Unusual pH-responsive fluid based on a simple tertiary amine surfactant: the formation of vesicles and wormlike micelles

Abstract

A novel pH-responsive viscoelastic micellar system was prepared by N,N-dimethyl oleoaminde-propylamine without the addition of hydrotropes. The micellar system undergoes a gradual transition from vesicles to spherical micelles to wormlike micelles by adding an HCl solution. Rheology, Cryo-TEM and dynamic light scattering (DLS) results revealed that the pH-responsive flow behavior is attributed to the microstructural transition among the spherical micelles, vesicles and worm-like micelles. In this paper, we found out that a vesicle is extremely pH sensitive because the degree of protonation and the concentration of Cl⁻ strongly affect the pH areas which could form worm-like micelles. The notable advantages of vesicles and worm-like micelles can be utilized to prepare pH-responsive viscoelastic fluids in the desired pH areas.

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1. Introduction

The self-assembly behavior between molecules is influenced by the magnitude of non-covalent bonding.^1,2 Reasonable control of the non-covalent interactions of molecular self-assembly could result in the control of the micellar structure according to the packing parameter.³ These non-covalent interactions include hydrogen bonds,^4–6 electrostatic forces,⁷ hydrophobic interactions,⁸ van der Waals forces,⁹ dipole–dipole interactions and π–π stacking.^10,11 The variations in non-covalent interactions between surfactants and counterions result in the polymorphism of their self-assemblies such as spherical micelles, vesicles, wormlike micelles and lamellar micelles.¹² Precise control of micellar construction is one of the biggest challenges. To obtain viscoelastic fluids, it is necessary to control the packing geometry of surfactant molecules under appropriate conditions. The macroscopic properties of stimuli-responsive materials can be dramatically changed with minor variation of the stimuli, and thus show an easy way to control the packing geometry.

Compared with some stimuli (such as photo,^13,14 CO₂,^15,16 redox^17,18 and temperature^19,20), pH is a simple and applicable approach to control viscoelastic fluids. Hitherto, there are two strategies that are generally employed to formulate pH-responsive viscoelastic fluids. Usually, a pH-responsive hydrotrope is introduced into surfactant solutions. Kalar and co-workers²¹ obtained pH-dependent mixtures of histidine and sodium dodecyl benzenesulfonate and reported a gradual transition from micelles to vesicles to bilayers to precipitate by manipulating the pH. Huang and co-workers²² introduced the pH-sensitive potassium phthalic acid into cetyltrimethylammonium bromide (CTAB) solutions and found that the microstructure of this pH-sensitive fluid can be transformed between wormlike micelles and short cylindrical micelles with a slight change in pH. Hassan and co-workers²³ reported the pH-responsive behavior of an amino acid which was used to control the electrostatic interactions on a cationic micellar surface and thereby alter the morphology of the self-assembled structures by adjusting the pH. Zakin and co-workers⁸ introduced a pH-responsive thread-like micellar system by mixing alkyl bis(2-hydroxyethyl)methylammonium chloride (EO12) and trans-o-coumaric acid (tOCA). The rheological response of this system to pH is revealed by viscoelasticity at both high and low pH levels, which is caused by the dual pK_a of tOCA. Moreover, we could also employ a pH-responsive surfactant. Maeda and co-workers⁵ demonstrated a reversible change from thread-like micelles to vesicles when the ionization degree of oleyldimethylamine oxide was increased by adjusting the pH. Feng and co-workers²⁴ developed a pH-switchable worm-like micellar system, which was prepared by mixing N-erucamidopropyl-N,N-dimethylamine and maleic acid via tuning the pH. According to these studies, pH-responsive fluids can be prepared by using pH-responsive hydrotropes or surfactants, and the pH area of the fluids is determined by the pK_a of pH-sensitive groups.

In this paper, we used DOAPA to develop a novel pH-responsive viscoelastic fluid, which shows viscoelasticity at both low and high HCl concentrations. We analyzed the mechanism of self-assembly behavior at both high and low HCl concentrations and found that the micellar structure is a vesicle at low HCl concentrations and is a worm-like micelle at high Cl⁻ concentrations. The pH-induced changes in the microstructures of the assemblies are investigated by Cryo-TEM and dynamic light scattering studies. Then, we examined the influence of pH and Cl⁻ concentration on the rheological behaviors of the obtained worm-like micellar fluids.

