From aggregative adsorption to surface depletion: aqueous systems of C_nE_m amphiphiles at hydrophilic surfaces

Adsorption of a short-chain nonionic amphiphile (C₆E₃) at the surface of mesoporous silica glass (CPG) was studied by a combination of adsorption measurements and mesoscale simulations. Adsorption measurements covering a wide composition range of the C₆E₃ + water system show that no adsorption occurs up to the critical micelle concentration, at which a sharp increase of adsorption is observed that is attributed to ad-micelle formation at the pore walls. Intriguingly, as the concentration is increased further, the surface excess of the amphiphile begins to decrease and eventually becomes negative, which corresponds to preferential adsorption of water rather than amphiphile at high amphiphile concentrations. The existence of such a surface-azeotropic point has not previously been reported in the surfactant adsorption field. Dissipative particle dynamics simulations were performed to reveal the structural origin of this transition from aggregative adsorption to surface depletion. The simulations indicate that this transition can be attributed to the repulsive interaction between head groups, causing depletion of the amphiphile in the region around the corona of the surface micelles.