The thermodynamic properties of AOT/Dynol-604 mixed surfactant solutions

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Highlights

  • Densities of mixed micelle system of AOT/Dynol-604 were measured.

  • Experimental values of CMC and volumetric properties were determined.

  • Excess thermodynamic properties of the surfactant mixtures were reported.

Abstract

Volumetric properties of mixed surfactants of Bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and Ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (Dynol-604) in aqueous solutions with various compositions have been studied at 298.15 K by density measurements. The apparent molar volumes, the partial molar volumes, the critical micelle concentrations, and the apparent molar volumes in the monomeric and micellar states of the binary mixed surfactants with various compositions were determined. Moreover, the results were used to calculate the micellar compositions, the excess free energies of mixing, the activity coefficients and the interaction parameter between two surfactants for the mixed micelles based on the Rubingh’s formulism.

Introduction

The mixtures of ionic and nonionic surfactants have been extensively studied in recent years because of their fundamental interest and industrial applications, where the synergistic effects of mixed surfactants can be used to minimize the amount of surfactant required, to optimize their performance and to reduce costs and side effects on environment [1]. Understanding the micellization behaviors and thermodynamic properties of mixed surfactant systems is helpful in designing their employment in specific applications [2]. The reliable knowledge of the volumetric behaviors of pure surfactants and their mixtures is of great importance in many fields of the basic research as well as in the industrial practice [3]. Density measurement is a superior method for investigating the volumetric properties of mixed surfactant systems, from which the critical micelle concentration (CMC) may be deduced and further used to obtain the thermodynamic properties of the mixed surfactant systems [4].

Bis(2-ethylhexyl) sodium sulfosuccinate (AOT) is a much-investigated versatile anionic surfactant, which forms micelles, oil-in-water microemulsions and water-in-oil microemulsions in pure water and water/oil mixed systems in different conditions. It is also very frequently used in the construction of the mixed micelles [5], [6], [7], [8]. Ethoxylated-2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (C24H46O6, Dynol-604) is a non-fluorous and non-siloxane acetylenic glycol based gemini nonionic surfactant, with a low-volatile, low-foam, nonionic wetting agent ideal for high performance waterborne applications [9], [10], [11]. The aggregation behavior of Dynol-604 in water has not been able to be explored since it is almost insoluble in water. However, as AOT is added, due to the salting-in effect in aqueous solution and the formation of the mixed micelles containing Dynol-604, an increase of the solubility of Dynol-604 in water may be expected, which will probably enhance the potential applications of AOT and Dynol-604.

The present paper reports the volumetric properties of a series of aqueous solutions of AOT/Dynol-604 mixed surfactants deduced from the density measurement. The apparent molar volumes (Vϕ) of the binary mixed surfactants with various compositions of the surfactants in aqueous solutions are calculated and used to determine the partial molar volumes (Vs¯) and the CMC of the mixed surfactants. Furthermore, the experimental density data are analyzed using the Rubingh equation [12] to obtain the molar fraction of Dynol-604 in the mixed micelle (x2), the excess free energies of mixing (ΔGex0), the activity coefficients for AOT (γ1) and Dynol-604 (γ2), and the interaction parameter (βm) between the two surfactants in the mixed micelle.

Section snippets

Chemicals

General structures of AOT and Dynol-604 are shown in figure 1(a) and (b). The purities, suppliers and the method for drying and storing of AOT and Dynol-604 are listed in table 1. The purity of Dynol-604 was claimed to be 1.00 in mass fraction by the supplier and checked by the elementary analyses before it was used to prepare samples, which was 66.74% and 10.55% for C and H, respectively. These were in agreement with theoretical data (66.88% for C and 10.68% for H). Double distilled water was

Results and discussion

The apparent molar volumes Vϕ of various surfactants in water with various concentrations have been determined for studies of the thermodynamic behaviors of their aqueous solutions [13], [14], [15], [16], which may be calculated from the densities of pure water and the aqueous solutions by the following equationVϕ=M/ρ+1000(ρw-ρ)/(msρwρ),where ρw, M and ms are the density of water (g · cm−3), the molar mass of the surfactant (g · mol−1) and the surfactant molalities (mol · kg−1) respectively. Taking

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Projects 20973061 and 21173080).

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