Antievaporative mechanism of wax esters: implications for the function of tear fluid

The tear film lipid layer (TFLL) is considered to act as an evaporation barrier and to maintain the tear film intact between blinks. In vitro methods have, however, failed to reproduce this evaporation-retarding effect. Wax esters (WEs) are a major component of the TFLL. Close to their bulk melting temperature, WEs have been found to retard the evaporation of water, but the nature of this mechanism has remained unclear. We studied the interfacial organization of WE films by measuring their isochors and isotherms and evaporation-retarding effect, and we imaged these films by Brewster angle microscopy (BAM). Behenyl palmitoleate (BP) was used as a representative WE because it resembles the WEs found in meibum. At low temperatures, BP forms solid monolayer crystals in which the molecules are organized in a bulk-like extended conformation. Within approximately 3 °C below the bulk melting temperature, these solid monolayer domains coexist with a fluid monolayer film. At temperatures above the bulk melting temperature, BP forms a completely fluid monolayer in which the molecules are in a hairpin conformation. A fluid hairpin monolayer of BP does not significantly retard evaporation, whereas a solid monolayer decreases evaporation by >50%. The results provide a molecular-level rationale for the evaporation-retarding properties of WEs close to their melting temperature.