pH-Responsive Anionic Wormlike Micelle Based on Sodium Oleate

Article pubs.acs.org/JPCB

pH-Responsive Anionic Wormlike Micelle Based on Sodium Oleate Induced by NaCl Hongsheng Lu,*,†,‡ Qianping Shi,† and Zhiyu Huang†,‡ †

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Xindu, Sichuan 610500, P. R. China Engineering Research Center of Oilfield Chemistry, Ministry of Education, Xindu, Sichuan 610500, P. R. China

‡

ABSTRACT: A pH-responsive anionic surfactant wormlike micellar system induced by NaCl has been developed. In this work, the anionic surfactant, sodium oleate (NaOA) solutions, transforms from low-viscosity fluid into high-viscoelastic solution induced by NaCl of 200 mM to 350 mM concentration. According to the above, the solution reversibility has been studied via changing pH value of the solution. This pH-responsive solution can be promptly switched between gel-like solution and waterlike fluid in a narrow pH value range. Steady and dynamic rheological measurements are employed to characterize the pH-responsiveness at different pH. The transformation between wormlike micelle and spherical micelle in the various pH solutions is demonstrated by dynamic light scattering tests, cryoTEM, and NMR measurements. The pH-responsive property of the system is attributed to the carboxylate ion contained by sodium oleate. With higher pH value, the ionized carboxylate combines with NaCl closely and thus forms wormlike micelles. On the contrary, sodium oleate converts to oleate acid when pH decreases. In this way, spherical micelles are transformed because of the weaker interaction between oleate aicd and NaCl.

1. INTRODUCTION During the past few years, public concern over wormlike micelles (WLMs)1−4 has continued to rise owing to their unique properties. These micelles with flexible and elongated characters can easily entangle with each other forming a transient network structure. This highly entangled structure results in solution phase change, and thus contributes to form a solution with great viscoelasticity. Hitherto, the fascinating properties of the WLMs render them extremely useful in many aspects of industrial applications including oil production,5,6 drag reduction agent,7,8 and drug delivery.9,10 Recently, reversible WLMs have attracted considerable interest due to their remarkable superiorities whose viscoelasticity can be readily controlled via common stimuli, such as pH,11−13 CO2,14,15 light,16,17 and temperature.18,19 Compared with others, pH-responsive WLMs are quite important, for their easy preparation and reversible control in both laboratory and industrial sites. To the best of our knowledge, using either pHresponsive surfactants or pH-responsive counterions is one of the most necessary factors to create pH-responsive WLMs. From now on, pH-responsive WLMs that have been reported can be prepared by cationic surfactants. The most common cationic surfactant used to prepare pH-responsive WLMs is cetyltrimethylammonium bromide (CTAB). Though CTAB does not have pH-responsive ability itself, using pH-responsive counterions can deal with this problem. Huang and coworkers20 mixed CTAB and potassium phthalic acid (PPA) to design pH-responsive WLMs. Such WLMs can be pHcontrolled, because PPA has different degree of ionization at different pH values. Then, Hassan’s team 21 preferred © 2014 American Chemical Society

anthranilic acid (AA) to be the pH-responsive counterions combining with CTAB and thus forming WLMs. Later, Feng’s group22 introduced a pH-responsive surfactant N-erucamidopropyl-N,N-dimethylamine combined with maleic acid with molar ratio of 2:1 to form a facile, rapid, and cost-effective reversible WLMs system. Lately, Jacques L. Zakin et al.8 demonstrated a reversible threadlike micelle by mixing alkylbis(2-hydroxyethy)methylammonium chloride (EO12) with trans-o-coumaric acid (tOCA), which has two high viscosity peaks at both high and low pH levels, but shows waterlike behavior at medium pH. However, pH-responsive WLMs created by anionic surfactant have rarely been reported. Though Feng’s group23 only used erucic acid to develop a novel pH-switchable anionic wormlike micellar system in the year of 2012, there are still hardly any reports on anionic surfactants with pH-responsive wormlike micelle structure. Compared with cationic surfactants, it is generally acknowledged that anionic surfactants were more biodegradable and less toxic.24,25 Moreover, anionic surfactants exhibit stability and popularity in many oil field applications such as enhanced oil recovery. Therefore, it is necessary to strive on novel pH-responsive WLMs based on anionic surfactants. Sodium oleate, one of the ultra-long-chain anionic surfactants, was often used to prepare micelles with salts, cationic surfactants, or other additives. Gokul C. Kalur25 and Received: July 8, 2014 Revised: October 9, 2014 Published: October 13, 2014 12511

