pH-Responsive Viscoelastic Fluids of a C22-Tailed Surfactant Induced by Trivalent Metal Ions

pH-responsive viscoelastic fluids are often achieved by adding hydrotropes into surfactant solutions. However, the use of metal salts to prepare pH-responsive viscoelastic fluids has been less documented. Herein, a pH-responsive viscoelastic fluid was developed by blending an ultra-long-chain tertiary amine, N-erucamidopropyl-N, N-dimethylamine (UC22AMPM), with metal salts (i.e., AlCl3, CrCl3, and FeCl3). The effects of the surfactant/metal salt mixing ratio and the type of metal ions on the viscoelasticity and phase behavior of fluids were systematically examined by appearance observation and rheometry. To elucidate the role of metal ions, the rheological properties between AlCl3− and HCl−UC22AMPM systems were compared. Results showed the above metal salt evoked the low-viscosity UC22AMPM dispersions to form viscoelastic solutions. Similar to HCl, AlCl3 could also protonate the UC22AMPM into a cationic surfactant, forming wormlike micelles (WLMs). Notably, much stronger viscoelastic behavior was evidenced in the UC22AMPM−AlCl3 systems because the Al3+ as metal chelators coordinated with WLMs, promoting the increment of viscosity. By tuning the pH, the macroscopic appearance of the UC22AMPM−AlCl3 system switched between transparent solutions and milky dispersion, concomitant with a viscosity variation of one order of magnitude. Importantly, the UC22AMPM−AlCl3 systems showed a constant viscosity of 40 mPa·s at 80 °C and 170 s−1 for 120 min, indicative of good heat and shear resistances. The metal-containing viscoelastic fluids are expected to be good candidates for high-temperature reservoir hydraulic fracturing.


Introduction
Viscoelastic fluids, based on surfactants, are typically "living polymers", with unique rheological properties, viz., viscoelasticity [1]. The peculiar viscoelasticity of the solution originates from the spontaneous assembly of the surfactants into wormlike micelles (WLMs) [1,2]. A striking advantage of viscoelastic surfactant fluids over regular watersoluble polymers is their reversibly shear-degradable characteristics because WLMs are connected by weak physical interactions that can continuously break and reform [3]. Among numerous viscoelastic fluids, viscoelastic fluids of C 22 -tailed surfactants are arguably the most attractive for the following reasons. (i) The viscoelastic fluids made of C 22 -tailed surfactants exhibit stronger viscoelasticity and better thermostability compared to their short-chain counterparts [4]. (ii) Unlike short-chain surfactants that are derived from crude oil-based products, C 22 -tailed surfactants are environmentally benign and sustainable because their feedstocks are natural, renewable raw materials, such as vegetable oil [5][6][7]. Owing to their fascinating rheological behavior, C 22 -tailed surfactant viscoelastic fluids

Phase Behavior of UC22AMPM-AlCl3 System
The macroscopic appearance photos of the UC22AMPM-AlCl3 system at diff (molar ratio = UC22AMPM:AlCl3) under CUC22AMPM = 50 mM are shown in Figure 1. find that, with increasing AlCl3 concentration, the UC22AMPM dispersion tran from an opalescent dispersion to a transparent solution, indicative of the incre water solubility of the UC22AMPM molecules. Remarkably, at α = 1:9, the mixed exhibited phase separation. To unveil the underlying reasons for the phase behavior variation, pH and 1 measurements were carried out. It can be seen from Figure 2A that the pH of the dramatically decreased from 8.17 to 3.73 as the AlCl3 concentration rose from 0 to The reducing pH was associated with the hydrolysis of AlCl3 in water, yielding sub amounts of H + in the solution. There is also no appreciable change in the pH valu further increase in AlCl3 concentration, indicating that the AlCl3 hydrolysis has equilibrium. Figure 2B compares the 1 H NMR spectra of UC22AMPM to those of a in which AlCl3 (α = 1:3) was added. In comparison to neat UC22AMPM, the chemic of protons neighboring amine groups (peaks a, b, and c) of UC22AMPM molecule presence of 150 mM AlCl3 were shifted from 1.74, 2.47, and 2.30 ppm to 1.96, 3.15, ppm, respectively, manifesting the protonation of the tertiary amine group. Co with the results of pH and 1 H NMR, it was suggested that the hydrolysis of AlCl3 yields substantial amounts of H + , leading to the protonation of UC22AMPM. quently, UC22AMPM behaves like a cationic surfactant, exhibiting good water sol As for phase separation, it may be due to the fact that the excessive AlCl3 sa UC22AMPM compounds out by dehydration, lowering their water solubility.

Phase Behavior of UC 22 AMPM-AlCl 3 System
The macroscopic appearance photos of the UC 22 AMPM-AlCl 3 system at different α (molar ratio = UC 22 AMPM:AlCl 3 ) under C UC22AMPM = 50 mM are shown in Figure 1. One can find that, with increasing AlCl 3 concentration, the UC 22 AMPM dispersion transformed from an opalescent dispersion to a transparent solution, indicative of the increment in water solubility of the UC 22 AMPM molecules. Remarkably, at α = 1:9, the mixed system exhibited phase separation.
were employed to unravel the intrinsic mechanism involved in the pH-responsive the UC22AMPM-AlCl3 mixed system. Finally, the properties of the UC22AMPM-Al coelastic solution as a fracturing fluid were evaluated in terms of temperature to and shear tolerance.

