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Properties of Citric Acid at the Glass Transition

1997; Elsevier BV; Volume: 86; Issue: 12 Linguagem: Inglês

10.1021/js970157y

1520-6017

Qun Lü, George Zografi,

Microencapsulation and Drying Processes

Abstract □ To better understand the properties of citric acid when used in solid dosage forms as an acid–base buffer, we initiated a study of its properties in the amorphous state. Such a state often arises during processes such as lyophilization and wet granulation. In view of inconsistencies in the literature concerning the glass transition temperature, T g , of citric acid in the dry and hydrated states, we measured the T g of samples formed by a melt-quench cool sequence in a DSC. We also used DSC to measure T g ', the glass transition temperature of the maximally freeze-concentrated solution. It was shown that dry citric acid has a T g of 11°C, while that containing 8.6% water (equimolar) has a value of –25°C. The T g ' of a frozen solution of citric acid is –53°C. Measuring T g for the dry and hydrated samples at various scanning rates allowed measurement of the activation energy for enthalpy relaxation at T g and enabled estimation of the degree of fragility (or the strength parameter) for both samples. It was shown that citric acid is a fairly fragile liquid expected to exhibit non-Arrhenius dynamic behavior and that the presence of residual water at a level of 8.6% causes a decrease in fragility.