LATERAL DIFFUSION OF SURFACTANT MONOLAYER MOLECULES CONFINED BETWEEN 2 SOLID-SURFACES

The molecular mobility at the contact region of two surfactant monolay er-coated solid surfaces was investigated as a function of relative hu midity (RH) by employing the fluorescence recovery after photobleachin g (FRAP) technique. Fluorescent probe (NBD-hexadecylamine) molecules i n myristic acid monolayers confined between silica surfaces are comple tely mobile at less than or equal to 45% RH. The measured lateral diff usion coefficient, D-s, increases by more than 1 order of magnitude wi thin 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 fluore scent 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 f luid phase fraction of relatively high mobility in the heterogeneous f ilm decreases in proportion to the growth of surfactant-enriched domai ns of relatively low mobility around the contacting surface microasper ities due to capillary (water) condensation, while the immobile fracti on arises presumably from the interdigitation of hydrocarbon chains of monolayer molecules between the contacting surface microasperities.