Temperature Dependence of Sol‐Gel Conversion Kinetics in Gelatin‐Water System

Abstract

The conversion kinetics of an aqueous gelatin solution to gel was studied by temperature modulated and regular DSC under isothermal and continuous cooling conditions. Isothermal runs revealed a decrease in the quasi-static heat capacity primarily associated with syneresis (phase separation) of the gel. Above 19 degrees C the isothermal process demonstrated negative effective activation energy that turned positive below 14 degrees C. Continuous cooling runs detected a reversing heat flow apparently related to the continuing formation and melting of new gel structures. Isoconversional kinetic analysis of continuous cooling measurements yielded negative activation energy for the whole range of conversions and temperatures suggesting that nucleation remained a rate controlling step throughout the whole gelation process.

Isoconversional Methodology

“…This expands the application area of the methods to such processes as crystallization [48,49], gelation [50], as well as some other phase transitions [51] that can be initiated by decreasing temperature. If a flexible method is based on integration with respect to temperature (e.g., Eq.…”

Physical Processes

“…For the range of faster cooling rates (7.5, 10, and 12.5 °C min −1 ), the average DSC peak temperature is ~ 11 °C so that the average value of E α is about − 90 kJ mol −1 . However, at slower cooling rates (0.5, 1.0, and 1.5 °C min −1 ), the average peak temperature is markedly higher (20 °C), i.e., closer to T gel so that the average E α estimated from the slower heating rates is around − 300 kJ mol −1 [165]. Therefore, isoconversional analysis of the continuous cooling DSC data demonstrates that under these conditions the rate of sol-gel conversion has a negative temperature coefficient, whose absolute value becomes larger when the process is measured closer to T gel .…”

“…The formation of hydrogen bonds during gelation of a gelatin solution produces sufficient amount of heat to follow the process by DSC (Fig. 3.62) [165]. On cooling, gelation becomes detectable below 40 °C.…”

“…Consecutively, the effective Arrhenius activation energy should be negative when estimated in the temperature range T max -T gel and positive when evaluated in the range T fr -T max . An actual example [165] of the Arrhenius plot for gelation of a gelatin solution at several temperatures is presented in Fig. 3.61.…”

Isoconversional Kinetics by Fast Scanning Calorimetry

“…This means that gelation that occurs on cooling should be expected to have a negative temperature coefficient just as the crystallization of a polymer melt. The first application of an isoconversional method to the process of gelation on cooling was by Chen and Vyazovkin [41].…”