Isotope effects on miscibility of 1-butyl-3-methylimidazolium tetrafluoroborate with butanols.

The liquid-liquid miscibility temperatures as a function of composition and deuterium substitution have been experimentally determined for the binary mixtures of 1-butyl-3-methylimidazolium tetrafluroborate with 1-butanol, isobutyl alcohol, 2-butanol, and tert-butyl alcohol and their deuterated forms (OH/OD substitution). All systems exhibit upper critical solution temperatures (UCSTs) with a visible effect of branching in alcohols. Deuteration of alcohols in the hydroxyl group results in a decrease of the UCST of the given system and the largest shift is observed for tert-butyl alcohol. These solvent isotope effects nicely correlate with the polarity expressed by dielectric constants or E(T)(30) parameters of alcohols. The effect of the isotope substitution on the miscibility of ILs with butanols can be rationalized by using the statistical-mechanical theory of the isotope effects coupled with a phenomenological g(E) model. Following this procedure one finds that the isotope shift of UCST is associated mainly with the zero-point energy contribution.