Thermotropic phase transitions of binary mixtures of wax and cholesteryl esters relevant to tears.

The most abundant lipids on the ocular surface are wax and cholesteryl esters that have long alkyl chains. The phase transition temperatures of some of these lipids including the most common cholesteryl and wax esters exceed the ocular surface temperature of 35 °C. However, the transition temperature of meibum is 30 °C. Limited infrared spectroscopic studies have shown that mixtures of some lipids from meibum may alter the phase transition and conformation of wax esters to stabilize the lipid layer of tears. However, binary mixtures of cholesteryl and wax esters have not been assessed with differential scanning calorimetry to compare the thermal transitions of individual components to the mix. Here, thermograms of binary mixtures of cholesteryl and wax esters show that phase transition temperatures of either single component are reduced by addition of the fellow component and/or new phase transitions appear. Enthalpy values of the binary mixtures were less than that of the sum of individual lipids. Increasing mole fractions of either component in the binary mixture resulted in thermographic peaks with greater width at half maximum as well as alteration in peak height. The phase transition of cholesteryl esters with high transition temperatures were not visible in mixtures with large excesses of wax esters. The results indicate that binary mixtures of wax and cholesterol esters reduce the enthalpic energy for transition, reduce cooperativity and lower the transition temperature. These changes may help to avert crystallization of high melting point lipids on the ocular surface.