Weakly to strongly structured mixtures

This paper addresses the question as to the differences between weakly and strongly structured mixtures of water (A), oils (Bₖ), and nonionic amphiphiles such as alkylpolyglycol ether (Cᵢ{E}ⱼ$). Starting with weak short-chain amphiphiles, it is shown that the three-phase body in such mixtures evolves from a tricritical point upon increasing either the carbon number k of the oil, or the amphiphilicity (i,j) of Cᵢ{E}ⱼ$. 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.