The properties and molecular structure of thin films. Part II.—Condensed films

Abstract

In this and a following paper, an account will be given of work done in continuation of the experiments on thin films of palmitic acid, described in Part I of this investigation (1), and, in particular, of the results obtained by extending the range of substances composing the films, and the range of temperature over which the experiments were made. As was there pointed out, the study of these films is a peculiarly attractive one, owing to the extremely simple manner in which the molecules are arranged; the films being of one molecule in thickness and the molecules in them being arranged not indiscriminately, but perpendicular to the surface and parallel to each other, as a rule. The experimental method (due in principle to Langmuir) has also a great advantage over those more commonly employed in capillary studies, that the forces tangential to the surface are measured directly. Exceptionally direct information is therefore obtainable as to some of the forces between the molecules composing the films, and by a study of the influence of chemical constitution on the properties of the films, much light should be shed upon the important problem of the relation between chemical and capillary forces. It will not be necessary to occupy space in proving that, for any of the substances here studied, the films are one molecule in thickness. Reasons for taking this view have been given by other workers, as well as on p. 344 of my preceding paper, and it is sufficient to say that the experiments described here afford evidence of the same character, which is equally conclusive for each one of the substances investigated.