Research ArticlesPhysical Properties of Lipids Used in Pharmacy II: Use of Heating-Cooling Curves to Study Lipid Materials

This report describes the use of heating-cooling curves for the study of polymorphism as it occurs in chemically pure or commercial samples of glyceryl mono-, di-, and tristearate, theobroma oil, spermaceti, white petrolatum, and cetyl alcohol. The interpretation of the data obtained by the heating-cooling method has been simplified by using a log temperature difference plot. With this plot, the usual temperature-time curve can be rationalized into a series of interconnecting straight lines. The effects of heating or cooling rates on crystal form and crystal transition temperatures were studied with the aid of the improved heating-cooling curve plots. Distinct polymorphic transitions were observed in the pure glyceryl stearates. On the other hand, in commercial materials there was little evidence that the observed crystal changes were associated with specific polymorphic transformations. The relationship between the transitions observed and the probable storage stability (i.e., the maintenance of film integrity, moisture permeability, etc.) is discussed.