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co-workers promoted the growth of WLMs by adding triethylammonium chloride and potassium chloride in sodium oleate solutions. Nawja El Kadi26 et al. mixed CTAB and sodium oleate to form micelles. According to this method, they systematically studied the critical micelle concentrations of such a mixture. Nevertheless, the pH-responsive ability of WLMs based on sodium oleate, worthy of further research, is little mentioned. According to previous work, we developed a novel pHresponsive anionic WLMs system that could be induced by metal counterion. The WLMs consist of anionic surfactant sodium oleate and metal salt NaCl. We examined its pHresponsive ability through the pH change by adding HCl or NaOH. This solution can be promptly switched between WLMs and sphere micelles within an extremely narrow pH change, and such transformation is demonstrated by dynamic light scattering tests, cryo-TEM, and NMR measurements.

3. RESULTS AND DISCUSSION 3.1. Wormlike Micelle Induced by NaCl. The zero-shear viscosities and phase behaviors of NaOA/NaCl system with different ratios are shown in Figure 1. The different sodium

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium oleate (C.P. grade) was used without any other processes; sodium chloride, hydrochloric acid, sodium hydroxide, all of A.R grade, were purchased from ChengDu Kelong Chemical Co.Ltd. All samples were prepared in water triply distilled by a quartz water purification system. 2.2. Sample Preparation. NaOA and NaCl were dissolved completely in distilled water individually. Then, the desired amount of NaCl solution was added to NaOA, and the sample was well-mixed followed by magnetic agitation for several minutes. NaOH and HCl were added to the NaOA/NaCl solution to regulate the pH values of samples. All these samples were prepared and stayed at 25 °C. 2.3. Rheological Measurements. The rheological properties of all the samples were measured on a HAAKE RS600 rational rheometer equipped with cone and plate geometry. All the samples were equilibrated at 25 °C over 20 min prior to experiments. The amplitude sweep measurements were used to determine the linear regime as the angular frequency was fixed at 1 Hz. A fixed stress was chosen in the linear regime where the amplitude of the deformations was very low, and the dynamic frequency spectra measurements were performed at this determined stress. 2.4. Dynamic Light Scattering (DLS) Measurements. The DLS measurements were performed on a Brookhaven BI200SM goniometer at 25 °C. A 450 nm membrane filter was used to filter the solution and was used in the scattering cell. The scattering angle was 90°. The operating procedure was programmed using software in such a way that the hydrodynamic diameter and size distribution were evaluated. 2.5. 1H NMR Measurements. Samples for 1H NMR were prepared in D2O. Experiments were performed at 25 °C with a Bruker Ascend 400 MHz NMR spectrometer. 2.6. Cryo-TEM Observation. Samples for cryo-TEM work were prepared in a controlled environment vitrification system at 25 °C. The 5 μL solutions were loaded on a TEM grid by a micropipette, and a thin film was produced by blotting off the redundant liquid with two pieces of filter papers. The samples were quickly plunged into a reservoir of liquid ethane, which was cooled by liquid nitrogen at −165 °C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder and examined with a JEOL JEM1400 TEM (120 kV) at about −174 °C. Digital images were recorded in the minimal electron dose mode by a Gatan multiscan cooled charge-coupled device (CCD) camera.

