Adsorption of 12-s-12 gemini surfactants at the silica-aqueous solution interface

The adsorption of gemini surfactants of the form alkanediyl-alpha.omega-bis (dodecyldimethylammonium bromide) to a silica substrate is investigated using optical reflectometry (OR) and atomic force microscopy (AFM). The adsorption isotherms and kinetics of adsorption have been determined for spacer sizes of 2, 3, 4, 6, 8, 10, and 12. The maximum surface excess correlates strongly with the size of the spacer group. The smallest spacer size yields the largest surface excess. Soft-contact AFM imaging has shown that flattened ellipsoidal aggregates are present on the surface at the shortest spacer lengths, Images of tile adsorbed layer cannot be obtained for spacer sizes greater than 3 because of a strongly attractive force regime. The origin of the attraction is attributed to proximal desorption induced by the approach of the tip toward the substrate. The linear increase in the area per adsorbed molecule with spacer size suggests that the aggregate structures become flattened with increasing spacer size but are otherwise similar.