Lateral diffusion of surfactant monolayer molecules confined between two solid surfaces

The molecular mobility at the contact region of two surfactant monolayer-coated solid surfaces was investigated as a function of relative humidity (RH) by employing the fluorescence recovery after photobleaching (FRAP) technique. Fluorescent probe (NBD-hexadecylamine) molecules in myristic acid monolayers confined between silica surfaces are completely mobile at ≤45% RH. The measured lateral diffusion coefficient, D_s, increases by more than 1 order of magnitude within the apparent contact area due to the adsorption of water vapor at the monolayer headgroup/silica interface with increasing RH from 0 to 45%. At higher RH values, however, a significant number of the fluorescent probe molecules become immobile within the apparent contact area. Furthermore, fluorescence micrographs show that the thin film within the apparent contact area is nonuniform at >45% RH, and an effective medium model well describes the complex dependence of D_s on RH. The fluid phase fraction of relatively high mobility in the heterogeneous film decreases in proportion to the growth of surfactant-enriched domains of relatively low mobility around the contacting surface microasperities due to capillary (water) condensation, while the immobile fraction arises presumably from the interdigitation of hydrocarbon chains of monolayer molecules between the contacting surface microasperities.