Phase Studies on Binary Systems of Cholesteryl Esters B. Three C18Ester Pairs

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Abstract
Three binary systems for the cholesteryl esters of C18 aliphatic acids, the stearate plus oleate, the stearate plus linoleate, and the oleate plus linoleate were studied in order to determine the phase transitions and the conditions for the cholesteric and smectic mesophases. Phase diagrams were determined using Differential Scanning Calorimeter (DSC-1B) and a polarizing microscope with hot stage. Both individual unsaturated esters exhibit a polymorphism in the solid state. The metastable modifications are formed from highly supercooled melts. Prolonged standing produces recrystallization in the stable form. The formation and stability of metastable modifications are enhanced in investigated binary systems and result in complex behavior with solid-solid transitions and regions of metastability. Both stearate systems are of a eutectic type with a limited solid solubility. A continuous series of a solid solutions over the entire composition interval was found in the oleate-linoleate system. The mesophases in binary blends are monotropic with respect to the crystalline state. The mesophase transition temperatures are linear functions of the composition of the mixtures. The cholesteric and smectic mesophases exist over the entire composition interval in both oleate systems. No sufficient supercooling was achieved to observe mesophases in the midconcentration range for the stearate-linoleate system. There are concentration regions in all three binary systems where mesophases can exist at body temperatures. The unsaturated and saturated cholesterol esters belong to the major components of artheriosclerotic deposits. From this standpoint, the mesomorphic behavior of mixtures of cholesteryl esters can be very important.