Pilgrim Trust Lecture, Molecular layers

If fragments of camphor are placed upon a clean water surface they move about vigorously and may even be made to propel toy boats. The late Lord Rayleigh (1890 a, 1890 c) found that these movements stopped rather abruptly if the surface tension of the water was lowered by 21 dynes/cm. by oily contamination of the surface. The amount of olive oil needed for this purpose was surprisingly small, corresponding to a thickness of only 16 A (16 x 10 -8 cm.). Miss Pockels (1891) proved that any amount of olive oil less than enough to give a critical thickness of about 10 A had no effect whatever on the surface tension of water, but above this limit the surface tension decreased rapidly as the amount of oil was increased. Only 5 g. of olive oil would be needed to cover an acre of water surface with a film of this critical thickness. Miss Pockels also showed that accidental contamination of the surface, which had previously complicated nearly all observations of surface-tension phenomena, could be eliminated by using a trough filled to the brim with water, and sweeping impurities off the surface by the motion of barriers which rested on the edges of the trough. This use of movable barriers to confine films, to compress them or to remove them from the surface, laid the foundation for nearly all the modern work with films on water. The early theories of surface tension had been developed by physicists (Thomas Young 1805; Laplace, Gauss, etc.) who either treated liquids as continuous fluids between whose elements of volume forces acted, or considered only spherical molecules which exerted upon one another forces that varied as a function of the distance between molecular centres. Such theories naturally could not take into account the wealth of knowledge that had been accumulated by organic chemists regarding the structures of organic molecules.

A rapid method for determining the lowering of tension of exposed water surfaces, with some observations on the surface tension of the sea and of Inland waters

Ordinary methods of determining surface tension require a certain amount of apparatus and time, and often disturb the surface, thereby altering the tension of a water surface covered by a film of oil. As it was desired to make a number of observations on the amount of invisible contamination on the sea, a method depending on the spreading power of drops of mixtures of different fatty substances has been worked out. An oil will just spread against an amount of contamination which lowers the tension by an amount equal to the spreading force, or “spreading coefficient”, of the oil. Pure long chain hydrocarbon oils do not spread on clean water; if small amounts of substances containing water-attracting groups in the molecule are dissolved in the hydrocarbon, spreading occurs, the spreading force depending on the amount of the second substance with the water-attracting group, in the solution. The theory has been treated particularly thoroughly by Langmuir (1933); this paper is, however, concerned only with the application of solutions of different concentrations of a substance such as dodecyl alcohol, in a pure, rather heavy, hydrocarbon oil, to the determination of the surface tension of either fresh or salt water. It is found that the behaviour of a single drop of each of three or four such solutions, observed for half a minute or so, indicates the value of the surface tension within an accuracy of 1 dyne/cm. Calibration against water surfaces with oily contamination depressing the tension by known amounts is of course required; this is however quickly done, once for all, and it is possible to find substances for the spreading solutions which behave similarly on waters differing widely in acidity and in salt content.