Wetting in mixtures of water, nonionic amphiphiles, and nonpolar solvents

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Abstract
As is well known, medium‐ and long‐chain alkanes do not spread across H2O–air interfaces, but shape a lens. In this paper it is shown that the same holds for the upper amphiphile‐rich phase in binary H2O–amphiphile mixtures with medium‐ and long‐chain nonionic amphiphiles that show a (closed) miscibility gap with water. This finding is somewhat unexpected because surfactants form monolayers at H2O–air interfaces which should facilitate the spreading of the amphiphile‐rich phase. This wetting behavior corresponds to that in ternary H2O–oil–nonionic amphiphile mixtures with a three‐phase body, in which the middle amphiphile‐rich phase does not spread across the H2O–oil interface. The results may stimulate further studies on critical‐point wetting [for a recent review see, e.g., S. Dietrich, in Phase Transitions and Critical Phenomena, edited by C. Domb and J. L. Lebowitz (Academic, London, 1988), Vol. 12, p. 1.], and may also help clarifying the properties of microemulsions [for a recent review see, e.g., M. Kahlweit, R. Strey, P. Firman, D. Haase, J. Jen, and R. Schomäcker, Langmuir 4, 499 (1988)]