Switchable Surfactant-Based CO2-in-water Foam Stabilized by

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Applied Chemistry

Switchable Surfactant-Based CO2-inwater Foam Stabilized by Wormlike Micelle Quanwu Tang, Zhiyu Huang, Cunchuan Zheng, Hongsheng Lu, and Dongfang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03103 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Industrial & Engineering Chemistry Research

Switchable Surfactant-Based CO2-in-water Foam Stabilized by Wormlike Micelle Quanwu Tang,† Zhiyu Huang,*, †, ‡, §, ǁǁ Cunchuan Zheng,† Hongsheng Lu,*, †, ‡, § and Dongfang Liu† †

College of Chemistry and Chemical Engineering, Southwest Petroleum University,

Chengdu 610500, People’s Republic of China ‡

Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Chengdu

610500, People’s Republic of China §

Engineering Research Center of Oilfield Chemistry, Ministry of Education, Chengdu

610500, People’s Republic of China ǁǁ

School of Materials Science and Engineering, Southwest Petroleum University,

Chengdu 610500, People’s Republic of China

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Utilizing CO2 switchable surfactant and formation of wormlike micelle, a potential CO2-in-water (C/W) foam fluid concept is proposed, which might give us a fantastic approach to recovering the surfactant composition. For this objective, CO2-responsiveness of N, N-dimethyl oleoaminde-propylamine (DOAPA) and surface activity of DOAPA in the presence of CO2 were verified. After bubbling CO2, aqueous solutions of DOAPA with NaSal at molar ratio 1:1 created of high apparent viscosity because of formation of wormlike micelle. The results from 1H NMR, DLS and cryo-TEM studies indicated that CO2-responsiveness of DOAPA@NaSal was related to DOAPA and a network wormlike structure formed in the solution. DOAPA@NaSal C/W foams stabilized with wormlike micelle displayed good thermal adaptability and thermal stability, which might be expected to be a practical fracturing fluid for coalbed methane extraction. Moreover, DOAPA could be recovered through adjusting pH and the recovery rate over 90% was obtained in the lab.

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1. INTRODUCTION Carbon dioxide (CO2)-in-water (C/W) foam has been of great interest in various applications, for instance, hydraulic fracturing, enhanced oil recovery, photoresist drying, microelectronic processing, dry cleaning, metal extraction, nanowire synthesis, etc.1-10 In general, C/W foam is a mixture of surfactant and CO2,1 in which the dispersion of CO2 gas phase in a continuous liquid phase is stabilized with surfactants. As is well-known, however, C/W foam will usually be not stable enough in the presence of a single surfactant. In fact, foamed fluid, an important hydraulic fracturing fluid, which can diminish formation damage and has less fluid loss to formation,2, 11-15 has been used in the oil and gas industry. Although the foam with a stabilizer (e.g., guar gum, xanthan gum and synthetic polymers) can get a long halflife, the presence of guar gum, xanthan gum and synthetic polymers would result in the worse formation damage.16 Meanwhile, it is worthwhile to note that nano- and micron-sized particles such as silica nanoparticles, particles of biological origin, polymer latex particles and particulate matter from coal combustion have been employed to improve the foam stability.2,

4, 17-20

Recent reports demonstrate that the foam stabilized with the

nanoparticles and surfactants not only could over the foam instability, for example, rupture of liquid films, foam drainage and interbubble gas diffusion but also showed a longer halflife, lower leakoff coefficient and core permeability damage, and better proppant-carrying ability than the surfactant-stabilized foam.2,

16

In short, the

nanoparticles absorbed at the air-liquid interface to generate a dense layer which could suppress the interbubble gas diffusion and Ostwald ripening, and the flow resistance between water and bubbles increased and liquid drainage decreased due to the interwoven distribution of non-adsorbed and adsorbed nanoparticles. In addition, the micron-sized particulate matter, an industrial byproduct with serious pollution risk to atmospheric environment, in combination with saponin, was used to stabilize the C/W foam, and it is found that the dispersion of particulate matter into the aqueous solution was promoted by saponin and the solidlike surface of the bubbles called an “armor” which could slow down the diffusion between CO2 and bubbles formed -3-



