Lipid Layer significantly Retards Evaporation (Fallacy #5)
The formation of an oily layer over water generally is a little understood process. So it may be of some benefit to discuss it in some detail.
Have you ever wondered what happens when oil is placed on water? [ Actually it could be any other liquid as long as it is immiscible with water and has less density so that it will float. ]
When an oil is placed on the surface of water one of three events may occur:
1. The oil will bead up and form a visible floating lens. When the water surface around the lens is tested, it will have the high surface tension of pure water. All nonpolar oils will exhibit such a behavior (e.g. paraffin or mineral oil. )
2. Some oils will appear the same as in the first event because floating lenses (one or more) will appear after a brief attempt at spreading. However, what appears as a clean water surface between the lenses, upon testing it will have a much lower surface tension than water does. It will even have a surface tension lower than the oil itself! And this holds the key to the explanation of this strange behavior. The molecules of the oil are quite polar with a hydrophilic (usually ionized) group at one end attached to a long hydrophobic hydrocarbon chain (tail). These molecules will cover the water surface with a monomolecular layer where the rod-like molecules will line up perpendicularly to the aqueous surface and the hydrophobic tails will be closely packed and held together by hydrophobic bonding while sticking out of the watery surface. The pure appearing water surface will actually consist of closely packed methyl groups (-CH3), the tail end of the hydrocarbon chain. Such a structured surface will have a surface tension even lower than the oil itself because at the oil surface the molecules are randomly oriented and will contain methylene (-CH2-) and some polar groups as well. So the interesting event occurs where the oil does not wet itself when it is in a highly oriented monolayer form. This is called an autophobic behavior. So the excess oil will collect in the form of lenses over the monolayer-coated water.
3. The third possibility is that the oil will spread over the water forming a film of uniform thickness over the water surface. Such a film was named a duplex film by Langmuir and it has a direct relevance to the superficial lipid layer of the tear film.
The formation of a duplex film sometimes is tricky. Either it always happens or some time it can never happen. ... Come again, Dr. Holly?
Well, when there is sufficient oil placed on water of a limited surface area so that the thickness of the resulting oily film is greater than 100 microns (cf. Fallacy #1) then gravity takes over and the weight of the oil will distribute it evenly at a constant thickness. This is not a genuine duplex film, however, because the spreading was not achieved by surface forces.
It has been proven by thermodynamic arguments some time ago that if the oil consists of only one component then it is unable to form a duplex film. One needs an oily phase of at least two components of different polarities to achieve that feat.
Everyone of us have observed this phenomenon after rain when the puddles on the road are coated with rainbow colored oil slicks. The interference colors signify that the oily films are duplex films. Quite thin so they are not affected by gravity and much thicker than a monolayer (at least 50 molecules thick). These slicks come from engine oil, used engine oils to be exact. If you try to do this using unused engine oil, it will exhibit the first behavior , thick oil lenses. It is the used engine oil partially oxidized in the engine that contains sufficient polar molecules to exhibit the rainbow colors.
By careful partial oxidation of engine oil one can create oils of with well defined spreading characteristics. Langmuir named these indicator oils. I could actually create some oils using Langmuir’s technique which had the almost identical film pressure and surface characteristics of normal meibomian secretion.
The superficial lipid layer of the tear film in the open eye is approximately 0.1 micron thick. It consists of many lipids mostly esters which have low polarity. It contains polar lipids at a low concentration. Approximately the ratio of polar to less polar fraction should be at least 1:50 (in molecular terms).
The polar fraction is located at the interface between the lipid layer and the aqueous tear layer. These molecules also interact with tear protein and glycoprotein molecules further stabilizing the lipid layer and the tear film.
Retardation of Evaporation.
I have already touched on this topic in the previous fallacy and posts.
Interestingly enough the greatest effect on evaporation is made by a well condensed monomolecular layer of lipids, even though its thickness is minute. Its thickness can be increased by increasing the length of the hydrocarbon chain of the molecule, and the retardation increases exponentially (!) with the length of the molecule. If the structure of the chains is such that no close packing of the molecules can be achieved , then the resistance to evaporation is considerably less.
Duplex oily films have minor or occasionally moderate effect on evaporation despite the monolayer of polar molecules at the interface. The reason is that the hydrophobic chains are diluted by the oily phase above the interface and no close packing can be achieved.
I emphasized more the role of the lipid layer in stabilizing the tear film during blinking than debating its role in the retardation of evaporation. The main reason is that this material briefly discussed her is not readily accessible anywhere and misconceptions are abound.
Pathology: If the meibomian lipids contain insufficient polar lipids unable to saturate the interface the superficial lipid layer may become unstable. Local variations in surface tension (film pressure) can create surface flow which phenomenon is also called Marangoni flow. Certain artificial tears contain polar lipids such as a phospholipid at a low concentration and can have a ameliorating effect.
If the polar lipid fraction is high due to lid infection, inflammation, etc. then at first the meibomian glands respond with increasing the secretion of the meibomian lipids often ten to a hundred fold. This is useful initially because it does dilute the polar lipid fraction.
However, if the polar fraction is high it can start to emulsify the lipid layer itself that again threatens the stability of the tear film. I already mentioned that under a bio-microscope one can see a similar breaking up the lipid layer when adding a minute amount of 0.1% benzalkonium chloride (a cationic surfactant) solution to the tear film surface. It is a fascinating (and convincing) view.