Specific Ion Effects on Water Structure and Dynamics beyond the First Hydration Shell

Abstract

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. 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. The best approach 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 (chaotrope) 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 considerably with the charge density on the solute surface. However, the challenge is to obtain a detailed understanding of those phenomenological observations by direct experimental microscopic 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). 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. 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. The two techniques proved to be complementary. The rotational dynamics of water molecules were measured with polarization-resolved anisotropy decay, while the low-frequency spectroscopy in the terahertz regime monitored intermolecular vibrations. Tielrooij et al. studied dissolved salts consisting of various combinations of ions that have different charge densities and water affinities such as LiCl, CsCl, MgCl2, Cs2SO4, Mg(ClO4)2, and MgSO4. 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 measurements Tielrooij et al. showed that only MgSO4 gives a very large reorientation component, whereas the individual ions