The structure of thin films. Part VIII.―Expanded films

Abstract

The present paper descrihes a closer investigation than has hitherto heen possible of the "expanded" state of surface films and of the relation of the films in this state to the condensed films on tire one hand, and the gaseous on the other. The expanded films were first reported by Labrouste (1) and examined more closely by one of us (2, Parts III to VI). They are formed from the close-packed condensed films by rise of temperature, the change being complete in a few degrees at constant pressure, and being of much the same nature, whatever the substance. It has been established in previous papers that the temperature of the expansion is raised regularly by an increase in the length of the chain of the molecule, the amount of this rise being practically the same in all homologous series where there is only one chain in the molecule; the absolute temperature of expansion depends, however, on many factors, not yet understood, dividends the expansion is a partial overcoming of the cohesion between the molecules in the condensed film. In the expanded films, however, there is still much cohesion. In hart VII we have shown that some of the expanded films possess, at low surface pressures, a definite vapour pressure of a few tenths of a dyne per cm., this region ending in a state of the dims which is analogous to a gas in turn dimensions. In the gaseous film, there is very little cohesion between the molecules. When there is a definite surface vapour pressure between the expanded film and the gaseous, we must clearly regard the expanded film as " liquid." We propose to call the expanded films which end in this vapour pressure region " liquid expanded films."