Calorimetric studies of cationic surfactant adsorption at low surface coverages on different silica samples

Calorimetric measurements of adsorption for the surfactant (benzyldimethyldodecylammonium bromide) and its polar head-group (benzyltrimethylammonium bromide) from aqueous solutions on two different silica surfaces (hydrophilic and hydrophobic one) allow a more detailed picture of the subsequent stages of the adsorption process to be drawn. It is possible to determine more precisely a boundary between the adsorption of individual molecules and the formation of surface aggregates. The local disruption of the structure of the interfacial water molecules by surfactant cations gives an endothermic contribution to the total enthalpy of displacement. This contribution depends on the length of alkyl chain as well as on the type and the origin of solid surface.