Analysis of phase transitions in frozen solutions

Lih-Min Her, Purdue University

Abstract

Thermal analysis of frozen solutions of non-crystallizing, non-electrolyte solutes (NNS) was carried out to define the effect of experimental parameters on qualitative and quantitative aspects of the thermograms in the glass transition regions. The intensity of enthalpy relaxations in the glass transition region is related to both heating and cooling rates. Plots of the logarithm of heating rate versus 1/T$\sb{\rm g}\sp\prime$ are linear, and activation enthalpies for structural relaxation are in the range of 210-350 kJ/mole. Both the activation enthalpies for structural relaxation and the heat capacity change accompanying the glass transition increase with increasing molecular weight of the solute. Molecular weight dependence of the T$\sb{\rm g}\sp\prime$ values agrees well with the Fox-Flory relationship. The electrical resistance (R) of the NNS frozen solutions was measured as a function of temperature to determine whether this technique can be applied for determination of T$\sb{\rm g}\sp\prime.$ Electrical thermal analysis (ETA) thermograms of frozen solutions containing the solute alone show no inflection point corresponding to T$\sb{\rm g}\sp\prime.$ However, addition of low levels ($\sim$0.1%) of electrolyte changes the shape of the thermogram into a biexponential function where the intersection of the two linear portions of the log (R) vs. T plot corresponds to T$\sb{\rm g}\sp\prime.$ The log (R) values over the temperature range studied increases as the ionic radius of the reporter ion increases. The sharpest inflection points in the log (R) vs. T curves, and the best correlation with DSC results, were obtained with ammonium salts. ETA is a more sensitive method than DSC, particularly at low solute concentrations and low heating rates. Addition of salts into an NNS solution tends to lower the T$\sb{\rm g}\sp\prime$ values. DSC study of fast frozen sucrose solutions indicate sodium chloride itself does not change T$\sb{\rm g}.$ The amount of unfrozen water of sucrose is $\sim$0.21 g H$\sb2$O/g solute measured by both DSC and nmr methods. $\sp1$H nmr study shows that addition of NaCl brings more unfrozen water into the freeze concentrated sucrose and the excess water is decreased by further lowering temperature, thus lower T$\sb{\rm g}\sp\prime$ value is observed. The rotational correlation times, $\tau\sb{\rm c},$ of the unfrozen water in sucrose solutions are of the order of $10\sp{-7}$sec.

Degree

Ph.D.

Advisors

Nail, Purdue University.

Subject Area

Pharmacology|Pharmaceuticals

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