Peculiarities of surface layer formation in binary aqueous solutions of surfactants and polymetacrylic acid

Abstract

The dependences of the surface tension of aqueous solutions of three surfactants: nonionic ‒ nonylphenoxypoly (ethoxy) ethanol, anionic ‒ sodium laureth sulfate, and cationic ‒ N-alkyl-1,3-propanediamine on their concentration have been studied. The use of a differential approach to the analysis of the obtained results helped to reveal certain peculiarities in the formation of surface layers in aqueous solutions of these substances. In particular, it has been found that the dependence of the reduction rate of the surface tension of these solutions on the concentration of the investigated surfactants has a maximum. Given this dependence, it is substantiated that the formation of surface layers takes place in two successive stages: the first ‒ diffusion movement of surfactant molecules to the phase boundary: aqueous solution – air interface; and the second one ‒ spatial structuring of these molecules in the surface layer. The reduction rate of the surface tension of these solutions is determined by the speed of the process of the stage at which it is less. At low concentrations of surfactants in solutions, the reduction rate of surface tension limits the diffusion movement of molecules to the phase boundary, and at high concentrations of surfactants ‒ the spatial structuring of molecules in the surface layer. For a nonionic surfactant, the transition from the restriction in the first stage to the restriction in the second stage occurs at lower concentrations compared to the anionic and cationic surfactants. It has been also shown that the presence of polymethacrylic acid macromolecules in solutions of these surfactants affects only the first stage of surface layer formation, namely, it facilitates the diffusion movement of surfactant molecules to the phase boundary. Keywords: aqueous solutions, intermolecular interaction, surface tension, surfactants, polymethacrylic acid.