Ultralow interfacial tensions in water–n-alkane–surfactant systems

Abstract

The interfacial tensions between water- and oil-rich phases in the presence of microemulsions have been measured for ternary systems of water, n -alkanes, and nonionic alkylpolyglycolether surfactants ( C i E j ). It is found that the minimum of the interfacial tension curve, which is observed for each system in conjunction with the well-known phenomenon of phase inversion, depends sensitively, but systematically, on the chemical nature of the oil and the surfactant. Specifically, the minimum value of the interfacial tension σ̄ ab decreases by 1 order of magnitude on decreasing either the carbon number of the alkane k by 6, or the number of oxyethylene groups j by 3, or by increasing the number of carbon atoms in the surfactant tail i by 2. The numerical values of the interfacial tensions as a function of temperature are presented along with an empirical description previously suggested [R. Strey, Colloid and Polymer Sci. 272, 1005 (1994)]. From the analysis, in terms of bending energy one obtains estimates for the bending and saddle-splay constants. The similar shape of the interfacial tension curves permits a superposition of the data for all 19 systems in support of a scaling relation recently derived [S. Leitao, A. M. Somoza, M. M. Telo da Gama, T. Sottmann, and R. Strey, J. Chem. Phys. 105, 2875 (1996)]. Furthermore, we note a striking coincidence of the numerical values of critical amplitude ratio R=σ 0 ξ 0 2 =0.37 kT in near-critical systems and the product σ̄ ab ξ̄ 2 =0.44(±0.10) kT where ξ̄ is the maximum length scale in the bicontinuous microemulsions.