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°C and, as a reference, at 30°C and 41°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°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°C the most effective retardants were BLN and BLNN decreasing evaporation by ~50%, whereas BP and BO decreased evaporation by only 5% to 10%. At 41°C, each lipid decreased evaporation by only 2% to 4%. The evaporation-retardant WEs all melted within 2°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.