The influence of electrolytes on gaseous monolayers II. Ionized films

An equation of state for a charged monolayer spread on an aqueous solution of a 1: 1 electrolyte at 20 degrees C in the absence of cohesive interactions has been derived, namely $\Pi $ = $\frac{2kT}{A-A_{0}}$ + 6$\cdot $10$\surd $c cosh sinh$^{-1}\frac{133}{A\surd c}$-6$\cdot $10$\surd $c. It has been found to apply to soluble charged films at hydrocarbon/water interfaces. In the case of insoluble monolayers of both alkyl quaternary ammonium halides and sodium alkyl sulphates at the hydrocarbon/water interface there is evidence of considerable ($\sim $ 50%) ion-pair association, and this appears to be even further enhanced at the air/water interface at least for sulphate films. Surface potentials, measured by both the radioactive electrode and vibrating plate condenser techniques have been found to be in good agreement. The surface potential of sulphate monolayers in contrast to the ideal behaviour of quaternary ammonium films has been found to be dependent not only on the electrolyte concentration and film area but also on the nature of the interface, the nature of the substrate and at the air/water interface on the chain length. This has been attributed to a reorientation of the film molecules at the interface. Differences in the surface potential behaviour of sulphate and quaternary ammonium films are explained by suggesting that whereas with the sulphate film restricted rotation about the C$\chembond{1,0} $O bond can lead to different configurations of the sulphate ion with respect to the alkyl chain, only one configuration is possible for the symmetrical quaternary ammonium ion. Surface potential evidence indicates that contrary to what has been suggested previously, sulphate monolayers are completely dissociated in the acid-base sense.