Effect of anion α-functionalization on the water affinity of thermo-responsive phosphonium acetate-derived ionic liquids

This study presents a facile method for assessing the effect of anion functional group substitution on the water affinity of phosphonium-acetate ionic liquids. By combining the same cation with three different α-substituted carboxylate anions, the effect of anion functional group substitution on the IL miscibility with water and hygroscopicity was experimentally evaluated and rationalized using COSMO-RS sigma profile analysis. Lower critical solution temperature (LCST) phase separation experiments and water vapor partial pressure measurements revealed that there is a stark α-substitution dependency on the affinity of water for acetate [AcO], pivalate [PivO], and trifluoroacetate [TFA] derived tributyl(octyl)phosphonium [P₄₄₄₈] ILs. It was found that the IL water affinities varied as: [P₄₄₄₈][AcO] > [P₄₄₄₈][PivO] > [P₄₄₄₈][TFA]. With the lowest water partial pressure and no LCST driven phase separation, [P₄₄₄₈][AcO] exhibited a high affinity for water. On the other hand, [P₄₄₄₈][PivO] and [P₄₄₄₈][TFA] exhibited LCST-driven phase separation and room temperature immiscibility, respectively, with higher water partial pressures, as a result of relatively weak water affinities. COSMO-RS sigma profile analysis revealed that the aliphatic carboxylate anions should incorporate apolar interactions commensurate with or greater than the polar hydrogen bond acceptor interactions for phase immiscibility of the corresponding IL with water. It was found that the relative magnitudes of apolar and polar interactions, assessed using the anion sigma profile peak intensity ratio (I_r), are directly correlated with the IL water affinity, and inversely correlated with the solution water activity coefficient. Therefore, it was concluded that I_r forms a quantitative link between the structural changes at the molecular level and the bulk water affinity exhibited by the ionic liquid as a whole.