Surface Chemistry and Interfacial Biophysics of Meibum and Meibum--Benzalkonium Chloride Films: Implications for Tear Film Lipid Layer

The tear film lipid layer (TFLL) prevents excessive evaporation, thereby maintaining normal ocular surface function. One characteristic of a healthy TFLL is a gel-like structure that resists disruptive mechanical forces; however, this has not been clearly demonstrated. Herein, the compressibility of TFLL was investigated via surface chemistry experiments of meibomian gland secretions (meibum), the source of TFLL. Meibum was collected post-mortem from bovine eyelids, and purity was verified by mass spectrometry. Meibum films on a water subphase in a Langmuir-Blodgett trough were studied. The changes in surface pressure ({pi}) versus area (A) during compression were recorded, and the corresponding compressive modulus (Cs-1) versus{pi} isotherms were determined. The meibum film was stable at a surface pressure as high as 32 mN/m and maintained a near-constant Cs-1 of 24-32 mN/m across a{pi} range of 8-30 mN/m, demonstrating gel-like behavior. Comparison of the maximum Cs-1 with those of representative individual meibum lipids supports the presence of a monolayer predominantly composed of low-polarity lipids, including wax esters, cholesteryl esters, and diesters. The average molecular areas determined for the film and reported for representative lipid species suggest that the monolayer is predominantly composed of saturated, ultralong-chain, low-polarity lipids and that the TFLL includes a dense monolayer and a loose overlying sublayer. This structural model is further supported by the lower Cs-1 (10 mN/m) and larger average molecular area (68 [A]2) observed for films containing a mixture of meibum and benzalkonium chloride (BAK), a preservative with surfactant properties.