2. Experimental

2.1 Materials

Oleic acid (>99%) and N,N-dimethyl-1,3-propanediamine (>99%) were purchased from Alfa Co., Ltd. Ammonium chloride (NH₄Cl, >99%), sodium hydroxide (NaOH, >99%) and hydrochloric acid (HCl, =37%) were purchased from Chengdu Kelong Chemical Factory and were used without further purification. All the solutions were prepared in distilled water. The pH was determined by a Leici PHS-25 pH meter.

2.2 Synthesis of N,N-dimethyl oleoaminde-propylamine

N,N-Dimethyl oleoaminde-propylamine was synthesized according to the reaction pathway shown in Scheme 1. An appropriate amount of oleic acid (42.37 g, about 150 mmol) was added to a three-necked flask. Then, N,N-dimethyl-1,3-propanediamine (16.55 g, about 162 mmol) was added dropwise under magnetic stirring and the temperature was increased gradually to a given temperature (160 °C). The mixture was refluxed at a reaction temperature for 7 h. Unreacted N,N-dimethyl-1,3-propanediamine was removed by vacuum distillation. The product was then washed by a saturated Na₂CO₃ solution three times to remove the unreacted oleic acid. The product was dried in a vacuum oven to a constant weight at 50 °C. Finally, the structure of N,N-dimethyl oleoaminde-propylamine was confirmed by ¹HNMR (ESI Fig. 1†). The ¹HNMR spectra was recorded on a Bruker Ascend 400 spectrometer at 400 MHz. ¹HMNR (400 MHz, CDCl₃): 0.9 (t, 3H, alkyl-CH₃), 1.26 (m, 20H, alkyl chain –CH₂), 1.56 (m, 2H, β-CH₂ to C=O), 1.65 (m, 2H, β-CH₂ to N), 2.0 (m, 2H, CH₂ to C=C), 2.16 (t, 2H, CH₂ to C=O), 2.24 (s, 6H, N–CH₃), 2.38 (t, 2H, N–CH₂), 3.3 (m, 2H, γ-CH₂ to N), 5.32 (t, 2H, –HC=CH–), 7.06 (t, H, –NH).

Scheme 1 Synthetic route of DOAPA.

2.3 Rheological measurements

The apparent viscosity of the samples was measured with a ZNN-D6B rheometer (Qingdao Tongchun Oil Instrument Co., Ltd, China) at a shear rate ranging from 5 to 1022 s⁻¹ with an experimental temperature of 20–80 °C. The measuring rotor and a measuring cup were made entirely from Hastelloy alloys.

The rheology experiment was carried out on a HAAKE RS600 rational rheometer equipped with a cone and plate geometry at 25 °C. The samples were equilibrated at 25 °C for at least 20 min prior to experimentation. A solvent trap was utilized to minimize water evaporation. Frequency spectra were conducted in the linear viscoelastic regime of the samples, as determined from the dynamic strain sweep measurements.

2.4 Dynamic Light Scattering (DLS) measurements

A Brookhaven BI-200SM goniometer was used in the dimension measurement at 25 °C. The solutions were filtered through a 0.45 μm membrane filter of hydrophilic MICRO PES into light scattering cells prior to the measurements. The scattering angle was 90°, and the intensity autocorrelation functions were analyzed using the CONTIN method.

2.5 Cryo-TEM observation

Cryo-TEM samples were prepared in a vitrification system (CEVS) at 25 °C. A micropipette was used to load 5 mL solution onto a TEM copper grid, which was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on mesh holes. After About 5 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by nitrogen) at −165 °C. The vitrified samples were then stored in liquid nitrogen until being transferred to a cryogenic sample holder (Gatan 626) and were examined with a JEOL JEM-1400 TEM (120 kV) at about −174 °C. The phase contrast was enhanced by under-focus. The images were recorded on a Gatanmultiscan CCD and were processed with a Digital Micrograph.

3. Results and discussion

3.1 Physical appearances

DOAPA is hardly solvated in water because of its ultra-long hydrophobic tail and behaves as an oily paste floating on the water surface under alkali or neutral conditions. Adding an HCl solution dropwise protonates DOAPA and forms N,N-dimethyl oleoaminde-propylaminehemihydrochloride (DOAPA·HCl). We found that the solution shows viscoelasticity at both low and high HCl concentrations when HCl is added dropwise.

