On the stability of unimolecular films. Part I.—The conditions of equilibrium

Abstract

Of the factors which determine the state of equilibrium of a unimolecular film on the surface of water the two which have been subjected to the most exhaustive examination are the effects of changes in temperature and pressure. In previous communications a generalisation of the effects of kinetic agitation on the state of a film, and the conditions of equilibrium imposed by the phase rule on the various states of such films have been described. As pointed out originally by Hardy, in film formation the magnitude and direction of the adhesional forces between the surface and the polar "heads" of the spreading molecules is one of primary importance. The effects of replacement of one type of polar head by another have been examined in detail by numerous investigators; but the effect on the state of a film of one substance of changing the magnitude of this adhesion, a factor clearly as important as the conditions of temperature and pressure, has not hitherto been considered. In these communications the results of an investigation on the effects of such a change are described. Unimolecular films of long chain fatty acids on the surface of water can exist in three well-defined states, the highly dispersed or vaporous, the expanded and the condensed. The expanded state, first noted by Labrouste, was considered originally by Adam to be vaporous in character. Schofield and Rideal ( loc. cit .), however, from an examination of the isothermals of the force area curves for a series of acids concluded that expanded that the true vapour pressure of a film of myristic acid at ordinary temperatures should be about 0∙2 dynes per centimetre. A similar suggestion was made independently by Langmuir and later accepted by Adam, when with a sensitive apparatus he effected the measurement of the vapour pressure of expanded films, which proved to be in good agreement with that predicted by Schofield and Rideal. It seemed probable that expanded films consist of some arrangement of tilted molecules with the chains still enmeshed and the heads free, as suggested by Schofield and Rideal and Langmuir. Adam, however, has concluded that the heads adhere to one another with sufficient tenacity to render this view improbable.