Exchange rates of surfactant at the solid-liquid interface obtained by ATR-FTIR

There is at present considerable variance in the rates of surfactant adsorption and desorption that are reported in the literature. We have used in situ ATR-IR to measure the rate of adsorption, desorption and exchange of tetradecyltrimethylammonium bromide (C(14)TABr) at the interface between silica and D2O solutions. The rate of exchange was monitored by exposing an adsorbed layer Of C(14)TABr to a C(14)TABr-d(38-) solution, while simultaneously monitoring the adsorption of both C(14)TABr and C(14)TABr-d(38-) We find that the exchange of monomers between the surface and the bulk is very fast. Complete exchange occurs within about 10 s, even when exchange at the surface is limited by transport to the surface. A simple model for exchange suggests that the desorption rate for a single monomer is greater than I s(-1), which is no more than 104 times slower than for the desorption of a monomer from a micelle in bulk solution. Above and way below the cmc, the rate of adsorption of surfactant (exchange with solvent or its dissociation products) is similarly rapid. The rate of desorption is rapid, but slow enough for us to discern a break in the desorption rate. The results are consistent with the transport-limited desorption of the surfactant that is held by hydrophobic association with other surfactant molecules, and activation limited desorption of the surfactant that is electrostatically held to the surface. The rates of adsorption, exchange, and desorption were almost the same when the experiment was performed 11 degreesC above and 8 degreesC below the Krafft temperature of the surfactant.