The 75 mM DOAPA solution shows different flow behaviors under varying pH values (Fig. 1). When the pH is adjusted to 7.41 (0.03 wt% HCl), the solution samples are similar as very low viscous emulsions. In contrast, the solution samples become transparent and have a comparatively higher viscosity when the pH is increased to 6.82 (0.16 wt% HCl), while their viscosity decreases dramatically under higher HCl concentrations (0.26 wt% pH 6.20). As the pH value reaches 0.45 (1.50 wt% HCl), they again increase significantly and reach the second maximum at pH 0.12 (3.0 wt% HCl). At this point, the solution can support its own weight when it is upside down. The pH-responsive flow behavior is attributed to the microstructural transition between spherical micelles, vesicles and worm-like micelles, which has been confirmed in detail below.

Fig. 1 Viscosity as a function of HCl concentration for DOAPA solutions at a shear rate of 170⁻¹.

3.2 pH-switchability of the 75 mM DOAPA micellar system

The measurement of pH-switchable ability was performed at a point where the DOAPA solution reaches the maximum viscosity at pH 0.12 and 6.82. Interestingly, these two samples can be repeated when HCl and NaOH are cyclically added; however, the point where the solution reaches the maximum viscosity at pH 0.12 changes to pH 6.20. The pH-switchability of the solutions at pH 6.82 is shown in Fig. 2a. When the pH is fixed at 7.41, the viscosity of the fluid is as small as ∼1 mPa s, while it increases to 2393 mPa s, which is approximately 10³ times than that of the solutions at pH 7.41 upon decreasing the pH to 6.82. Then, the viscosity drops to 3 mPa s at the pH value of 6.20. Viscosity can be switched between ∼10³ and ∼3 mPa s by varying the pH for at least two cycles and the sample can be turned between gel-like and water-like states. The high pH sensitivity and reversible control of the rheological properties may facilitate the applications of such a responsive viscoelastic fluid.

Fig. 2 Switchable viscosity of the “75 mM DOAPA” micellar system start at (a) pH 7.41 and (b) pH 0.12.

In general, the macroscopic properties originate from microscopic structures. Cryo-TEM observation was adopted to display the microstructures of the micellar solutions in the high and low viscosity status. As shown in Fig. 3, only some small spherical micelles were found in the micellar solutions at pH 6.20. On the contrary, vesicles were observed at pH 6.82. Therefore, such a significant change in the viscosity of the 75 mM DOAPA micellar system under different situations can be clearly illustrated by the reversible transformation between vesicles and spherical micelles.

Fig. 3 Cryo-TEM images of 75 mM DOAPA at pH 6.82 and 6.20.

The self-assembly behavior of micelles is conventionally indicated by the packing parameter,²⁵ P, which is a geometric quantity defined as V/al, where a is the effective headgroup area and V is the volume of the lipophilic chain with the maximum effective length l. When P < 1/3, spherical micelles are formed; when 1/3 < P < 1/2, wormlike micelles are found; and when 1/2 < P < 1, vesicles or bilayers can be obtained. In the current work, the effective area of the headgroup is the only variable. The morphology of DOAPA, between nonionic (DOAPA) and cationic (DOAPAH⁺), is depended on the pH of the solutions. More DOAPAH⁺ can be obtained with the decreasing value of the pH. The solution becomes viscoelastic at pH 6.82 and the degree of protonation X = 0.6 (Fig. 4). Hydrogen bonds are formed between the DOAPAH⁺ and DOAPA molecules (–N⁺–H⋯N–). The hydrogen bonding was confirmed by the IR data (ESI Fig. 2†). The DOAPAH⁺ and DOAPA can form a stable tertiary amine-cation with each other through unusually strong hydrogen bonds and hydrophobic interactions. The protonation of DOAPA was entirely finished at pH 6.20, and the vesicles changed into spherical micelles, which resulted in the solution looking like water. A proposed mechanism for the self-assembly of DOAPA and DOAPAH⁺ into vesicles is shown in Scheme 2.

Fig. 4 Species distribution of DOAPA solution within pH range of 5.5–8.5.

Scheme 2 Transformation between a vesicle and spherical micelle induced by pH.