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surrounding the bubbles because of the migration of particulate matter at the CO2/liquid interface.4 Remarkably, the ultradry C/W foam which can be stabilized with a viscoelastic aqueous phase composed of entangled wormlike micelles offered a possible approach of a massive reduction in the amount of wastewater to hydraulic fracturing, and the observations confirm that the foam lamella drainage rates were low and the low rates of Ostwald ripening were caused because viscous thick lamellae formed.21 In particular, a series of novel insights show that the viscoelastic zwitterionic surfactants stabilized ultralow water content C/W foam can over a wide temperature range, and viscoelastic diamine surfactant stabilized C/W foams were suitable for application in a wide range of salinity and temperature.22, 23 However, despite significant studies on C/W foam, few reports focusing on CO2 switchable surfactant-based C/W foam could be found in the literature. Unlike traditional cationic, anionic, nonionic and zwitterionic surfactants like 3-tetradecyloxy-2-hydroxypropyltrimethylammonium bromide (R14HTAB),24 sodium dodecyl benzenesulfonate (SDBS),25,

26

sodium dodecyl sulfate (SDS),3, 27-29 alpha

olefin sulfonate (AOS),2 nonionic surfactant C12–14(EO)2230 and zwitterionic amidopropylcarbobetaines

(R-ONHC3H6N(CH3)2CH2CO2),22

the

switchable

surfactant which can be reversibly switched itself “on” and “off” via external stimuli,31 has attracted much attention in recent years. Using the appropriate trigger such as UV light,32 pH,33 temperature,34, 35 redox reactions,36 and CO2,37-39 switchable surfactants can undergo reversible conversions. CO2 triggered-surfactant, in contrast, is currently preferred due to some advantages of CO2 trigger such as relatively nontoxic, inexpensive, nonflammability and easy availability.37-39 Besides, it is reported that self-assembly of surfactants can form aggregates so as to exhibit unique shapes (for example, micelle, liquid crystal and vesicle).34, 37, 38, 40-42 As far as we are aware, existence of wormlike micelles in the aqueous solution can make the viscosity increased, which is attributed to the effect of wormlike micelles.21-23 More fortunately, a viscoelastic aqueous phase composed of entangled wormlike micelles which can improve C/W foam stability has been investigated and proved.21 Therefore, based on the CO2 switchable surfactant and formation of wormlike micelle, a new C/W foam -4-


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fluid concept is proposed in this paper, which might give us a fantastic approach to recovering the surfactant composition from the fracturing flow-back fluid as well. In our previous work,39-41 we have focused on exploring the switchable surfactant and its application (such as viscoelastic fluids and nanoemulsions), in which N, N-dimethyl oleoaminde-propylamine (DOAPA) has been synthesized and reported. In the present study, CO2-responsiveness of DOAPA and surface activity of DOAPA in the presence of CO2 were further surveyed. The aim of this paper was to describe that the formation of NaSal-induced wormlike micelles in the DOAPA solution depending on the CO2 stimuli can stabilize the C/W foam. Moreover, recovery of the DOAPA through adjusting the pH was investigated. Finally, the possible mechanism of C/W foam stabilization in the DOAPA@NaSal solution via the CO2 stimuli was presented as a result of the optimal condition study in the lab. 2. MATERIALS AND METHODS 2.1. Materials. Octanoic acid, N,N-dimethyl-1,3-propanediamine, sodium salicylate (NaSal), sodium hydroxide (NaOH) and potassium chloride (KCl) were purchased from Chengdu Kelong Chemical Co. Ltd (Chengdu, China). All reagents were of analytical reagent grade in this work. CO2 (>99.99%) was obtained from Jinli Air Co. Ltd (Chengdu, China). Deionized water (resistivity being18.25 MX cm) was used throughout. 2.2. Preparation and CO2-Responsiveness Evaluation of DOAPA. Scheme S1 (see the Supporting Information) illustrates that preparation of DOAPA has been conducted according to the method described previously.40 In addition, pH changes of aqueous solution with and without DOAPA (120 mmol/L) was investigated by pH Meter (Shanghai Dapu Instrument Co. Ltd., Shanghai, China) under the condition of bubbling CO2. CO2 was bubbled into DOAPA aqueous solution at a flow rate of 0.2 L/min (0.15 MPa) in this process. 2.3. Surface Activity Test. DOAPA, a long chain tertiary amine, can become an effective surfactant on conversion to the charged tertiary amine bicarbonate by exposure to CO2 and water, as shown in Scheme S2 (see the Supporting Information). Appropriate concentrations -5-



