Evolution of water-in-oil emulsion controlled by droplet-bulk ion exchange: acoustic, electroacoustic, conductivity and image analysis

Abstract

Water-in-kerosene emulsion stabilized with SPAN surfactant exhibits a slow transition (on scale of hours) from an emulsion to a mini-emulsion state. We continuously monitor this transition in the relatively concentrated samples (5 vol.% water), without dilution, using acoustic, electroacoustic and conductivity measurements. Continuous stirring prevents sedimentation. We confirm our measurements with microscopic image analysis and by comparing with a stable water-in-car oil microemulsion stabilized with AOT. Acoustic measurements yield information about the droplet size evolution in time. The original droplets, having a size of about 0.4 μm, slowly coalesce into larger droplets. After 10 h the droplet size has increased to about 5 μm. At this point a mini-emulsion fraction appears with a droplet size of only 25 nm and the droplet size distribution becomes bimodal. It takes another 24 h for the emulsion droplets to completely transform into a mini-emulsion state. The conductivity exhibits a rapid change during the first 10 h of emulsion coalescence, but the rate becomes much slower as the mini-emulsion fraction begins to grow. Electroacoustic measurements shows that the original emulsion droplets carry a substantial surface charge, which we are able to calculate using Shilov's theory for overlapped DLs. The measured electroacoustic signal gradually decays with time. In the final state the mini-emulsion droplets generate practically no electroacoustic signal and appear uncharged. This fact, combined with conductivity measurements, indicates a strong role for electrostatic factors in emulsion stability and its transition to mini-emulsion state. We suggest that ion exchange between the exterior and interior diffuse layers leads to a gradual collapse of the exterior DL and explains all the experimental observations.