Figure 1. Zero-shear viscosity and phase variation of NaOA/NaCl samples with different ratios.

oleate (NaOA) concentration curves varying with NaCl concentration are quite similar except that the viscosity increases as the NaOA concentration grows. One can easily find that the mere NaOA solution without NaCl exhibits an extremely low viscosity no matter what NaOA concentration is. However, when the NaCl concentration increases to 300 mM, all the solutions form transparent micelles accompanied by sharp viscosity increase. With further NaCl concentration increments up to 350 and 400 mM, the viscosity decreases slightly and remarkably, respectively, indicating that the highviscoelastic structure could be gradually destroyed by excessive NaCl. All in all, a possible mechanism of NaOA forming micelles induced by NaCl is illustrated in Figure 2. The added NaCl

Figure 2. Chemical principles of NaOA foming micelles induced by NaCl.

provides metal counterion Na+ to NaOA, which is the crucial factor to let micelles entangle with each other. A certain amount of Na+ in the system can compress the electric double layers of the interface and can screen the electrostatic repulsion between OA−, the charged head-groups, which results in more OA− going into micelles followed by micelle entanglement.27 The flowing properties of NaOA/NaCl solutions are further confirmed by rheological results at a fixed NaOA concentration of 65 mM. Figure 3a shows the variation of steady shear viscosity as a function of shear rate at 25 °C. The shear viscosity of the solution without NaCl is as small as water and stays unchangeable on any condition of the shear rate or shear stress, 12512

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frequencies and the G′ and G″ are related to the shear frequency, ω, by the following relations: G′(ω) =

G″(ω) =

(ωτR )2 1 + (ωτR )2

ωτR 1 + (ωτR )2

G0

G0

(1)

(2)

In the relations, G0 is the plateau modulus given by G′ at high ω, and the relaxation time τR is estimated as 1/ωc, where ωc is the crossover frequency that G′ and G″ meet. A linear plot of the imaginary part G″ and the real part G′ reveals a semicircular shape of the Cole−Cole plot characteristic of a Maxwell fluid: ⎛ G ⎞2 ⎛ G ⎞2 G″2 + ⎜G′ − 0 ⎟ = ⎜ 0 ⎟ ⎝ ⎝ 2 ⎠ 2 ⎠

(3)

Figure 3c illustrates the Cole−Cole plots of the 65 mM NaOA with different NaCl concentrations shown in Figure 3b. The curves represent the best fits for the semicircular behavior in the range of low frequencies obtained with eq 3, but deviations occurs from semicircles at high frequencies. This suggests that viscoelastic wormlike micellar system can be described by the Maxwell model with a single relaxation time over a considerable frequency range. 3.2. Phase Behavior and Zero-Shear Viscosity Variation Induced by pH. The macroscopic appearances of the 65 mM NaOA with 300 mM NaCl at different pH values are shown in Figure 4. There is no doubt that the sample forms

Figure 3. Effect of NaCl concentration on (a) steady rheology, (b) dynamic rheology, and (c) Cole−Cole plot of the 65 mM NaOA micellar solution at 25 °C.

which is a typical behavior of Newtonian fluids.27 Nevertheless, the samples with different NaCl concentrations exhibit a viscosity plateau at low shear rates and shear-shinning behavior at high shear rates, respectively. The variety of values in viscosity plateau exhibits a significant enhancement of the viscosity when NaCl concentration increases from 0 to 350 mM, and whose low-shear viscosity is nearly 104 times bigger than water. Furthermore, the shear-thinning at critical value indicates the structural change of WLMs, revealing arrangement of the long micelles at high shear rates.28,29 Dynamic rheological measurements (Figure 3b) denote a strong viscoelasticity of the 65 mM NaOA with different NaCl concentrations. The elastic or storage modulus (G′) is always smaller than the viscous or loss modulus (G″) at low frequencies. The curves of G′ and G″ exhibit a crossover point at a specific high frequency point, and this point, representing an inverse phenomenon beyond crossover, is often utilized to determine the relaxation time. Interestingly, increase in NaCl concentration leads to a lower crossover frequency, implying that higher NaCl concentration results a longer relaxation time. With a single stress relaxation time (τR), viscoelastic micellar system generally follows Maxwell fluid model at low shear

