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 c...