SUBSTRATE-INDUCED STRUCTURAL-CHANGES IN ELECTRODE-ADSORBED LIPID LAYERS - A SELF-CONSISTENT FIELD-THEORY

In this paper, the adsorption and self-assembly of lipid molecules on solid-water interfaces are modelled to produce thermodynamic explanations for the observed phase transitions. In particular, structural changes are studied in the adsorbed layer when, at a constant amount of lipids at the surface, the surface properties are altered. A Self-Consistent Field approach is used in which interfacial equilibrium conformations of model-lipid molecules are found with Flory-Huggins-type approximations. This treatment involves Markov-type chain statistics (Rotational Isomeric State scheme) combined with a local Mean Field approximation. Such a theory is applicable well above the gel-to-liquid phase transition temperature. Taking the mercury-water interface as an example, decreasing or increasing the applied potential with respect to the pzc renders the surface less hydrophobic. This changes the interaction energies between the surface and the various components of the lipid and solution. As a result a complicated reorientation process takes place for constant lipids on the surface which depends, among other things, on the type of lipid used. For head groups with a rather high surface affinity, two transitions are predicted. The first one originates from a displacement of tail segments by head group segments at a potential where the tail segments still have a high surface affinity compared to the water leaving an inhomogeneous adsorbed layer. At the second transition the surface affinity of the tails is overtaken by that of the water molecules which leads to a partially almost bare (i.e. water covered) surface in equilibrium with adsorbed bilayer structures. For head groups with a low surface affinity at the pzc, the first and second phase transitions coincide. © 1990.