Dipole orientational order at the critical interface - art. no. 011507

We present experimental evidence that dipoles exhibit orientational order a t the critical interface of mixtures of polar plus nonpolar liquids using t he technique of ellipsometry. In this technique the ellipticity <(<rho>)ove r bar> at the critical interface for all nonpolar or weakly polar fluids or fluid mixtures diverges as t(beta-nu) where t=(T-c - T)/T-c is the reduced temperature relative to the critical temperature T-c and beta= 0.328, nu= 0.632 are critical exponents. For polar fluids, however, the dipole-image d ipole interaction at the interface can cause long-range orientational order resulting in deviations from this power-law divergence. Theoretical result s predict that the surface orientational order parameter alpha (2)(z)simila r tom*(4)[d(2)nu (z)/dz(2)], where m* is the reduced dipole moment and nu ( z) is the local composition at position z within the interface. We find qua ntitative agreement with these predictions for two different critical binar y liquid mixtures composed of a highly polar plus nonpolar component.