Further experiments on surface films of sterols and their derivatives

MOST of the compounds of the sterol group form stable unimolecular films when spread on a water surface.As with the long-chain aliphatic substances, the two principal stabilising forces for such films are (a) the anchorage to the water afforded by the strong attraction perpendicular to the surface between the water-soluble or "polar" groups and the underlying water, and (b) the lateral adhesion between the molecules of the film, which is largely, but not wholly, due to the hydrocarbon part of the molecules.In the great majority of cases, surface films of sterols are of the "coherent" type, i.e. they show a very small surface vapour pressure, or tendency of the molecules to escape along the surface to form a film of the gaseous or vapour type.R 12 Me 1 e 2 8 3 7 4 6 Diagram I.Sterols containing a single hydroxyl group in the position occupied by that in cholesterol, i.e. at 3 on the modern formula, frequently form very incompressible films with areas per molecule near to 40 sq. A. This has been shown by Leathes [1923], Adam [1928], Knight [1928], Adam and Rosenheim [1929], Fosbinder [1933], and Danielli and Adam [1934].In such films there is little doubt that the molecules are standing very closely packed and perpendicular (or very nearly perpendicular) to the surface, the hydroxyl groups at 3 being at the lowest point of the molecules.Measurements on models show that the area of cross-section of a sterol ring system perpendicular to its longest axis is of this order of magnitude; moreover, in the vertical position it is the carbon atom at 3 which is at the lowest point of the molecule, hence as the water- attracting group always attempts to approach the water as closely as possible, a vertical position is more probable with a soluble group at position 3 than at any other.If the molecules are vertical, a low compressibility is to be expected, as no reduction of area can be obtained by altering the tilt of the molecules to the surface.Slight differences in area have been found between various sterols ( 1786 )