Dipole orientational order at the critical interface

Modification of a supported lipid bilayer by polyelectrolyte adsorption

Addition of a weak polyelectrolyte, poly(methacrylic acid) (PMA), to a supported phospholipid bilayer made from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) depresses the melting temperature and alters the morphology of the bilayer in the gel phase. Ellipsometry measurements show that PMA adsorption lowers the phase transition temperature by 2.4 degrees C. Atomic force microscopy (AFM) showed no visible contrast in the fluid phase (above the melting temperature) but a rich morphology in the gel phase. In the gel phase, adsorption leads to formation of significantly less mobile phospholipid islands and other defects. One consequence of this lower mobility is a decrease in the implied cooperativity number of the phase transition, N, when polymer is added. Additionally, AFM images of the gel-phase bilayer show a highly defected structure that anneals significantly more slowly than in the absence of adsorbed polymer. Tentatively, we suggest that PMA preferentially decorates island and defect edges of the DMPC bilayer.

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) approximately -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(s)(o+)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(s)(o+)>xi(b)(o+) (the bulk correlation length amplitude).

Dipole orientational order at liquid/vapor surfaces

We present ellipsometric observations of the orientational order alpha(2) of highly polar molecules at the noncritical liquid/vapor surface of critical polar+nonpolar mixtures. The dipoles, which are repelled from the interface via interactions with their image dipoles, are preferentially oriented with their axes parallel to the surface and possess an orientational order which is well described by alpha(2) approximately -t(2beta)D+(z/xi), where t=[T-T(c)]/T(c) is the reduced temperature, beta=0.328 is a critical exponent, and D+ is a universal function of the dimensionless depth z/xi with surface correlation length xi.

Computer simulation study of the interface width of the liquid/liquid interface

Four molecular dynamics computer simulations have been performed to study the intrinsic width and the width due to thermal fluctuations of the water/carbon tetrachloride interface. We observed that thermal fluctuations have a capillary wave character. The surface tension calculated by using capillary wave formalism shows a very good agreement with the value obtained from the components of the pressure tensor.