Effect of elastic bending energy on the emulsification failure in a microemulsion

Abstract

Small-angle neutron scattering experiments have been carried out to study the phase stability of a very dilute water-in-oil microemulsion. At the fixed water to surfactant molar ratio of ω = 30, we observed a decrease in the microemulsion droplet radius and an increase in the polydispersity when the dispersed-phase volume fraction, φ, was dilute to less than 1%. Upon further dilution to below 0.1%, the droplets disappeared. This suggested that in the concentration range, 0.1% ⩽ φ ⩽ 1.0%, a single-phase microemulsion was unstable, and a phase separation, known as the emulsification failure, occurred. To understand the interesting phase behavior, we have formulated an analytic solution for the droplet size distribution function for a two-phase microemulsion based on the theory of Borkvec et al. ( J. Colloid Interface Sci. 131, 366 (1989)). We found that the distribution function attained a simple Gaussian form, and the width of the distribution function was inversely proportional to the renormalized elastic bending energy of the microemulsion droplets. Other features predicted by the theory were also in good agreement with the small-angle neutron scattering measurements.