Adsorption of Cationic Surfactants on a Hydrophilic Silica Surface at Low Surface Coverages: Effects of the Surfactant Alkyl Chain and Exchangeable Sodium Cations at the Silica Surface

The influence of the alkyl chain length and the concentration of exchangeable sodium cations at the solid/water interface on the energetics of cationic surfactant adsorption on a hydrophilic silica surface at free pH has been studied. Adsorption isotherms, electrophoretic mobilities of the silica particles, and differential molar enthalpies of displacement for benzyldimethyldodecylammonium bromide (BDDAB) on the original silica sample (SilNa) at different ionic strengths (pure deionized water and 0.1 M NaBr solution) were compared, as well as those on a washed silica sample (SilH). The stability of silica suspensions, as characterized by the changes in turbidity, was analyzed with the intention of shedding light on the possible orientation of the adsorbed surfactant ions. Adsorption of BDDAB was also compared with that of benzyldimethyloctylammonium bromide (BDOAB) on SilNa. Individual surfactant adsorption occurs by ion pairing or cation exchange mechanisms, depending on the purity of silica sample and composition of the bulk phase. A significant endothermic contribution to the total enthalpy of displacement upon individual surfactant adsorption derives from desorption of the very structured interfacial water due to the specific adsorption of a hydrophobic surfactant moiety. In the presence of sodium cations at the silica surface, this endothermic contribution dominates over others because the enthalpy of adsorption for surfactant ionic heads and the enthalpy of desorption for sodium cations cancel each other out to a great extent. For moderate interfacial concentrations of sodium, the overall enthalpy change is endothermic in a certain adsorption interval, whereas for high concentrations it is always exothermic. Individual surfactant adsorption at free pH causes the pH of the equilibrium bulk solution to decrease, indicating an increase in the surface charge of silica.