Phase Transitions in Condensed Monolayers of Normal Chain Carboxylic Acids

Abstract

INVESTIGATIONS using a thermostated double-acting recording surface balance of the Wilhelmy-Dervichian type^1,2 reveal an interesting series of phase changes in condensed monolayers of normal chain fatty acids of high molecular weight. As an example, we may take the surface pressure – area curves for n-docosanoic (behenic) acid³ spread on acid (0·01 N hydrochloric acid) substrate shown in Fig. 1. At the lowest temperature the solid film gives a smooth pressure – area curve and the area can be reduced down to about 18·6 sq. A. per molecule before a gradual collapse sets in. At this area the chains are evidently vertical and as closely packed as in the normal three-dimensional hydrocarbon crystal, in which the cross-section of the chains is about 18·4 sq. A. This close-packed solid phase may be denoted by CS. When the temperature is raised, another phase, L₂, appears⁵, the stability range of which increases with temperature. At 8·2° a new phase, which we may call L'₂, appears and is quite marked in the curve for 10° in Fig. 1. At 13·8° the high-pressure phase S appears and the curve for 14·8° shows a transition S &rlarr2; CS which is rapidly displaced towards higher pressures with raising temperature. Above 18°, CS has disappeared and S shows a sharp collapse at about 55 dynes pressure. The smallest area at which the phase S is stable is about 19·3 sq. A. It seems likely, therefore, that the molecules are vertical and perform strong torsional oscillations⁶ or complete rotations, as the area per chain corresponds to that found in hydrocarbon crystals near the melting point⁴. (A few degrees below the melting point some hydrocarbons show a gradual, others an abrupt, change from a state of lower symmetry into one of hexagonal symmetry, in which the vertical chains are rotating about the long axis.)