Figure 4. Phase behaviors and zero-shear viscosiy variation of 65 mM NaOA with 300 mM NaCl induced by pH.

a high viscoelastic system in initial pH value 9.68. The high viscoelastic system switches to “water” instantaneously after tuning pH to 9.43, and the transparent solution turns a turbid one simultaneously. Moreover, phase separation appears while continually adding HCl to adjust pH value to 9.33. The reason for these phenomena is that adding HCl leads NaOA to turn to oleic acid. Only the transparency of the sample is affected at first because NaOA and dissolved oleic acid coexist in the solution. Then, phase separation occurs since more oleic acid is transformed which is insoluble in water and even separated at lower pH value. On the other hand, at higher pH range of 9.53−10.18, NaOH provides more Na+ to screen the electrostatic repulsion and reduce distances between the head groups of OA−, which is more likely to promote wormlike micelle growth and hence causes a big increase in viscosity in a certain range. After that, Na+ exhibits smaller influence on viscosity and makes the viscosity curve stable. 12513

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In addition, zero-shear viscosities are plotted as a function of pH values to examine the effect of pH value on viscosity also shown in Figure 4. The zero-shear viscosity of the sample presents a dramatic change since pH value is altered. Two pH values, 9.43 and 9.53, are the critical values of the low and high viscosity, respectively. The 9.46 pH value of viscosity changes sharply, at which point more HCl or NaOH is needed to change the value of pH from 9.46, proving that a large amount of sodium oleate and oleic acid is mutually transformed at such pH value. When pH is no more than 9.43, the viscosity of the fluid is quite low (2 mPa s), while the viscosity increases at the same time since the pH value is raised by adding NaOH. The viscosity increases rapidly at pH value of 9.46, and the viscosity exhibits a continued slight enhancement with the pH increasing to about 10.18, and then stabilizes. These phenomena can be seen that sodium oleate and oleic acid can easily and mutually transform into each other at an extremely narrow pH value. When pH value is lowered slightly, the gel-like structure is broken because the sodium oleate switches to the oleic acid, and phase separation occurs because oleic acid is nearly insoluble in water. While the pH value is added a little, the separated phase disappears because of the oleic acid changing into sodium oleate and dissolving into water. Then, micelles entangle and form WLMs induced by NaCl. 3.3. Effect of pH on Rheological Behaviors. As above, the samples at different pH values have different properties. To further prove this point, steady and dynamic rheological measurements were employed to investigate the flowing properties of the 65 mM NaOA with 300 mM NaCl at different pH values. Figure 5a shows shear rate is the function of the variation of steady shear viscosity at different pH values, and all the samples were measured at 25 °C. It clearly shows that the flow behavior is highly affected by the pH value of solution. A Newtonian fluid is obtained at pH 9.43, whereas the solution regains viscoelastic behavior at pH 9.53 and 10.18. In the nonNewtonian pH values, these two samples show a low shear Newtonian plateau, followed by shear thinning at higher shear rates, which has been taken as evidence of the formation of WLMs. The pH effect of the viscoelasticity of the 65 mM NaOA with 300 mM NaCl micellar system is shown in Figure 5b where the storage modulus (G′) and loss modulus (G″) are plotted as a function of oscillatory shear frequency (ω) at 25 °C and different pH values. It is clearly seen that G′ shows a plateau at high frequencies and exceeds over viscous G″ in the whole frequency range of both samples, indicating the strong elastic behaviors. However, one can deduce that G′ will first cross over and then will drop below G″ at a lower ω, showing an obvious viscous behavior. More interestingly, a longer relaxation time and better viscoelastic behavior are proven by a lower cross frequency (ω0) at higher pH value. Here, for the different pH values of the viscoelastic solutions as shown in Figure 5b, one can find the data of G′ and G″ fit well with the Maxwell fluid model at lower frequencies, and the radius of the semicircle increases with the enhancement of pH value (Figure 5c). While at higher shear frequencies, the data presents a significant deviation on account of nonreptation effects and Rouse model.30 3.4. Effect of pH on the Average Hydrodynamic Diameter of the Aggregates. The molecular state variation of the solution with different pH values also reflects the evolution of the microstructures in the system. As we can see,

Figure 5. Effect of pH on (a) steady rheology, (b) dynamic rheology, and (c) Cole−Cole plot of the 65 mM NaOA with 300 mM NaCl micellar solution at 25 °C.