Phase Behavior of UC22AMPM-AlCl3 System
The macroscopic appearance photos of the UC22AMPM-AlCl3 system at diff (molar ratio = UC22AMPM:AlCl3) under CUC22AMPM = 50 mM are shown in Figure 1. O find that, with increasing AlCl3 concentration, the UC22AMPM dispersion transf from an opalescent dispersion to a transparent solution, indicative of the increm water solubility of the UC22AMPM molecules. Remarkably, at α = 1:9, the mixed exhibited phase separation. To unveil the underlying reasons for the phase behavior variation, pH and 1 H measurements were carried out. It can be seen from Figure 2A that the pH of the dramatically decreased from 8.17 to 3.73 as the AlCl3 concentration rose from 0 to 5 The reducing pH was associated with the hydrolysis of AlCl3 in water, yielding subs amounts of H + in the solution. There is also no appreciable change in the pH value further increase in AlCl3 concentration, indicating that the AlCl3 hydrolysis has r equilibrium. Figure 2B compares the 1 H NMR spectra of UC22AMPM to those of a in which AlCl3 (α = 1:3) was added. In comparison to neat UC22AMPM, the chemica of protons neighboring amine groups (peaks a, b, and c) of UC22AMPM molecules presence of 150 mM AlCl3 were shifted from 1.74, 2.47, and 2.30 ppm to 1.96, 3.15, a ppm, respectively, manifesting the protonation of the tertiary amine group. Com with the results of pH and 1 H NMR, it was suggested that the hydrolysis of AlCl3 in yields substantial amounts of H + , leading to the protonation of UC22AMPM. quently, UC22AMPM behaves like a cationic surfactant, exhibiting good water solu As for phase separation, it may be due to the fact that the excessive AlCl3 sal UC22AMPM compounds out by dehydration, lowering their water solubility. To unveil the underlying reasons for the phase behavior variation, pH and 1 H NMR measurements were carried out. It can be seen from Figure 2A that the pH of the sample dramatically decreased from 8.17 to 3.73 as the AlCl 3 concentration rose from 0 to 50 mM. The reducing pH was associated with the hydrolysis of AlCl 3 in water, yielding substantial amounts of H + in the solution. There is also no appreciable change in the pH values with further increase in AlCl 3 concentration, indicating that the AlCl 3 hydrolysis has reached equilibrium. Figure 2B compares the 1 H NMR spectra of UC 22 AMPM to those of a sample in which AlCl 3 (α = 1:3) was added. In comparison to neat UC 22 AMPM, the chemical shifts of protons neighboring amine groups (peaks a, b, and c) of UC 22

Rheological Properties of the UC22AMPM-AlCl3 Mixed System
To further shed light on the effect of AlCl3 on the UC22AMPM solution, the rheological properties of UC22AMPM-AlCl3 with different α were studied. As depicted in Figure  3A, in the cases of α = 1:0, 9:1, and 6:1, the viscosities (η) of the solutions were close to that of water, and they were independent of shear rate. That proved that these fluids were typical Newtonian fluids, implying the existence of only spherical micelles in the above solutions. Meanwhile, the UC22AMPM-AlCl3 samples at α = 3:1, 1:1, 1:3, and 1:6 presented a Newtonian plateau and shear-shinning behavior at low-and high-shear rate regions, respectively, reflecting the formation of WLMs. To gain insight into the rheological properties of the UC22AMPM-AlCl3 solution, we performed dynamic rheological scans at 25 °C on the UC22AMM-AlCl3 sample at α = 1:3. As indicated in Figure 3B, it can be seen that the sample showed a gel-like response, i.e., the G′ exceeds G″ over the frequency range. Similar rheology behavior has been extensively observed in the WLMs of C22-tailed surfactants by both Raghavan [22,23] and Feng [22], mirroring the extremely long relaxation times of WLMs of UC22AMPM-AlCl3 complexes. For comparison, we also investigated the steady rheology of the UC22AMPM systems in the presence of HCl. As is exhibited in Figure 4A, at pHs of 7.14 and 6.88, the UC22AMPM-HCl mixtures behaved as water-like Newtonian fluids with a constant η of ~3 mPa·s. At pH below 6.88, the mixtures displayed Newtonian plateau and shear- As for phase separation, it may be due to the fact that the excessive AlCl 3 salted the UC 22 AMPM compounds out by dehydration, lowering their water solubility.

