Evaporation and Hydrocarbon Chain Conformation of Surface Lipid Films

Purpose: The inhibition of the rate of evaporation (R-evap) by surface lipids is relevant to reservoirs and dry eye. Our aim was to test the idea that lipid surface films inhibit Revap. Methods: Revap were determined gravimetrically. Hydrocarbon chain conformation and structure were measured using a Raman microscope. Six 1-hydroxyl hydrocarbons (11-24 carbons in length) and human meibum were studied. Reflex tears were obtained from a 62-year-old male. Results: The Raman scattering intensity of the lipid film deviated by about 7 % for hydroxyl lipids and varied by 21 % for meibum films across the entire film at a resolution of 5 mu m(2). All of the surface lipids were ordered. R-evap of the shorter chain hydroxyl lipids were slightly (7%) but significantly lower compared with the longer chain hydroxyl lipids. R-evap of both groups was essentially similar to that of buffer. A hydroxyl lipid film did not influence R-evap over an estimated average thickness range of 0.69 to > 6.9 mu m. Revap of human tears and buffer with and without human meibum (34.4 mm thick) was not significantly different. R-evap of human tears was not significantly different from buffer. Conclusions: Human meibum and hydroxyl lipids, regardless of their fluidity, chain length, or thickness did not inhibit R-evap of buffer or tears even though they completely covered the surface. It is unlikely that hydroxyl lipids can be used to inhibit R-evap of reservoirs. Our data do not support the widely accepted (yet unconfirmed) idea that the tear film lipid layer inhibits R-evap of tears.