Abstract
The ability of a porous material to uptake and release molecules is determined by the thermodynamic state of preadsorbed surfactants in pore space. However, the effect of spatial confinement on the thermodynamic state of the surfactants is poorly understood. In this study, we use a combination of small-angle neutron scattering experiments and all-atom molecular dynamics simulations to develop a better understanding of the effect of confinement on self-assembly and temperature-induced liquid-liquid phase separation of surfactant solutions. Here, we study the demixing of a model nonionic surfactant triethyleneglycol monohexyl ether (C6E3) in tubular nanopores of the SBA-15 silica material (pore diameter of 8.6 nm). We find that the nonionic surfactants show an enhanced in situ aggregation behavior in silica pores upon increasing the temperature above the lower critical solution temperature (LCST). The surfactant molecules directly bound to the pore walls act as anchor sites for subsequent adsorption, resulting in the increased uptake of nonanchored surfactants upon increasing temperature and driving an increase in the size of assemblies. The nonanchored surfactant molecules can be released into a bulk solvent by decreasing the temperature below LCST. This ability of C6E3 surfactant allows reversible partitioning of secondary molecules (here dye) dissolved in the cores of surfactant micelles between the pore space and bulk solvent. The findings presented in the article provide fundamental insights into the surfactant assembly in confinement, which is crucial in the development of new multiresponsive materials for isolating and recovering molecules.
Original language | English |
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Pages (from-to) | 9957-9966 |
Number of pages | 10 |
Journal | Journal of Physical Chemistry C |
Volume | 123 |
Issue number | 15 |
DOIs | |
State | Published - Apr 18 2019 |
Funding
The authors thank Prof. Kerry Dooley (LSU) for assistance with gas adsorption measurements, Dr. Monojoy Goswami (Oak Ridge National Laboratory) for useful suggestions for MD simulations, and Prof. Gerhard Findenegg for discussions. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support with the material synthesis of the project. The material characterization and simulations were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under EPSCoR grant no. DE-SC0012432 with additional support from the Louisiana Board of Regents. Experiment planning and measurements of experimental data and contribution to manuscript preparation by Gernot Rother were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.