18O/16O isotope effect on miscibility of isobutyric acid with water

The miscibility of isobutyric acid with water and the influence of the isotope substitution of oxygen (O-16/O-18) in water over a broad concentration range are reported. The system exhibits a phase diagram with, an upper critical solution temperature (UCST) and a visible isotope effect thereon. The oxygen isotope substitution decreases the UCST, leading to better miscibility of isobutyric acid with water. The isotope shift, Delta T-c = T-c(O-18) - T-c(O-16), extrapolated to the 100% enrichment in 180, is equal to-1.15 K; hence it is much smaller and opposite to that found for deuterium substitution. The origin of the observed miscibility isotope effect has been qualitatively discussed in terms of the condensed-phase isotope effect theory.