Molecular interaction in monolayers: I—complexes between large molecules

Abstract

The course of chemical actions in monomolecular films can be followed by measurements of change in phase boundary potentials, showing the interaction of polar hydrophylic groups; and of changes in surface tension which ensue on interaction of the hydrophobic groups in the molecules by Van der Waal''s forces. These 2 interactions are necessary for penetration of a liquid-air film of a substance by solute injected below it, and when penetration is complete the resultant film complex is a molecular association distinct in its chemical and physical properties from either of its constituents. Its phase boundary potential corresponds to that of an equimolecular mixed film subjected to maximum compression. The stability of the film complex depends on the strength of association of its hydrophilic and hydrophobic groups, e.g., a saponin-cholesterol film becomes solid, with an increase in surface tension of 60 dynes/cm. and a fall in surface potential of 100 mV; its formation is not prevented by prior compression of the cholesterol film to 30 dynes/cm. But the formation of more-weakly associated saponin-calciferol, -lecithin, -oleyl or -cetyl alcohol films is prevented by this pressure. Digitonin is more successful in penetrating films, Na-cetyl sulphate still more. The chief characteristic of the molecular association is its sensitivity to either small changes in polar reactivity, such as the change from -SO3 to -COO to -C00H, or sterol OH to cyclic OH to primary OH, or the insertion of a double bond; or to changes in the hydrophobic group saturated long chains show a strong interaction with condensed ring systems or saturated long chains, but a weak interaction with un-saturated condensed ring systems or unsaturated long chains with double bonds. The reactivity of the groups was graded by haemolysis experiments on red blood corpuscles, using equimolecular mixtures of Na-cetyl sulphate and the film-forming materials used above. Haemolysis was prevented in those mixtures which had produced stable films, i.e., showed strong association, not prevented in mixtures which had shown weak or negligible association. An exception occurred in the case of the alcohols, which formed definite complexes with the cetyl sulphate, but did not prevent lysis, probably due to the stronger association of the sulphate with the proteins or cholesterol in the corpuscle membrane.