Terahertz spectroscopy of aqueous ionic solutions to understand the role of hydrogen-bond network breaking and strengthening in the Hofmeister

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In 1888, Franz Hofmeister in his paper 'Zur Lehre von der Wirkung der Salze' discovered a series of salts -- later named the Hofmeister series -- that have an increasing propensity to precipitate proteins. It became clear over time that the Hofmeister series is related to the propensity of various ions to solvate water. The classical view is that ions may be classed as kosmotropes or chaotropes: kosmotropes are thought to induce a more icelike ordering in the surrounding water whereas chaotropes disrupt the local tetrahedral network. However, this tidy picture was thrown into disarray by ultrafast infrared studies pertaining to show that the water structure remains unaltered beyond the first solvation shell around solvated ions (the 'solventberg' model). Here we propose to use terahertz Raman and infrared spectroscopy combined with time-resolved two-dimensional infrared spectroscopy (in collaboration with Robin Hochstrasser) to study femtosecond hydrogen-bond dynamics of solvated water molecules and picosecond diffusive dynamics around ions. Preliminary results indicate that the terahertz Raman data is at odds with previous pump-probe experiments and various theoretical studies, requiring an overhaul of theoretical models. Our collaboration with Casey Hynes will allow us to influence the development of theoretical work on ionic solvation.