POLAR-MOLECULES IN PLANAR INTERFACES - ROLE OF THE TRIPLET DIRECT CORRELATION-FUNCTION IN THE ASYMPTOTIC-BEHAVIOR OF THE PROFILE

We investigate the density-orientation profile of a polar liquid in planar interfaces. This liquid, which represents one side of the interface, is coupled with a second medium which can be an ideal solid characterized only by its dielectric constant or another polar liquid. We derive an expression for the asymptotic behavior of the profile which is exact provided the molecules interact by an additive pairwise potential. As a consequence of the renormalization of bonds, we show that the result does not depend explicitly on the particular form of the short range part of the pair potential. Moreover, the asymptotic behavior of the profile does not depend on the physical origin of the second medium provided it behaves as a dielectric. In addition, we show that the result remains exact in presence of a soft wall and can be extended to the liquid-vapor interface. As established in a previous paper, we show that the isotropic part of the profile is intimately connected with the electrostriction phenomena observed in bulk phase when a strong electrical field is applied. In order to clarify this point, we derive the expression of the electrostriction constant from the relation density-one body direct correlation function which has been used for studying the profile. We point out that the electrostriction and the long tail of the profile represent two ways for investigating some orientational components of the triplet direct correlation function in bulk phase. More generally, we show that the triplet direct correlation function largely determines the orientational structure of the profile. © 1990 American Institute of Physics.