Particular Behavior of Surface Tension at the Interface between Aqueous Solution of Surfactant and Alkane.

Valentin B Fainerman,Reinhard Miller,Alexander V Makievski,Eugene V Aksenenko,Emanuel Schneck,Aliyar Javadi,Mykola V Nikolenko

doi:10.1021/acs.langmuir.9b02579

Abstract

A two-component interfacial layer model was employed to describe the experimental results obtained for various surfactants. In contrast to the previous works, here it is shown that the adsorption activity of alkane depends on its interaction with the adsorbed surfactant and is proportional to the surface coverage by this surfactant. Also, it is assumed that this increase of the adsorption activity parameter is limited by some maximum value. This model provides a good description of the influence of hexane and dodecane, which results in the decrease of surface tension by 2-5 mN/m at very low surfactant concentrations. The adsorbed amounts of the surfactant and alkane molecules in this low surfactant concentration range have been calculated. The reorientation model of surfactant adsorption predicts a smaller number of alkane molecules per one surfactant molecule than that which follows from the Frumkin model.

Highlights

The adsorption of surfactants at fluid interfaces has been widely investigated for all types of surfactants and their mixtures with polymers or particles
For the sake of comparison, we show the adsorbed amounts of the surfactant calculated from the slope of the interfacial tension curve, using the fundamental adsorption equation of
In many studies on surfactants adsorption at their aqueous solution interface with oil, an unexpected phenomenon was reported a decreased interfacial tension at very low surfactant concentrations, i.e., 49−45 mN/m instead of 52−51 mN/m. These decreased tensions could not be attributed to the adsorption activity of the surfactant

Summary

■ INTRODUCTION

The adsorption of surfactants at fluid interfaces has been widely investigated for all types of surfactants and their mixtures with polymers or particles. At a very low surfactant concentration, the interfacial tension decreases by several mN/m, which cannot be explained by the mentioned increased adsorption activity This phenomenon was treated in ref 32 in the framework of a theoretical model that assumed the formation of a mixed adsorption layer comprised of surfactant and oil molecules. The total interfacial coverage by the two layers is constant and equal to the maximum monolayer coverage θ01 for the case that only alkane molecules form the interfacial layer, i.e., the θ1 value calculated from eq 4 for θ2 = 0 This approach, similar to the originally proposed model, does not take into account the influence of the surfactant molecules on the interfacial activity of the alkane molecules. The parameter α accounts for the different adsorption activities of molecules in different adsorption states.[31]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES