Studies on the Interaction of Surface Films with Solute in Solution. IV. Effect of Congo Red Dissolved in Aqueous Substrate upon the Monolayers of Octadecylamine, Cetyl Alcohol, Ethyl n-Hexadecyl Ether, Ethyl Stearate and Cholesterol

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Abstract
Effect of dissolved Congo red upon the monolayers of octadecylamine, cetyl alcohol, ethyl n-hexadecyl ether, ethyl stearate and cholesterol spread on the substrate containing the dye has been studied, under varying pH of the solution. It has been observed generally that all monolayers expanded when they were spread on aqueous solution, the pH of which was in such a region that the molecules of dye were almost in zwitterion form. Dipolar and van der Waals’ attraction were tentatively postulated to explain the nature of the cohesive force by which the molecules of dye were bond together so that the colloidal aggregates were formed in the solution. The expansion of the monolayer can be accounted for by the binding of the aggregate to film materials. This explanation could successfully be applied to the films of such nonionic substance as cetyl alcohol, ethyl n-hexadecyl ether and ethyl stearate. The largest expansion of the monolayer of octadecylamine was observed when the substrate containing dye showed acidity so that the molecules of dye were partly of negative charge while those of octadecylamine were of positive charge. In this case we had better take account of the possibility of ion-ion attraction between octadecylamine and dye, in addition to the forces which appeared in the cases of nonionic substances.