Magnesium ions slow water dynamics on short length scales

The presence of ions affects the structure and dynamics of water on a multitude of length and time scales. New results from ultrafast spectroscopy and theoretical analyzes show that the water environment of specific pairs of magnesium and sulfate ions has a decisive impact on the dynamics of aqueous solutions.

Liquid water, the native medium of biochemical and cellular processes, is made up of a complex network of polar molecules connected by hydrogen bonds. Water reacts to the presence of a solute by modifying its local structure. The influence of negatively and positively charged ions on liquid water is generally classified via the Hofmeister series which classifies ions according to their ability to structure the water around them or to disrupt the structure of water. The microscopic origin and molecular mechanisms of the Hoffmeister series are controversial, despite many years of research. However, the Hoffmeister series has great relevance because it characterizes the influence that ions exert on biomolecules dissolved in water.

Recent experiments and simulations have now revealed a much more complex influence of ions on the dynamics of surrounding water molecules. The study, published in the journal ACS Physical Chemistry Au, combines spectroscopic experiments with in-depth theoretical analyzes of molecular interactions and dynamics. Researchers from the Max Born Institute in Berlin, the Freie Universität Berlin and the Ludwig-Maximilian University of Munich used the asymmetric stretching vibrations of sulfate (SO42-) as locally responsive probes to map dynamic properties of the environment. In this context, hydrated sulfate ions are a prototypical model system, as they are commonly found in minerals and are of great importance in physiology and biochemistry. In order to obtain information about the local hydration environment, the researchers used the experimental technique of two-dimensional infrared spectroscopy in the femtosecond time domain and compared the behavior of sulfate ions in the presence of sodium (Na+) or magnesium (Mg2+) ions at low ion concentrations. They find that the presence of Mg2+ reduces ultrafast fluctuations of the water layer around a sulfate ion, resulting in a specific slowing of the solvation dynamics of hydrated MgSO4 relative to aqueous Na2SO4 solutions.

Extensive simulations provide a microscopic view of the observed dynamics and reveal a molecular picture in which the slowing of water dynamics stems from the structural characteristics of SO42- –Mg2+ ion pairs that share water molecules in their hydration shells. Contrary to what is widely reported in the literature, the described effects are short range and limited to the first 1-2 layers of water around the sulfate ion. Unlike the Hoffmeister series, which classifies the effect of ion types on the structure of water, the new results demonstrate a particular relevance of the individual solvation geometries of specific ion pairs for the dynamics of dilute aqueous systems.

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Material provided by Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI). Note: Content may be edited for style and length.

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