Investigations of first adsorption step of cationic dimeric (Gemini) surfactants onto silica surfaces by analytical and calorimetric methods

This paper reports new results on the adsorption of cationic dimeric surfac tants (12-s-12 surfactants, where s is the carbon number of the polymethyle ne spacer) on adsorbents with different surface functions, namely raw and H Cl-treated silica. These results were obtained by traditional methods (adso rption isotherms, electrophoretic mobility, and chemical analysis of the eq uilibrated supernatant) and microcalorimetry. The results showed that the s toichiometry of the first step of the adsorption (ion-exchange step) varies strongly with the spacer carbon number. The binding of one surfactant to t he surface brings about the release of between 1 and 2 sodium ions as the s pacer carbon number is increased from 2 to 10. Thus the surfactant binding to the surface involves one head group for the 12-2-12 surfactant (short sp acer) and two head groups for the 12-10-12 surfactant (long spacer). These results suggest the use of dimeric surfactants as molecular rulers to study the distribution of charged sites on surfaces. The microcalorimetric exper iments clearly showed the two adsorption steps. The ion-exchange step gives rise to an endothermal effect having an amplitude that depends strongly on the spacer carbon number. The second adsorption step associated with the f ormation of surfactant aggregates gives rise to an exothermal effect that a lso depends on s. (C) 2001 Academic Press.