Molecular Interaction in Mixed Monolayers II. Unstable Mixtures with Unsaturated Acids

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Abstract
This investigation concerns the effect of the form of a molecule, with otherwise identical composition and structure, on its binding to the acidic or basic water of the subphase and to the molecules of the film itself. The binding to the water by the carboxyl group should be approximately the same for stearic acid and for the unsaturated acids, oleic and elaidic acid, with the double bond in the middle of the hydrocarbon chain. By compression of the film oleic acid may be squeezed out practically completely, from a mixed monolayer in which a saturated long chain alcohol, acid, or amine is the other constituent. Thus the energy of binding of the oleic acid to the other molecules in the film is weaker than that between the saturated molecules. However, earlier work in this laboratory has shown that the presence of the double bond does not decrease, but very slightly increases, the attraction between molecules. Thus it seems to be the shape of the molecule which reduces the energy of binding of oleic acid in the film. Oleic acid is a cis form, so a nine carbon atom chain R2 is bent backward with respect to the other 9C chain R1 which ends in the carboxyl group. It may be assumed that the whole molecule is free to rotate, with the carboxyl group in contact with the water, and that the group R2 is free to vibrate or ``flagellate.'' Thus the oleic acid molecule would occupy a larger area than a saturated acid and this increased distance together with a lessening of the length of the contact between this and the other molecules in the film, decreases greatly the energy of binding. Space models of oleic and elaidic acid have been used in the study of these relations. By a greater straightening of the chain, as in the transcompound, elaidic acid, the energy of binding should become intermediate between that of oleic and stearic acid, and much above that for oleic acid. On this basis compression should segregate elaidic acid slightly from the mixed film, but by no means completely, and this is what the experiments demonstrate.