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Abstract:
Thermodynamic parameters of mixed monolayer formation of aliphatic amines CnH2n+1NH2 and carboxylic acids CnH2n+1COOH (n = 6–16) are calculated using the quantum chemical semiempirical PM3 method. Four types of mixed dimers and tetramers amine–acid are considered. The total contribution of interactions between the hydrophilic parts of amine and acid into clusterization Gibbs energy is slightly lower than the corresponding interactions for individual surfactants. It suggests a synergetic interaction between the regarded amphiphilic compounds as proved by experimental data in the literature. Two types of competitive film formation are possible: mixed 2D film 1, where the molecules of the minor component are single distributed among the molecules of the prevailing second component (mixture of components on molecular level), and 2D film 2 with a domain structure comprised of pure component “islands” linked together. The dependence of the Gibbs energy of clusterization per monomer for 2D film 1 on the component mole fraction shows that the maximum synergetic effect is typical for the case that both surfactants have the same even number of carbon atoms in the hydrocarbon chain and form an equimolar mixture. Formation of 2D film 1 is more preferable than that of 2D film 2, if the difference of the hydrocarbon chain lengths is not larger than 5 methylene units. The limiting mole fraction of carboxylic acids in such mixed monolayers is 66.7%.
