Adsorption characteristics of sugar-based monomeric and gemini surfactants at the silica/aqueous solution interface

The adsorption behavior of sugar-based surfactants at the silica/aqueous solution interface has been characterized on the basis of the adsorption isotherm, ζ potential and in situ atomic force microscopy (AFM) data. The surfactants used in the current work are monomeric Glu(n) (N-alkyl-N-methylgluconamide) and gemini Glu(n)-2-Glu(n) (N,N′-dialkyl-N,N′-digluconamide ethylenediamine), where n is the hydrocarbon chain length of 10 and 12. The adsorption isotherm reveals that, for all the surfactants, the adsorbed amount increases even above the critical micelle concentration (cmc) and an adsorption plateau region appears at the equilibrium concentration well above the cmc. The driving force for this adsorption is deemed to be a combination of the hydrogen bonding between the sugar moieties and the surface silanol groups and the intermolecular hydrophobic interaction of free monomers to the surfactants pre-adsorbed on silica. In the adsorption plateau region, the occupied area per molecule adsorbed on silica is calculated to be generally larger for the gemini surfactants than for the monomeric ones. This reflects the bulky structure of the headgroups of the sugar-based gemini surfactants and/or the relatively less favorable interaction of the headgroups with the silica surface. The combination of the soft-contact AFM and force curve data suggests that a structural transformation of the adsorbed layer occurs with an increase in the surfactant concentration, depending on the packing parameter of each surfactant. At the concentration above the cmc, the monomeric surfactants form either worm-like surface micelles or a patchy bilayer whereas the gemini surfactants only form a patchy bilayer.