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Effect of Humidity and Temperature on Molecular Mobility in Surfactant Monolayers Confined between Two Solid Surfaces

The lateral diffusion coefficient (D s ) of fluorescent-labeled probe molecules in surfactant monolayers of fatty acids confined between two silica surfaces was directly measured as a function of relative humidity (RH) andtemperature by the fluorescence recovery after photobleaching (FRAP) technique. The RH dependence of D s in myristic acid monolayers showed a general trend that the molecules become more mobile with increasing RH. Two distinct RH regimes of different mobility behavior were observed. The molecular mobility characteristics in each regime are explained in terms of the formation of a hydration layer and the capillary condensation of water vapor from the ambient atmosphere. The RH dependences of D s in stearic acid monolayers and oleic acid monolayers showed similar trends to those in myristic acid monolayers, except that stearic acid monolayers yielded a much smaller D s and no immobile fraction, whereas oleic acid monolayers gave a somewhat larger D s and immobile fraction than those in myristic acid monolayers. As the temperature is raised, D s increases in an exponential manner under dry (0% RH) conditions. At 45 and 55% RH, D s initially decreases upon heating, and then increases and appears to remain constant above 65 °C. This trend was proposed to be because of the combined effect of RH reduction and thermal energy increase upon heating. Finally, upon repeated heating and cooling, D s becomes lower, which implies that the monolayers undergo some irreversible structural change. These findings provide new information on the general mobility behavior of surfactant molecules in confined geometry relevant to boundary lubrication.