Anti‐evaporative mechanism of wax esters: implications for the function of tear fluid

Abstract

AbstractPurpose Our aim was to explain the molecular level basis of the ability of wax ester (WE) films to retard evaporation close to their bulk melting temperature. We studied the surface behavior of behenyl palmitoleate (BP), a WE representing the most abundant meibum WEs.Methods Isotherm and isochor measurements coupled with imaging by Brewster angle microscopy (BAM) were used to study BP films at the air‐water interface. In addition, evaporation rates through BP films were measured.Results Close to room temperature, BP formed solid, non‐spreading islands when applied to the air‐water interface. Between 35 and 38 °C, a fluid, effectively spreading monolayer phase coexisted with solid domains of BP. At higher temperatures, BP formed completely fluid films. Mean molecular areas of the fluid and solid films were determined to be approximately 50 Å2 and 18 Å2, respectively. The thickness of a solid BP monolayer was determined to be 42 ± 6 Å by BAM measurements. A fluid BP monolayer only had minimal evaporation resistance, while a solid monolayer retarded evaporation by > 50%.Conclusion Below 35 °C, BP assumes an extended conformation at the air‐water interface, similar to the bulk crystal structure of WEs. The extended conformation allows tight packing of hydrocarbon chains, which hinders the permeation of water molecules and leads to the evaporation retarding effect. Although the solid islands effectively retard evaporation, they do not spread at low temperatures and water evaporation proceeds through the uncovered areas of the surface. Coexistence of fluid and solid monolayer phases between 35 and 38 °C allows the film to spread, leading to a complete coverage of the interface by the solid phase and effective evaporation retardation.