As shown in Scheme 2, at pH 6.82, the stable tertiary amine-cation has a bigger volume for the lipophilic chain and an unchanged effective length, which induces an increase in P approaching the range of 1/2–1, indicating that the formation of vesicles occurred more easily. The pH value and the degree of ionization play important roles in the formation and stability of vesicles. This explains the phenomenon that the enhancement of viscosity only occurs at specific pH values.

The pH-switch at another point where the DOAPA solution reaches the maximum viscosity imparts full reversibility to the process (Fig. 2b) and could be repeated for several cycles. The apparent viscosity can be switched between ∼200 and ∼2 mP s by varying the pH for at least four cycles and the sample can be turned between gel-like and water-like states. However, the point of maximum viscosity of the solution changes from 3.0 wt% HCl to pH 6.2. In the switching process, NaCl is produced by repeatedly adding HCl and NaOH. Compared with the viscosity at pH 6.20 (Fig. 2a), the DOAPA system experiences a transition from the water-like states to the gel-like states when Cl⁻ concentration is increased, which has considerable effect on the pH-responsiveness and rheology behavior.

3.3 Effect of Cl⁻ concentration on pH-responsiveness and rheology behavior

We investigated the effect of Cl⁻ concentration on the rheology of a 75 mM DOAPA solution with the pH fixed between 1.37 and 8.51 to ensure the optimal pH effect. Fig. 5 shows that the apparent viscosity of the 75 mM DOAPA solutions is a function of the concentration of NH₄Cl at pH 6.20 and 25 °C. As the NH₄Cl concentration increases, the apparent viscosity attains a maximum value and then decreases. Samples close to the maximum value of viscosity are extremely viscoelastic and almost gel-like, and the structures of the samples change from spherical micelles to worm-like micelles (Fig. 6). The growth of the micelles is displayed in Scheme 3; Cl⁻ ions could reduce the distance between the headgroups of DOAPAH⁺, and can induce the transition from spherical micelles to rod-like micelles, which leads to the increase in viscosity with the addition of NH₄Cl. On the other hand, lengthening the hydrophobic chains will promote hydrophobic interactions, leading to a linear structure which entangles together to form a dense network, resulting in a rapid increase in viscosity. However, the superfluous absorbance of Cl⁻ on the micellar surface leads to further reduction in the electrostatic repulsions, and the linear micelles may become huddled. Ultimately, the micelles undergo a transition from linear ones to branched ones whose framework is slightly weaker than the linear micelles.

Fig. 5 Effect of NH₄Cl concentration on the viscosity of a solution containing 75 mM DOAPA.

Fig. 6 Cryo-TEM images of 75 mM DOAPA and 75 mM DOAPA and 470 mM NH₄Cl at pH 6.20.

Scheme 3 Transition from a spherical micelle to a wormlike micelle with increasing Cl⁻.

We now describe the rheology of typical DOAPA–NH₄Cl samples as a function of pH. In Fig. 7, data are presented for apparent viscosity as a function of pH for four samples with c(DOAPA) = 75 mM and c(NH₄Cl) = 280 mM, 420 mM, 470 mM and 580 mM. For the 280 mM sample, the apparent viscosity increases slightly at pH 6.81, and then drops monotonically over the entire range of pH (Fig. 7a). When the degree of protonation, X, is 0.6 at pH 6.81, the microstructures of the assemblies transform from spherical micelles to vesicles because of strong hydrogen bonds and hydrophobic interaction, resulting the increase of the apparent viscosity. This is the expected trend in apparent viscosity vs. pH for vesicle micellar fluids, and a similar behavior is seen for all the samples below a 280 mM salt concentration. On the other hand, the 420 and 470 mM samples show a qualitatively different behavior. For the 420 mM sample (Fig. 7b), the apparent viscosity again increases to pH 1.57. For the 470 mM sample (Fig. 7c), the apparent viscosity increases again at pH 2.35, and the second peaks distinctly move to the right side of the x-axis compared with the 420 mM sample. The microstructures of the assemblies transform from spherical micelles to vesicles at pH 6.81, but the protonation of DOAPA entirely finishes at pH 6.20. The vesicles are broken simultaneously, which leads to the decrease in the apparent viscosity. With the decreasing pH, the Cl⁻ concentration gradually increases. The worm-like micelle can be induced by Cl⁻, which leads to the increase in the apparent viscosity. For the 470 mM sample (Fig. 7d), the apparent viscosity increases significantly at pH 7.25. The microstructures of the assemblies directly transform from spherical micelles to worm-like micelles under high Cl⁻ concentrations. Fig. 7 reveals an unusual increase in viscosity with pH in some DOAPA–NH₄Cl samples. As the NH₄Cl concentration increases above 370 mM, the increase in viscosity occurs over a wider range of pH. As shown in Fig. 7, the second value peak shifts close to or even overlaps the first one at higher pH values by adding NH₄Cl. An abundance of Cl⁻ can induce viscoelastic worm-like micelles in the solutions. It also can be provided by HCl and NH₄Cl, and thus this is a simple way to control the pH range through tuning the NH₄Cl concentration.

