Calorimetric investigation of the adsorption of cationic dimeric surfactants with a hydrophilic spacer group onto silica

The adsorption of dimeric cationic surfactants, made up of two ammonium species linked via a hydrophilic spacer group, i.e. Br^-n-C ₁₂H₂₅N⁺Me₂-CH₂(CH ₂OCH₂)_xCH₂N⁺Me ₂-n-C₁₂H₂₅Br^-, referred to as 12-EO_x-12, onto silica has been studied in aqueous solution at 298 K and free pH. The effect of the molecular structure on the adsorption of these cationic surfactants onto raw silica (SiNa) and HCl-treated (SiH) silica was investigated. For this purpose, batch microcalorimetry measurements of the differential molar enthalpy of dilution and that of adsorption were performed at 298 K. The enthalpy data were supplemented by the classical observations such as the adsorption isotherms. The microcalorimetry results show that the adsorption mechanism and the aggregation at the solid-liquid interface do not depend on the spacer group length. Then, important quantitative differences are observed between the two silica surfaces. The investigated surfactants provide useful molecular probes for the characterization of the surface active sites onto treated and raw silicas. The amounts adsorbed allow the surface sites present in SiNa and SiH to be quantified.