SUBSTRATE-INDUCED STRUCTURAL-CHANGES IN ELECTRODE-ADSORBED LIPID LAYERS - EXPERIMENTAL-EVIDENCE FROM THE BEHAVIOR OF PHOSPHOLIPID LAYERS ON THE MERCURY WATER INTERFACE

A theoretical model for substrate induced structural changes in adsorbed lipid layers is tested experimentally in this paper. The behaviour of phospholipids with varying chain length, chain saturation and head group surface affinity at the electrified mercury-water interface has been studied. The capacitance of the adsorbed phospholipid was measured by ac voltammetry. The potential dependence and the resolution of the capacitance peaks which correspond to two phase transitions has been found to depend on the surface affinity of the lipid head groups and the hydrophobicity and/or unsaturation of the hydrocarbon tails. Increasing the hydrophobicity and/or unsaturation of the tails shifts the phase transitions to a more extreme potential and in some cases increases their potential separation. Decreasing the surface affinity of the heads also shifts the phase transitions to a higher potential. For the most polarisable head groups with the weakest affinity for the interface, the two transitions coincide. Increasing the coverage of the electrode over that required to form a compact monolayer increases the significance of the first phase transition over the second and at twice monolayer coverage the second phase transition begins to disappear. When coverage is reduced below that required to form a monolayer, this is reversed. These phenomena support the theoretical prediction that the first phase transition represents a competition between the heads and the tails for access to the electrode interface and the second phase transition relates to a desorption of the tails from the surface. As a result, it is proposed that the mechanistic basis behind the two capacitance peaks is a two step conversion of a monolayer into a pored bilayer. At more negative potentials this lipid film desorbs since the lipid aggregates do not have enough adsorption energy gain to remain on the mercury surface. © 1990.