Rheological Properties of the UC 22 AMPM-AlCl 3 Mixed System
To further shed light on the effect of AlCl 3 on the UC 22 AMPM solution, the rheological properties of UC 22 AMPM-AlCl 3 with different α were studied. As depicted in Figure 3A, in the cases of α = 1:0, 9:1, and 6:1, the viscosities (η) of the solutions were close to that of water, and they were independent of shear rate. That proved that these fluids were typical Newtonian fluids, implying the existence of only spherical micelles in the above solutions. Meanwhile, the UC 22 AMPM-AlCl 3 samples at α = 3:1, 1:1, 1:3, and 1:6 presented a Newtonian plateau and shear-shinning behavior at low-and high-shear rate regions, respectively, reflecting the formation of WLMs. To gain insight into the rheological properties of the UC 22 AMPM-AlCl 3 solution, we performed dynamic rheological scans at 25 • C on the UC 22 AMM-AlCl 3 sample at α = 1:3. As indicated in Figure 3B, it can be seen that the sample showed a gel-like response, i.e., the G exceeds G over the frequency range. Similar rheology behavior has been extensively observed in the WLMs of C 22 -tailed surfactants by both Raghavan [22,23] and Feng [22], mirroring the extremely long relaxation times of WLMs of UC 22 AMPM-AlCl 3 complexes.

Rheological Properties of the UC22AMPM-AlCl3 Mixed System
To further shed light on the effect of AlCl3 on the UC22AMPM solution, the rheological properties of UC22AMPM-AlCl3 with different α were studied. As depicted in Figure  3A, in the cases of α = 1:0, 9:1, and 6:1, the viscosities (η) of the solutions were close to that of water, and they were independent of shear rate. That proved that these fluids were typical Newtonian fluids, implying the existence of only spherical micelles in the above solutions. Meanwhile, the UC22AMPM-AlCl3 samples at α = 3:1, 1:1, 1:3, and 1:6 presented a Newtonian plateau and shear-shinning behavior at low-and high-shear rate regions, respectively, reflecting the formation of WLMs. To gain insight into the rheological properties of the UC22AMPM-AlCl3 solution, we performed dynamic rheological scans at 25 °C on the UC22AMM-AlCl3 sample at α = 1:3. As indicated in Figure 3B, it can be seen that the sample showed a gel-like response, i.e., the G′ exceeds G″ over the frequency range. Similar rheology behavior has been extensively observed in the WLMs of C22-tailed surfactants by both Raghavan [22,23] and Feng [22], mirroring the extremely long relaxation times of WLMs of UC22AMPM-AlCl3 complexes. For comparison, we also investigated the steady rheology of the UC22AMPM systems in the presence of HCl. As is exhibited in Figure 4A, at pHs of 7.14 and 6.88, the UC22AMPM-HCl mixtures behaved as water-like Newtonian fluids with a constant η of ~3 mPa·s. At pH below 6.88, the mixtures displayed Newtonian plateau and shear-  For comparison, we also investigated the steady rheology of the UC 22 AMPM systems in the presence of HCl. As is exhibited in Figure 4A, at pHs of 7.14 and 6.88, the UC 22 AMPM-HCl mixtures behaved as water-like Newtonian fluids with a constant η of~3 mPa·s. At pH below 6.88, the mixtures displayed Newtonian plateau and shear-thinning behavior. The similarity in rheological behavior of the above two systems signified that AlCl 3 plays a comparable role to HCl in UC 22 AMPM. That is, UC 22 AMPM can be protonated by adding either HCl or AlCl 3 , converting the cationic state and self-assembling into WLMs. protonated by adding either HCl or AlCl3, converting the cationic state and self-assembling into WLMs.
To further compare the effects of AlCl3 and HCl on the rheological properties of UC22AMPM solution, the zero-shear viscosity (η0) of the UC22AMPM solution was plotted as a function of AlCl3 concentration or pH ( Figure 4B). It was found that η0 increased by five orders of magnitude when the AlCl3 concentration increased from 15 to 30 mM. Following an increase in AlCl3 concentration above 30 mM, the η0 almost remained unchanged at 10 5 mPa·s. In contrast, the η0 of UC22AMPM solution increased by three orders of magnitude as HCl increased. Apparently, AlCl3 is more efficient in thickening the UC22AMPM dispersion as compared to HCl. A plausible explanation could be that the tertiary amine of UC22AMPM can coordinate with trivalent metal ions, i.e., Al 3+ , leading to cross-linking of the wormlike micellar chains, enhancing the solution viscosity [24][25][26]. Similar results were reported by Hao et al. [25], who found the presence of trivalent metal ions (Fe 3+ and Al 3+ ) increased the viscosity of WLMs of tetradecyldimethylamine oxide and amphiphilic short peptides by 10-and 25-fold, respectively.

