Volumetric determination of reverse micelle structural properties and the validity of commonplace approximations

Abstract

Molecular volumes within reverse micelles are important in determining many aggregate properties. Molecular volumes also depend upon differences in molecular environment and thereby might also be capable of detecting changes in molecular structure and interactions within an aggregate. However, approximations are typically used in their place along with approximations of aggregate radius, aggregation number, surfactant tail length, and reverse micelle shape particularly for Aerosol OT systems. Consequently, volumetric measurements have not been thoroughly tested as a measure of aggregate properties. Volumetric methods do not require the commonly used approximations. Therefore, comparison with previous approaches will demonstrate volumetric measurements’ utility in measuring aggregate structural properties and provide a useful assessment of the applicability of these commonplace approximations. Partial molar volumes of water and surfactant, evaporation rate, and reverse micelle radius are measured over the w0 = 0–40 range for water/Aerosol OT/isooctane solutions. These results are used to calculate how aggregation number, surfactant layer thickness, head group area, and water droplet structure change with aggregate radius. Volumetric measurements reveal a surfactant layer structural transition at w0 = 5, an aggregate shape change at w0 = 25, and reproduced the general trends in structural properties. Important variations from approximated methods are noted. Molecular volume and aggregate radius approximations were found valid in many but not all situations. However, approximations of aggregation number and tail length created important miscalculations of some properties when treating reverse micelles as spherical. It was found that consideration of an ellipsoidal reverse micelle shape for w0 < 10 could account for previously identified changes in surfactant layer thickness and head group area that were based on a spherical geometry.