Evaluation of carboxylic acid-induced formation of reverse micelle clusters: comparison of the effects of alcohols on reverse micelles

https://doi.org/10.1016/j.bej.2004.04.012Get rights and content

Abstract

The effects of the addition of various carboxylic acids on the formation of AOT-reverse micelle clusters have been studied by quantitative analysis of their percolation phenomena. The results were compared with those of various alcohols based on the functional groups, namely, the hydrocarbon group, hydroxyl group, halogen substituents, and carboxyl group. Among these groups, the hydrophobic hydrocarbon group and halogen substituents suppressed the formation of micellar clusters, depending on their chain number or species. However, the hydrophilic carboxyl and hydroxyl groups enhanced the formation of micellar clusters. Furthermore, the effect of a hydroxyl groups on the formation of micellar clusters was stronger than that of carboxyl groups. These results can be explained by the additive contribution of each carboxylic acid and alcohol molecule, and they can be interpreted as a simple correlation based on the accessible surface area (ASA). These results suggest that the formation of AOT-reverse micelle clusters by the addition of carboxylic acids and alcohols can be explained by the hydrophobicity of the molecule, as well as by the formation of micelle-like structures with both carboxylic acid and alcohol molecules.

Introduction

A reverse micellar solution is a thermodynamically stable mixture of water, oil, and surfactant, in which regions of water are separated from those of oil by a monolayer of surfactants. One of the surfactants often used to form reverse micelles and microemulsions is sodium di [2-ethylhexyl] sulfosuccinate, usually referred to as AOT or Aerosol OT. Reverse micellar systems (RVMS) have the ability to solubilize a variety of biomolecules such as proteins and enzymes into a water pool with nanometer-size. Therefore, RVMSs have a wide range of potential uses, e.g. in protein extraction systems [1], [2], [3], [4], [5], [6], [7], to create hydrophobic reaction media for enzymes [8], [9], [10], and in preparation media to obtain functional materials [11], [12].

When using an AOT–RVMS in the protein extraction process, the distribution of proteins between the micellar organic phase and the aqueous phase is largely determined by the conditions in the aqueous bulk phase and organic phase, e.g. the pH, ionic strength, types of salt, concentration and type of surfactant, the presence of a co-surfactant, and the type of solvent [13]. By controlling these parameters, the extracted fraction can be varied via variations of electrostatic, hydrophobic, and steric interactions between proteins and micelles. Among these interactions, electrostatic interactions were considered as the main driving force in the protein extraction process. The protein extraction process when using an RVMS is also influenced by the properties and/or structures of reverse micelles. Dungan et al. have reported that micellar–micellar interactions can be regarded as an important factor in reverse micellar extraction processes, particularly in the case of the back-extraction process [14]. In our previous papers [15], [16], we demonstrated that because the properties and/or structures of reverse micelles are reflected in the interactions between micelles, these interactions could be easily evaluated by the analysis of the percolation phenomenon observed in RVMS.

The percolation phenomenon reflects the formation of micellar clusters caused by micellar–micellar interactions, and it can be easily quantified by the measurement of the electrical conductivity of the RVMS. Electrical conductivity measurements have been used to assess the formation of micellar clusters and to probe the structural changes occurring in such systems [15], [16], [17], [18], [19]. Individual droplets maintain low conductivity, whereas a sharp increase in electrical conductivity is caused by the percolation phenomenon, which is demonstrative of the micellar–micellar interactions. It is generally accepted that conductivity percolation in AOT–RVMS with a spherical droplet structure is the result of reverse micellar droplet clustering [19]. The conductivity of RVMS has been measured as a function of water content (W0) or temperature. By increasing the water content or the temperature in a reverse micellar solution, the properties of the reverse micellar solution, e.g. electric conductivity and viscosity, are drastically changed. Furthermore, various additives such as protein and synthetic polymers can change the percolation threshold. It has also been demonstrated that the solubilization of proteins or synthetic polymers clearly affects the percolation process, with a rapid increase in conductivity occurring at lower or higher water contents or temperatures, suggesting stronger or weaker attractive interactions between micelles in the presence of proteins or synthetic polymers [15], [16], [20], [21], [22], [23]. The percolation threshold of RVMS is significantly altered by the addition of a co-surfactant such as an alcohol molecule [24], [25]. Generally, amphiphilic additives can be distributed between the organic solvent and the reverse micelles in relation to their amphiphilic properties. The following questions identified by Luisi et al. [2] have been raised in this context: (a) what is the extent of additive partitioning between the reverse micelles and the external solvent? (b) Where do the additives reside when they are associated with reverse micelles? (c) What are the driving forces behind the phenomenon of additive–micelle association? These questions have only partially been answered regarding reverse micellar systems [26], [27], [28]. However, the reasons for changes in the structure and properties of reverse micelles induced by additives have not been sufficiently discussed, therefore, it remains an important topic to study.

In this study, RVMS percolation processes involving various carboxylic acids were investigated, and the effects of the presence of various alcohols on reverse micelle cluster formation were compared. The present experiments were conducted in order to understand the effects of additives on structural changes in reverse micelles. The roles played by carboxylic acids and alcohols in the formation of reverse micelle clusters were discussed in relation to the additive contribution of each constituent group of the carboxylic acids and alcohols; the accessible surface area (ASA) of each of the constituent groups, i.e. that of the hydrocarbon group, hydroxyl group, halogen substituents, and the carboxyl group, was considered in the present study.

Section snippets

Materials

Sodium di [2-ethylhexyl] sulfosuccinate (AOT) (95% purity) was purchased from Tokyo Kasei Co. (Tokyo, Japan) and was used without further purification. 2,2,4-Trimethylpentane (isooctane) was purchased from Ishizu Seiyaku Ltd. (Osaka, Japan). The abbreviations used to designate the carboxylic acids are summarized in Table 1. PrCOOH, BuCOOH, PenCOOH, HexCOOH, HepCOOH, and OctCOOH were purchased from Wako Pure Chemical (Osaka, Japan). Bu(COOH)2, Pen(COOH)2, Hex(COOH)2, Hep(COOH)2, BrEtCOOH,

Effect of carboxylic acids on the percolation process

The percolation process is reflective of micellar–micellar interactions and it can easily be quantified by measuring the electrical conductivity of the RVMS. Fig. 1 shows the effect of the addition of carboxylic acids on the percolation processes of RVMSs. In the case of BuCOOH added to the RVMS, the electrical percolation thresholds (φt) decreased with an increase in the BuCOOH concentration. This result indicates that the attractive interaction between micelles (i.e. the formation of reverse

Conclusions

We quantitatively examined the effects of carboxylic acids and alcohols on the percolation phenomena by utilizing the percolation process of an RVMS. The percolation process of an RVMS was found to be highly dependent on the species and concentration of added carboxylic acids. Depending of the species of molecule, the βt values (defined as the variation in the percolation threshold per unit concentration of added carboxylic acid) reflected the stability of the RVMS and they also were reflective

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