The dissolution of polyols in salt solutions and ionic liquids at molecular level: Ions, counter ions, and hofmeister effects

Abstract

The dissolving process of polyols in salt solutions (TBAF, TBAC, TBAB, TBAI, TMAF) and imidazolium-based ionic liquids ([C2mim][OAc], [C2mim][Et2PO4], [C2mim][EtSO 4], [C2mim][SCN]) is exemplarily studied by IR spectroscopy. Vibrational bands and their shifts in the OH stretch region reveal crucial information for the dissolved polyol interacting with the anions of the salt solutions and ionic liquids. The well-chosen set of ionic solutions confirms the linear relation between the OH-stretch frequencies and the solubility capacity of the salt solutions. Likewise, it also provides an explanation of the dissolving process at molecular level. Notably, the solubility capacities of the anions in the salt solutions follow the well-known Hofmeister series. This phenomenon can be understood on the basis of the disruption power of the anions and the specific size ratio of the anion/cation combinations. All good things come in threes! The dissolving power of traditional solvents and ionic liquids for polyols such as cellulose depends on three terms: sufficiently strong interacting anions, anion/cation combinations including small sized anions and large sized cations, as well as small water concentrations. The anions can only develop their disruption power if they are absorbed by smaller cations and not hydrated by water molecules (see picture). This behavior is related to Hofmeister effects. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.