of the DOAPA·CO2 (tertiary amine bicarbonate) aqueous solution was prepared and the quiescent time of samples before each test was 10 min. For the purpose of calibration, the surface tension of deionized water (72.1 mN/m) was used. Then, DT-102A Tensiometer (Zibo Huakun Electronic Instrument Co. Ltd., Zibo, China) with a platinum ring tensiometer was applied to measure the surface tensions of DOAPA·CO2 aqueous solutions at 25±0.1°C. 2.4. Exploration of NaSal-Induced Wormlike Micelle 2.4.1. Preparation and Viscosity of DOAPA@NaSal Solution. When DOAPA concentration was 100 mmol/L, NaSal concentrations used were 20, 40, 60, 80, 100 and 120 mmol/L and when NaSal concentration was 100 mmol/L, DOAPA concentrations used were 20, 40, 60, 80, 100 and 120 mmol/L. Besides, solution samples with the fixed molar ratio of DOAPA and NaSal (1:1) were obtained and the solution concentrations were 20, 40, 60, 80, 100 and 120 mmol/L. After bubbling of CO2, the viscosity of samples was measured by Brookfield DV-III rotational viscometer (Brookfield Engineering Laboratories Inc., USA) at 7.34 s-1. 2.4.2. 1H NMR Spectroscopy. To confirm the CO2-responsiveness of DOAPA@NaSal solution, 1H NMR spectra of DOAPA, NaSal and DOAPA@NaSal were recorded by a Bruker AC-E 200 spectrometer (Bruker BioSpin, Switzerland) with 1H frequency of 400 MHz at 25 °C. 2.4.3. Dynamic Light Scattering (DLS). Before and after bubbling of CO2, the change of particle size on the DOAPA@NaSal solution was studied. BI-200SM (Brookhaven, Holtsville, New York, USA) with a 900 back-scattering angle and He-Ne laser (λ=532 nm) was used to perform the DLS measurement and intensity autocorrelation functions were analyzed by using the CONTIN method. 2.4.4. Cryogenic Transmission Electron Microscopy (cryo-TEM). Before and after bubbling of CO2, aggregate morphology of the DOAPA@NaSal solution was investigated by cryo-TEM. The sample (5 µL) was loaded onto a TEM copper grid to form a thin liquid film, which was rapidly plunged into the liquid ethane. Whole process was cooled at −165 °C using the liquid nitrogen. The vitrified -6-


