Aggregation behaviour and solubilization capability of mixed micellar systems formed by a gemini lipoamino acid and a non-ionic surfactant

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Abstract

In this work we carried out the physicochemical characterization of mixtures of a non-ionic surfactant and a gemini lipoamino acid one. The mixed systems were studied by surface tension measurements, fluorescence quenching and SAXS determinations. Both surfactants formed mixed micelles in different relationships and the properties of the mixtures were more similar to those of their major component. SAXS analysis showed that the micelles were triaxial ellipsoids, type core shell. The mixtures exhibited non-ideal behaviour, as indicated by critical micelle concentrations (CMC) lower than those of the individual surfactants, and negative interaction parameters, β.

The mixed surfactant systems produced enhanced water solubilization of polycyclic aromatic hydrocarbons, viz, naphthalene, phenanthrene and pyrene, quantified in terms of the molar solubilization ratio and the micelle-water partition coefficient. Synergism due to the presence of both surfactants in the mixture was observed for the solubilization of naphthalene and phenanthrene, but not for pyrene. This difference in behaviour might be related to the CMC and structure of the surfactants, the polarity of the solutes, and their association constants with the micellar phase.

Introduction

Over the last years many efforts were made to design and develop new amphiphilic molecules or new systems of amphiphiles with adequate properties for application in different areas or technologies [1], [2], [3].

In this respect, gemini surfactants are an interesting type of surfactants consisting of two hydrophobic chains and two hydrophilic heads covalently attached through a spacer [4], [5]. These surfactants have attractive properties such as low critical micelle concentrations (CMC), higher surface activity, better solubilizing power and viscoelastic properties than those of monomeric ones [6], [7], [8]. However, the synthesis of these surfactants usually requires several steps and, in some cases, it is more expensive than certain commercial monomeric surfactants for application to large volumes. Some mixtures of surfactants show synergism, with better properties than those of the individual components [9], [10]. In the presence of synergic effect, the study of mixtures of surfactants could lead to the use of lower amounts of the more expensive or difficult to obtain surfactant, in combination with one more accessible, for different possible applications.

In our laboratory we synthesised, according to the literature [11], a gemini lipoamino acid surfactant named 3,3′-disulfanediyl bis 2-decamido propanoic acid (SDDC), Fig. 1, derived from the amino acid cystine and a fatty acid, decanoic acid. The use of surfactants from renewable sources is also particularly interesting due to the improved properties of these compounds, including enhanced biodegradability and biocompatibility, and a variety of chemical functionalities, with minimal environmental impact [12], [13].

Surfactants are important tools for solving many hazardous environmental problems caused by toxic water-insoluble organic compounds [14]. There are many examples in the literature of applications of surfactants to increase the solubilization of insoluble pollutants in water, by partition of the hydrophobic organic compounds in the micellar core [15], [16], [17]. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants formed by incomplete combustion of organic matter due to natural and anthropogenic activities [18]. They display low water solubility and high adsorption capacity, thus their removal from soils depends largely on the desorption of the pollutant from the soil surface and its incorporation into the micelles. A variety of mixed surfactant systems was studied with the aim of obtaining better media to enhance the remediation of PAH contaminated soils, finding synergism in some mixtures [19], [20], [21]. A new and not so explored field is the application of gemini surfactants [22], [23] or binary systems formed by gemini-gemini [24] or gemini-conventional surfactants on the solubilization of PAHs [25], [26]. Synergism is also sometimes shown in these systems.

We had previously studied a mixture of a non-ionic surfactant (Tween 80) with an anionic one (sodium laurate), finding synergism for the solubilization of naphthalene in water [27] and for the desorption of naphthalene and phenanthrene from solid matrices [28]. The behaviour observed in that system was attributed to the formation of mixed micelles between both surfactants. In this work, we studied a new mixture of surfactants formed by the same non-ionic surfactant previously used (Tween 80) and the anionic gemini surfactant, SDDC, synthesized in our laboratory (Fig. 1). The aim of this work was twofold: first, to characterize the new mixed micellar system, where the previous anionic surfactant was changed for a compound with two hydrophobic chains and two anionic charges in the working conditions. The studies included surface tension, fluorometric determinations, and small-angle X-ray scattering measurements (SAXS). The changes in the structure of the anionic surfactant influenced considerably the characteristics of the mixed micellar system, providing it with different properties. Second, we proposed to analyse the application of these systems in the solubilization of naphthalene, phenanthrene, and pyrene (as representative PAHs) in water, seeking synergism in the mixture of surfactants.

Section snippets

Materials

Aqueous solutions were prepared from Milli Q water (Millipore purification equipment). Methanol (HPLC grade, J. T. Baker) was used as received.

PAHs were obtained from Sigma-Aldrich with purity higher than 98%.

The non-ionic surfactant, Tween 80, was obtained from Sigma-Aldrich and used without purification. SDDC was synthesised in the laboratory according to the literature by the reaction of cystine with decanoic chloride [11].

The solutions were prepared in buffer of pH 9.16 (0.01 M Na2B4O7, 0.02 

Micellar properties of mixed surfactant systems

The CMC of the pure surfactants and the Tween 80/SDDC mixtures were determined by surface tension measurements (Table 1). The break points in the plots of surface tension against the logarithm of the concentration of surfactants were taken as CMC values. Fig. 2 shows a representative plot. From Table 1 it can be seen that the experimental CMCs at different compositions of the mixture range between the CMC values of gemini surfactant and Tween 80, except the value for αTW80 = 0.8 that is even

Conclusions

We investigated the properties of mixed surfactant solutions formed by a commercial surfactant, Tween 80, and the lipoamino acid derivative SDDC. The mixed micelles had triaxial ellipsoid shape with properties in between both pure surfactants. The mixtures showed a non-ideal behaviour as indicated by a lower CMC than that of the individual surfactants, and negative interaction parameter, β, values.

The solubility of naphthalene, phenanthrene, and pyrene was measured in solutions of the

Acknowledgements

This research was supported in part by Agencia Nacional de Ciencia y Tecnología (FONCyT, PICT 2010–2001), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP 112-201101-00441), Ministerio de Ciencia y Tecnología de la Provincia deCórdoba (MINCyT, PID 2009–2011) and Universidad Nacional de Córdoba (UNC), Argentina (05/C579). We thank the Brazilian Synchrotron Light Laboratory (Beamline SAXS-1) for beamline under project 20150067. MAF and RGO hold researcher positions at

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