Contributions of Polar and Nonpolar Lipids to the Evaporation Resistance of the Tear Film Lipid Layer

Purpose: To quantify the respective contributions of polar and nonpolar lipids to the evaporation resistance of the tear film lipid layer under physiologically relevant conditions. Methods: A physiologically relevant model TFLL composed of behenyl oleate, cholesteryl lignocerate, triolein, and (O-acyl)-ω-hydroxy fatty acid (OAHFA) (40:40:10:10, mol%) was investigated using a controlled-area droplet evaporimeter, which enables direct quantification of evaporation resistance while simultaneously controlling surface pressure, droplet surface area, relative humidity, and surface temperature. The nanoscale organization of OAHFA monolayers and the model TFLL was characterized using atomic force microscopy. Results: The evaporation resistance of the model TFLL increased from 8.8% at a surface pressure of 20 mN/m to 35.1% at 35 mN/m. Quantitative analysis demonstrated that the relative contributions of polar and nonpolar lipids to evaporation resistance depended strongly on surface pressure. At 20 mN/m, nonpolar lipids contributed approximately 60% of the total evaporation resistance. At 35 mN/m, polar lipids became the dominant contributor, accounting for approximately 60% of the total evaporation resistance. Conclusions: The evaporation barrier function of the TFLL is governed by the cooperative, surface pressure-dependent interactions of polar and nonpolar lipids. Although nonpolar lipids dominate evaporation resistance at low surface pressures, polar lipids become the primary determinant as the TFLL approaches physiological compression. These findings establish a direct relationship between TFLL nanoscale organization and evaporation resistance, provide experimental evidence supporting the duplex TFLL model, and identify restoration of the polar lipid layer as a promising therapeutic target for evaporative dry eye disease.