SCANNING OF THE CATIONIC SURFACTANT ADSORPTION ON A HYDROPHILIC SILICA SURFACE AT LOW SURFACE COVERAGES

The thermodynamic aspect of adsorption of benzyldimethyldodecylammonium ions on the hydrophilic surface of silica has been investigated, especially in the region of low surface coverage ratios where surfactant ions are adsorbed through cation exchange with the preadsorbed sodium ions and protons. Calorimetric measurements of the enthalpy of displacement have been correlated with adsorption and electrophoretic measurements, and supplemented by the analysis of ion concentration in the supernatant. The comparison of adsorption data for the surfactant and its polar headgroup, the benzyltrimethylammonium ion, allows a more detailed picture of the subsequent stages of adsorption to be drawn. In particular, it is possible to (a) determine more precisely a boundary between the adsorption of individual molecules and the formation of surface aggregates, (b) evaluate the number of cationic surfactants adsorbed as individual ions, (c) identify the interfacial origin of cations released to the bulk phase upon adsorption, and (d) estimate the endothermic contribution to the enthalpy of displacement due to the disruption of the structure of the interfacial water by the presence of a hydrophobic chain in the surfactant molecule.