Surface Adsorption and Orientation Near the Critical Point of Binary Liquid Mixtures

Critical binary liquid mixtures have proven to be extremely useful for quantitatively studying wetting and adsorption phenomena that occur at the surfaces of these systems. In this paper recent experimental developments in our understanding of adsorption gained via the study of critical mixtures are reviewed. At the noncritical liquid/vapor surface the component possessing the smallest surface tension completely saturates this surface for sufficiently large surface tension differences σ between the two components. This complete (or strong) adsorption is described by a universal scaling function P which depends upon the dimensionless depth z/ξ where ξ is the correlation length. Weakly polar and nonpolar binary liquid mixtures have been used to define the scaling function P. If the surface energy difference σ between the two components is small, then the surface is no longer completely saturated by one component. In this weak adsorption regime the adsorption behavior is now described by a universal function G of both z/ξ and σ. The functional dependence of G is elucidated by examining a homologous series of critical binary AB liquid mixtures where component A is an n-alkane while component B (methyl formate) is fixed. For this system σ changes sign with increasing n-alkane chain length. In addition to adsorption, dipole surface orientational order at the liquid/vapor surface of highly polar + nonpolar mixtures is also discussed. This orientational order is induced by the dipoleimage dipole iInteraction in the vicinity of a surface.