Marcel J. Vogel

The effect of purity on the thermodynamic properties of cholesteryl heptadecanoate

Abstract The thermodynamic properties of two samples of cholesteryl heptadecanoate have been measured and contrasted. As little as 2% impurity (presumably oxidized cholesterol and free acid) profoundly effects all of the thermodynamic properties. The mesophases are the most sensitive to alteration and distortion by impurities. A transition diagram has been developed for this ester.

Effect of Substituent Location on the Thermodynamic Properties of Cholesteryl Halobenzoates and Halocinnamates

Abstract A series of substituted aromatic esters of cholesterol has been examined by DSC. Temperatures and heats of transition were measured. A special modification of the cholesteric mesophase was noted in the 3-chloro and 4-bromo benzoate esters by microscopy. A series of statements has been formulated from these observations which permit the prediction of transition temperatures as well as entropies as a function of the modification of a parent compound whose properties are known within the cholesteryl ester series.

Polymorphism in Cholesteryl Esters: Cholesteryl Palmitate

Abstract The thermal transitions in cholesteryl palmitate have been evaluated and compared to previously reported data. A sample synthesized from carefully purified cholesterol and palmitic acid using p-toluene sulfonic acid compared directly with previously reported data. A sample of recrystallized ester from a commercial source showed a slightly depressed solid to mesophase transition temperature. However, the mesophase proved to be smectic, commonly reported as monotropic with respect to the solid phase. Both smectic to cholesteric and cholesteric to isotropic liquid transitions were very sharp, indicating a very pure material. It is postulated that a specific impurity, an antioxidant, is responsible for this previously unreported effect. This effect of related compound interaction has been reported for nematic mesophase forming compounds. Thin layer chromatography, microscopy, depolarized light intensity analysis, differential scanning calorimetry and NMR spectroscopy data are discussed.

Effect of Solvent Type on the Thermodynamic Properties of Normal Aliphatic Cholesteryl Esters

The effects of purity on the phase transitions in cholesteryl esters have been observed by several workers (1, 2, 3). In addition, recent calorimetric studies have indicated that cholesteryl esters have more than one stable room temperature solid form (3). Barrall, Porter and Johnson found that for certain esters the transition temperature of the solid→mesophase conversion was very sensitive to the way in which the solid phase had been formed, from ethanolic solution or from the melt (3). Indeed, the propionate ester exhibited two overlapping endotherms for the solid→mesophase transition when the solid had been formed from the melt. The same solid formed from ethanolic solution exhibited only one endotherm. These and other reported effects indicate that the esters of cholesterol have an extensive solid-phase polymorphism.