Complex fluids. Weakly to strongly structured mixtures

Abstract

This paper addresses the transition from weakly to strongly structured liquid mixtures of water, alkanes (Bk), and nonionic amphiphiles (CiEj). Starting with weak short-chain amphiphiles, it is shown that the three-phase bodies in such mixtures evolve from tricritical points (tcp) upon increasing either the carbon number k of Bk at fixed amphiphilicity (i,j) of CiEj, or upon increasing (i,j) at fixed k. In the first case, the three-phase bodies grow monotonically with increasing distance from the tcp, whereas in the second case, they first grow, pass through a maximum in the range of medium-chain amphiphiles, and shrink again as one proceeds to long-chain amphiphiles. In that range in which the three-phase bodies pass their maximum, one observes the gradual evolution of properties that distinguish weakly from strongly structured mixtures, such as the evolution of cmc surfaces, non-wetting of the water/oil interface by the amphiphile-rich middle phase, an o/w → w/o dispersion inversion near the mean temperature of the three-phase body, and a correlation peak in the I(q) curves of SANS. This permits drawing a rather well defined border line between weakly and strongly structured mixtures in (i,j)–k space.