Effect of Other Trivalent Metal Salts on UC22AMPM
It is well known that properties of the coordinated ion, such as ionic radius and crystal field stabilization energy, profoundly affect the strength of ion-ligand coordination bonds and the solution viscoelasticity [24,27]. To clarify the influence of these different trivalent metal ions on the rheological behavior of UC22AMPM, two trivalent metal salts (FeCl3 and CrCl3) were separately added to 50 mM UC22AMPM dispersion, and the resulting mixed samples were examined by a combination of rheometer and visual observations.
As depicted in Figure 5A,B, both mixtures underwent a transformation from orange or white dispersions to red or black translucent solutions as the metal salt concentration (CMCl3) increased. The different solution colors originated from the nature of Fe 3+ or Cr 3+ ions [28]. Steady rheology spectra of the UC22AMPM-FeCl3 and UC22AMPM-CrCl3 systems are shown in Figure 5C,D, and it was demonstrated that both systems also exhibited similar transitions from a water-like, low-viscosity fluid to a translucent viscoelastic fluid with increasing β (molar ratio = UC22AMPM:FeCl3) and χ (molar ratio = UC22AMPM:CrCl3). This finding demonstrated that the trivalent metal ions could induce UC22AMPM dispersion to form viscoelasticity fluid, regardless of the type of trivalent metal ions. To further compare the effects of AlCl 3 and HCl on the rheological properties of UC 22 AMPM solution, the zero-shear viscosity (η 0 ) of the UC 22 AMPM solution was plotted as a function of AlCl 3 concentration or pH ( Figure 4B). It was found that η 0 increased by five orders of magnitude when the AlCl 3 concentration increased from 15 to 30 mM. Following an increase in AlCl 3 concentration above 30 mM, the η 0 almost remained unchanged at 10 5 mPa·s. In contrast, the η 0 of UC 22 AMPM solution increased by three orders of magnitude as HCl increased. Apparently, AlCl 3 is more efficient in thickening the UC 22 AMPM dispersion as compared to HCl. A plausible explanation could be that the tertiary amine of UC 22 AMPM can coordinate with trivalent metal ions, i.e., Al 3+ , leading to cross-linking of the wormlike micellar chains, enhancing the solution viscosity [24][25][26]. Similar results were reported by Hao et al. [25], who found the presence of trivalent metal ions (Fe 3+ and Al 3+ ) increased the viscosity of WLMs of tetradecyldimethylamine oxide and amphiphilic short peptides by 10-and 25-fold, respectively.

Effect of Other Trivalent Metal Salts on UC 22 AMPM
It is well known that properties of the coordinated ion, such as ionic radius and crystal field stabilization energy, profoundly affect the strength of ion-ligand coordination bonds and the solution viscoelasticity [24,27]. To clarify the influence of these different trivalent metal ions on the rheological behavior of UC 22 AMPM, two trivalent metal salts (FeCl 3 and CrCl 3 ) were separately added to 50 mM UC 22 AMPM dispersion, and the resulting mixed samples were examined by a combination of rheometer and visual observations.
As depicted in Figure 5A,B, both mixtures underwent a transformation from orange or white dispersions to red or black translucent solutions as the metal salt concentration (C MCl3 ) increased. The different solution colors originated from the nature of Fe 3+ or Cr 3+ ions [28]. Steady rheology spectra of the UC 22  To explore the efficiency of viscosity enhancement, the effect of the above two cases of CMCl3 on η0 of the UC22AMPM solution was further investigated. From the results shown in Figure 6A, it was clear that the η0 of both cases first remained constant when the CMCl was lower than 10 mM, and then it sharply rose in the CMCl3 range of 10-25 mM, and finally, it reached a viscosity plateau once the CMCl3 exceeded 25 mM. Note that the above two solutions achieved a η0 of 10 5 mPa·s within the studied salinity scope, suggesting the identical thickening capability of both metal salts (i.e., FeCl3 and CrCl3). Interestingly, a relatively lower amount of FeCl3 (20 mM) was required to reach such a high η0 compared to CrCl3 (24 mM), meaning that the efficiency of viscosity enhancement of Fe 3+ is superio to that of Cr 3+ . In Figure 6B, the pH for the above two cases is compared as a function o CMCl3. Overall, the pH of the UC22AMPM-metal salt mixtures showed a decreasing trend within the studied salinity scope, which can also be interpreted in relation to the hydrol ysis of metal salts. It is noteworthy that the pH of the UC22AMPM-CrCl3 sample was higher than that of the UC22AMPM-FeCl3 sample under identical CMCl3, signifying tha FeCl3 is more adequately hydrolyzed than CrCl3. Therefore, we attributed the preferable viscosity-enhancing efficiency of Fe 3+ to the more adequate hydrolysis of FeCl3. To explore the efficiency of viscosity enhancement, the effect of the above two cases of C MCl3 on η 0 of the UC 22 AMPM solution was further investigated. From the results shown in Figure 6A, it was clear that the η 0 of both cases first remained constant when the C MCl3 was lower than 10 mM, and then it sharply rose in the C MCl3 range of 10-25 mM, and, finally, it reached a viscosity plateau once the C MCl3 exceeded 25 mM. Note that the above two solutions achieved a η 0 of 10 5 mPa·s within the studied salinity scope, suggesting the identical thickening capability of both metal salts (i.e., FeCl 3 and CrCl 3 ). Interestingly, a relatively lower amount of FeCl 3 (20 mM) was required to reach such a high η 0 compared to CrCl 3 (24 mM), meaning that the efficiency of viscosity enhancement of Fe 3+ is superior to that of Cr 3+ . In Figure 6B, the pH for the above two cases is compared as a function of C MCl3 . Overall, the pH of the UC 22 AMPM-metal salt mixtures showed a decreasing trend within the studied salinity scope, which can also be interpreted in relation to the hydrolysis of metal salts. It is noteworthy that the pH of the UC 22 AMPM-CrCl 3 sample was higher than that of the UC 22 AMPM-FeCl 3 sample under identical C MCl3 , signifying that FeCl 3 is more adequately hydrolyzed than CrCl 3 . Therefore, we attributed the preferable viscosity-enhancing efficiency of Fe 3+ to the more adequate hydrolysis of

