was between -0.96 and 1.04. Within this range, a negative HLD_mix resulted in a greater spontaneity to form o/w emulsion, and a w/o emulsion was more likely to form when the HLD_mix was positive. When the HLD_mix was close to 0 (between -0.22 and 0.56 in our systems), emulsions were formed in both the oil and aqueous phases with high spontaneity. A combined effect of ultralow interfacial tension, Span-80 micelle swelling, and interfacial turbulence due to Marangoni effects is likely the main mechanism of the spontaneous emulsification observed in this study. A synergistic reduction in interfacial tension was observed between Span-80 and SLES (<1 mN/m). When the HLD of the system was close to 0, a bicontinuous emulsion phase was formed at the oil-water interface. The bicontinuous emulsion broke-up over time due to the ultralow interfacial tension and interfacial turbulence, forming dispersed oil and water droplets. Results from this work provide a practical method to suggest what surfactant composition, salinity, and oil type could promote (or eliminate) the conditions favorable for spontaneous emulsification.