Browsing by Author "Paananen, Riku Oskari"

The anterior surface of the eye is covered by a thin tear film, which lubricates and protects the ocular surface as well as provides a smooth optical interface for light to enter the eye. The outermost layer of the tear film is a lipid layer produced mainly by the Meibomian glands. Tear film lipid layer (TFLL) is thought to stabilize the tear film and prevent the ocular surface from drying by retarding evaporation of the aqueous tear fluid. A deficient TFLL leads to increased evaporation and is a common cause of dry eye syndrome. Dry eye syndrome is a disease of the ocular surface that causes discomfort, disturbance of vision and possible damage to the ocular surface. It is one of the most common diagnoses among ophthalmologic patients, with tens of millions of people suffering from moderate-to-severe symptoms worldwide and many more from milder or more periodic symptoms. The molecular organization responsible for the evaporation retarding properties of TFLL is not well understood. Some lipids, like saturated fatty acids and fatty alcohols are known to be efficient evaporation retarding lipids. However, TFLL consists mostly of wax esters, cholesteryl esters and unsaturated polar lipids, which have not been typically considered to be effective in retarding evaporation. According to recent studies, pure wax ester films do retard evaporation, but only close to their bulk melting temperature. In this Master's thesis, the properties of behenyl palmitoleate (BP), a wax ester closely resembling the most abundant wax esters found in tear fluid, were studied at the air-water interface. The aims of the study were to characterize the phase behaviour of BP at the air-water interface and determine the molecular basis of its evaporation retarding properties. Isotherms and isochors were measured, coupled with imaging by Brewster angle microscopy. In addition, the evaporation resistance of BP films were measured. BP was found to exist in a fluid state that spreads efficiently but does not retard evaporation and a solid state that does not spread but efficiently retards evaporation. Approximately 3 °C below bulk melting temperature, solid and fluid monolayer phases coexist, allowing a solid monolayer to cover the water surface. Furthermore, BP was found to assume an extended conformation in the solid phase, which allows tight packing of the molecules and prevents the permeation of water. Taken together, these results provide a molecular level explanation for the evaporation retarding properties of wax esters several degrees below their bulk melting temperature.