Interaction of a biosurfactant, Surfactin with a cationic Gemini surfactant in aqueous solution

Abstract

The interaction between biosurfactant Surfactin and cationic Gemini surfactant ethanediyl-1,3-bis(dodecyldimethylammonium bromide) (abbreviated as 12-3-12) was investigated using turbidity, surface tension, dynamic light scattering (DLS) and small angle neutron scattering (SANS). Analysis of critical micelle concentration (CMC) values in Surfactin/12-3-12 mixture indicates that there is synergism in formation of mixed Surfactin/12-3-12 micelles. Although Surfactin and 12-3-12 are oppositely charged in phosphate buffer solution (PBS, pH7.4), there are no precipitates observed at the concentrations below the CMC of Surfactin/12-3-12 system. However, at the concentration above CMC value, the Surfactin/12-3-12 mixture is severely turbid with high 12-3-12 content. DLS and SANS measurements follow the size and shape changes of mixed Surfactin/12-3-12 aggregates from small spherical micelles via elongated aggregates to large bulk complexes with increasing fraction of Gemini surfactant.

Introduction

The mixtures of different types of surfactants have attracted significant interest since mixtures provide a synergistic enhancement of performance and functionality which cannot take place in single surfactant systems [1], [2], [3]. Synergism increases with the degree of charge difference [4], [5], [6] meaning that the higher level of synergism is obtained by mixing anionic and cationic surfactants. There have been plenty of studies of anionic/cationic surfactants mixtures. The behavior and physicochemical properties of mixtures of a highly branched cationic and sodium alkyl sulfate have been widely studied by Yu and co-workers [7], [8], [9]. And, it has been reported that the asymmetrically double-tailed cationic surfactants fail to fit well into a crystal lattice structure and hence minimize precipitation [10]. Bergström and co-workers have studied the structures in cationic/anionic surfactant including Gemini systems using the small angle neutron scattering technique [11], [12], [13]. However, precipitation is a common phenomenon that can occur in all of the anionic/cationic surfactants mixture mentioned above and in most cases precipitation is undesirable because it renders the surfactant ineffective in solution. Therefore, a better understanding of such system is necessary to broaden the horizon for their application.

Surfactin, a biosurfactant produced by various Bacillus subtilis strains, is cyclic lipopeptide built from a heptapeptide (Glu-Leu-d-Leu-Val-Asp-d-Leu-Leu) and a β-hydroxy fatty acid with variable chain lengths of 13–15 carbon atoms [14], [15], [16] (Fig. 1(a)). Surfactin has been receiving attention for industrial, biotechnological, and therapeutical applications [17], [18] because of its high surface activity, which could reduce the surface tension of water to 27 mN/m [17] by that concentration as low as 10⁻⁵ M. It can be seen that Surfactin has two –COOH groups in l-Glu1 and l-Asp5. The pK_a values [19] of Asp and Glu are around 4.3 and 4.5, respectively, which means it exists as an anionic molecule in neutral solution.

Gemini surfactants contain two single-chain surfactant moieties joined by a spacer group [20], [21], [22]. The two amphiphilic moieties are close to each other, especially when the spacer group is short. Owing to this, on the one hand, the interaction between the hydrophobic chains is enhanced; on the other hand, repulsion between the hydrophilic groups (particularly the electrostatic between the ionic head groups) will be greatly reduced due to the chemical bonds connection. This novel class of surfactants has many unique properties that are superior to those of conventional single-chain surfactants, such as remarkably low critical micelle concentration (CMC), much higher surface activity, and better wetting properties. Moreover, it has unusual aggregation morphologies. Since the surfactant micellization is driven by hydrophobic interactions but opposed by the repulsion of charged head groups (for ionic surfactants) and hydration (for nonionic surfactants), Gemini surfactant is more readily to form aggregates. The quaternary ammonium Gemini surfactants are widely investigated about their properties and applications [23], [24]. For the 12-s-12 (12 carbon atoms on the hydrophobic alkyl chain and ‘s’ represents the number of carbon atoms of the spacer) series, at the concentration close to CMC value, spherical micelles are formed. Increasing the surfactant concentration, for the short spacer, such as s = 2, 3, elongated rod-like micelles are formed; for the medium length like s = 4, 6, 8, 10, 12, spherical micelles are formed, and for the longer spacer, s = 16, 20, vesicles are readily to form [24].

There are a lot of studies on Surfactin [14], [15], [16], [17], [18], [19] and Gemini [20], [21], [22], [23], [24] from various aspects. However, to the best of our knowledge, there is no research about their mixtures. The dynamic interfacial tension between the Surfactin/12-3-12 binary aqueous system and crude oil has been carried out in our group, which revealed that Surfactin/12-3-12 mixtures can reduce the interfacial tension to an ultralow level in alkaline environment (see details in Supporting Information) which can be used in the application in oil recovery. The better understanding of the fundamental physicochemical property of the mixture of Surfactin/Gemini surfactant system is the key point for their practical application. In our previous work, we have investigated the micelle formation and interfacial behavior of Surfactin [25] in PBS buffer. A further research [26] has also been carried out on the interaction of Surfactin with betaines which revealed that there was a synergistic effect between them, and the configuration of mixed micelles changed with the head-groups of betaines. Then, the interactions among neutral polymer PAM, Surfactin and four betaines in PBS buffer were further studied. Transition from spherical to rodlike aggregates (micelles) has been observed in solutions of Surfactin and Surfactin/SDDAB (N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate) with addition of 0.8 wt% of PAM [27]. Wang et al. [28] studied the interaction between surfactin and cationic surfactant Cetyltrimethylammonium Bromide (CTAB) in mixed micelle, There exited synergism between anionic Surfactin cationic surfactant CTAB, and the mixed system might be able to form vesicle spontaneously at high molar fraction of surfactin. However they did not study the precipitation condition and the aggregates properties (shape and size) of the anionic/cationic mixed surfactant aqueous system. A continuation of the aggregation behavior of Surfactin/12-3-12 mixtures were investigated in present paper by surface tension, small angle neutron scattering and dynamic light scattering, which will broaden the potential application for both Surfactin and Gemini surfactants.