BINAEY MXXTDEEB OF CHOLESTERYL ESTERS
' ~ a ~ ~ and ~ : ~Phase ~ i ~Relationships n s for Binary Mixtures
y ~ ~ t h V. o Galanti n ~ LkparSrnent of General Engineering, Pennsylvania State University, Hazleton Campus, .Madeton, Pmnaylvaniu
andl Roger S . Porter* i'olyme~ Science and Engineering, University of Massachusetts, Amherst, Massachusetts
(Received December 13, 1971)
P u b h a l i o n costs assisted by Roger 8.Porter
Investigations have been undertaken on defined blends of pure esters of cholesterol with the purpose of revealing compositional dependencies for the equilibrium thermodynamic conditions for the formation of the isotropic state, the smectic and the cholesteric mesophases, and the stable crystal structure, Measurements have been made by differential scanning calorimetry. The systems investigated have been mixtures of cholesterql myristate with the stearate and with the nonanoate esters. The full range of compositions have been covered including the most pure esters obtainable. The heats and temperatures of transition have been correlated as a function of composition for each of the multiple transitions. New conclusions are drawn concerning the application of the van't Hoff equation to the binary ester systems. Results have implications comeruing the nature of the various states in regard to costructure formation and in regard to the associaticm which lead to mesophase structures. This information provides the basis for understanding the use of ester mixtures as temperature indicators.
A significant amount of research has been performed on the therrrodynamic properties of the fatty acid esters of cholesternl.2-8 This work has been warranted because these materials have both fundamental and applied intereats For example, these esters represent the principal series far one of the three basic classes of liquid crystals. the cholesteric mesophase. Indeed, many of these e s w s are polymorphic liquid crystals which exhibit both smectic and cholesteric nzosophases. From a basic viewpoint, the effectsof structural modifications on the mesomorphic properties of related sterols (as in a homologous series) have been successfully ~ n ~ e ~ t ~ g ~ The t e dhealth ~ ' ~ aspecl ~ ~ , ~ as ,related ~ to heart disease in such materials as the cholesteryl esters of the Gs aliphatic acids is under current and extensive research.8 These include several significant works on esters of c%ioltstercd.2--8Eowever, these investigations bavc been dei~otedto the thermal behavior of the pure compounds. To be sure, various mixtures for other mesophase types h a w been investigated in relevant tvC)rl99% pure. This is confirmed by previous purification tests and transitional analyses on these esters.2bl*-G The cholesteryl nzyristate was a sample ivbich Davis6recrystallized from n-pentyl alcohol, a solvent demonstrated to be most effective for purification of these esters. The composition of blends was caieulated o n the as(I) Part XXXI of a series on Order and Flow of Liquid Crystals. ( 2 ) (a) G. W. Gray, J . Chem. Sac., 3733 (1956); (b) E. M . Barrall, 11, R. S. Porter, and J. F. Johnson, J . Phys. Chem., 71, 1224 (1967). (3) R. D. Ennulat, Mol. Cryst., 8, 247 (1969). (4) G. J. Davis, R. S. Porter, and E. M. Barrall, 11, ibid., 10, I
