No abstract

Mixed distearoylphosphatidylethanolamine (DSPE) and dioleoylphosphatidylethanolamine (DOPE) monolayers and bilayers have been deposited on mica using the Langmuir-Blodgett (LB) technique, as a model system for biomembranes. Investigation with atomic force microscopy revealed phase-separation for both monolayers in air and bilayers in water in the form of microscopic DSPE domains embedded in a DOPE matrix. For the monolayers in air, the step height measured between the higher DSPE phase and the lower DOPE phase was larger than expected from the molecular lengths, and a significant contrast in adhesion and friction was observed despite identical lipid end groups. This unexpected behavior resulted primarily from a difference in the film mechanical properties, the DOPE phase being inelastically deformed by the probe. For the bilayers in water, similar trends were found in terms of height, adhesion, and friction, but an additional short-range repulsive hydration/steric force over the DSPE phase contributed to the observed differences.

This paper explores the ability of n-alkanethiolates chemisorbed at Au(111) to function as boundary lubricants at microscopic length scales as probed by scanning force microscopy (SFM). Through an examination of the influence of alkyl chain length, we show that the macroscopic structure of this system, as developed from insights into the chain-packing density via infrared reflection spectroscopy, greatly influences the observed friction and wear. That is, the longer chain monolayers exhibit a markedly lower friction and a reduced propensity to wear than the shorter chain monolayers, a situation that reflects the more extensive cohesive interactions between chains. From the combined weight of these findings, we examine the frictional process within the context of an activation mechanism that involves pressure and shear activation volumes. The ability of longer chain alkanethiolate monolayers to lubricate features that arise from changes in substrate topography is also presented, and the resulting mechanistic issues are discussed.