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specimen was transferred into JEOL JEM-1400 cryo-microscope (JEOL Ltd., Japan) and the image of aggregate morphology was recorded by the Gatanmultiscan cooled charge-coupled device (CCD) camera. 2.5. Stability Measurement of C/W Foams. C/W foams were prepared according to the Waring Blender method. DOAPA and DOAPA@NaSal solutions (100 mL) were stirred to obtain the C/W foam in CO2 atmosphere for 3 min at 7000 rpm by using a GJ-3S blender (Qingdao Senxin Machinery Equipment Co., Ltd., Qingdao, China), respectively. The above C/W foam was transferred to a 500mL cylinder and the original foam volume was recorded. In addition, the time of the 50 mL liquid drained from the C/W foam was recorded, which was defined as the half-life time of foam. All measurements were carried out under the atmospheric ambient and the employed temperature was controlled by a warm oven. The morphological change of C/W foams was studied by using a microscope (50I, Nikon, Japan). 2.6. Recovering the DOAPA. Recovery of DOAPA from the DOAPA@NaSal solution system by adding NaOH was investigated. The pH change of the solution was recorded using pH Meter (Shanghai Dapu Instrument Co. Ltd., Shanghai, China) and the recovery rate was calculated by the ratio of the mass of recovered DOAPA to the mass of incipient DOAPA. 1H NMR spectrum of recovered DOAPA was recorded by a Bruker AC-E 200 spectrometer (Bruker BioSpin, Switzerland) with 1H frequency of 400 MHz at 25 °C. 3. RESULTS AND DISCUSSION 3.1. CO2-responsive Behavior of DOAPA. Under the condition of bubbling CO2, the study of pH changes of aqueous solution with and without DOAPA was conduced to further understand the CO2-responsive behavior of DOAPA. Alkalescence expression for the aqueous solution of DOAPA can be observed (Figure 1), despite DOAPA is poorly soluble in water. As shown in Figure 1, with the bubbling CO2, the pH of aqueous solution in the presence of DOAPA decreases and the pH is 7.02 after 3min, which suggests DOAPA reacts with -7-



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CO2 in the water and the positive charged cationic tertiary amine namely DOAPAH+ could form (Scheme S2, see the Supporting Information).31, 39 Once in the water, CO2 reacts to generate carbonic acid (H2CO3), thus the pH of aqueous solution decreases. Figure 1 shows that while the bubbling CO2 continues, the pH of aqueous solution in the presence and absence of DOAPA declines and a weak acid system forms finally. Furthermore, apparent viscosity of aqueous solution in the presence of DOAPA was measured at different pH, as given in Table S1 (see the Supporting Information). Table S1 reveals that the apparent viscosity of aqueous solution in the presence of DOAPA depends on the pH of weak acid system and the maximum apparent viscosity (i.e., 190.3 mPa·s at 170s-1) can be obtained at the pH 6.82 which is similar with our previous study.35 All the information obtained from our previous and current work has verified DOAPA with CO2-responsive ability.39,

40

Actually, CO2-responsive behavior of DOAPA is mainly related to the pH.40 3.2. Surface Activity of DOAPA·CO2 Analysis. DOAPA·CO2 (tertiary amine bicarbonate) is the bicarbonate of DOAPA and illustration of formation procedures of DOAPA·CO2 is represented in Scheme S2 (see the Supporting Information). Figure 2 shows the scatter diagram of surface tension (γ) versus the logarithm of DOAPA·CO2 concentration (logC) in the solution. As shown in Figure 2, the value of critical micelle concentration (CMC) and the surface tensions at CMC (γCMC) can be confirmed. Meanwhile, on the basis of the surface tension plot, the surface excess concentration of DOAPA·CO2 (Γmax) was calculated by the Gibbs Adsorption Equation (eq 1).43, 44

Γmax = -

 ∂γ  1   2.303nRT  ∂ log C  T

(1)

where the value of n is 2 for the ionic surfactant,43 R represents the gas constant, T represents the absolute temperature and C is the concentration of DOAPA·CO2 in the solution. The minimum surface area per surfactant molecule (Amin) was calculated by following the Gibbs equation (eq 2).43, 44

-8-


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Amin =

1 NAΓmax

(2)

