Water DOI: 10.1002/anie.201004501 Specific Ion Effects on Water Structure and Dynamics beyond the First Hydration Shell** Dietmar Paschek* and Ralf Ludwig* Dedicated to Professor Manfred Zeidler on the occasion of his 75th birthday hydration · hydrogen bonding · salt effects · structure elucidation · water Our ongoing interest in the puzzling physical properties of liquid water arises from waters presence in daily life, and its importance in technical, chemical, and biological processes. As water is already interesting alone, the addition of solutes considerably broadens the spectrum of observed phenomena. For this reason the structure and dynamics of water in the vicinity of solutes have been studied for decades. One of the most challenging phenomena in this respect is the so-called Hofmeister effect, first reported by Franz Hofmeister in 1888. [1, 2] He made the observation that different salts have different efficiencies in salting-out proteins, while some salts have no effect. Most importantly, the effectiveness of the anions and cations seems to assume a particular specific order. Moreover, these specific ion effects are ubiquitous in chemistry and biology, and similar ordering of the ions is observed for numerous macroscopic properties including surface tension, chromatographic selectivity, colloid stability, and protein-denaturation temperatures. [3–7] The best ap- proach to understanding these ion effects is to focus on the simple solvation of the ions. Consequently, the Hofmeister series has been speculated to reflect different ordering powers of ions, usually anions, on the surrounding water molecules. Hence the ionic sequence has been thought as ranging from stabilizing “kosmotropes” to disruptive “chaotropes”. The structure-making (kosmotrope) and structure-breaking (cha- otrope) influence of ions on the hydration water has been basically understood as arising from a balance between the water–water and ion–water interactions, which vary consid- erably with the charge density on the solute surface. However, the challenge is to obtain a detailed understanding of those phenomenological observations by direct experimental mi- croscopic examination of what the different ions do to water. In particular, it seems to be important to understand whether the alteration of the water structure extends beyond the first hydration shell (Figure 1). [8, 9] In two very recent studies new types of spectroscopy (along with computer simulations) provide valuable new insight into the rotational and translational motion of water molecules in solution. [10, 11] These studies set out to challenge the notion that the Hofmeister effect can be explained solely by direct ion interactions and that salts affect the structure of water molecules only in their immediate surroundings. Tielrooij et al. studied the effect of ions on water by means of femtosecond time-resolved infrared (fs-IR) spectroscopy and terahertz dielectric relaxation (DS) spectroscopy. [10] The two techniques proved to be complementary. The rotational dynamics of water molecules were measured with polariza- tion-resolved anisotropy decay, while the low-frequency spectroscopy in the terahertz regime monitored intermolec- ular vibrations. Tielrooij et al. studied dissolved salts consist- ing of various combinations of ions that have different charge densities and water affinities such as LiCl, CsCl, MgCl 2 , Cs 2 SO 4 , Mg(ClO 4 ) 2 , and MgSO 4 . In the DS experiments they found that ions with a larger charge density affect the dynamics of a larger number of water molecules than ions with a lower charge density. Obviously, small and multivalent ions give higher hydration numbers. From the fs-IR measure- ments Tielrooij et al. showed that only MgSO 4 gives a very large reorientation component, whereas the individual ions Figure 1. Water molecules in the first (left) and beyond the first (right) hydration shells of cations (Na + , red) and anions (Cl À , green) in an aqueous sodium chloride solution as taken from a snapshot of molecular dynamics simulations. New experiments show that the structure and dynamics of these water molecules is ion specific and different from bulk water. [*] Dr. D. Paschek, Prof. Dr. R. Ludwig Institut für Chemie, Abteilung Physikalische Chemie Universität Rostock, 18051 Rostock (Germany) Fax: (+ 49) 381-498-6518 Fax: (+ 49) 381-498-6524 E-mail: dietmar.paschek@uni-rostock.de ralf.ludwig@uni-rostock.de Homepage: http://www.chemie1.uni-rostock.de/pci/ludwig/ Prof. Dr. R. Ludwig Leibniz-Institut für Katalyse an der Universität Rostock e.V. (Germany) [**] This work was supported by the Deutsche Forschungsgemeinschaft, the State of Mecklenburg Vorpommern, and the BMBF (Spitzen- forschung und Innovation in den neuen Ländern). Highlights 2  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49,2–4 Ü Ü These are not the final page numbers!