(23e) Adsorption and Self-Assembly of Surfactants in Nanopores

Adsorption phenomena of surfactants in narrow pores are of great importance to a wide range of technological applications, such as surfactant-mediated ad-solubilization phenomena or the removal of hydrophobic organics from aqueous media by adsorption processes. Adsorption isotherms of non-ionic surfactants at hydrophilic surfaces commonly exhibit a sigmoid shape, with an initial low-affinity region followed by a steep increase of the adsorption at a concentration somewhat below the CMC, which is attributed to surface aggregation of the surfactant. Depending on the relative strength of the two competing interactions in the system (viz., polar interactions of surfactant heads with the surface, and hydrophobic interactions among the surfactant tails), two different aggregate structures can occur: (a) an adsorbed surfactant (bi-)layer covering the surface as a film of uniform thickness, or (b) separate surface micelles which may be only loosely attached to the surface. In order to assess the influence of confinement and of surface curvature on the self-assembly at interfaces we have studied the adsorption of non-ionic surfactants of the alkyl ethoxylate (CnEm) and alkyl glucoside (CnG1 and CnG2) type in two kinds of nanoporous silica: (i) MCM-41 and SBA-15 with cylindrical pores of uniform width in a range from 3 to 10 nm; (ii) controlled-pore glass (CPG-10) materials comprising an interconnected mesopore system of relatively low mean curvature. Pronounced effects on the adsorption are found when the pore width is approaching the diameter of the surfactant micelles or the thickness of bilayer aggregates. There are also pronounced differences in the temperature dependence of the adsorption depending on the nature of the surfactant head groups. The experimental findings are analysed in terms of a theoretical model of the temperature dependence of surfactant self-assembly at surfaces [1] and an extension of this model to account for effects of confinement on the self-assembly in narrow pores.

[1] H. Bock, K. Gubbins, Phys. Rev. Lett. 92 (2004) 135701.