where NA is Avogadro’s number and Γmax is the surface excess concentration. Table 1 gives the adsorption parameters of DOAPA·CO2 on the interface of gas-liquid at 25 °C. The slightly lower value of CMC and γCMC was obtained, indicating the excellent surface activity of DOAPA·CO2.44 Moreover, a higher value of Γmax and a lower value of Amin could reveal a higher compactness of the aggregation in the solution.44 The pC20 value is widely used to estimate the adsorption of surfactant and the larger pC20 value obtained means DOAPA·CO2 with higher adsorption efficiency.44 3.3. Formation of NaSal-Induced Wormlike Micelle. 3.3.1. Determination of Molar Ratio between DOAPA and NaSal. The applicable molar ratio between DOAPA and NaSal was determined by using variation of apparent viscosity with the increasing DOAPA in the NaSal solution and with the increasing NaSal in the DOAPA solution (Figure 3). Figure 3a shows that with the concentration of DOAPA increasing, the apparent viscosity rises continuously until a maximum value reaches at molar ratio 1:1. In addition, after the maximum value has reached, the apparent viscosity decreases gradually, because the micellar branching may occur at high salt.45, 46 On the other hand, it can be seen (Figure 3b) that the homologous variation of apparent viscosity with NaSal in the DOAPA solution is exhibited and a maximum value appears at molar ratio 1:1. Moreover, the apparent viscosity begins to decrease after reaching the maximum value, the electrostatic attraction between NaSal and DOAPA might weaken while increasing DOAPA continues. There is no doubt, therefore, that the variation of apparent viscosity is attributed to a strong interaction between DOAPA and NaSal in this weak acid system and high viscous fluids could form while the applicable molar ratio between DOAPA and NaSal is 1:1. 3.3.2. Influence of DOAPA@NaSal Concentration on Apparent Viscosity. The influence of DOAPA@NaSal (molar ratio 1:1) concentration on apparent viscosity was studied, as shown in Figure 4. Figure 4 shows the apparent viscosity -9-



increases with increasing DOAPA@NaSal concentration, which implies that the spherical micelles could be gradually transformed into the flexible wormlike micelles in the DOAPA@NaSal solution after bubbling CO2 and the wormlike micelle growth continues when the DOAPA@NaSal concentration increases. As we all know, the self-assembly aggregate of wormlike micelles can form a network wormlike structure, which is a better explanation for the change of apparent viscosity. 3.3.3. 1H NMR Analysis. Figure 5 shows 1H NMR spectra of DOAPA, NaSal and DOAPA@NaSal before or after bubbling CO2 in CD3OD/D2O. By comparing the 1H NMR spectra of DOAPA before and after bubbling CO2, the chemical shift value of H9 (1.61 ppm), H10 (2.29 ppm) and H11 (2.17 ppm) has shifted to a higher field after bubbling CO2, but other characteristic peaks could be unchanged nearly, which suggests DOAPA with CO2-responsiveness due to the protonation of terminal tertiary amine site.40 Moreover, it can be observed from the 1H NMR spectra of DOAPA, NaSal and DOAPA@NaSal after bubbling CO2, that the characteristic peaks (H1-6, 8 and H9-11) of DOAPA@NaSal are consistent with of DOAPA and the chemical shift value of H3’, 5’ (6.95 ppm), H4’ (7.45 ppm) and H6’ (7.82 ppm) in the 1H NMR spectra of DOAPA@NaSal and NaSal has not changed. This indicates that CO2-responsiveness of DOAPA@NaSal is similar to the finding of DOAPA in which protonation of terminal tertiary amine site happens in the mixture system of DOAPA and NaSal. 3.3.4. Microstructure Analysis. To further explore the viscosity change of DOAPA@NaSal solution from the micro perspective, the DLS and cryo-TEM measurements were carried out, as displayed in Figure 6. Figures 6a, 6b and 6c show the size distribution of aggregates in the DOAPA@NaSal solution at 25°C. The average hydrodynamic diameter value of aggregates in the 30, 60 and 100 mmol/L DOAPA@NaSal solution before bubbling CO2 is 3.5, 3.7 and 3.7 nm, respectively. It can be seen (Figures 6a, 6b and 6c), however, that an increase in the average hydrodynamic diameter is indicating the micelles growth after bubbling CO2. Furthermore, as shown in Figures 6d, 6e and 6f, some spherical micelles can be found in the 30, 60 and 100 mmol/L DOAPA@NaSal - 10 -


