Apparent Molar Volume of Solubilized Water in AOT/Isooctane/Water Reverse Micellar Aggregates

Abstract

States of the solubilized water in reverse micellar solution of aerosol-OT (AOT, bis(2-ethylhexyl) sodium sulfosuccinate) in isooctane were investigated by determining the apparent molar volume of water, V w, through density measurements at different temperatures and water contents w o ([H 2 O]/[AOT]) of the solution. The low V w values in the range up to w o = ∼ 2 are due to the strong interaction of H 2 O molecules with the ionic headgroup of AOT. Then, V w exhibits a steep rise with water content in the range of w o = 2−6 owing to the swelling of the spherical reverse micelle, which is caused by the weaker binding of the additional water molecules to the ester group of AOT. In the range of w o = 6−12, V w exhibits a complex behavior as a function of the water content with a distinct minimum in the vicinity of w o = 10. This nonmonotonic course of V w as a function of water content in this range cannot be rationalized in terms of monotonic changes of the characteristics of the solubilized water in the micelle, or in terms of simple swelling of the reverse micelle. The minimum in the molar volume of the solubilized water is interpreted in terms of a structural rearrangement of the micellar aggregate from spherical to another (probably cylindrical) geometry, thus providing a larger interface for strong interaction between H 2 O molecules and the compact ionic headgroups of the surfactant molecules. Beyond the minimum (10 < w o ≤ 40), V w increases and displays slight undulations with increasing w o, which raises the possibility of further structural changes in the aggregate.