Melting Points-The Key to the Anti-Evaporative Effect of the Tear Film Wax Esters

PURPOSE. We examined in vitro the evaporation-retarding effect of wax esters (WEs). The WEs resembled closely the most abundant WE species in meibum. METHODS. A custom-built system was used to measure the evaporation rates through WE layers applied to the air-water interface at 35 degrees C and, as a reference, at 30 degrees C and 41 degrees C. Additionally, the melting points of the WEs were determined. The organization and stability of the WE layers were assessed using Brewster angle microscopy (BAM) and Langmuir film experiments, respectively. RESULTS. Four of 19 WEs retarded evaporation at 35 degrees C: behenyl palmitoleate (BP), behenyl oleate (BO), behenyl linoleate (BLN), and behenyl linolenate (BLNN) decreased evaporation by 20% to 40%. BP was the most effective evaporation retardant. At 30 degrees C the most effective retardants were BLN and BLNN decreasing evaporation by similar to 50%, whereas BP and BO decreased evaporation by only 5% to 10%. At 41 degrees C, each lipid decreased evaporation by only 2% to 4%. The evaporation-retardant WEs all melted within 2 degrees C of physiological temperature. BAM images showed that the evaporation-retardant WE layers spread somewhat uniformly and possibly exhibited areas of condensed lipid. The isotherms suggested that WE layers were surface pressure tolerant but unstable under compression-relaxation cycles. CONCLUSIONS. The evaporation-retarding effect is dependent on the physicochemical properties of the WEs at given temperature, and therefore, the effect most likely arises from a certain phase of the WE layer. However, WEs as such are poor surfactants and need to be accompanied by polar lipids to form stable lipid layers.