Formation of Worm-Like Micelles in Mixed N-Hexadecyl-N-Methylpyrrolidinium Bromide-Based Cationic Surfactant and Anionic Surfactant Systems

Through the descriptive and rheological characterization of worm-like micelles formed by N-hexadecyl-N-methylpyrrolidinium bromide and sodium laurate, the formation and properties of the worm-like micelles were affected by the concentrations of sodium laurate and temperature. Additionally, cryogenic transmission electron microscopy images further validated the formation of worm-like micelles.


Introduction
Numerous self-aggregates, such as micelles, bicelles, micro emulsions, liquid crystals and vesicles, are able to form by the association of surfactant molecules [1][2][3][4][5]. Among these diverse microstructures, worm-like micelles have attracted extensive attention because of excellent performance in terms of viscoelastic behavior [6,7]. Worm-like micelles have found applications in various areas over the past two decades, such as clear fracturing fluids, heat-conducting fluids, personal care products and templates for nanomaterial synthesis [8][9][10][11].
In dilute solutions, surfactants generally form small spherical micelles spontaneously when the concentration of surfactant is higher than the critical micelle concentration (CMC) [12,13]. Concurrently, under appropriate conditions of surfactant concentration, salinity and temperature, the spherical micelles may grow in one-dimension and form entangled worm-like structures that present viscoelastic performance similar to that of polymer solutions [14,15]. In agreement with the Israelachvili prediction [16], the introduction of additives facilitates the self-assembly of surfactants via electrostatic repulsion force screening [17,18]. However, in contrast to polymers that are connected by covalent bonds, worm-like micelles are also known as ''living polymer systems'' [19] because they join together through ionic bonds and can break and intertwine reversibly.
Ionic liquids (ILs) are also known as organic molten electrolytes due to their distinctive physical and chemical properties, such as high conductivity, low melting temperature, outstanding catalytic properties, and so on. ILs, therefore, have earned close attention from scientific researchers. Similar to ionic surfactants, surface active ionic liquids (SAILs) exist as a hydrophilic head and a hydrophobic chain simultaneously; hence, the self-assembled aggregates can be formed in aqueous solution. In recent years, the aggregation behaviors of SAILs and their structures, which can be manipulated by changing the cations, anions, substituent, etc., have been reported frequently [20,21]. Zhao and Zheng synthesized N-alkyl-N-methylpyrrolidinium bromide (C n MPBr) and studied its multiple assembly behaviors [22,23]. Dong and Zheng synthesized N-alkyl-N-methylimidazolium bromide (C n mimBr) and investigated its phase behavior [24]. Geng and Zheng studied the mutual interaction effects between bovine serum albumin and N-tetradecyl-N-methylimidazolium bromide [25].
Therefore, we were motivated to explore worm-like micelles formed by C 16 MPBr in the presence of sodium laurate (SL). Their molecular structures of C 16 MPBr and SL are shown in Figure 1. To study the shape and viscoelastic properties of worm-like micelles, rheometer and cryogenic-transmission electron microscopy (cyro-TEM) were utilized. These techniques were used to observe microscopic configuration, to calculate the rheological parameters and the flow activation energy. In addition, the effect of temperature on the morphology of worm-like micelles were also studied.

Materials
Cationic surfactant C 16 MPBr was synthesized and purified as described previously [22,23]. Anionic surfactant SL was an AR grade product of the Aladdin Chemistry Company and was used without purification. Deionized water was used to prepare all solutions.

Sample preparation
The solutions to be analyzed were prepared by simply mixing a variable concentration of SL with a fixed concentration of C 16 MPBr (70 mM). Samples were homogenized by mild heating and vortex mixed and then placed in a thermostatic bath at 25uC for at least one week to equilibrate before investigation.

Rheological measurements
The rheological properties were performed on a Physica MCR301 rheometer made by Anton Paar GmbH with a Rotor CC27 system. The shearing rate ranged from 0.01 to 1000 s 21 in the steady shear experiment. For the dynamic oscillatory measurements, the frequency region was set to a range of 0.01-100 rad?s 21 , and the linear viscoelastic region was identified depending on a dynamic strain sweep test, in which the frequency was fixed at 1.0 Hz.
The Maxwell-fluid model with a single relaxation time was usually used to explain the aggregation behavior of worm-like micelle solutions. The elastic modulus (storage modulus) G9, the viscous modulus (loss modulus) G0 and the complex viscosity Dg Ã D are given by the following equations [1,2]: Here, v is the frequency. t R is the relaxation time and is approximately equal to the reciprocal of v c , which is the crossover frequency when G9 and G0 intersect. G 0 is the plateau modulus, which is estimated by reaching a plateau at high frequency.

