Adsorption of dodecyltrimethylammonium bromide and sodium bromide on gold studied by liquid chromatography and flow adsorption microcalorimetry.

Here, we report on a new aspect of the adsorption of Br- on the surface of gold. The adsorption of dodecyltrimethylammonium bromide (C12TABr) from aqueous solutions onto macroporous gold particles was studied by continuous flow frontal analysis solid/liquid chromatography and flow adsorption microcalorimetry. The material balance and enthalpy balance of adsorption and the change in the solution pH were measured simultaneously. Initially, Br- is irreversibly bound to high-affinity surface sites counterbalanced by the adsorption of H+ from the aqueous phase. The surface speciation is accompanied by the formation of C12TAOH, which in turn results in a significant pH increase in the bulk solution. The net process was found to be strongly exothermic (-280 kJ.mol(-1)), which is indicative of the occurrence of chemisorption. The specific adsorption of Br- is followed by the reversible adsorption of C12TABr to produce a firmly bound monolayer in a head-to-surface arrangement (-53 kJ.mol(-1)). In a relatively narrow range of the surface coverage, various composite structures may develop on the top layer and eventually transform to full-cylindrical surface aggregates. The surface aggregation was found to be reversible, with an enthalpy change of -11 kJ.mol(-1). The importance of the specific binding of Br- to the surface of gold was confirmed by measurement of the initial adsorption of NaBr on the microparticles. The initial adsorption was found to be irreversible, with an enthalpy change of approximately -240 kJ.mol(-1). This process involved the formation of an AuBr-/H+ electric double layer at the gold/water interface, accompanied by a dramatic increase in the solution pH due to the release of a copious amount of OH- in the bulk liquid phase.