pH Responsiveness of the UC22AMPM-AlCl3 Mixed System
As stated, the metal ion-ligand coordination bonds are sensitive to external pH variations because both metal ions and H + ions compete to combine with the ligand. Therefore, it is expected that the UC22AMPM-AlCl3 blends would exhibit tunable viscoelasticity by altering the pH of the solution. To prove this concept, the UC22AMPM-AlCl3 mixed system at α = 1:3 was chosen as a representative sample and characterized concerning its rheological behavior at different pH values. As observed in Figure 7A, the η0 of the UC22AMPM-AlCl3 mixed system increased slightly, then rapidly reduced, and finally remained intact as pH increased. From the insets of Figure 7A, the sample was the bluish transparent solution and milky turbid fluid at the examined pH levels (pH 3.75 and 7.05), respectively, which were related to the protonation and deprotonation of UC22AMPM. To be specific, the UC22AMPM was protonated and thus it easily dissolved in water in acidic conditions. Under alkaline conditions, UC22AMPM preferred non-ionic species and showed poor solubility due to its extremely long hydrophobic tail, resulting in the formation of turbid solutions. Impressively, unlike regular pH-responsive systems, which switch between gel-like and water-like states [29,30], the η0 of UC22AMPM-AlCl3 varied by merely one order of magnitude at the examined pH scope (1. 30-8.78). According to our previous studies [29], it was demonstrated that the nonionic UC22AMPM would self-assemble vesicles in basic conditions. Here, we proposed that the vesicles could also coordinate with Al 3+ ions to form metal ions-vesicle complexes, blocking a substantial solution viscosity reduction. Figure 7B describes the curve of pH-stimulated reversibility for the UC22AMPM-AlCl3 mixed system. As the cycle number increased, the η0 of the blends diminished slightly at both pH 7.05 and 3.75, reflecting their poor switchability. We attributed this to the formation of by-products during the repeated addition of acids and bases, which would deteriorate the viscoelasticity of fluids.
To reveal the underlying reasons for the variation of macroscopic properties, the morphology of the UC22AMPM-AlCl3 mixed system with α = 1:3 at different pH values was directly visualized by cryo-TEM. As shown in Figure 7C, high-density, long, and flexible WLMs were observed in the UC22AMPM-AlCl3 mixed system at pH 3.75, and it is difficult to identify where they begin and end. These WLMs overlapped and entangled with each other into three-dimensional network structures, accounting for the gel-like response and high η0 of this sample. On the contrary, when increasing pH to 7.05, only spherical vesicles are observed ( Figure 7D), consistent with our previous inference ( Figures 3A and 7A).