Cryo-TEM
Specimens for examination were stored in a controlled environment vitrification system with temperature and humidity control functions. The preparation of the cryo-TEM samples was performed with the following sequence of operations. First, 5 mL of the sample was placed onto a perforated polymer film held by tweezers to insure that the formation of the thin film spanned the mesh hole. Second, after 10 seconds, the samples were immediately immersed into liquid ethane just above its freezing point of 2 183uC. Third, the samples were then stored in liquid nitrogen to protect against contamination before they were moved to a Gatan cryo Holder 626 and examined with a FEI Tecnai 20 TEM (200 kV) at about 2174uC. The images were captured with a Gatan US1000 894 CCD and processed with Leginon software.

Results and Discussion
The formation of worm-like micelles The shear viscosity of 70 mM C 16 MPBr with different concentrations of SL is measured by the rheometer and the results are plotted in Figure 2a. At low SL concentrations, the samples show the typical characteristics of a Newtonian fluid; the steady-shear viscosity does not change with the shear rate. When the concentration of SL exceeds 20 mM, the samples show a non-Newtonian flow phenomenon; the viscosity maintains a constant value (Newtonian plateau value) at low shear rates and decreases dramatically at high shear rates, deviating from the simple Maxwell Model. Flow instability resulting in shear-banding that occurred in the solutions is the main reason why differences appeared from low to high shear rates. Usually, this phenomenon is identified as evidence of the formation of worm-like micelles [26]. When the concentration of SL exceeds 50 mM, the color of the solutions change from transparent to milky due to the formation of precipitate, which is generally called the salting-out effect [12].
The variation of zero-shear viscosity g 0 , which can be calculated from the average viscosity of the low shear rate, as a function of SL concentration is shown in Figure 2b. At the beginning, the values of g 0 are approximately zero, indicating Newtonian fluid behavior. With the increase of SL concentration, the g 0 decreases sharply after reaching the peak value of 23.5 Pa?s at 30 mM. The value of the maximum g 0 was typical for SAILs. An increase in the curvature energy of surfactant aggregates is induced by an increase in the salt concentration, causing the flexible worm-like micelles to form from the original spherical micelles, which schematic illustrate in figure 2b. The decrease in viscosity is generally attributed to a faster time-scale for reversible breakage which allows disentanglement to happen faster and the formation of branched micelles [27].

Rheological model of worm-like micelles
The variation profile of shear stress s on the shear rate c with different concentrations of SL at 70 mM C 16 MPBr is shown in Figure 3. The worm-like micellar solutions belong to the class of viscoelastic fluids with no yield stress, which can be inferred from the fact that all curves pass through the origin and show a linear relationship [28].

The viscoelasticity of worm-like micelles
To characterize the viscoelastic behavior, dynamic measurements were employed to test the C 16 MPBr/SL solution system. Before dynamic rheological experiments, the linear viscoelastic zone where G* (complex modulus) is independent of the applied stress was first determined through the stress sweep measurement. s = 1.0 Pa was chosen in subsequent frequency sweep measurements according to the stress scanning of the 70 mM C 16 MPBr/ 30 mM SL solution. Figure 4a shows the frequency (v) dependence of the storage (G9) and loss (G0) modulus for the sample containing 70 mM C 16 MPBr/30 mM SL. With the increase of v, both G9 and G0 increased in the low frequency region, and G9 continued to rise, while G0 declined after the crossover frequency v c . The sample shows liquid-like behavior (G9,G0) before v c and solid-like behavior (G9.G0) thereafter. Finally, G9 reaches a plateau (G 0 ), while G0 increases again after reaching a minimum (G0 min ). This trend fits with the viscoelastic characteristics of worm-like micelles that follow the single-element Maxwell's model [29,30]. The moduli G9 and G0 vary as v 2 and v in the low frequency region, and deviation from the rules occurs in the high frequency region. These are two pieces of evidence that the formation of worm-like micelles and the deviation can be explained by breaking/ recombining or the dynamic equilibrium theory [31].
To uncover how well the data are in accord with the semicircle characteristic of the Maxwell-fluid model, ''Cole-Cole'' plots of G0  Formation of Worm-Like Micelles PLOS ONE | www.plosone.org versus G9 are described by [6]: As shown in Figure 4b, the shape of the curve follows semicircular behavior perfectly, which indicates the formation of worm-like micelles at low and medium frequency, whereas the degree of deviation from the semicircular behavior at high frequency can be obtained by measuring the deviation of the data points.
In general, the steady-shear viscosity (g) and the complex viscosity (Dg Ã D) of surfactant solutions with network structures, such as worm-like micelles, are equal or near-equal when shear rate (c) and frequency (v) are equivalent values. When Dg Ã D is significantly larger than g, if the structures can survive small oscillatory deformations but are ruptured by large deformations, we can draw the conclusion that the Cox-Merz empirical rule does not apply. The steady-shear viscosity as a function of g and the complex viscosity as a function of Dg Ã D for the 70 mM C 16 MPBr/30 mM SL solution is shown in Figure 5. The coincidence of g and Dg Ã D indicates that worm-like micelles that possess rigid network structure have already formed [32]. The curves deviate from the Cox-Merz empirical rule at high frequencies, a result that can be attributed to faster breakdown and reconnection of the aggregates.
The rheological parameters related to the microstructures of C 16 MPBr solutions with different SL concentrations have been calculated according to the following formulas [1,2], and the results are listed in Table 1.
Here, l e is the entanglement length, l p is the persistence length, L L is the average contour length, k b is the Boltzmann constant, T is the absolute temperature, E scis is the scission energy, and G 0 ? is obtained by the extrapolation of the Cole-Cole plots at high frequencies to the X-axis intercept or from the equation G 0 ?~2 G'' max , where G'' max is the viscosity modulus at shear frequency v c . Here, we set l p = 15 nm according to the literature [28]. From Table 1, it can be observed that L L is largest and v c is smallest for the sample at 35 mM SL. The 70 mM C 16 MPBr/ 35 mM SL solution has the longest worm-like micelles and the longest kinetic lifetimes [2].

