Effect of fluorocarbon surfactants on the adsorption of hydrocarbon surfactants mixture at the water-air interface

Taking into account the great practical importance of the surface tension of the aqueous solution of multicomponent surfactant mixtures their adsorption at the water-air interface and the composition of the adsorbed monolayer were considered. For this consideration the mixture of the p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol) Triton X-165 (TX165) and hexadecyltrimethylammonium bromide (CTAB) with FC6E014 as well as with FC5E010 was applied. The surface tension of the studied solution at the constant CTAB and TX165 binary mixture concentration corresponding to its solution surface tension equal to 60 and 50 mN/m and changing fluorocarbon surfactant concentration was measured. The obtained results were described by the exponential function of the second order and predicted by the modified Fainerman and Miller equation. To solve the Fainerman and Miller equation the fraction of adsorbed mixed monolayer at the water-air interface occupied by particular surfactants was determined. For its determination, the contribution of particular surfactants in the mixture to reduction of water surface tension was taken into account. The fraction occupied by the given surfactant in the mixed monolayer was compared to that determined using the Hua and Rosen equation obtaining good agreement between them. The fraction of the mixed monolayer occupied by a given surfactant was also successfully used to solve the Gibbs and Frumkin equations for determination of surface concentration of a particular surfactant in the mixed monolayer. Based on the isotherms of the surface excess concentration as well as the Hua and Rosen theory the packing of the mixed monolayer and the presence of the synergetic effect in the reduction of water surface tension were deduced. This effect was also reflected in the standard Gibbs free energy of adsorption of surfactant mixtures at the water-air interface. It was found that this energy can be predicted based on this energy for particular surfactants of the mixture and Gibbs free energy of mixing. (C) 2021 Elsevier B.V. All rights reserved.