Elucidating the mechanism of selective ion adsorption to the liquid water surface
Date
2012
Authors
Otten, D.E.
Shaffer, P.R.
Geissler, P.L.
Saykally, R.J.
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Journal article
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Proceedings of the National Academy of Sciences of the United States of America, 2012; 109(3):701-705
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Abstract
Adsorption of aqueous thiocyanate ions from bulk solution to the liquid/vapor interface was measured as a function of temperature by resonant UV second harmonic generation spectroscopy. The resulting adsorption enthalpy and entropy changes of this prototypical chaotrope were both determined to be negative. This surprising result is supported by molecular simulations, which clarify the microscopic origins of observed thermodynamic changes. Calculations reveal energetic influences of adsorbed ions on their surroundings to be remarkably local. Negative adsorption enthalpies thus reflect a simple repartitioning of solvent density among surface, bulk, and coordination regions.
A different, and much less spatially local, mechanism underlies the concomitant loss of entropy. Simulations indicate that ions at the interface can significantly bias surface height fluctuations even several molecular diameters away, imposing restrictions consistent with the scale of measured and computed adsorption entropies. Based on these results, we expect an ion's position in the Hofmeister lyotropic series to be determined by a combination of driving forces associated with the pinning of capillary waves and with a competition between ion hydration energy and the neat liquid's surface tension.
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Data source: Supplementary data, http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116169109/-/DCSupplemental
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Copyright The Authors 2012. Beginning with articles submitted in Volume 106 (2009) the author(s) retains copyright to individual articles, and the National Academy of Sciences of the United States of America retains an exclusive License to Publish these articles and holds copyright to the collective work. (http://www.pnas.org/site/aboutpnas/rightperm.xhtml)