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Title: Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water

Abstract

The adsorption of ions to aqueous interfaces is a phenomenon that profoundly influences vital processes in many areas of science, including biology, atmospheric chemistry, electrical energy storage, and water process engineering. Although classical electrostatics theory predicts that ions are repelled from water/hydrophobe (e.g., air/water) interfaces, both computer simulations and experiments have shown that chaotropic ions actually exhibit enhanced concentrations at the air/water interface. Although mechanistic pictures have been developed to explain this counterintuitive observation, their general applicability, particularly in the presence of material substrates, remains unclear. Here we investigate ion adsorption to the model interface formed by water and graphene. Deep UV second harmonic generation measurements of the SCN-ion, a prototypical chaotrope, determined a free energy of adsorption within error of that for air/water. Unlike for the air/water interface, wherein repartitioning of the solvent energy drives ion adsorption, our computer simulations reveal that direct ion/graphene interactions dominate the favorable enthalpy change. Furthermore, the graphene sheets dampen capillary waves such that rotational anisotropy of the solute, if present, is the dominant entropy contribution, in contrast to the air/water interface.

Authors:
ORCiD logo; ORCiD logo; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1375817
Alternate Identifier(s):
OSTI ID: 1530315
Grant/Contract Number:  
AC02-05CH11231
Resource Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 114 Journal Issue: 51; Journal ID: ISSN 0027-8424
Publisher:
Proceedings of the National Academy of Sciences
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; specific ion effects; graphene; SHG spectroscopy; molecular dynamics; adsorption

Citation Formats

McCaffrey, Debra L., Nguyen, Son C., Cox, Stephen J., Weller, Horst, Alivisatos, A. Paul, Geissler, Phillip L., and Saykally, Richard J. Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water. United States: N. p., 2017. Web. doi:10.1073/pnas.1702760114.
McCaffrey, Debra L., Nguyen, Son C., Cox, Stephen J., Weller, Horst, Alivisatos, A. Paul, Geissler, Phillip L., & Saykally, Richard J. Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water. United States. doi:10.1073/pnas.1702760114.
McCaffrey, Debra L., Nguyen, Son C., Cox, Stephen J., Weller, Horst, Alivisatos, A. Paul, Geissler, Phillip L., and Saykally, Richard J. Mon . "Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water". United States. doi:10.1073/pnas.1702760114.
@article{osti_1375817,
title = {Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water},
author = {McCaffrey, Debra L. and Nguyen, Son C. and Cox, Stephen J. and Weller, Horst and Alivisatos, A. Paul and Geissler, Phillip L. and Saykally, Richard J.},
abstractNote = {The adsorption of ions to aqueous interfaces is a phenomenon that profoundly influences vital processes in many areas of science, including biology, atmospheric chemistry, electrical energy storage, and water process engineering. Although classical electrostatics theory predicts that ions are repelled from water/hydrophobe (e.g., air/water) interfaces, both computer simulations and experiments have shown that chaotropic ions actually exhibit enhanced concentrations at the air/water interface. Although mechanistic pictures have been developed to explain this counterintuitive observation, their general applicability, particularly in the presence of material substrates, remains unclear. Here we investigate ion adsorption to the model interface formed by water and graphene. Deep UV second harmonic generation measurements of the SCN-ion, a prototypical chaotrope, determined a free energy of adsorption within error of that for air/water. Unlike for the air/water interface, wherein repartitioning of the solvent energy drives ion adsorption, our computer simulations reveal that direct ion/graphene interactions dominate the favorable enthalpy change. Furthermore, the graphene sheets dampen capillary waves such that rotational anisotropy of the solute, if present, is the dominant entropy contribution, in contrast to the air/water interface.},
doi = {10.1073/pnas.1702760114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 51,
volume = 114,
place = {United States},
year = {2017},
month = {8}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
DOI: 10.1073/pnas.1702760114

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