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:

McCaffrey, Debra L.  ^[1];

• Search DOE PAGES for author "McCaffrey, Debra L."
• Search DOE PAGES for ORCID "0000-0002-3624-6358"

Nguyen, Son C.  ^[2];

• Search DOE PAGES for author "Nguyen, Son C."
• Search DOE PAGES for ORCID "0000-0001-7713-4195"

Cox, Stephen J. ^[3]; Weller, Horst ^[4]; Alivisatos, A. Paul ^[5]; Geissler, Phillip L. ^[1]; Saykally, Richard J. ^[1]

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+ Show Author Affiliations

1. Department of Chemistry, University of California, Berkeley, CA 94720,, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
2. Department of Chemistry, University of California, Berkeley, CA 94720,, The Hamburg Centre for Ultrafast Imaging, University of Hamburg, 22761 Hamburg, Germany,
3. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
4. The Hamburg Centre for Ultrafast Imaging, University of Hamburg, 22761 Hamburg, Germany,, Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany,
5. Department of Chemistry, University of California, Berkeley, CA 94720,, Department of Materials Science and Engineering, University of California, Berkeley, CA 94720,, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,, Kavli Energy Nanoscience Institute, University of California, Berkeley, CA 94720

Publication Date:

Mon Aug 21 00:00:00 EDT 2017

Research Org.:

Lawrence Berkeley National Laboratory (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