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Microscopic Probing of the Size Dependence in Hydrophobic Solvation
Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
LBL, Adv Light Source, Berkeley, CA 94720 USA .
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2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 7, 074507- p.Article in journal (Refereed) Published
Abstract [en]

A dependence on solute size of the hydrophobic effect has been proposed based on theory and simulations, such that small apolar solutes leave the hydrogen (H-) bonding network in water intact or even strengthened, whereas hydration of larger, nanometer-sized apolar solutes breaks hydrogen bonds and creates a liquid-vapor-like interface around the solutes. Here we report the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in aqueous solutions of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N + (TAA) cations with increased side chain length by probing the H-bonding network in water through O K-edge x-ray absorption spectroscopy and the solute-solute interaction using small angle x-ray scattering. These results open for unique experimental opportunities to investigate hydrophobic effects for a range of important processes in chemistry and biology.

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N+ (R4N+) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.

Place, publisher, year, edition, pages
2012. Vol. 136, no 7, 074507- p.
Keyword [en]
hydrogen bonds, hydrophobicity, molecular dynamics method, organic compounds, positive ions, solvation, solvent effects, X-ray absorption spectra, X-ray scattering
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Other Physics Topics
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Chemical Physics
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URN: urn:nbn:se:su:diva-47127DOI: 10.1063/1.3684893ISI: 000300551000021OAI: oai:DiVA.org:su-47127DiVA: diva2:373053
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9Available from: 2010-11-29 Created: 2010-11-29 Last updated: 2017-12-12Bibliographically approved

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