Kinetics and thermodynamics study of ultrasound-assisted depolymerization of κ-carrageenan in acidic solution

Abstract

K-carrageenan is a natural polymer with high molecular weight ranging from 100 to 1000 kDa. The oli-gocarrageenan with low molecular weight is widely used in biomedical application. The aim of this work was to depolymerize κ-carrageenan in an acidic solution with the assistance of ultrasound irradi-ation. The ultrasonication was conducted at various pH (3 and 6), temperatures (30-60 °C), and depoly-merization time (0-24 minutes). The results show that the depolymerization reaction follows pseudo-first-order kinetic model with reaction rate constant of 1.856×10-7 to 2.138×10-6 s-1. The reaction rate constant increases at higher temperature and lower pH. The Q10-temperature coefficients of the depoly-merization are 1.25 and 1.51 for pH 6 and 3, respectively. The enthalpy of activation (ΔH‡) and the Gibbs energy of activation (ΔG‡) are positive, while the entropy of activation (ΔS‡) is negative, indicat-ing that the activation step of the ultrasound-assisted depolymerization of κ-carrageenan is endother-mic, non-spontaneous, and the molecules at the transition state is more ordered than at the ground state. The ΔH‡ and the ΔS‡ are not affected by temperature, while the ΔG‡ is a weak function of tem-perature. The ΔH‡ and ΔS‡ become smaller at higher pH, while the ΔG‡ increases with the increase of pH. The kinetics and thermodynamics analysis show that the ultrasound-assisted depolymerization of κ-carrageenan in acidic solution is possibly through three mechanisms, i.e. bond cleavage due to cavita-tional effect of microbubbles, hydroxyl radical and hydrogen peroxide, as well as proton.