Adsorption microcalorimetry used to study interfacial aggregation of quaternary ammonium surfactants (zwitterionic and cationic) on powdered silica supports in dilute aqueous solutions

Titration microcalorimetry has been used to compare interfacial aggregation of some zwitterionic and cationic quaternary ammonium surfactants on powdered silica substrates with micellisation in free aqueous solution. Enthalpy of dilution measurements allowed determining the molar enthalpies of micellisation for two zwitterionic amphiphiles, (dodecyldimethylammonio)butanoate and (dodecyl dimethylammonio)-1-propanesulfonate, and two cationics, benzyldimethyldodecylammonium bromide (BDDAB) and tetradecyltrimethylammonium bromide (TTAB). The effect of temperature and ionic strength on adsorption of zwitterionic surfactants on a macroporous, hydrophilic silica surface was studied so as to prove the micelle-like character of surface-bound aggregates. Interfacial aggregation of cationic surfactants on a non-porous, a macroporous, and a mesoporous solid sample in dilute aqueous solutions was inferred from adsorption and calorimetry measurements at room temperature. Special attention was given to the control of the solid surface purity. Interfacial aggregation in such systems results from the interplay of the same molecular interactions that drive the formation of micelles in free solution, with the influences and constraints of the solid–solution interface.