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solution before bubbling CO2, some wormlike micelles can be observed in the 30 and 60 mmol/L DOAPA@NaSal solution after bubbling CO2, and a network wormlike structure has formed in the 100 mmol/L DOAPA@NaSal solution after bubbling CO2. Therefore, the microstructure transition of aggregates in DOAPA@NaSal solution which can be beneficial to increasing apparent viscosity has been determined. 3.4. Evaluation of C/W foams. 3.4.1. Influence of KCl on Apparent Viscosity. 1.0wt% KCl was used to simulate formation water and can always be a clay control agent to improve the anti-swelling ability of the liquid in the core.47, 48 Figure 7a shows the apparent viscosity of the 100 mmol/L DOAPA@NaSal solution in the absence and presence of KCl. The addition of KCl can lead to a slight increase for the apparent viscosity. However, when a maximum value of apparent viscosity appears, the apparent viscosity begins to decrease with the continuously increasing KCl. The −

main reason might be described as follows. The synergistic effect of Cl and Sal

−

could further neutralize the charges of the surfactant headgroups to weaken the electrostatic repulsions among them, so the apparent viscosity increases slightly. The addition of KCl continues, whereas the micellar branching may occur because of high salt, which probably refers to a huge loss of apparent viscosity.45, 46 As shown in Figure 7b, the result obtained from DLS might support this guess. In particular, Figure 7b reveals that the average hydrodynamic diameter value of aggregates in the 100 mmol/L DOAPA@NaSal solution without and with 1.0wt% KCl after bubbling CO2 would change a little bit, despite there is a slight increase for the apparent viscosity. This means that the microstructure of the DOAPA@NaSal solution in the presence of 1.0wt% KCl is not damaged and 1.0wt% KCl is preferred in 100 mmol/L DOAPA@NaSal solution. 3.4.2. Stability of C/W Foams Analysis. The initial foam volume was measured to study the foaming ability. As shown in Figure 8, the foam volume of the DOAPA C/W foam is much higher than that of the DOAPA@NaSal C/W foam, which reveals that the DOAPA@NaSal system exhibits low foaming ability compared with the DOAPA. Moreover, it can be observed that - 11 -



the foam volume increased originally with increasing temperature and the increasing temperature will result in the decrease of foam volume after a maximum foam volume appears. This could be illustrated by the following explanations. Firstly, the increasing temperature would cause a series of changes such as liquid expanding and an increase in the liquid intermolecular distance and the kinetic energy of surfactant molecules. Especially, owning to the kinetic energy of surfactant molecules increasing, the surfactant molecules do easy escape from liquid phase and come into gas-liquid interface and its adsorption amount on the surface rises, so that the surface tension decreases. Therefore, the foaming ability of the surfactants improves with increasing temperature, which can enlarge the foam volume. Secondly, the foam drainage accelerates, as the temperature becomes higher. So, the foam formed at high agitation speed ruptures and the foam volume reduces. Indeed, there is the maximum foam volume of the DOAPA C/W foam at 50 °C, whereas that of the DOAPA@NaSal C/W foam exists at 60 °C, meaning that the DOAPA@NaSal C/W foam may display good thermal adaptability. The temperature, which refers to depth of formation, can impact the fracturing performance of foam fluid, because foam stability might deteriorate at a high temperature into the target formation. It is absolutely essential to investigate the half-life of foam at different temperatures (Figure 8). Figure 8 indicates that the half-life time of the DOAPA C/W foam is much lower than that of the DOAPA@NaSal C/W foam. This verifies that the DOAPA@NaSal C/W foam shows excellent thermal stability compared with the DOAPA C/W foam. The morphology of DOAPA@NaSal and DOAPA C/W Foams was investigated. As shown in Figure 9, the bubble size of DOAPA@NaSal C/W foam remains relatively unchanged within 10 min but the bubble size of DOAPA C/W foam increased signally with time. More particularly, by comparison of the morphology of DOAPA@NaSal and DOAPA C/W Foams after 15 min, it could be found that the bubble film thickness of DOAPA C/W Foam decreases signally so that its bubble film ruptures. This microscopic analysis supports that the DOAPA@NaSal C/W foam is quite stable. - 12 -


