Adsorption and depletion regimes of a nonionic surfactant in hydrophilic mesopores: An experimental and simulation study.

Adsorption and aggregation of nonionic surfactants at oxide surfaces has been studied extensively in the past, but only for concentrations below and near the critical micelle concentration. Here we report an adsorption study of a short-chain surfactant (C6E3) in porous silica glass of different pore sizes (7.5 to SO nm), covering a wide composition range up to 50 wt % in a temperature range from 20 degrees C to the LCST. Aggregative adsorption is observed at low concentrations, but the excess concentration of C6E3 in the pores decreases and approaches zero at higher bulk concentrations. Strong depletion of surfactant (corresponding to enrichment of water in the pores) is observed in materials with wide pores at high bulk concentrations. We propose an explanation for the observed pore-size dependence of the azeotropic point. Mesoscale simulations based on dissipative particle dynamics (DPD) were performed to reveal the structural origin of this transition from the adsorption to the depletion regime. The simulated adsorption isotherms reproduce the behavior found in the 7.5 nm pores. The calculated bead density profiles indicate that the repulsive interaction of surfactant head groups causes a depletion of surfactant in the region around the corona of the surface micelles.