DOI: 10.1126/science.1074481 , 1540 (2002); 297Science et al.Uri Raviv,Fluidity of Bound Hydration Layers www.sciencemag.org (this information is current as of April 11, 2007 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/297/5586/1540version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/297/5586/1540#otherarticles, 1 of which can be accessed for free: cites 17 articlesThis article 50 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/297/5586/1540#otherarticles 3 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/chemistryChemistry : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. c 2002 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on April 11, 2007 www.sciencemag.orgDownloaded from11. Materials and methods are available as supportingmaterial on Science Online.12. L. M. Demers et al., Anal. Chem. 72, 5535 (2000).13. R. L. McCreery, Raman Spectroscopy for ChemicalAnalysis ( Wiley, New York, 2000).14. G. C. Schatz, R. P. Van Duyne, in Handbook of Vibra-tional Spectroscopy, J. M. Chalmers, P. R. Griffiths,Eds. (Wiley, New York, 2002), pp. 759–774.15. A. Campion, P. Kambhampati, Chem. Soc. Rev. 27,241 (1998).16. S. R. Emory, S. Nie, J. Phys. Chem. B 102, 493 (1998).17. M. D. Musick, C. D. Keating, M. H. Keefe, M. J. Natan,Chem. Mater. 9, 1499 (1997).18. A. M. Michaels, M. Nirmal, L. E. Brus, J. Am. Chem.Soc. 121, 9932 (1999).19. S. J. Park, T. A. Taton, C. A. 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R.J. isgrateful for the support of the American ChemicalSociety Cognis Fellowship in Colloid and SurfaceChemistry.Supporting Online Materialwww.sciencemag.org/cgi/content/full/297/5586/1536/DC1Materials and MethodsSchemes S1 and S2Figs. S1 to S312 June 2002; accepted 31 July 2002Fluidity of Bound HydrationLayersUri Raviv1and Jacob Klein1,2*We have measured the shear forces between solid surfaces sliding past eachother across aqueous salt solutions, at pressures and concentrations typical ofnaturally occurring systems. In such systems the surface-attached hydrationlayers keep the compressed surfaces apart as a result of strongly repulsivehydration forces. We find, however, that the bound water molecules retain ashear fluidity characteristic of the bulk liquid, even when compressed down tofilms 1.0 ⫾ 0.3 nanometer thick. We attribute this to the ready exchange (asopposed to loss) of water molecules within the hydration layers as they rub pasteach other under strong compression.The presence of water molecules tightlybound to ions or ionized surfaces in aqueouselectrolytes leads to strong repulsion whenthey approach each other to within a fewnanometers or less (1–4 ). This effect isthought to arise from the reluctance of theions or surfaces to shed their hydration sheath(3–6 ). It can dominate the double-layer re-pulsion/van der Waals attraction mechanisms[accounted for in the classic DLVO (Der-jaguin, Landau, Verwey, and Overbeek) the-ory (7 )] and is particularly important at thehigh salt concentrations (⬃0.1 M salt) foundin nature. The way in which the properties ofsuch hydration layers differ from those ofbulk water has for decades excited muchdebate (8–11). At issue here is a simple ques-tion: Is the hydration layer surrounding suchhighly confined bound ions fluid, or is ithighly viscous? The difference is crucial andis directly implicated in areas ranging fromclay plasticity (12) and biolubrication (13)togating of charge migration in DNA (14 ). Inaddition, many biological processes requireshear and displacement of the final sub-nanometer layers of bound hydration layersbefore molecular contact or passage. Theseinclude interactions between ligands and re-ceptors, transport within the very crowdedintracellular environment (15) or through ionchannels (16 ), and protein folding (17 ).Extensive direct measurements, as well asmodeling (1–4, 6 ), have shed much light onequilibrium interactions of such bound hydra-tion layers. In contrast, few direct measure-ments have been reported concerning theirfluidity (18–22), particularly in the regime ofthe hydration sheaths, i.e., films of thicknessD ⫽ 7to10Å(23–25). We used a surfaceforce balance (SFB) with extreme sensitivityin measuring shear interactions to probe di-rectly the fluidity of aqueous electrolytescompressed and sheared between molecularlysmooth mica surfaces. While our results con-firm the long-established equilibrium hydra-tion repulsion, they reveal at the same timethat the bound water in the hydration layersremains extremely fluid under shear. Thisfluidity persists down to films in the rangeD ⫽ Dc⫽ 1.0 ⫾ 0.3 nm, a thickness com-parable to the size of hydrated ions in solu-tion. Within such films, most of the confinedwater molecules are expected to be in boundhydration layers.The SFB used has been described in detail(26 ). Its main features are schematically out-lined in Fig. 1. We focus here on investiga-tions of NaCl