A voltammetric study of monolayers and bilayers self-assembled on metal electrodes

Three types of aliphatic thiols, n-dodecanethiol, n-hexadecanethiol and n-octadecanethiol, differing in their physical state in self-assembled monolayers at ambient temperature were tested from the point of view of their integrity and usefulness as the sub-layer for the second, adjacent layer of phospholipids. These self-assembled monolayers were formed on mercury and polycrystalline gold electrodes in order to assess their electrochemical behavior as monolayers passivating the electrodes against various redox probes present in aqueous phase, such as hexaamineruthenium(III) chloride and benzoquinone, differing in their heterogeneous rate constants and hydrophobic-hydrophilic character. Subsequently, as such a type of hydrophobic alkanethiol monolayer is frequently used as a part of alkanethiol/phospholipid asymmetric bilayer membranes, chemically bound to the metallic surface, the monolayer was covered with the adjacent phosphatidylcholine monolayer, and the electrochemical behavior of such an asymmetric system was again tested. The results obtained show that the liquid-like monolayers, particularly those formed on mercury, possess better passivating properties (much smaller number of defects) as compared to crystalline thiols, also providing a better driving force for the attachment of the second liquid-crystalline phosphatidylcholine monolayer. An interesting observation was also noted that the outer phospholipid monolayer imposes a larger energy barrier to the penetration of benzoquinone that hexaamineruthenium(III) cations.