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3.5. Recovery of the DOAPA through adjusting the pH. DOAPA could undergo the reversible conversion between ionic state and molecular state at different pH. There mainly exists the hydrophilic cationic state (DOAPAH+) at pH7.80, and both states coexist at pH ranged from 6.20 to 7.80.40 Figure 10 shows recovery rate of DOAPA at the different pH value. As shown in Figure 10, it can be observed that DOAPA could not recover at the pH ranged from 5.82 to 6.82, because the DOAPA appears at pH=6.20 but the strong hydrogen bonds between DOAPA and DOAPAH+ (−N+−H···N−) might make it impossible to remove the DOAPA.40 However, the DOAPA has gradually separated from the solution at pH>6.82 so that the solution becomes turbid. The turbid solution is divided two sections at pH=7.51, in which the upper layer oily liquid should be DOAPA. Furthermore, it is found that the maximum recovery rate is about 98% at pH=7.81 and the recovery rate will not change with the pH value increasing (Figure 10). On the other hand, 1H NMR analysis was implemented to confirm the upper layer oily liquid. As shown in Figure 11, the data of the recovered DOAPA such as 6.82 (H7), 5.26 (H4), 3.25 (H8), 2.31 (H10), 2.16 (H11), 2.05 (H6), 1.94 (H3), 1.65(H9), 1.56 (H5), 1.21(H2) and 0.83(H1), are in agreement with of the initial DOAPA.39 In other word, it is possible to recover the DOAPA by adjusting the pH. 3.6. Mechanism of C/W Foam Stabilization Depending on the Wormlike Micelle Figure 12 shows the mechanism of C/W foam stabilization in the DOAPA@NaSal solution via the CO2 stimuli. There is no denying the fact that CO2 would act the dual role in the C/W foam, meaning that CO2 as the gas phase is dispersed in a continuous liquid phase and also as the trigger can make DOAPA switchable to be the cationic surfactant. As shown in Figure 12a, the C/W foam could be not stable enough only in the presence of the switchable surfactant. Fortunately, however, the network wormlike structure in the DOAPA@NaSal solution (Figure 12b), which should mainly be attributed to the formation of NaSal-induced wormlike micelle (Figure 12c), can result in the apparent viscosity increasing, thereby the C/W foam drainage slows and the CO2 gas diffusion through foam film would be suppressed. As a result, the - 13 -



DOAPA@NaSal C/W foam is endowed with the excellent stability and thermal adaptability. 4. CONCLUSIONS CO2-responsiveness of N, N-dimethyl oleoaminde-propylamine (DOAPA) was verified and the result indicated that the apparent viscosity of aqueous solution in the presence of DOAPA depended on the pH of weak acid system and the maximum apparent viscosity (i.e., 190.3 mPa·s at 170s-1) can be obtained at the pH 6.82 which was similar with our previous report.40 The surface activity test has demonstrated the excellent surface activity of DOAPA in the presence of CO2, which could reveal the DOAPA with good foaming ability. After bubbling CO2, the aqueous solution of DOAPA with NaSal at molar ratio 1:1 created of high apparent viscosity because of formation of wormlike micelle. The 1:1 DOAPA@NaSal (100 mmol/L) solution was investigated by 1H NMR, DLS and cryo-TEM. The results of 1H NMR spectra showed that CO2-responsiveness of DOAPA@NaSal was related to DOAPA. DLS and cryo-TEM observations revealed that a network wormlike structure formed in the solution. Moreover, it was found that the microstructure of the DOAPA@NaSal solution in the presence of 1.0wt% KCl did not damage and 1.0wt% KCl was preferred in 100 mmol/L DOAPA@NaSal solution. It was found that the half-life time of the DOAPA C/W foam is much lower than that of the DOAPA@NaSal C/W foam and the maximum foam volume of the DOAPA C/W foam and the DOAPA@NaSal C/W foam exists at 50 and 60 °C, respectively. This revealed that DOAPA@NaSal C/W foam stabilized with wormlike micelle displayed good thermal adaptability and excellent thermal stability. In addition, DOAPA could be recovered through adjusting the pH and the recovery rate over 90% was obtained in the lab. In summary, the concept discussed in the present work could help extend the application of switchable surfactants and promote the development of C/W foam fracturing fluids.

