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18O/16O Isotope Effect on Miscibility of Isobutyric Acid with Water

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journal contribution
posted on 11.10.2007 by Agnieszka Siporska, Anna Makowska, Jerzy Szydłowski
The miscibility of isobutyric acid with water and the influence of the isotope substitution of oxygen (16O/18O) 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, ΔTc = Tc(18O) − Tc(16O), extrapolated to the 100% enrichment in 18O, 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.