Switchable-Hydrophilicity Triethylamine: Formation

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Nov 30, 2018 - and determined at 25 ◦C, Et3N/H2O volume ratio of 1:2 and CO2 injection rate at 300 mL/min. When it was used to extract heavy oil from ...
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Switchable-Hydrophilicity Triethylamine: Formation and Synergistic Effects of Asphaltenes in Stabilizing Emulsions Droplets Xingang Li 1,2,3 , Jinjian Hou 1,2, * , Hong Sui 1,2,3, *, Lingyu Sun 1,2 and Lin Xu 1,2,4 1 2 3 4

*

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; [email protected] (X.L.); [email protected] (L.S.); [email protected] (L.X.) National Engineering Research Centre for Distillation Technology, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China China Academy of Safety Science and Technology, Beijing 100012, China Correspondence: [email protected] (J.H.); [email protected] (H.S.); Tel.: +86-022-27404701 (J.H.)

Received: 18 October 2018; Accepted: 29 November 2018; Published: 30 November 2018

 

Abstract: In this study, SHT (switchable-hydrophilicity triethylamine, [Et3 NH]·[HCO3 ]) has been synthesized and instrumentally characterized by Fourier transform–infrared spectroscopy (FTIR) and 13 C nuclear magnetic resonance (NMR). The operational synthesis conditions of SHT were optimized and determined at 25 ◦ C, Et3 N/H2 O volume ratio of 1:2 and CO2 injection rate at 300 mL/min. When it was used to extract heavy oil from unconventional oil ore, it was found that it could break maltenes-in-water emulsions. When asphaltenes were present in the oil phase, it was observed that SHT could cooperate with asphaltenes. These results indicated that SHT works with asphaltenes, leading to synergistic effects in stabilizing oil–water (o/w) emulsions. Keywords: switchable-hydrophilicity triethylamine (SHT); synergistic effect; emulsions stabilization; asphaltene

1. Introduction Switchable hydrophilicity solvents (SHSs) are organic solvents with switchable miscibility between hydrophilicity and hydrophobicity [1,2]. Due to this property, SHS can be removed from the product and recycled without requiring distillation, potentially saving energy. Durelle et al. proposed a mathematical model to describe the behavior of CO2 -triggered switchable-hydrophilicity solvents in terms of their basicity and hydrophilicity [3]. The mathematical model facilitates the optimization of the synthesis of switchable-hydrophilicity solvent by adjusting pressure and the solvent/water volume ratio. The application of these switchable hydrophilicity chemicals as process aids in oil extraction has been widely reported [4–12]. For instance, SHS could efficiently extract and recovery diesel from oil-based drill cuttings [8]. SHS could extract oil from Jatropha curcas L. oil seeds to produce biodiesel, and the extraction ratio is higher than hexane [10]. Some researchers found that SHS could extract phenols from lignin bio-oil, and 91% SHS could be recovered [12]. Besides, SHSs have been used in oil–sand separation as solvents or interfacial active materials [5–7,13]. During this oil–solid separation, the SHSs play important roles in modifying the oil–water (o/w) interface and oil–solid interface, resulting in the formation of oil–water emulsions [5]. The formation of emulsions during oil extraction makes the separation much more difficult [14,15]. However, little information has been reported on the exact mechanisms of SHS in stabilizing oil–water emulsions, especially its role together with natural interfacial active materials, such as asphaltenes [16–22].

