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

ABSTRACT

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.

KEYWORDS:

• Adsorption
• mesoscale simulations
• nanopores
• surface azeotrope
• surfactant

Acknowledgments

This work was supported by the German Research Foundation (DFG) in the framework of IRTG 1524 ‘Self-Assembled Soft-Matter Nanostructures at Interfaces’. Analysis of experimental data and contribution to manuscript preparation by GR was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

German Research Foundation (DFG) [Grant Number IRTG 1524]; for GR by Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy; Oak Ridge National Laboratory [Grant Number ERKCC72].