THERMOKINETICS OF REACTIONS WITH LIQUID-LIQUID PHASE-SEPARATION

Isothermal heat flow calorimetry with controlled thermoelectric coolin g resp. heating is applied to study the critical slowing down of the k inetics of reactions with liquid-liquid phase separation. Results (i) on the acid-catalysed hydrolysis of chloromethyloxirane by water, and (ii) on the ring-opening and esterification of bromomethyloxirane by d ichloroacetic acid in the presence of cyclohexane as an inert solvent, both at 298.15 K, are presented. The heat production w(t) which is pr oportional to the reaction rate partial derivative zeta/partial deriva tive t (where i is the reaction coordinate) exhibits a funnel-like fal l-off when the reaction path intersects the binodal curve at the criti cal solution point CP. From the shape of this part of the w(t) curves an average value of the critical exponent phi = 0.72 +/- 0.003, which refers to the change of zeta with t at constant T and p, is calculated . This result is to be compared with the theoretical value of phi = 0. 708 obtained from a modification of the Griffiths and Wheeler theory o f critical points in multicomponent systems to chemically reacting sys tems far from equilibrium by introducing zeta as an additional extensi ve variable. This allows the critical slowing down of the reaction rat es to be interpreted as resulting from the divergence of the correlati on length xi resp. from the convergence of the transport phenomena to zero at the CP.