Surface orientational order at liquid-vapor interfaces induced by dipole-image-dipole interactions

Highly dipolar molecules become orientationally ordered in the vicinity of a surface due to the electrostatic interaction with their image dipoles. We study this orientational order alpha(2) at the noncritical liquid-vapor interface of critical dipolar+nonpolar mixtures using ellipsometry. The dipolar molecules, which are strongly desorbed from the interface, possess an orientational order which is well described by alpha(2)similar to-t(2beta)D(+/-)(z/xi), where t=\T-T-c\/T-c is the reduced temperature relative to the critical temperature T-c, D+/- is a universal function of the distance z and the surface correlation length xi=xi(o+)(s)t(-nu), while beta=0.328 and nu=0.632 are standard critical exponents. The dipoles, which are preferentially oriented parallel to the surface, are repelled by their image dipoles, where xi(o+)(s)>xi(o+)(b) (the bulk correlation length amplitude).