Influence of the propanol on the behaviour of binary mixture of nonionic surfactants at the water–air interface

Abstract

The influence of propanol on the behaviour of mixed nonionic/nonionic surfactant solutions, that is p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols) Triton X-100 (TX-100) and Triton X-165 (TX-165) at the water–air interface was studied by measuring the surface tension. The obtained results of the surface tension measurements were compared with those calculated from the von Szyszkowski and Connors equations. It appeared that using the Connors equation, it is possible to describe the dependence between the surface tension of studied solutions as a function of propanol concentration. However, in the case of the Szyszkowski equation it was possible to describe this dependence with good accuracy only in the range of propanol concentration from 0 to that corresponding to the propanol molecules aggregation (CAC). The constant b in the Szyszkowski equation is equal to that in the Langmuir one and allows calculating the values of the standard Gibbs free energy of propanol adsorption which is nearly the same as those determined from the Gu and Zhu equation. The standard Gibbs free energy of adsorption of propanol and surfactant mixture indicates that propanol decreases the surfactant mixture adsorption to a larger extent than vice versa which is confirmed by the comparison of the surface layer composition. At the concentration of propanol higher than its CAC, the surface tension of the solution in the presence and absence of surfactants mixture is the same which suggests that the adsorption of surfactants is very small or practically close to zero.