pH Responsiveness of the UC 22 AMPM-AlCl 3 Mixed System
As stated, the metal ion-ligand coordination bonds are sensitive to external pH variations because both metal ions and H + ions compete to combine with the ligand. Therefore, it is expected that the UC 22 AMPM-AlCl 3 blends would exhibit tunable viscoelasticity by altering the pH of the solution. To prove this concept, the UC 22 AMPM-AlCl 3 mixed system at α = 1:3 was chosen as a representative sample and characterized concerning its rheological behavior at different pH values. As observed in Figure 7A, the η 0 of the UC 22 AMPM-AlCl 3 mixed system increased slightly, then rapidly reduced, and finally remained intact as pH increased. From the insets of Figure 7A, the sample was the bluish transparent solution and milky turbid fluid at the examined pH levels (pH 3.75 and 7.05), respectively, which were related to the protonation and deprotonation of UC 22 AMPM. To be specific, the UC 22 AMPM was protonated and thus it easily dissolved in water in acidic conditions. Under alkaline conditions, UC 22 AMPM preferred non-ionic species and showed poor solubility due to its extremely long hydrophobic tail, resulting in the formation of turbid solutions. Impressively, unlike regular pH-responsive systems, which switch between gel-like and water-like states [29,30], the η 0 of UC 22 AMPM-AlCl 3 varied by merely one order of magnitude at the examined pH scope (1. 30-8.78). According to our previous studies [29], it was demonstrated that the nonionic UC 22 AMPM would self-assemble vesicles in basic conditions. Here, we proposed that the vesicles could also coordinate with Al 3+ ions to form metal ions-vesicle complexes, blocking a substantial solution viscosity reduction. Figure 7B describes the curve of pH-stimulated reversibility for the UC 22 AMPM-AlCl 3 mixed system. As the cycle number increased, the η 0 of the blends diminished slightly at both pH 7.05 and 3.75, reflecting their poor switchability. We attributed this to the formation of by-products during the repeated addition of acids and bases, which would deteriorate the viscoelasticity of fluids.
To reveal the underlying reasons for the variation of macroscopic properties, the morphology of the UC 22 AMPM-AlCl 3 mixed system with α = 1:3 at different pH values was directly visualized by cryo-TEM. As shown in Figure 7C, high-density, long, and flexible WLMs were observed in the UC 22 AMPM-AlCl 3 mixed system at pH 3.75, and it is difficult to identify where they begin and end. These WLMs overlapped and entangled with each other into three-dimensional network structures, accounting for the gel-like response and high η 0 of this sample. On the contrary, when increasing pH to 7.05, only spherical vesicles are observed ( Figure 7D), consistent with our previous inference (Figures 3A and 7A). Based on the above results, we proposed the following mechanism to account for the effect of metal salts on the UC22AMPM (Scheme 2). The addition of metal salts (AlCl3) was first hydrolyzed to generate abundant trivalent metal ions (Al 3+ ) and H + (Equations (1) and (2)). In this scenario, the resulting H + first protonated the UC22AMPM molecules to their cationic form (Equation (3)), inducing the formation of WLMs. Entanglement of these WLMs into a transient network imparts high η to solutions. More importantly, Al 3+ was tightly associated with the headgroups of UC22AMPM to form metal-WLM ligand-coordinated systems by coordinating interaction, further enhancing the η of the solution [31]. These reactions could be expressed as follows: Upon decreasing pHs (by adding HCl solution), the UC22AMPM molecules still maintained protonation states, and thus the corresponding wormlike micellar structure remained unchanged. Meanwhile, the reaction of H + with Al(OH)3 yields a larger amount of Al 3+ (Equation (4)). There is no doubt that the increment in the amount of Al 3+ further boosted the formation of metal-WLM ligand coordination, enhancing the entanglement density of the WLMs network and, thereby, leading to a remarkable enhancement of the solution η.
Al(OH)3 + 3H + → Al 3+ + 3H2O (4) Conversely, the UC22AMPM molecules converted into nonionized forms upon increasing pH (addition of NaOH into the aqueous solution), leading to a transformation of the aggregate structure from wormlike to the vesicle. More importantly, the OH − induced Al(OH)3 to produce a larger amount of AlO2 − (Equation (5)), reducing the amount of Al 3+ . Based on the above results, we proposed the following mechanism to account for the effect of metal salts on the UC 22 AMPM (Scheme 2). The addition of metal salts (AlCl 3 ) was first hydrolyzed to generate abundant trivalent metal ions (Al 3+ ) and H + (Equations (1) and (2)). In this scenario, the resulting H + first protonated the UC 22 AMPM molecules to their cationic form (Equation (3)), inducing the formation of WLMs. Entanglement of these WLMs into a transient network imparts high η to solutions. More importantly, Al 3+ was tightly associated with the headgroups of UC 22 AMPM to form metal-WLM ligand-coordinated systems by coordinating interaction, further enhancing the η of the solution [31]. These reactions could be expressed as follows: Upon decreasing pHs (by adding HCl solution), the UC 22 AMPM molecules still maintained protonation states, and thus the corresponding wormlike micellar structure remained unchanged. Meanwhile, the reaction of H + with Al(OH) 3 yields a larger amount of Al 3+ (Equation (4)). There is no doubt that the increment in the amount of Al 3+ further boosted the formation of metal-WLM ligand coordination, enhancing the entanglement density of the WLMs network and, thereby, leading to a remarkable enhancement of the solution η.
It is worth noting that the η0 of UC22AMPM−AlCl3 mixtures was reduced by only one order of magnitude as pH increased from 3.75 to 7.05, which was due to the presence of metal-vesicle coordinated complexes. Scheme 2. Schematic illustration of the pH-responsive mechanism of the UC22AMPM-AlCl3 mixed system.

Temperature Tolerance and Shear Tolerance of the UC22AMPM−AlCl3 Mixed System
At present, viscosity loss is the most prominent defect in clean fracturing fluids at high temperatures and high shear rates. Therefore, temperature-and shear-resistant properties have been considered the major criteria for evaluating the applicability of viscoelastic fluids in hydraulic fracturing [15].
To evaluate the potential of UC22AMPM-AlCl3 mixed systems as clean fracturing fluids, high temperature and high shear measurements were performed under simulated fracturing conditions. As shown in Figure 8A, the η at 170 s −1 of UC22AMPM-AlCl3 mixed system at α = 1:1 gradually decreased and then maintained a constant viscosity of 60 mPa·s with increasing temperature from 25 to 65 °C. Over the temperature range from 65 to 95 °C, the UC22AMPM-AlCl3 mixed system further declined to 40 mPa·s, reflecting its good heat tolerance. Again, this can be interpreted by the fact that Al 3+ ions are very tightly bound to the UC22AMPM by coordination interaction, which greatly promotes the thermal stability of WLMs.
Moreover, at temperatures of 80 and 60 °C, the UC22AMPM-AlCl3 mixed system can keep a stable viscosity around 40 mPa s and 60 mPa s for 120 min ( Figure 8B), respectively, indicating the good shear tolerance of the UC22AMPM-AlCl3 mixed system. More importantly, it is also higher than the viscosity requirements (>25 mPa s) for clean fracturing fluid [32]. Therefore, it was believed that the UC22AMPM-AlCl3 mixed system can satisfy the demand for clean hydraulic fracturing in the vast majority of oil fields, from middlelow-temperature reservoirs to high-temperature reservoirs.

