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 R(evap). METHODS: R(evap) 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 μm(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 μm. R(evap) of human tears and buffer with and without human meibum (34.4 μm 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.