Rheological investigation on anionic wormlike micelles at high temperature

Highlights

• The present paper sytematically investigated the rheological properties of wormlike micelles at 80 °C.

• Inorganic salt KCl induces extra-long anionic surfactant sodium erucate to form strong viscoelastic and thermo-stable wormlike micelles.

• KCl shows higher ability to promote micelles growth than tetramethyl ammonium bromide.

• Molecular interaction between salts and surfactants are detailed analysed.

Abstract

Rheological experiments were carried out on wormlike micelle formed by C22-tailed anionic surfactant sodium erucate (NaOEr) in the presence of KCl or tetramethyl ammonium bromide (TMAB). The effects of salt and surfactant concentration are systematically investigated at 80 °C. Two kinds of salt-based micelle systems both maintain strong rheological properties at high temperature. By comparison, KCl–NaOEr system possesses lower critical overlapping concentration, stronger viscosity, and threshold salt concentration. Obviously, KCl shows high efficiency than TMAB in favoring micelle growth and improving the viscoelasticity of solution. The steric hindrance should be responsible for the inefficiency of TMAB compared to KCl. The extra-long hydrophobic chain of NaOEr is responsible for the desirable thermostability.

Introduction

Surfactants can self-assemble into well-organized structures in aqueous media, such as spherical micelles, rodlike micelles, wormlike micelles (WLMs), and vesicles, due to unfavorable contact with water [1]. Among those aggregates, WLMs have attracted wide interests and have been extensively investigated both in fundamental research and practical applications, mainly due to admirable rheological properties like polymer solutions [2], [3]. However, its viscosifying mechanism is quite different from polymer solutions. Self-assembled by small surfactant molecules, WLMs can be reversibly disorganized and reconstructed under shear destroying. Therefore, they are also called “living” or “equilibrium” polymers [4]. Considering the unique rheological properties, WLMs can find industrial applications in drag reduction [5], enhanced oil recovery [6], and oil fracturing [3].

The typical WLMs systems are constructed by cetyltrimethylammonium halides (C16TAX) and their analogues with the assistance of salts, such as salicylate, tosylate, or dichlorobenzoate [7], [8], [9], [10], [11], [12]. The emphasis has been mostly put on studying the influence of hydrotrope or counterion on the formation of cationic WLMs. They facilitate the growth of micelles either from electrostatic repulsion or hydrophobic association, realized via interaction with surfactants. Researchers never stop exploiting new systems with high viscosifying efficiency. C22-tailed surfactant systems begin to be intriguing subject in recent years, and those surfactants include quaternary ammonium cationic surfactants [13], [14], zwitterionic surfactants [15], and anionic surfactant [16], [17]. The long-chain surfactant-based systems exhibit enhanced rheological properties [13], lower critical overlapping concentration and threshold salt concentration [13], [14], [16], and better tolerance of hydrocarbon and high temperature [13], [17]. The hydrophobic interaction between surfactants tails is the main drive force for the growth of WLMs, and therefore elongation on surfactant tail is favorable for hydrophobic aggregation.

Comparatively speaking, anionic surfactants have received fewer attentions than cationic surfactants, although they are more bio-compatible and degradable [18], [19]. Alkyl sulfate and carboxylate possessing moderate hydrophobic tails (no more than C18), are investigated frequently in WLMs field. However, these surfactants form WLMs usually at relatively high surfactant and salt concentration, and furthermore the rheological properties are not so strong and thermostable [20], [21], [22]. Feng's research group firstly introduced C22-tailed anionic surfactant, sodium erucate (NaOEr), to prepare strong viscoelastic WLMs [16], [17]. The dissolution of NaOEr is the prerequisite for further study due its poor solubility. Quaternary ammonium ions (TAA⁺) effectively enhance the solubility of NaOEr due to salt-in effect and meanwhile promote the micelle growth acting as hydrotrope [16], [17], [19], [23]. The dual contributions of TAA⁺ cooperatively promote the formation of NaOEr WLMs and they become an exclusive hydrotrope for NaOEr so far. As far as we known, inorganic salts have seldom been used to induce NaOEr to form WLMs. It is a promising approach to exploit matched counterions for NaOEr to enrich anionic WLMs systems.

NaOEr bears a C22 tail with a cis unsaturation bond at the 13-carbon position and the molecule structure is shown in Scheme 1. KCl and tetramethyl ammonium bromide (TMAB) are respectively investigated on the influence of phase behavior and rheological properties of NaOEr solution. The rheological properties were systematically investigated from the aspects of salt concentration effect and surfactant concentration effect at 80 °C.