Anatomy of a Coacervate

Abstract

Coacervates, prepared by adding inorganic and organic salts to aqueous solutions of Aerosol OT (AOT), were analyzed for their AOT, salt, and water content. In addition, the volumes of the coacervates generated under standard conditions were determined. It was found that for inorganic salts, there is a “critical salt concentration” below and above which the volume of coacervate is very small. For example, NaCl has a critical concentration of 0.3 M, a concentration that will convert 50 mL of 0.02 M AOT into more than 7 mL coacervate (the remainder being equilibrium liquid that floats above the immiscible coacervate phase). At 0.2 or 0.6 M NaCl, the coacervate volume is reduced to 2 mL or less. The coacervate formed at the critical salt concentration has an [AOT] = 0.2 M and a [NaCl] = 0.3 M. Thus, a coacervate of 0.2 M AOT and 0.3 M NaCl is immiscible with 0.3 M NaCl despite the highly aqueous nature of both. This is attributed to the enthalpic requirements for breaking up three-dimensional AOT structures composed of bilayers. Critical concentrations of the alkali metals Li+, Na+, K+, Rb+, and Cs+ are 1, 0.3, 0.1, 0.1, and 0.1 M, respectively. Organic coacervators, such as n-octylammonium chloride and used in place of NaCl, have a profound effect upon the coacervation process. Thus, n-octylammonium ion has no critical salt maximum. Instead, the coacervate volume remains small until about 0.25 M added salt, whereupon the insoluble layer increases dramatically in volume to become the dominate phase. Coacervation is rationalized in terms of positive-to-negative changes in the spontaneous mean curvature of bilayers.