Weakly to strongly structured mixtures

Abstract

This paper addresses the question as to the differences between weakly and strongly structured mixtures of water (A), oils (${\mathit{B}}_{\mathit{k}}$), and nonionic amphiphiles such as alkylpolyglycol ether (${\mathrm{C}}_{\mathit{i}}$ ${\mathrm{E}}_{\mathit{j}}$). Starting with weak short-chain amphiphiles, it is shown that the three-phase body in such mixtures evolves from a tricritical point upon increasing either the carbon number k of the oil, or the amphiphilicity (i,j) of ${\mathrm{C}}_{\mathit{i}}$ ${\mathrm{E}}_{\mathit{j}}$. In the first case, the three-phase bodies grow monotonically with increasing distance from the tricritical point, with the mixtures, however, remaining weakly structured, whereas in the second case, the three-phase bodies first grow, then pass through maxima 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 maxima, one observes a gradual evolution of properties that distinguish weakly from strongly structured mixtures. This permits drawing a rather well-defined border line between weakly and strongly structured mixtures in (i,j)-k space.