Molecular interaction in monolayers III. Complex formation in lipoid monolayers

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Abstract
2 new methods are described for investigating the process of penetration of monolayers by substances injected into the substrate. In one the mono-layer is confined by a movable barrier and maintained at constant pressure by means of a non-reactive piston fluid, the area change on injection being noted. In the other an electrically operated Langmuir trough is employed, the slide retaining the monolayer travelling automatically so as to retain the monolayer at constant pressure during the process of penetration or ejection of one constituent. Molecules containing ring systems such as digitonin, on penetrating a ring structure monolayer such as cholesterol, form only 1:1 complexes. A saturated chain molecule such as Na cetyl sulphate penetrating a ring structure monolayer such as cholesterol, forms a 1:1 complex. The surface pressure of the complex is found to be dependent on the conc, of the reactant in the substrate and to be in equilibrium with it. The difference in surface-free energies of formation of the complex with different bulk concs, of reactant can be evaluated both from the surface-pressure differences as well as from the variations in the bulk cones., thus showing an exptl. justification of Gibbs'' Law and that the complexes behave as chemical compounds. Other examples of this type were found to form complexes of composition both 1:2 and 1:1. Saturated chain reactants, e.g., Na cetyl sulphate on penetrating saturated chain mono-layers, e.g., cetyl alcohol, form stable complexes of composition 1:3 and 1:1, while an unstable 1:2 complex can be identified under certain conditions. The rate of decomposition of a complex monolayer at constant pressure follows the unimolecular law. The presence of 1:3 and 1:1 complexes giving solid mixed monolayers with saturated chains and of 1:2 and 1:1 complexes with irans-unsaturated chains suggests that the monolayer lattices of these complexes correspond to different "crystal" symmetries or types, and if this proves to be the case on crystallographic examination, Patterson''s classification appears to be applicable. Marked differences, due to cis and trans isomerism, are obtained on penetration, supporting the concept of interlocking chains.

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