Hydration force, steric force or double-layer force between zwitterionic surfaces?

Abstract

The force between two parallel planar zwitterionic surfaces has been calculated using Monte Carlo computer simulations. The zwitterions are modelled as two oppositely charged hard spheres joined by a string of length L with parameters chosen to mimic a phospholipid system. All centres interact by a homogeneous Coulomb interaction and by a hard-sphere exclusion. The negative centres are anchored to the surface by a parabolic potential.For distances D between the surfaces, as defined by the location of the negative centres, that are larger than 2L there is an attractive force of the classical van der Waals type. When, on the other hand, D < 2L a strong repulsive force appears, which in the limit D≪ 2L is analogous to a double-layer force.Recently it was suggested (Israelachvili and Wennerstrom, Langmuir, 1990, 6, 873) that the repulsive so-called hydration force observed for biological lipid systems had its origin in confinements on surface excitations induced by a second surface. Here we demonstrate how this mechanism works in a particular microscopic model of the surface. Although still simplistic, several qualitative features of the force in the phospholipid systems are reproduced in the calculations. For example, a reduction of the size of the charged centres leads to a decrease in both the attractive and more significantly the repulsive forces. This mimics the observed difference between phosphatidyl choline and phosphatidyl ethanolamine.