How is an even density of oil molecules maintained?
Oil molecules raise surface pressure as molecules come close together. This property, called surface film pressure, is the reason oil molecules spread over water in a monolayer. (See Fig. 1.) The relation between the area occupied by oil and the surface film pressure follows a constant curve. (See Fig. 2.) By specifying the surface film pressure, an even density of oil molecules can be maintained. How do you measure Surface Film Pressure? When the bottom of a detecting glass plate is immersed in subphase water, a meniscus (a phenomena of subphase wetting up to the plate surface) is formed as shown in Fig. 3. The oil molecules spread over water are arranged on the water surface and apply pressure to each other. This pressure pushes the glass plate upward.