The bend+libration combination absorption band of liquid water appears in all aqueous samples at approximately 2130 cm<sup>-1</sup> with a width of over 200 cm<sup>-1</sup>, in a region largely free of any other signals. This band’s frequency shifts substantially in electrolyte solutions (as well as solutions of other solutes). The frequency of the combination band is a direct and collective reporter of the rigidity of the H-bond network of liquid water, and thus it is sensitive to Hofmeister (chaotropic vs kosmotropic) effects of aqueous ions as well as other ion-water interactions. From time to time, narrow and structured vibrational signals in the same frequency region (especially in systems with low water content and systems with surface-attached or otherwise confined water molecules) have been reported, and we demonstrate that those narrow signals almost always come from solutes and not water.  Applications of the combination band as a (very easily collected) probe to better understand both bulk and surface-confined electrolyte solutions will be discussed.

Electrolyte effects on the infrared combination band of liquid water

2019 Spring National Meeting

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