X-ray Absorption Spectroscopy Study of the Hydrogen Bond Network in the Bulk Water of Aqueous Solutions Lars-Åke Na 1 slund, ²,‡ David C. Edwards, § Philippe Wernet, ‡,| Uwe Bergmann, ‡ Hirohito Ogasawara, ‡ Lars G. M. Pettersson, ² Satish Myneni, ⊥ and Anders Nilsson* ,²,‡ Fysikum, AlbaNoVa, Stockholm UniVersity, SE-106 91 Stockholm, Sweden, Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, California 94309, U.S.A., Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, U.S.A., BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany, and Department of Geosciences, Princeton UniVersity, Princeton, New Jersey 08544, U.S.A. ReceiVed: January 24, 2005; In Final Form: April 21, 2005 We utilized X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS) in order to study the ion solvation effect on the bulk hydrogen bonding structure of water. The fine structures in the X-ray absorption spectra are sensitive to the local environment of the probed water molecule related to the hydrogen bond length and angles. By varying the concentration of ions, we can distinguish between contributions from water in the bulk and in the first solvation sphere. We show that the hydrogen bond network in bulk water, in terms of forming and breaking hydrogen bonds as detected by XAS/XRS, remains unchanged, and only the water molecules in the close vicinity to the ions are affected. 1. Introduction The interaction between ions and water molecules influences macroscopic properties of water, for example, boiling point, freezing point, surface tension, and viscosity. It also affects other important properties in many chemical and biological processes. In electrochemical processes, the rate of charge transfer is controlled by the mobility of the ions in solutions. 1 Within the living body, even a small amount of ions influences the conformation of proteins and nucleic acid strands and their functions. 2,3 It is generally assumed that the interactions between water and ions, which result in effects on the macroscopic properties, affect the long range ordering of the hydrogen bonding network in the bulk liquid, for example, by breaking or forming hydrogen bonds (HB). The magnitude of the effect follows the Hofmeister series given in Chart 1. 4,5 In connection to measured macroscopic properties of aqueous solutions, ions that exhibit strong interactions with water molecules, for example, small di- and/or trivalent ions, are often called structure breakers, while ions that exhibit weaker interac- tions with water than water itself, for example, large monovalent ions, are called structure makers. Another view, which gives an opposite definition of the structure-maker/structure-breaker concept, is the correlation to the Jones-Dole viscosity B coefficient; 6 a positive (negative) B coefficient in the Jones- Dole expression of viscosity indicates increased (reduced) viscosity relative to pure water. 7 Strongly hydrated ions give positive B coefficients and are thus called structure makers, while weakly hydrated ions reduce the viscosity leading to a negative B coefficient and are called structure breakers. A third approach to the structure-breaker/structure-maker concept is based on the effect on the entropy of ion solvation, which also has been considered to indicate water structuring. 8 By separating the entropy into ion and hydration water contributions, the latter can describe the change in entropy of the water due to the presence of ions. The structure makers have negative hydration entropy, whereas the structure breakers increase the entropy associated with the hydration waters. The structure-breaker/structure-maker nomenclature is thus ambiguous and is based on macroscopic observations. The important result is, however, that strongly hydrated ions affect the properties of the liquid. Recent neutron diffraction studies of water in ionic solutions suggest that a strongly hydrated ion increases the difference between the HB donating and accepting capacity of the linked water molecules, resulting in a breakdown of the HB network (i.e., an effect similar to increased temperature or pressure). 9 The effect on the pair-correlation functions from the presence of ions is, however, only seen for very concentrated solutions where most water molecules are within the first or second coordination spheres. Recent time-resolved infrared spectroscopy studies of the rotational dynamics of water by Omta et al. 10,11 also indicated that the ions affect only the water molecules in the first coordination sphere. Moreover, molecular dynamics (MD) simulations have not provided a clear, conclusive result regarding the effects of ions on the HB network in the bulk liquid. Both supportive and negative pictures of the structure- maker/-breaker concepts have been obtained. 12-18 In previous work, we have used X-ray absorption spectros- copy (XAS) and X-ray Raman scattering (XRS) at the oxygen K edge (O 1s) to demonstrate the ability to probe the HB structure in liquid bulk water. 19-22 The comparison between liquid water and ice showed a large difference in the spectral features related to specific hydrogen-bonded configurations. The O 1s XA spectrum of bulk ice has a pronounced structure around 541 eV (post-edge), while the ice surface and liquid water have additional strong spectral features seen as a pre-edge peak at * Corresponding author. Phone: +1 (650) 926 2233. Fax: +1 (650) 926 4100. E-mail: nilsson@slac.stanford.edu. ² Stockholm University. ‡ Stanford Synchrotron Radiation Laboratory. § Department of Chemistry, Princeton University. | BESSY. ⊥ Department of Geosciences, Princeton University. 5995 J. Phys. Chem. A 2005, 109, 5995-6002 10.1021/jp050413s CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005