Scheme 2.
Schematic illustration of the pH-responsive mechanism of the UC 22 AMPM-AlCl 3 mixed system. Conversely, the UC 22 AMPM molecules converted into nonionized forms upon increasing pH (addition of NaOH into the aqueous solution), leading to a transformation of the aggregate structure from wormlike to the vesicle. More importantly, the OH − induced Al(OH) 3 to produce a larger amount of AlO 2 − (Equation (5)), reducing the amount of Al 3+ . As a result, the metal-WLM ligand-coordinated and entangled WLM networks were broken, rendering a decrease in viscosity.
It is worth noting that the η 0 of UC 22 AMPM−AlCl 3 mixtures was reduced by only one order of magnitude as pH increased from 3.75 to 7.05, which was due to the presence of metal-vesicle coordinated complexes.

Temperature Tolerance and Shear Tolerance of the UC 22 AMPM−AlCl 3 Mixed System
At present, viscosity loss is the most prominent defect in clean fracturing fluids at high temperatures and high shear rates. Therefore, temperature-and shear-resistant properties have been considered the major criteria for evaluating the applicability of viscoelastic fluids in hydraulic fracturing [15].
To evaluate the potential of UC 22 AMPM-AlCl 3 mixed systems as clean fracturing fluids, high temperature and high shear measurements were performed under simulated fracturing conditions. As shown in Figure 8A, the η at 170 s −1 of UC 22 AMPM-AlCl 3 mixed system at α = 1:1 gradually decreased and then maintained a constant viscosity of 60 mPa·s with increasing temperature from 25 to 65 • C. Over the temperature range from 65 to 95 • C, the UC 22 AMPM-AlCl 3 mixed system further declined to~40 mPa·s, reflecting its good heat tolerance. Again, this can be interpreted by the fact that Al 3+ ions are very tightly bound to the UC 22 AMPM by coordination interaction, which greatly promotes the thermal stability of WLMs.

Preparation of UC22AMPM-Metal Salts Mixtures
A stock dispersion with 50 mM UC22AMPM was prepared by adding designed amounts of power-like samples and deionized water to a sealed Schott-Duran bottle equipped with a magnetic bar inside, followed by gentle agitation at 60 °C, yielding a lowviscosity emulsion-like dispersion. Then, the desired amount of inorganic metal salts was added into a 50 mM UC22AMPM dispersion, followed by mechanical agitation for 12 h. The samples were left at 25 °C for at least 24 h prior to measurements. The dilute NaOH and HCl solutions were employed to adjust the pH of the UC22AMPM-metal salt mixtures; the pH was determined by a Sartorius basic pH meter PB-10 (±0.01).

Rheology Measurement
Rheological properties of mixture solutions were performed on a Physica MCR 302 (Anton Paar, Graz, Austria) rotational rheometer equipped with a concentric cylinder geometry CC27 (ISO3219). Samples were equilibrated at the testing temperature for no less than 20 min prior to the experiments. During steady-shear tests, the ̇ and test time (t) parameters were varied logarithmically from 1 × 10 −4 to 1.5 × 10 3 s −1 and 1 × 10 4 to 1 s, respectively, according to the relationship ̇ × t ≥ 1. The extrapolation of η to zero-shear rate in the steady-shear measurement yields the zero-shear viscosity, η0.
For temperature sweep measurements, solution viscosity was recorded at a shear rate of 170 s −1 at various temperatures, ranging from 25 to 95 °C. The heating rate was fixed at Moreover, at temperatures of 80 and 60 • C, the UC 22 AMPM-AlCl 3 mixed system can keep a stable viscosity around 40 mPa s and 60 mPa s for 120 min ( Figure 8B), respectively, indicating the good shear tolerance of the UC 22 AMPM-AlCl 3 mixed system. More importantly, it is also higher than the viscosity requirements (>25 mPa s) for clean fracturing fluid [32]. Therefore, it was believed that the UC 22 AMPM-AlCl 3 mixed system can satisfy the demand for clean hydraulic fracturing in the vast majority of oil fields, from middle-low-temperature reservoirs to high-temperature reservoirs.

Preparation of UC 22 AMPM-Metal Salts Mixtures
A stock dispersion with 50 mM UC 22 AMPM was prepared by adding designed amounts of power-like samples and deionized water to a sealed Schott-Duran bottle equipped with a magnetic bar inside, followed by gentle agitation at 60 • C, yielding a low-viscosity emulsion-like dispersion. Then, the desired amount of inorganic metal salts was added into a 50 mM UC 22 AMPM dispersion, followed by mechanical agitation for 12 h. The samples were left at 25 • C for at least 24 h prior to measurements. The dilute NaOH and HCl solutions were employed to adjust the pH of the UC 22 AMPM-metal salt mixtures; the pH was determined by a Sartorius basic pH meter PB-10 (±0.01).

