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

The noncritical liquid/vapor surface is completely saturated with the component of smaller surface tension for sufficiently large surface tension difference between the two components of a critical liquid binary mixture. This complete (or strong) adsorption is described by a universal scaling function of the volume fraction order parameter. We have determined the universal function P; the models provide excellent descriptions of the ellipsometric data for a number of weakly polar and nonpolar binary liquid mixtures. In the limit of a small surface energy, the surface preference, however, depends strongly on the strength of the surface field and the surface is no longer completely saturated. We also studied the universal function G for this weak critical adsorption using a homologous series of critical liquid mixtures where their surface tension difference changes from negative to positive as the length of the carbon chain increases. The experimental results agree with theoretical predictions in both one-and two-phase regions. In addition to the surface adsorption, we will also discuss a dipole orientational order at the liquid/vapor surface of highly polar + nonpolar mixtures. Consideration of an electrostatic model provides a reasonable description of this effect.