Modeling heat and degree of gelation for methyl cellulose hydrogels with NaCl additives

Abstract

Methyl cellulose (MC) hydrogels are thermoreversible physical hydrogels and their gelation is an endothermic process. A model consisting of a generalized expression for two bell-shape curves was formulated to describe and capture enthalpy changes that take place during the gelation of an aqueous solution of MC, SM4000, in the presence of sodium chloride, NaCl, in different concentrations. The procedure followed in obtaining the necessary constants for the model using the differential scanning calorimetric (DSC) measurements is elaborated. The developed model described the salt-out effects of NaCl in various % on the MC gelation very well. One of the two bell-shape curves mapped most part of the DSC thermograms. The secondary bell-shape curves portrayed the minor enthalpy changes. The possible mechanisms and molecular bonding processes driven by the energy represented by the area under these two individual curves are discussed. Subsequently, a sigmoidal growth model for the degree of gelation was introduced, and its development is explained in the paper. The import of various constants for these two models, the bell-shape curves and the sigmoidal growth models, in terms of gelation kinetics is identified. The need for a specific term of the sigmoidal model for depicting the effect of the salt additive onto the gelation is recognized. The comparison between the results obtained using these two models is discussed.