Dipole orientational order at the critical interface

We present experimental evidence that dipoles exhibit orientational order at the critical interface of mixtures of polar plus nonpolar liquids using the technique of ellipsometry. In this technique the ellipticity $\overline{\rho }$ 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 $\beta =0.328,\nu =0.632\mathrm{}$ are critical exponents. For polar fluids, however, the dipole-image dipole interaction at the interface can cause long-range orientational order resulting in deviations from this power-law divergence. Theoretical results predict that the surface orientational order parameter ${\alpha }_2\mathrm{}\left(z\right)\mathrm{}\sim m^{{*}^4}\left[d^2{v\left(z\right)/dz}^2\right],$ where $m^{*}$ is the reduced dipole moment and $v\left(z\right)\mathrm{}$ is the local composition at position z within the interface. We find quantitative agreement with these predictions for two different critical binary liquid mixtures composed of a highly polar plus nonpolar component.