Surface excitations involving headgroup conformations and molecular protrusions. a monte carlo study of surface forces.

Abstract

The force between two parallel 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 centers interact by a homogeneous Coulomb interaction and by a hard sphere exclusion. The anchoring of the negative centers to the surfaces has been treated within two different models. They were either anchored by a parabolic potential or by a protrusion potential, i.e., a potential proportional to the distance between the center and the surface. The latter model gives a more realistic picture of the interaction between amphiphilic surfaces in aqueous solution but here only repulsive forces could be calculated. The first model also allowed the calculation of attractive forces. For distances D between the surfaces, as defined by the location of the negative centers, 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 Wennerström, Langmuir, 6 (1990) 873 ) that the repulsive so called hydration force observed for biological lipid systems has its origin in confinements of surface excitations induced by a second surface. Here we demonstrate how this mechanism works in a particular microscopic model of the surface and we show that it gives an important contribution to the total force. 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 centers leads to a decrease in the repulsive force. This mimics the observed difference between phosphatidyl choline and phosphatidyl ethanolamine.