Proton transport at the monolayer-water interface

Abstract

It is shown that when monolayers of stearic acid, palmitic acid, DPPC, or DPPS are compressed above some critical area A c a lateral conduction mechanism is initiated at the monolayer/water interface. The interfacial conductance increases on further increasing the molecular packing density in the monolayer. All compounds also show major changes in surface potential at A c the potential becoming more positive in all cases. It is argued that this is a consequence of structural reorganisation at the headgroup/water interface causing a significant reduction in the local permittivity. The critical area, A c, is approximately double the molecular areas estimated from the pressure-area isotherm, and experiments with stearic acid monolayers show that A c decreases significantly when the chaotropic ion SCN −, which is known to disrupt the molecular structure of water, is added to the subphase. It is likely, therefore, that the structural changes occurring at A c involve the formation of a hydrogen bonded network between monolayer headgroups and adjacent water molecules at the monolayer/water interface. It is suggested that the conduction mechanism initiated at A c arises from proton hopping along this hydrogen-bond network.