Critical exponents of a fluid mixture in the presence of isotope exchange: Isobutyric acid/D2O

Abstract

Experiments on phase diagrams and critical opalescence of a fluid mixture, isobutyric acid in D₂O, indicate that the presence of isotope exchange reactions can change the critical behavior of such a system from that of a simple binary fluid mixture. Appreciable amounts of additional species due to isotope exchange distort the coexistence curve, shift the critical solution concentration y_c away from the concentration (y_I*) where the maximal phase separation temperature T_p,max occurs, and make the critical exponents γ and ν in the one‐phase region (T≳T_c) different from those of the coexisting two‐phase region (T<T_c). We obtained y_c (in mass fraction of isobutyric acid) =0.358 at T_c=44.90 °C differing from y_I*=0.310 at T_p,max=45.11 °C. In the one‐phase region, γ=1.25, ν=0.633, and ξ₀=3.13 Å, in excellent agreement with γ=1.24 and ν=0.633 of simple fluid systems. However, in the coexisting two‐phase region, the critical exponents appear to be renormalized with γ_x =1.39, ν_x?0.76, and ξ₀?0.6 Å. These results are in agreement with the renormalized critical exponents γ_x=1.40±0.02 and ν_x =0.73±0.04 near the plait point of a ternary liquid mixture: ethanol–water–chloroform.