Rheology Measurement
Rheological properties of mixture solutions were performed on a Physica MCR 302 (Anton Paar, Graz, Austria) rotational rheometer equipped with a concentric cylinder geometry CC27 (ISO3219). Samples were equilibrated at the testing temperature for no less than 20 min prior to the experiments. During steady-shear tests, the . γ and test time (t) parameters were varied logarithmically from 1 × 10 −4 to 1.5 × 10 3 s −1 and 1 × 10 4 to 1 s, respectively, according to the relationship . γ × t ≥ 1. The extrapolation of η to zero-shear rate in the steady-shear measurement yields the zero-shear viscosity, η 0 .
For temperature sweep measurements, solution viscosity was recorded at a shear rate of 170 s −1 at various temperatures, ranging from 25 to 95 • C. The heating rate was fixed at 1 • C/min to ensure that the sample was equilibrated. Sufficient time was allowed before data collection at each temperature to ensure the viscosities reached their steady values.
The oscillatory measurements were conducted in the fixed stress (linear viscoelastic region), as determined from prior dynamic stress sweep measurements. The frequency varied from 0.01-100 rad·s −1 . All measurements were carried out in stress-controlled mode, and Canon standard oil was used to calibrate the instrument before measurements. The temperature was controlled at 25 ± 0.1 • C using a Peltier device, and a solvent trap was used to minimize water evaporation during the measurements. For all experiments, flow curves were registered in a stress-controlled mode, and the data were acquired by the software Rheoplus TM .

Micellar Structure Observation
The specimens of the UC 22 AMPM-AlCl 3 mixed solution were prepared in a controlled environment vitrification system. The temperature of the chamber was maintained at about 25 • C, and the relative humidity was maintained close to saturation to prevent evaporation during preparation. Typically, 5 µL of sample solution was deposited on a copper grid and gently blotted with a piece of filter paper to obtain a thin liquid film (20-400 nm) on the grid. Next, the grid was plunged rapidly into liquid ethane (−183 • C) and transferred into liquid nitrogen (<−160 • C) for storage. Finally, the vitrified specimen was transferred into a FEI Talos F200C using a Gatan 626 cryo-holder and observed at an acceleration voltage of 200 KV and a temperature of −170 • C. The images were recorded digitally with a charge-coupled device camera under low-dose conditions with an under-focus of approximately 3 mm.

Phase Behavior Observation
Different amounts of melt salts were added to aliquots of UC 22 AMPM solution (50 mM) to give a range of final melt salt concentrations (C s = 5.5 − 450 mM). The resulting colloidal dispersions were kept in a sealed glass vial at 25 • C for equilibration. Phase behavior was recorded by visual observation, following the previously reported procedure [34].

1 H NMR Experiments
An amount of 4.0 mg of sample, including the neat UC 22 AMPM and the mixtures of UC 22 AMPM and AlCl 3 , was dissolved in 0.6 mL of CD 3 OD and D 2 O (V/V = 5:1) mixed solvent. 1 H NMR spectra were registered in a Bruker AC 400 spectrometer (Bruker Instruments, Mannheim, Germany) at a proton resonance frequency of 400 MHz. Chemical shifts were reported on the δ (ppm) scale. The accuracy of the chemical shift reading was ±0.01 ppm.

Conclusions
In this work, we developed a pH-responsive viscoelastic fluid by complexing a C 22tailed surfactant, UC 22 AMPM, with AlCl 3 . For molar ratios of UC 22 AMPM to AlCl 3 = 3:1, 1:1, 1:3, and 1:6, the UC 22 AMPM dispersion could form a transparent viscoelastic fluid, similar to that of the UC 22 AMPM-HCl mixed system. Due to the metal-ligand coordination between the Al 3+ and N atoms of UC 22 AMPM, the viscosity of the UC 22 AMPM-AlCl 3 mixed system was higher than that of the UC 22 AMPM-HCl system under identical pH values. By tuning the pH of the solution, the macroscopic behavior of the UC 22 AMPM-AlCl 3 mixed system switched between milky dispersion and a clear viscoelastic solution, accompanied by changes in viscosity. The pH responsiveness was caused by the morphology transition between WLMs and spherical vesicles at different pHs. The temperature and shear resistance test revealed the UC 22 AMPM-AlCl 3 system attained a viscosity of 40 mPa·s at 80 • C and 170 s −1 for 120 min, featuring good thermal and shear resistance. These benefits indicated that such viscoelastic fluids can be a promising clean fracturing fluid for the exploration of high-temperature reservoirs. In summary, this study successfully constructed a pH-responsive viscoelastic fluid by introducing trivalent metal ions in a C 22 -tailed surfactant and revealed the role of trivalent metal ions in viscoelastic fluids, enriching the methodology for the preparation of pH-responsive viscoelastic fluids.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.