Fourier transform infrared investigation on the state of water in reverse micelles of quaternary ammonium gemini surfactants C12-s-C12⋅2Br in n-heptane

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Abstract

The state of water in the reverse micelles of C12-s-C12⋅2Br homologues has been investigated by Fourier transform infrared spectroscopy. The results showed that the solubilized water had four states: the quaternary ammonium head-group-bound, the Br-bound, the bulklike, and the free water. With increasing W0, the number of bulklike water per surfactant (nb) rapidly increased, which indicated swelling of the reverse micelle. The number of the head-bound water per surfactant (nN+) gradually increased. This was attributed to a reduction of the interfacial curvature, which permitted more water molecules to associate with the ionic heads of surfactants and also led to a part of n-hexanol being expelled from the interface and thus water filled up. Owing to the existence of n-hexanol in the interface, the head-bound water of the present system was smaller than that of AOT system at the same W0. The number of counterion-bound water per surfactant (nBr) remained unchanged with W0. This was due to much smaller dissociation of the head of C12-2-C12⋅2Br than that of AOT. With increasing s, unchanged nN+ is attributed to the comprehensive effects of enlarged head, which promotes the hydration, increased ionization degree, and reduced size of the water pool. Owing to increased ionization degree, nBr increases with s.

Graphical abstract

The states of water solubilized in the cores of the reverse micelles of C12-s-C12⋅2Br homologues have been investigated by Fourier transform infrared spectroscopy. The results showed that the water in the reverse micelles of C12-s-C12⋅2Br had four states: the quaternary ammonium head-group-bound, the bulklike, the Br-bound, and the free water.

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Introduction

It is well known that water in living organisms exists in various states with different properties, and plays important roles in biological functions. It is of interest to study the state of water in reversed micelles as a model of specific water in biological systems because reversed micelles can easily produce water of various states by changing the W0 value, where W0 means molar ratio of water to surfactant [1], [2], [3], [4], [5], [6], [7], [8], [9]. For instance, Li et al. [7] examined the states of water in the reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane by FT-IR and NMR and proposed a four-component model, i.e., the head-group-bound, the Na+(counterion)-bound, the bulklike, and the free water coexisting in the reverse micelles. Similarly, Zhou et al. [8] and Gonzalez-Blanco et al. [9] also reported four water species in the reversed micelles formed by AOT in isooctane or toluene. For these water species, the free water corresponds to those dispersing among long hydrocarbon chains of surfactant and is very little compared with the other three types of water. The amount of the bulklike water depends on the size of the water pool. The water located near the inner surface of the reversed micelle has a specific property by binding with the head groups of surfactant and with their counterions. The hydration ability and the dissociation of the head group of surfactant determine the number of the bound water.

Different from AOT that has one head group and two alkyl chains, gemini surfactant is made up of two hydrophilic head groups, two hydrophobic chains, and a spacer linked at or near the head groups via covalent bond [10], [11]. The size of the head of gemini surfactant depends on the length of the spacer chain, which induces the alteration of the hydration and the dissociation of the head group [12]. Thus the reversed micelles formed by gemini surfactant can be used as a good system to study the effect of the head group on the state of water.

Recently, we have investigated the aggregation of a homologous series of gemini surfactants, alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) (designated as C12-s-C12⋅2Br) in n-heptane with the assistance of n-hexanol [13], [14], [15]. A part of n-hexanol participates to construct a film of water droplet together with C12-s-C12⋅2Br, where the hydroxyl groups of n-hexanol molecules interact with the quaternary ammonium heads of surfactants. This undoubtedly affects the bound water associated with the head groups.

Infrared spectroscopy (IR) is a noninvasive technique suitable for detecting hydrogen bonds and has often been employed to study the states of water solubilized in reverse micelles in many laboratories [7], [8], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The purposes of this paper is to reveal the states of water in the reversed micelles formed by C12-s-C12⋅2Br in n-heptane by FT-IR techniques and to discuss the effects of the molecular structure of the gemini surfactant on the properties of water. To our knowledge, this is the first time that the characteristics of water in the reverse micelles of gemini surfactants were examined.

Section snippets

Materials

The molecular structure of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) (designated as C12-s-C12⋅2Br, s=2, 3, 4, 5, 6, 8) is represented as follows: These compounds were synthesized in our laboratory with the method reported by Zana et al. [12], where C12-2-C12⋅2Br was synthesized by the reaction of N,N,N,N-tetramethylethylenediamine with 1-bromo-n-dodecane, and the other compounds were synthesized by the reaction of N,N,N-dodecyldimethylamine with α,ω-dibromoalkanes.

For n-heptane

O–H stretching band of water in the reverse micelles

It is well known that the IR bands over the region of 3050–3750 cm−1 are attributed to the O–H stretching vibrations of water in reversed micellar solutions [16], [18], [19], [21], [25]. For the reversed aggregate formed by quaternary ammonium surfactant in aliphatic solvent, alcohol is required to construct the mixed interfacial film together with surfactant [13], [27], [28], [29], [30], [31], [32], [33], [34]. The O–H stretching vibration at 3536 cm−1 corresponding to hydration alcohol

Conclusions

Water in the reverse micelle of C12-s-C12⋅2Br homologues is determined as four states: the quaternary ammonium head-group-bound, the Br-bound, the bulklike, and the free water.

The water associated with the head group of C12-2-C12⋅2Br is smaller than that of AOT at the same water content owing to the existence of n-hexanol at the interface. The water associated with the counterion in C12-2-C12⋅2Br system is unchanged with W0, whereas that increases in AOT system. This is due to the dissociation

Acknowledgements

Support from the National Natural Science Foundation of China (20173010, 20373012), the Foundation of MOE of China (20050386007), and the Fujian Provincial Natural Science Foundation of China (E0510005) is gratefully acknowledged.

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