Cation Functionalization Alters Cohesion, Not Interaction Type, in Ionic Liquids: Evidence from Infinite Dilution Activity Coefficients

The effect of cation functionalization on solute-solvent interactions in ionic liquids (ILs) was investigated through infinite dilution activity coefficients (IDACs) of 34 molecular solutes in 1-ethyl-1-methylpyrrolidinium diethyl phosphate and 4-ethyl-4-methylmorpholinium diethyl phosphate. New experimental IDAC data were obtained by gas-liquid chromatography over the temperature range from (308.15 to 358.15) K. The results show that insertion of an oxygen atom into the saturated cation ring systematically increases IDACs and thus reduces solvent affinity, with the effect being strongest for nonpolar solutes and progressively smaller for polar compounds. This behavior indicates that cation functionalization primarily raises the baseline thermodynamic cost of solvation rather than altering the fundamental hierarchy of solute-IL interactions. For both ILs, nonpolar solvation is dominated by the free-energy cost of cavity formation and is both enthalpically and entropically unfavorable, whereas polar solutes are stabilized mainly by favorable dipolar and hydrogen-bonding interactions. Temperature-dependent analysis further shows that the cation substitution modifies both enthalpic and entropic contributions to solvation while preserving the overall interaction pattern. Linear solvation energy relationship analysis, regular-solution treatment, and COSMO-RS calculations support this interpretation, showing that the two ILs differ mainly in overall solvent affinity rather than in interaction type. Overall, the results identify cation functionalization as a rational means of tuning solvation thermodynamics in ILs through controlled modification of the liquid-state interaction balance.