Figure 6 reveals the hydrodynamic diameter and size distribution of the aggregates in solution containing 65 mM NaOA with 300 mM NaCl at various pH values. The average hydrodynamic radius of 48 nm is obtained when pH is 9.53. Also, the radius of the particles in the solution is progressively decreased to 3.6 nm while regulating the pH to 9.43, which has been previously verified to be spherical micelles. However, the

Figure 6. Effect of pH on the average hydrodynamic diameter of the aggregates at 25 °C. 12514

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Na+. On the contrary, with the decrease of pH value, nonionic carboxylic group displaces the ionized carboxylate group in NaOA. In this way, the aggregate structure of the system transforms to spherical micelles from wormlike micelles due to the weaker binding with the counterions. 3.7. pH-Responsive Ability. In real applications, the switchability of viscoelasticity is expected to repeat several times. The pH-responsive ability upon pH change was tested for 4 cycles. Interestingly, the viscosity shows an apparent transition via a facile way of tuning the pH through the minor addition of acid or base. Two pH values (9.43 and 9.53) were chosen to verify the pH-responsive ability owing to their low and high viscosity. As is exhibited in Figure 9, when pH is 9.43,

peak turns to move to the right slightly with increasing pH, indicating the further growth of the micelles due to the more Na+ provided by NaOH as mentioned in Figure 4 3.5. Cryo-TEM. To further gain the transformation of the micellar solutions, the microstructures in the NaOA/NaCl mixtures at pH 9.53 and 9.43 described above were probed by using cryo-TEM. As shown in Figure 7a, we start with sample at

Figure 7. Cryo- TEM images of the 65 mM NaOA with 300 mM NaCl at pH 9.53 (a) and 9.43 (b), respectively.

pH 9.53. Flexible elongated micelles entangle with each other, and high-density WLMs appear in the solution. In contrast, viscosity decreases extremely when tuning pH to 9.43. According to Figure 7b, compared with the sample at pH 9.53, we could clearly see that the high-density WLMs had been substituted for spherical micelles. Obviously, the existence of several spherical micelles is consistent with such low viscosity. Therefore, the samples at “on” and “off” status are clearly determined by the reversible transformation between entangled WLMs and spherical micelles. 3.6. 1H NMR Measurements. NaOA contains carboxylate ion which has different structural forms in water under a various pH value environment. Also, this may strongly affect the interactions between amphiphile and counterion. NMR is a powerful tool for studying the interactions in the system. The proton resonances of NaOA/NaCl system with pH value of 9.53 and 9.43 are shown in Figure 8. We can easily find that the

Figure 9. Zero-shear viscosity of the 65 mM NaOA with 300 mM NaCl against pH cycles between 9.53 and 9.43 at 25 °C.