Materials 2018, 11, 2431; doi:10.3390/ma11122431

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In this paper, SHT will be formed and optimized. Its role in the stabilization of maltenes-in-water emulsions will be studied. Accordingly, the objectives of this study are to (i) find the optimal conditions for the formation of SHT; (ii) to understand the synergistic effect of SHT in stabilizing oil-water emulsions together with asphaltenes. 2. Experimental 2.1. Materials Chemicals reagents, including toluene, n-heptane, Et3 N (triethylamine), hydrochloric acid, sodium hydroxide, and anhydrous sodium sulfate, were analytical grade purchased from Tianjin Jiangtian Technology Co. Ltd. (Tianjin, China). The Canadian oil sands samples were obtained from Athabasca, Canada. The composition of Athabasca oil sands was analyzed using the standard method to give 10.72 wt.% bitumen, 1.79 wt.% water, and 87.49 wt.% solids. The N2 and CO2 (of 99.9% purity) were purchased from Tianjin Liufang Technology Co. Ltd., China. 2.2. Optimization of Switchable-Hydrophilicity Triethylamine (SHT) Formation Ten milliliters Et3 N and 10 mL deionized water were added into to a 100 mL three-neck round-bottom flask at 20 ◦ C in a water bath. Then, a thermometer, 5 mm-diameter gas duct and exhaust pipe were placed in the flask. After the gas duct was inserted below the surface of the water, the CO2 was injected into the liquid at a rate of 500 mL/min at atmospheric pressure. With the injection of CO2 into the flask, the pH of the water phase decreased accordingly. The pH of the solution was measured using a pH meter in real time. Initially, there were two phases due to the immiscibility of the tertiary amines in water. Continued bubbling of CO2 caused the phase interface to disappear, finally producing a homogeneous solution. The SHT concentration was highly dependent on the pH of the solution at a given temperature. When the pH remained stable, CO2 injection was stopped. This meant that the homogenous solution was formed when the pH value change was within ±0.02. Then, the SHT solution was poured into a screw-neck glass bottle, and the anhydrous sodium sulfate was put into the SHT solution until the particles floated, which indicated that the water had been removed. The process was repeated by altering the temperature, triethylamine/water volume ratio and CO2 injection rate according to a single factor experiment allowing the process parameters to be optimized. 2.3. Characterization Fourier transform–infrared (FT–IR) spectra of SHT and Et3 N were measured using a FT–IR spectrometer (Bruker, Karlsruhe, Germany). For nuclear magnetic resonance (NMR) sample preparation, 200 µL Et3 N was dissolved in 800 µL of deuterated chloroform (CDCl3 99.8%-d) containing tetramethylsilane (TMS 0.3% v/v), and 200 µL SHT was dissolved in 800 µL D2 O. Subsequently, 600 µL of the mixture was transferred into a standard 5 mm NMR tube for direct measurement. The reagent was purchased from Aladdin (Shanghai, China). All one-dimensional 13 C-NMR spectra were recorded at 500 MHz and 298 K on a Bruker AV 600 spectrometer (Bruker Corporation, Faellanden, Switzerland) equipped with a cryoprobe and a z-gradient. 2.4. Emulsion Stability Experiment The experimental procedures of the preparation of asphaltenes, maltenes, emulsions and demulsification are shown in Figure 1. The bitumen was extracted from 150 g oil sands through the toluene solvent extraction process. After extraction, the bitumen (2 g) was added to a conical flask, and 90 mL n-heptane was added. The bitumen was sonicated at 50 ◦ C for 30 min. After sonication, the mixture was centrifuged at 8500 rpm for 15 min. After the centrifugation, the supernatant was transferred into a 500 mL conical flask. The insoluble solids were deposited back into the conical flask. The centrifugation and precipitation processes were repeated with additional 100 milliliters n-heptane

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until the supernatant was colorless. The n-heptane was removed by a rotary evaporator, leaving the maltenes. thePEER insoluble solids were dried in a vacuum oven at 80 ◦ C, and the solids were Materials 2018,Then, 11, x FOR REVIEW 3 ofthe 10 asphaltene samples. The maltenes were transferred into a flask to mix with the toluene, obtaining a mixture with maltenes at maltenes a concentration of 1.2%. Theoforganic phase was adjusted by adding obtaining a mixture with at a concentration 1.2%. The organic phase was adjustedSHT, by asphaltenes or Et N. Twenty milliliters deionized water and 30 mL organic phase were mixed and adding SHT, asphaltenes or Et3N. Twenty milliliters deionized water and 30 mL organic phase were 3 transferred into a 100 mL flask, and stirred at 900 rpm for 220rpm min.for Then, mixture mixed and transferred intoround-bottom a 100 mL round-bottom flask, and stirred at 900 220 the min. Then, wasmixture quickly was transferred a measuring and the precipitated nonaqueous phase volume or the quicklyinto transferred intocylinder, a measuring cylinder, and the precipitated nonaqueous aqueous phaseorvolume were recorded. phase volume aqueous phase volume were recorded.

Figure 1. Schematic of the emulsion stability experiment procedures. (a) Preparation of asphaltenes Figure 1. Schematic of the emulsion stability experiment procedures. (a) Preparation of asphaltenes and maltenes; (b) preparation of emulsions and the demulsification process. and maltenes; (b) preparation of emulsions and the demulsification process.

2.5. Interfacial Tension (IFT) Measurements 2.5. Interfacial Tension (IFT) Measurements The interfacial tension (IFT) between the non-aqueous phase and deionized water was measured The interfacial (IFT) between theKino, non-aqueous and deionized watertension was measured by the pendant droptension shape method (SL200B, Winslow,phase AZ, USA). The interfacial between by the pendant drop shape method (SL200B, Kino, Winslow, AZ, USA). The interfacial tension the oil phase and water phase was measured based on the shape of a pendant drop using the between the oil phase and water phase was measured based on the shape of a pendant drop using asymmetric drop shape analysis technique, the similar experiment procedures could be found in the drop shape technique, the◦ C. similar experiment procedures could bethe found in the asymmetric literature [6,23,24]. Theanalysis temperature was 25 The organic phase was injected into water the literature The temperature °C. The organic was injected into the water phase using a[6,23,24]. U-shape bent needle fixed was on a 25 micro-syringe. The phase experimental instrument could fit phase using a U-shape bent needle fixed on a micro-syringe. The experimental instrument could fit a a theoretical Young–Laplace curve according to the drop shape. The drop got bigger when the organic theoretical Young–Laplace according to the until drop the shape. The dropwas gotthe bigger whenand thethe organic phase injected by manual. curve The injection stopped drop shape biggest, drop phase injected by manual. The injection stopped until the drop shape was the biggest, and the drop would fall off if we continued increasing liquid when the drop shape was the biggest. Until the fitted would fall off if we continued increasing liquid the IFT. dropThe shape the biggest. Until themN/m, fitted curve was stable and unchanged, the fitted valuewhen was the IFT was of toluene/water is 36.1 curve stable and the fitted value was the IFT.measurement The IFT of toluene/water 36.1 mN/m, whichwas indicated that unchanged, the IFT measurement is accurate. Each was repeatedisat least three which indicated that the IFT measurement is accurate. Each measurement was repeated at least three times until the results were constant. times until the results were constant. 3. Results and Discussion 3. Results and Discussion When Et3 N is exposed to CO2 , it is hydrophilic and even has complete miscibility in water. Et3N is exposed to CO 2, it is and eventhe hastwo complete miscibility in water. The The When CO2 injection is stopped until thehydrophilic interface between immiscible phases disappears CO2 injection is stopped until the interface between the two immiscible phases disappears (Figure 2b), which means that the reaction reached the equilibrium state and the SHT solution formed. The equation of SHT formation is [1,24]: ⎯⎯ → [ (CH 3CH 2 )3 NH ][ HCO 3 ] (CH 3CH 2 )3 N+H 2 O+CO 2 ←⎯ ⎯

