the head groups by a spacer group which can be hydrophilic The adsorption on silica of four alkanediyl-a,v-bis(dodecyldior hydrophobic, flexible or rigid (1-3). They constitute a methylammonium bromide) dimeric surfactants with the alkanenew class of surfactants that is arousing considerable interest diyl spacer groups C 2 H 4 , C 4 H 8 , C 6 H 12 , and C 10 H 20 (the correin view of some of their properties. Thus dimeric surfactants sponding surfactants are referred to as 12-2-12, 12-4-12, 12have cmcs (critical micelle concentrations) that are 10 to 100 6-12, and 12-10-12, respectively), has been investigated by meatimes smaller than those of the corresponding monomeric suring the amount of surfactant adsorbed, the sodium and bromide surfactants. Likewise, their surface activity, characterized by ion concentrations in the supernatant solution along the binding the value of C 20 , surfactant concentration for which the surisotherm, and the electrophoretic mobility of the silica particles.
face tension of water is lowered by 20 mN/m, is much
The maximum amount of adsorbed surfactant was found to increase in the sequence 12-10-12 Γ΅ 12-6-12 Γ΅ 12-4-12 Γ΅ 12-greater than that of the corresponding monomeric surfac-2-12. In the range of very low equilibrium concentration the adtants. Besides, dimeric quaternary ammonium halide surfacsorption isotherms showed a rapid but small increase of the tants with short polymethylene spacers, (CH 2 ) s groups, give amount of adsorbed surfactant, G, followed by an adsorption plarise to assemblies that are less curved than those formed by teau corresponding to a G value that was nearly independent of monomeric surfactants (4, 5). As a result the solutions of the spacer carbon number, s. The point of zero charge was reached some of these surfactants show remarkable rheological propat the beginning of this plateau. In this first adsorption step, the erties (6). Last, dimeric surfactants have been shown to be residual sodium ions present at the silica surface are released upon superior to their monomeric counterparts in several properbinding of dimeric surfactant ions to the silica surface (exchange ties on which surfactant utilizations are based: foamability, process). The length of the adsorption plateau depends strongly wetting, solubilization, antibacterial activity, etc. (1, 2). on the nature of the surfactant, being the shorter for the most
The adsorption of dimeric surfactants at the air/solution hydrophobic surfactant, i.e., 12-10-12. At the end of the first adsorption plateau the value of G shows a second very rapid in-interface has been much investigated (1-3, 7). Their behavcrease when plotted against the concentration of free surfactant. ior at a solid/solution interface has been very little investi-This increase is accompanied by a decrease of the supernatant gated thus far. To our knowledge the only reported studies pH, from 6.5 to about 4.0, and an increase of the concentration are those of Esumi et al. that concern the dimeric surfactant ratio [ free bromide ion]/[ free surfactant ion]. This second adethane-1,2-bis(dodecyldimethylammonium bromide) at the sorption step corresponds to the formation of surfactant aggregates solution/silica interface (8) and its adsorption by laponite at the silica surface. Part of the bromide ions are shown to be clay (9). This first study focused mainly on the amount of bound to these aggregates which are, however, more ionized than adsorbed surfactant and on some microscopic properties of the free micelles formed by the dimeric surfactants investigated.
the adsorbed surfactant aggregates. The amount of dimeric α§ 1998 Academic Press surfactant adsorbed on silica was lower than for its mono-