Fig. 7 pH dependence of viscosity in the systems of 75 mM DOAPA and different NH₄Cl concentrations: (a) 280 mM NH₄Cl; (b) 420 mM NH₄Cl; (c) 470 mM NH₄Cl; (d) 580 mM NH₄Cl.

3.4 Effect of pH on rheological behavior

The pH-responsive behaviors of the 75 mM DOAPA and 470 mM NH₄Cl solutions were further confirmed by rheological results. Fig. 8a shows the variation of steady shear viscosity (η) as a function of shear rate at 25 °C and different pH values. At pH 7.41, the shear viscosity of the solutions is as small as water (∼1 mPa s) and is constantly maintained regardless of the shear rate or the shear stress, which is a typical behavior of Newtonian fluids. Nevertheless, as the pH decreases to 6.91, the rheogram shows a Newtonian plateau at low shear rates and a shear-thinning slope when the shear rate reaches a critical value, indicating the presence of worm-like micelles, which undergo structural change-alignment of long micelles at high shear rates. Upon further pH decrease, the viscosity increases and all the viscosity curves display the coexistence of a Newtonian plateau and shear-thinning zone; the critical shear rate starts to shift to lower values.

Fig. 8 Effect of pH on (a) steady rheology, and (b) zero-shear viscosity of 75 mM DOAPA and 470 mM NH₄Cl micellar solutions.

Zero-shear viscosities (η₀), obtained from the extrapolation of shear viscosity along the Newtonian plateau to zero-shear rate, are plotted as a function of pH to examine the effect of pH on viscosity. When the pH is 7.41, the value of η₀ for the solution is very small, about 3–4 mPa s, and the aqueous DOAPA–NH₄Cl solution is obscure. The pH of the solution decreases with the addition of HCl, which causes the solution to become transparent immediately and the viscosity increases sharply. When the pH drops from 6.61 to 5.00, η₀ drops from 234 300 to 123 900 mPa s. The viscosity of the solution increases when its pH continually decreases below 5.00. The abovementioned parameters follow a similar trend as that observed in viscosity, as shown in Fig. 7c.

The pH-induced viscoelastic nature of the micellar solutions can also be inferred from the frequency-dependent oscillatory-shear measurements. In general, entangled worm-like micelles exhibit Maxwellian behavior under oscillatory shear experiments. For a Maxwell fluid, the frequency-dependent elastic and viscous moduli (G′ and G′′ respectively) are given by

where τ_R and ω denote the relaxation time and frequency in rad s⁻¹, respectively, and G₀ is the plateau modulus. At a given temperature, G₀ is a measure of the degree of entanglements, whereas the relaxation time gives information regarding the average micellar length. For a Maxwell fluid, the relaxation time and plateau modulus are related to the zero-shear viscosity, η₀, of the fluid by the relation,

η₀ = G₀τ_R (3)

At very high frequencies, the storage modulus G′ approaches the plateau modulus G₀ and the Maxwell fluid behaves like an elastic body. However, at low frequencies, G′ and G′′ become proportional to ω² and ω, respectively, and the fluid behaves like a simple liquid.²³

The variation of the elastic or storage modulus (G′) and the viscous or loss modulus (G′′) as a function of oscillation frequency (ω) for the DOAPA–NH₄Cl mixed micelles at different pH values are shown in Fig. 9a. At pH 6.91, the low frequency behavior of the data is similar to that of a Maxwell fluid. The solid lines in Fig. 9a are the best fit to the data using a single relaxation time Maxwell model. Such a viscoelastic behavior indicates the formation of an entangled worm-like micellar network. At high frequencies, a deviation from the Maxwell model is observed. It can be interpreted as a transition in the relaxation mode from reptation at longer time scales to “breathing” or Rouse mode at short time scales.²⁶

Fig. 9 Effect of pH on (a) dynamic rheology, (b) Cole–Cole plots, and (c) plateau modulus and relaxation time of 75 mM DOAPA and 470 mM NH₄Cl micellar system.

