Water solvation proceeds more slowly than expected

Given how common water is, you’d think by now we would know just about everything there is to know about it. Not so. A new study shows that when it comes to orienting around a solute, water molecules are a lot more sluggish than generally thought (J. Chem. Phys. 2018, DOI: 10.1063/1.5034225). Years of spectroscopy experiments using lasers with short pulses, led researchers to consider water to be among the fastest-acting solvents, with a response time in the subpicosecond range. Those studies have generally been done by probing electronic properties of a water-dissolved solute, often an excited dye molecule. But rather than study water solvation from the solute’s point of view, Saima Ahmed, Peter Hamm, and coworkers at the University of Zurich used a fast terahertz (THz) vibrational spectroscopy method to carry out the experiment from water’s perspective. THz, or far-infrared, light uniquely probes collective motions of groups of water molecules—water cluster dances, in effect. By using this method, the team found that water solvated an excited dye molecule rather slowly—on the 10-ps timescale. Now the team plans to conduct related experiments on liquid water cooled below its freezing point.