the viscosity is similar to that of the water (∼2 mPa s). While increasing pH to 9.53, the viscosity increases to nearly 104 times bigger than pH of 9.43. The viscosity gives an agile transition in such a narrow pH value range. The high viscosity decreases dramatically via dropping HCl; however, the lowviscosity fluid returns back to a high viscoelastic solution through adding NaOH. Even after 4 cycles of pH altering, the change of viscosity is still in existence with an invariable magnitude level. The self-assembly behavior of amphipathic molecules forms various microstructures such as spherical micelles, wormlike micelles, flexible bilayers vesicles, planar bilayers, and inverted micelles which are derived from parameter, p,32,33 which is defined as V/al, where V is the volume of hydrophobic group, a is the effective area of hydrophilic group, and l is the length of lipophilic chain. Spherical micelles are found in comparatively lower p values (smaller than 1/3) and entangled into WLMs when p is between 1/3 and 1/2, and then vesicles and bilayers are observed in the p range 1/2 to 1, while the inverted micelles are dominant in existence in solution if p is bigger than 1. When a certain surfactant is adopted, the transition from spherical micelle to WLMs depends on the size of effective head group area which is usually affected by additives in the solution. In this work, as shown in Figure 10, medium amounts of NaCl can effectively bind to the NaOA head group. As a result, the electrostatic repulsion between NaOA head groups is shielded, leading to the size of head groups and the distance between groups both decreasing, thus forming wormlike micelles. Then, when pH decreases, NaOA prefers nonionic species (−COOH) which cannot combine with NaCl to ionic species (−COO−),23 and forms spherical micelles, or even insoluble in water at an extremely lower pH value. On the contrary, the ionic species (−COO−) predominate in the solution and

Figure 8. Proton resonances for NaOA/NaCl system at pH 9.53 and 9.43.

chemical shifts of most protons (H1−H15) in the NaOA remain nearly unchangeable in these two solutions except the peaks of H16 and H17 protons move downfield at lower pH value. A downfield shift manifests a weaker electron density of the system.31 Therefore, when pH is 9.53, WLMs are formed because the ionized carboxylate has a strong combination with 12515

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(3) Acharya, D. P.; Kunieda, H. Formation of Viscoelastic Wormlike Micellar Solutions in Mixed Nonionic Surfactant Systems. J. Phys. Chem. B 2003, 107, 10168−10175. (4) Pei, X.; Xu, Z.; Song, B.; Cui, Z.; Zhao, J. Wormlike Micelles Formed in Catanionic Systems Dominated by Cationic Gemini Surfactant: Synergistic Effect with High Efficiency. Colloids Surf., A 2014, 443, 508−514. (5) Boek, E. S.; Jusufi, A.; L Wen, H.; Maitland, G. C. Molecular Design of Responsive Fluids: Molecular Dynamics Studies of Viscoelastic Surfactant Solutions. J. Phys.: Condens. Matter 2002, 14, 9413−9430. (6) Nasr-El-Din, H A.; Samuel, E.; Samuel, M. Application of a New Class of Surfactants in Stimulation Treatments; SPE 84898; Malaysia, 2003. (7) Shi, H.; Wang, Y.; Fang, B.; Talmon, Y.; Ge, W.; Raghavan, S. R.; Zakin, J. L. Light-Responsive Threadlike Micelles as Drag Reducing Fluids with Enhanced Heat-Transfer Capabilities. Langmuir 2011, 27, 5806−5813. (8) Shi, H.; Ge, W.; Wang, Y.; Fang, B.; Huggins, J. T.; Russell, T. A.; Talmon, Y.; Hart, D. J.; Zakin, J. L. A Drag Reducing Surfactant Threadlike Micelle System with Unusual Rheological Responses to pH. J. Colloid Interface Sci. 2014, 418, 95−102. (9) Cai, S.; Vijayan, K.; Cheng, D.; Lima, E. M.; Discher, D. E. Micelles of Different MorphologiesAdvantages of Worm-like Filomicelles of PEO-PCL in Paclitaxel Delivery. Pharm. Res. 2007, 24, 2099−2109. (10) Afifi, H.; Karlsson, G.; Heenan, R. K.; Dreiss, C. A. Solubilization of Oils or Addition of Monoglycerides Drives the Formation of Wormlike Micelles with an Elliptical Cross-Section in Cholesterol-Based Surfactants: A Study by Rheology, SANS, and Cryo-TEM. Langmuir 2011, 27, 7480−7492. (11) Kawasaki, H.; Souda, M.; Tanaka, S.; Nemoto, N.; Karlsson, G. R.; Almgren, M.; M, H. Reversible Vesicle Formation by Changing pH. J. Phys. Chem. B 2002, 106, 1524−1527. (12) Johnsson, M.; Wagenaar, A.; Stuart, M. C. A.; Engberts, J. B. F. N. Sugar-Based Gemini Surfactants with pH-Dependent Aggregation Behavior: Vesicle-to-Micelle Transition, Critical Micelle Concentration, and Vesicle Surface Charge Reversal. Langmuir 2003, 19, 4609− 4618. (13) Sakai, K.; Nomura, K.; Shrestha, R. G.; Endo, T.; Sakamoto, K.; Sakai, H.; Abe, M. Wormlike Micelle Formation by Acylglutamic Acid with Alkylamines. Langmuir 2012, 28, 17617−17622. (14) Su, X.; Cunningham, M. F.; Jessop, P. G. Switchable Viscosity Triggered by CO2 Using Smart Worm-like Micelles. Chem. Commun. 2013, 49, 2655−2657. (15) Zhang, Y.; Feng, Y.; Wang, J.; He, S.; Guo, Z.; Chu, Z.; Dreiss, C. A. CO2-Switchable Wormlike Micelles. Chem. Commun. 2013, 49, 4902−4904. (16) Kumar, R.; Ketner, A. M.; Raghavan, S. R. Nonaqueous Photorheological Fluids Based on Light-Responsive Reverse Wormlike Micelles. Langmuir 2010, 26, 5405−5411. (17) Oh, H.; Ketner, A. M.; Heymann, R.; Kesselman, E.; Danino, D.; Falvey, D. E.; Raghavan, S. R. A Simple Route to Fluids with Photo-Switchable Viscosities Based on a Reversible Transition Between Vesicles and Wormlike Micelles. Soft Matter 2013, 9, 5025−5033. (18) Davies, T. S.; Ketner, A. M.; Raghavan, S. R. Self-Assembly of Surfactant Vesicles that Transform into Viscoelastic Wormlike Micelles upon Heating. J. Am. Chem. Soc. 2006, 128, 6669−6675. (19) Lin, Y.; Qiao, Y.; Yan, Y.; Huang, J. Thermo-Responsive Viscoelastic Wormlike Micelle to Elastic Hydrogel Transition in Dualcomponent Systems. Soft Matter 2009, 5, 3047−3053. (20) Lin, Y.; Han, X.; Huang, J.; Fu, H.; Yu, C. A Facile Route to Design pH-Responsive Viscoelastic Wormlike Micelles: Smart Use of Hydrotropes. J. Colloid Interface Sci. 2009, 330, 449−455. (21) Verma, G.; Aswal, V. K.; Hassan, P. pH-Responsive SelfAssembly in an Aqueous Mixture of Surfactant and Hydrophobic Amino Acid Mimic. Soft Matter 2009, 5, 2919−2927.