In Fig. 9b, the loss modulus (G′′) is plotted as a function of the storage modulus (G′) for the DOAPA–NH₄Cl mixed micelles at different pH (Cole–Cole plot). The solutions at pH 6.91 have a good fit for the Cole–Cole model in the low to medium frequency range, but deviation occurs at high frequencies due to non-reptative effects.²⁷ The single-mode Maxwell constitutive equation can be used to describe the dynamic rheological behaviors of the DOAPA–NH₄Cl solutions over a comparable frequency range, implying the formation of WLMs. When the pH decreases to 6.71 or 2.5, the shape of the curves obviously deviates from a semicircle because of the strong elastic behaviors, as displayed in Fig. 9b.

The estimated G₀ and τ_R obtained after fitting the experimental data to the Maxwell model are plotted in Fig. 7c as a function of pH. The rheological parameters G₀ and τ_R follow a similar trend to that observed in η₀, as shown in Fig. 6b. G₀ and τ_R can be used to estimate the network density of entangled worm-like micelles or the mesh size of the network²⁸ and the length of the micelles,²⁹ respectively. When the pH value is decreased, the protonation of DOAPA is increased. The increase of G₀ and τ_R at pH 6.71 could be understood as the growth of the vesicles and wormlike micelles. At pH 5, the complete protonation of DOAPA led to a break in the vesicles, and G₀ and τ_R decreased. Below pH 5, G₀ and τ_R increase monotonously, indicating an increase in the network density of the worm-like micelles.

3.5 Micelle transition induced by pH

The change in the molecular states of DOAPA with decreasing pH value or increasing NH₄Cl is also a reflection of the evolution of the self-assembled structures in the system. To verify this, dynamic light scattering measurements were carried out. As demonstrated in Fig. 10, the size of the particles in the 75 mM system progressively increases with decreasing pH value or increasing NH₄Cl concentration. Without the addition of NH₄Cl, at pH 6.20, the DOAPAH⁺ self-assemble into particles with an average hydrodynamic radius of around 2 nm, which have been previously verified to be spherical micelles (Fig. 1b). In contrast, upon changing the pH to 6.71, another group of particles with an average hydrodynamic radius of around 14 nm were found, which have been previously verified to be spherical micelles (Fig. 1a). When 470 mM NH₄Cl was added to the solution at pH 6.20, only an average hydrodynamic radius of 36.5 nm was obtained, which is the size range for worm-like micelles. The DOAPA solution exhibits a micelle to vesicle transition with varying pH; meanwhile, it exhibits a micelle to worm-like micelle transition with varying Cl⁻ concentration. The mechanism for this transition has been discussed above.

Fig. 10 Diameter sizes of the aggregates at various conditions.

4. Conclusion

In summary, we have reported the fabrication of a pH-responsive molecular self-assembling system by using a simple tertiary amine surfactant. The microstructure of the system transforms from vesicles to spherical micelles, and finally forms worm-like micelles by adding HCl such that the viscosity of the micellar system significantly increases at low and high HCl concentrations. The vesicle is extremely pH sensitive because of the degree of protonation. The worm-like micelle can be induced by Cl⁻. We can control the formation of wormlike micelles at expected pH values by adding suitable concentrations of NH₄Cl. Strong hydrogen bonds and hydrophobic interactions induce the change from spherical micelles to vesicles. Electrostatic repulsions and hydrophobic interactions induce the change from spherical micelles to worm-like micelles. We could manipulate the responsive pH-range upon understanding the rationale behind the special responsiveness of these materials, which provides a significant advantage in the fabrication and development of oilfield materials with desired pH areas.

Acknowledgements

The authors acknowledge the experimental support from the Engineering Research Center of Oilfield Chemistry and the financial support from the Education Department of Sichuan Province (13TD0025) supported by scientific research starting project of SWPU (No. 373).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08004a

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