Adsorption of dimeric (gemini) surfactants at the aqueous solution silica interface

The adsorption on silica of four alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) dimeric surfactants with the alkane-diyl spacer groups C2H4, C4H8, C6H12, and C10H20 (the corresponding surfactants are referred to as 12-2-12, 12-4-12, 12-6-12, and 12-10-12, respectively), has been investigated by measuring the amount of surfactant adsorbed, the sodium and bromide ion concentrations in the supernatant solution along the binding isotherm, and the electrophoretic mobility of the silica particles. The maximum amount of adsorbed surfactant was found to increase in the sequence 12-10-12 < 12-6-12 < 12-4-12 < 12-2-12, In the range of very low equilibrium concentration the adsorption isotherms showed a rapid but small increase of the amount of adsorbed surfactant, Gamma, followed by an adsorption plateau corresponding to a Gamma value that was nearly independent of the spacer carbon number, s. The point of zero charge was reached at the beginning of this plateau. In this first adsorption step, the residual sodium ions present at the silica surface are released upon binding of dimeric surfactant ions to the silica surface (exchange process). The length of the adsorption plateau depends strongly on the nature of the surfactant, being the shorter fbr the most hydrophobic surfactant, i.e., 12-10-12. At the end of the first adsorption plateau the value of Gamma shows a second very rapid increase when plotted against the concentration of free surfactant. This increase is accompanied by a decrease of the supernatant pH, from 6.5 to about 4.0, and an increase of the concentration ratio [free bromide ion]/[free surfactant ion], This second adsorption step corresponds to the formation of surfactant aggregates at the silica surface, Part of the bromide ions are shown to be bound to these aggregates which are, however, more :ionized than the free micelles formed by the dimeric surfactants investigated. (C) 1998 Academic Press.