Effect of temperature
We also studied whether temperature has an impact on the formation of worm-like micelles. To show this effect quantitatively, rheological measurements of the 70 mM C 16 MPBr/30 mM SL solution at different temperatures, from 15uC to 30uC, and related rheological parameters are calculated in Table 2. The values of t R , g 0 and L L clearly decrease, but G 0 alone and l e are constant with temperature, albeit in an opposite manner. In the case of heating, the constant of G 0 indicates that the degree of entanglement of the micellar network is unchanged, and a decline of g 0 shows a lower viscosity of the sample. We speculate that the viscosity decrease is not only on account of the decrease of L L but also because of the amount of branching joints growing as a consequence of the rise in temperature. The growth of branching joints aggravate slide effects that accompany applied force, hence allowing a fast relaxation process and a decrement in viscosity under the condition of a maintained network structure [33].  The relaxation time fits the Arrhenius equation [34], resulting in a linear relationship, which can be observed from Figure 6a, which shows an Arrhenius plot of ln t R versus 10 3 /T as given by [1,2]: Here, E a is the flow activation energy, R is the gas constant and A is a constant. The value of E a calculated from the slope is approximately 120 kJ?mol 21 , approximately equal to the values of worm-like micelle systems reported previously [35]. Figure 6b presents the plot of ln (G0 min /G 0 ) as a function of the reciprocal of the absolute temperature, according to Eq. (7). The value of E scis is equal to 228 kJ?mol 21 for the 70 mM C 16 MPBr/30 mM SL solution indicates that it is more favorable to form elongated micelles with kinetic lifetime longer than conventional ionic surfactants such as CTAB [2,12].

Cryo-TEM
Cryo-TEM is employed for the direct observation of the micelle morphology and micelle joints of C 16 MPBr/SL solutions. As above, the morphology of aggregation undergoes a transformation process from spherical aggregates to worm-like micelles. During this process, the value of g 0 achieves a maximum value. Figure 7a and Figure 7b show that three-dimensional networks of worm-like micelles are observed in the 70 mM C 16 MPBr/30 mM SL solution and 70 mM C 16 MPBr/35 mM SL solution, respectively. The contour length could not be obtained on account of the unclear ends of the micelles. These worm-like micelles are found to overlap with each other and entangle into three-dimensional network structures, which corroborate the results of the rheology measurements, i.e., the appearance of a maximum g 0 is attributed to the presence of worm-like micelles. Other than the entangled worm-like micelles, a few branched micelles with joints were also observed that can explicitly explain the temperature effect on the character of worm-like micelles.

Conclusions
In summary, with the aid of sodium laurate, the aggregation behavior of worm-like micelles that fit the Maxwell model of a single stress relaxation mode has been observed in aqueous solution. These were formed by a novel surface active IL, Nhexadecyl-N-methylpyrrolidinium bromide. Rheological measurements revealed that the main factor affecting the viscoelastic properties of worm-like micelles are temperature and the concentrations of sodium laurate. Cryo-TEM imaging visually observed the formation of worm-like micelles and verified the influencing factors on the network structure of worm-like micelles. We believe that this work further explained the mechanism of formation for solutions of worm-like micelles with high viscoelasticity and serves as a possible utilization of ILs in colloidal systems.