The role of micellar catalysis from kinetic and thermodynamic investigations of the reaction between bromhexine drug with para-dimethylaminobenzaldehyde

Abstract

Kinetic study of the condensation reaction of bromhexine with para-dimethylaminobenzaldehyde (DAB) in weakly acidic EthOH/H 2O has been investigated. An opposite first order reaction with respect to bromhexine and zero order with respect to DAB has been observed. The rate constants, activation energies, frequency factors, and other related thermodynamic functions of activation for both forward and reverse reactions have been determined. Standard thermodynamic parameters including equilibrium constant, Δ G°, Δ H°, and Δ S° have been calculated. The effect of the presence of anionic surfactant (sodium dodecyl sulfate, SDS) upon these kinetic and thermodynamic parameters of this reaction in aqueous solution has been examined. The results indicate that the presence of 4 × 10 −3 M SDS increase the forward and reverse rate constants together with equilibrium constant by 104.3, 19.48, and 5.33 times, respectively. A substantial increase in the values of frequency factors of 1 × 10 5 and 2 × 10 4 for forward and reverse reactions, respectively, due to the presence of this amount of SDS has been detected. The observed positive sign of Δ S° in presence and absence of SDS has been confirmed through conductivity measurements. The rate constants in micellar phase, binding constants between each bromhexine and DAB with SDS, and the change in standard chemical potential have been determined at different temperatures. Activation energy, frequency factor, and other related thermodynamic functions of activation in micellar phase have also been calculated. It has been concluded that the increases in the number of collisions between reactants molecules caused by presence of micelles plays a major role in the physicochemical properties of surfactants solutions.