ACKNOWLEDGMENTS The authors were very grateful for the financial support of China Postdoctoral Science Foundation (Grant No. 2018M631100). - 14 -


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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. The synthesized process of N, N-dimethyl oleoaminde-propylamine (DOAPA) [Scheme S1], proposed schematic drawing on the formation of DOAPA·CO2 (tertiary amine bicarbonate) [Scheme S2] and changes of apparent viscosity with pH (due to bubbling CO2) at 170 s-1 [Table S1]. (PDF) AUTHOR INFORMATION Corresponding authors *(Zhiyu Huang) E-mail: [email protected]. Tel.: +86 02883037400. *(Hongsheng Lu) E-mail: [email protected] or [email protected]. Tel.: +86 02883037330. ORCID Zhiyu Huang: 0000-0002-9115-5571 Hongsheng Lu: 0000-0003-3201-0855

Notes The authors declare no competing financial interest.

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Table Captions: Table 1. Surface Properties of DOAPA·CO2 on the Interface of Gas-liquid at 25 °C

Figure Captions: Figure 1. The pH changes of aqueous solution with and without DOAPA at 25 °C under the condition of bubbling CO2. Figure 2. Surface tension versus logC for DOAPA·CO2 at 25 °C. Figure 3. Variation of apparent viscosity (a) with DOAPA in the NaSal solution and (b) with NaSal in the DOAPA solution after bubbling of CO2 at 25 °C. Figure 4. Influence of concentration changes on viscosity of DOAPA @NaSal after bubbling of CO2 at 25 °C. Figure 5. 1H NMR spectra of DOAPA, NaSal and DOAPA@NaSal before or after bubbling CO2 in CD3OD/D2O (volume ratio 5:1). Figure 6. The size distribution (a, b and c) and cryo-TEM (d, e and f) of the DOAPA@NaSal solution at 25 °C. Figure 7. The apparent viscosity of the 100 mmol/L DOAPA@NaSal solution without and with KCl (a) and the size distribution of 100 mmol/L DOAPA without and with 0.5wt%, 1wt%, 2wt% and 3wt% KCl (b). Figure 8. Foam volume and half-life time at different temperatures (DOAPA@NaSal C/W foam, 100 mmol/L DOAPA@NaSal + 1.0wt% KCl + CO2; DOAPA C/W foam, 100 mmol/L DOAPA + 1.0wt% KCl + CO2). Figure 9. Microscopic analysis of C/W foams at 25 °C. Figure 10. Recovery rate of DOAPA versus the different pH value at 25 °C. Figure 11. 1H NMR spectra of DOAPA and recovered DOAPA in CDCl3. Figure 12. Schematic drawing of proposed mechanism of C/W foam stabilization depending on the wormlike micelle.

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Table 1: Surfactant

CMC / (mM)

γ / (mN/m)

Γmax / (µmol/cm2)

Amin / (Å2)

PC20

DOAPA·CO2

0.32

27.70

7.20

23.10

5.00

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The TOC graphic

For Table of Contents Only

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Figure 1 272x220mm (150 x 150 DPI)



Figure 2 272x228mm (150 x 150 DPI)


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Figure 3 347x233mm (150 x 150 DPI)



Figure 4 282x226mm (150 x 150 DPI)


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Figure 5 279x215mm (96 x 96 DPI)



Figure 6 543x245mm (150 x 150 DPI)


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Figure 8 294x225mm (150 x 150 DPI)


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Figure 10 279x215mm (150 x 150 DPI)


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Figure 11 279x215mm (150 x 150 DPI)



Figure 12 254x137mm (96 x 96 DPI)


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Switchable Surfactant-Based CO2-in-water Foam Stabilized by

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