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The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na+, K+, NMe4 +) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion-water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.
Bibliographical noteFunding Information:
We gratefully acknowledge the financial support for this work from the U.S. Department of Energy Basic Energy Sciences (DOE BES) grant DE-SC0010328. G.L.J. acknowledges a National Defense Science and Engineering Graduate (NDSEG) Fellowship from the U.S. Department of Defense. Synchrotron SAXS analyses were conducted at Sector 12 of the Advanced Photon Source at Argonne National Laboratory, which is supported through the U.S. DOE contract DE-AC02-06CH11357 under GUP-48102 and GUP-42048. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We thank the Rick Goyette for logistical support during our BASIS measurement, Antonio Faraone for useful discussions regarding QENS data analysis, and Jose M. Borreguero for MANTID program support. This work also utilized University of Wisconsin−Madison instrumentation facilities funded in part by NSF CHE-9974839 and CHE-1048642, and materials characterization facilities funded by DMR-0832760 and DMR-1121288. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the University of Minnesota NSF MRSEC (DMR-1420013). Research reported in this publication was also supported by facilities funded by the Office of the Director, National Institutes of Health of the National Institutes of Health under award number S10OD011952. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Copyright © 2018 American Chemical Society.
PubMed: MeSH publication types
- Journal Article
- Research Support, N.I.H., Extramural
- Research Support, U.S. Gov't, Non-P.H.S.