DEPENDENCE OF THE RATE OF DIFFUSION OF ENTRAINED WATER ON THE DEGREE OF ION INCORPORATION IN LANGMUIR-BLODGETT-FILMS

We develop an integrated form of Fick's first law to model the charact eristic drying of entrained subphase through mixed behenic acid/metal behenate monolayers deposited vertically on a quartz crystal microbala nce using the Langmuir-Blodgett (LB) technique. The model combines the first-order kinetics of diffusion through fatty acid regions with tha t through salt regions to describe accurately the drying purely in ter ms of the transport of water molecules outward perpendicular to the la yers of the film. We invoke no lateral migration of water molecules in the plane of the layers toward pinholes in the LB film, or the film e dges, where the water would be seen to evaporate at a much faster rate . The ratio of acid to salt molecules in the LB film has previously be en gained using a model derived from Poisson-Boltzmann-Stern electroch emical theory. Results show that diffusion through fatty acid regions is slower than through salt regions, the diffusion coefficient for the acid films being an order of magnitude smaller than for the latter. T he initial amount of entrained subphase is shown to be dependent on th e degree and type of metal ions incorporated in the LB film.