Abstract
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 50 nm), covering a wide composition range up to 50 wt % in a temperature range from 20 °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.
Original language | English |
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Pages (from-to) | 11406-11416 |
Number of pages | 11 |
Journal | Langmuir |
Volume | 33 |
Issue number | 42 |
DOIs | |
State | Published - Oct 24 2017 |
Funding
It is a pleasure to dedicate this paper to Professor Keith E. Gubbins in celebrating his 80th birthday and in appreciation of long-standing professional and personal interactions. 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 U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division.