Showing papers by "Richard J. Saykally published in 2012"

Elucidating the mechanism of selective ion adsorption to the liquid water surface

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.

Pinning Down the Water Hexamer

Abstract: Water comes in numerous forms, from isolated clusters to at least one liquid form and 17 known forms of ice. The detailed characterization of water clusters is essential for developing more accurate and detailed models for describing all these different forms. The comparatively simple hydrogen-bond structures in small water clusters help to elucidate the much more complicated cooperative hydrogen bonding in liquid water ( 1 ), which is difficult to reproduce quantitatively from simplified models. The water hexamer has received particular attention as the smallest representative of multicyclic three-dimensional hydrogen-bonded structures. On page 897 of this issue, Perez et al. ( 2 ) report a landmark experimental study that resolves some of the controversies regarding the relative energies of water hexamer isomers.