The rigidity of solid unimolecular films

Abstract

On evaporation at low temperatures of a benzene solution of palmitic or stearic acid on the surface of water an apparently solid film is left. The experiments of I. Langmuir and N. K. Adam have shown that these films are unimolecular in character, and inasmuch that they possess a fairly well defined melting point, although this varies with the acidity of the underlying solution, we may regard these films as unimolecular sheets of orientated solid acid. Anyone who examines these films even superficially cannot fail to notice their exceptional strength, all the more extraordinary when we consider their extreme thinness. Whilst the compressibility of such films, a property which can be readily determined by examination of the slope of the force area curve obtained with the well-known Langmuir trough apparatus, does not present any unusual features, being of the order anticipated for a hydrocarbon, yet we may expect that the coefficient of rigidity will be unexpectedly high. It seemed a matter of some importance to attempt to measure the rigidity of such films, by applying a suitable torque and determining the displacements effected, a method suggested to us by Prof. G. I. Taylor. In our preliminary experiments we endeavoured to employ a simple static method of placing a disc at the centre of a large circular film and applying torsion by means of a torsion head and wire on the disc to which a mirror is attached. We have to express our thanks to Prof. G. I. Taylor for the loan of an excellent and finely-calibrated head for this purpose. After numerous attempts with various modifications of the method we were reluctantly compelled to abandon it. The results were invariably the same; on applying a small torque to the disc no motion was visible on the image of the mirror attached to the disc. If the torque be increased the film is ruptured and the disc breaks loose and slips, generally forcing itself entirely from attachment to the film. Even with discs coated with wax or corrugated and milled on the circular edge no better results were obtained. Evidently the grip on the disc being only of one molecule thick is not sufficient to hold the slightest movement on the part of the shearing disc.