Effects of some anions of the Hofmeister series on the rheology of cetyltrimethylammonium-salicylate wormlike micelles
Graphical abstract
Introduction
Their remarkable tunable viscoelastic characteristics have led wormlike micelles, WLM, to be exploited to large number of applications. In the semi-dilute regime, the WLM chains entangle, forming viscoelastic systems useful to be used from cosmetic and pharmaceutical products to oil exploration fields [1], [2], [3]. In dilute regime, WLMs promote large hydrodynamic drag reduction, saving energy to pump water for long distances [4], [5]. The non-covalent nature of WLMs is unique in this case, because, contrary to polymeric agents, they are immune to undergo mechanical degradation at high shear rates [6].
According to Lerouge and Berret [7], the first description of the micelle growing can be found in the pioneer work of Debye and his group [8] during the investigation of their dissymmetry by light scattering. They observed that addition of KBr to cetyltrimethylammonium bromide, CTAB, induced a transition from spherical to rod-like aggregates.
Usually, the growth of micelles takes place at high concentration of inorganic ions. This is because the screening of the electrostatic charges leads to favor decreasing in the micelle curvature, forming long cylindrical structures. However, for cationic surfactants, this growth can be induced in dilute regime with addition of aromatic anions, such as salicylate. The association is usually very strong due to the incorporation of the aromatic ring at the micelle palisade [9], [10], [11].
In semi-dilute regime, the entanglements of the WLM chains produce remarkable viscoelastic properties and unique rheological behavior since the micelles break and reform within a characteristic lifetime [12]. For this reason, they were referred to as ‘living polymers’ by Cates and Candau to describe their linear rheological behavior [13]. According to this model, two dynamic processes coexist (reptation and breaking) where each of them present a specific relaxation time, λrep and λbreak, respectively. When λbreak ≪ λrep, the stress relaxation is described by a unique relaxation time (Eq. (1)):
The storage and loss moduli, G′ and G″, respectively, are described by the Maxwell model, according to Eqs. (2), (3), being ω the frequency of the oscillation:
The viscoelastic properties of some WLM systems can be tuned by light [14], [15], pH [16] and addition of hydrophobic solute [17]. Therefore, this is an interesting system in which macroscopic properties can be switched via simple molecular triggers [18].
Particularly, we are interested on the effects of anions with different nature on the rheology of WLM. In this sense, WLM formed by cetyltrimethylammonium salycilate, CTASal, was used as a reference to investigate the effect of some anions of the Hofmeister's series. The series: F− ≈ SO42− > HPO42− > acetate > Cl− > Br− > NO3− > ClO4− > SCN−, was proposed by Franz Hofmeister (in 1888) according to their crescent ability to precipitate egg-white proteins [19]. Ions on the left of Cl− reduce the solubility of proteins by inducing their crystallization and are called salting-out ions, water-structures-makers or cosmostropic ions. The opposite is observed for ions on the right of Cl−, known as salting-in, water-structure-breakers or chaotropic ions [20], [21].
WLMs of CTASal at low ionic strength present excess of positive charges at their surface. The adsorption of anions on the micelles surface reduces the electrostatic repulsion and changes the rheology of the systems. Based on this aspect, the present study describes the effects of the ions Cl−, Br−, NO3−, SCN− and even Salicylate, Sal−, on solutions formed by CTASal. The characteristics of these systems were investigated by using oscillatory rheology and their microstructure was probed by small-angle X-ray scattering (SAXS) measurements. The results correlate with the Hofmeister series, with important implications to tune viscoelastic properties of WLMs formulated with cationic surfactants and salicylate.
Section snippets
Materials
CTAB and the sodium salts: bromide, chloride, nitrate, thiocyanate and salicylate were obtained from Sigma–Aldrich and used without previous purification. Ultra-pure water milli-Q (18 MΩ cm) was used in the preparation of the solutions.
Sample preparations
CTASal was synthesized using the protocol described by Svensson et al. [22]. Firstly, CTAOH was obtained from CTAB through an ion exchange process (by using Dowex monosphere 550A (OH) anion exchange resin – Sigma–Aldrich). Finally, CTASal was obtained by titration
Results and discussion
The results for shear oscillatory studies of pure CTASal solutions at different concentrations are shown in Fig. 1. In the concentration range between 25 and 300 mmol L−1, the elastic moduli, G′ (Fig. 1 – left), are always higher than the correspondent viscous moduli, G″ (Fig. 1 – right), and G′ is practically constant in the studied frequency range. The solutions of CTASal behavior have very long relaxation times, which cannot be experimentally determined with the present experimental set up.
In
Conclusions
The present study demonstrates that the rheological behavior of CTASal solutions is strongly affected by addition of salts (NaCl, NaBr, NaSal and NaSCN) leading to a transition from long to short relaxation time. Therefore, the rheological properties of the WLM of CTASal can be tuned by using salts with different anions and at different concentrations. It was demonstrated that the Hofmeister series is obeyed. However, salicylate is a special case due to the threading of the aromatic ring into
Acknowledgments
The authors thank the Brazilian agency CNPq, and PETROBRAS-CENPES for financial support and fellowships. We also thank the National Synchrotron Laboratory (LNLS) for the SAXS experiments.
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