Figure 10. Chemical principles of the pH-responsive wormlike micelles.

closely combine with NaCl when pH increases, which benefits the formation of WLMs.

4. CONCLUSIONS In conclusion, a novel pH-responsive wormlike micellar system has been designed. In this paper, NaCl was introduced to provide metal counterion Na+ to induce sodium oleate entangling with each other and thus to form WLMs. Medium concentrations of NaCl induce sodium oleate to change solution property from waterlike to gel-like, but with excess NaCl, micelle breaks and phase separation occurs. Two pH values, 9.53 and 9.43, are the critical values of the high and low viscosity, respectively. The wormlike micelle and spherical micelle are mutually transformed in this range of pH. With higher pH value, the ionized carboxylate combines with NaCl closely and thus forms wormlike micelles. On the contrary, sodium oleate converts to oleate acid when pH decreases. In this way, spherical micelles are transformed because of the weaker interaction between oleate acid and NaCl. Unlike other cationic surfactants with pH-responsive wormlike micelle structures which have been largely reported, the anionic surfactant we used is low-cost, common, and environmentally friendly. Moreover, this may provide one way to develop pHresponsive wormlike micellar system which is based on anionic surfactant and is induced by metal counterion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-28-83037305. Fax: +86-28-83037305. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the Engineering Research Centre of Oilfield Chemistry, Ministry of Education Key, for experiment condition support.

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