(1)

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(Figure 2b), which means that the reaction reached the equilibrium state and the SHT solution formed. The equation of SHT formation is [1,24]:   (CH 3 CH2 )3 N + H2 O + CO2  (CH 3 CH2 )3 NH [HCO3 ]

(1)

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Figure (a) The phase and and water coexist in the systemsystem with an with Figure 2. (a)2. The Et3Et N3Noiloilphase waterphase phase coexist in trimethylamine-water the trimethylamine-water interface between them; (b) one homogenous phase appears in the SHT solution after CO 2 addition. an interface between them; one homogenous phase appears in the SHT solution after CO2 addition.

3.1. Characterizations of SHT 3.1. Characterizations of SHT Figure 2. (a) The Et3N oil phase and water phase coexist in the trimethylamine-water system with an interface between them; (b) one homogenous phase appears in the SHT solution after CO2 addition.

3.1.1.3.1.1. Fourier Transform–Infrared (FT–IR) Fourier Transform–Infrared (FT–IR)Analysis analysis 3.1. Characterizations of SHT

To further detect whether has been successfully, Et3N and SHTEt were To further detect whetherSHT SHT has synthesized been synthesized successfully, and SHT were 3 N characterized by FT–IR, as shown in Figure 3. Obviously, the characteristic bands for Et 3 N were mainly located at 3000– 3.1.1. Fourier Transform–Infrared (FT–IR) analysis characterized by FT–IR, as shown in Figure 3. Obviously, the characteristic bands for Et3 N were mainly −1 (the stretching −1 (the stretching vibration − 1 2800 cm vibration of saturated C–H bonds) and 1385–1060 cm located at 3000–2800 (the stretching vibration ofsuccessfully, saturatedEt3C–H 1385–1060 cm−1 To furthercm detect whether SHT has been synthesized N andbonds) SHT wereand characterized −1, 3000–2800 cm−1, of C–N characteristic bands forThe SHT were mainly located 3500–3400 cm bybonds). FT–IR, asThe shown in Figure Obviously, the characteristic bands for Etat 3Nfor wereSHT mainly located at 3000–located at (the stretching vibration of C–N3.bonds). characteristic bands were mainly −1 and 1500–1000 cm−1. The strong and broad band at 3445 cm−1−1was associated with the –N– −1 1700–1600 cm 2800−cm (the stretching vibration of saturated C–H bonds) and 1385–1060 − cm (the stretching vibration 1 − 1 − 1 1 3500–3400 cm , 3000–2800 cm , 1700–1600 cm and 1500–1000 cm−1 . The strong and broad band at −1 H stretching vibration [(CH3CH2)bands 3NH]+for . The bands at about 2974atcm and 2941 of C–N bonds). Thein characteristic SHT were mainly located 3500–3400 cm−1cm , 3000–2800 cm−1, the + . represented 3445 cm−11700–1600 was associated with the –N–H stretching vibration in [(CH CH ) NH] The bands at 2 3 the band cm−1 stretching and 1500–1000 cm−1. The broad band at 3445 cm was associated with the –N– cm−1about C–H asymmetrical vibration ofstrong –CH3,and –CH 2–, respectively. In−13addition, at 1631 − 1 − 1 + −1 −1 2974 was cm due 2941vibration cm C=O represented the. C–H asymmetrical stretching vibration of –CH Hand stretching in [(CH 3CH2)3NH] The bands at was aboutfrom 2974the cm and 2941 cm weak represented the 3 , –CH2 –, to asymmetric stretching vibration, which [HCO 3]−. The bands at about 1 was asymmetrical stretching vibration of –CH 3− , –CH 2–, respectively. In addition, the bandstretching at 1631 cm−1 vibration, respectively. addition, the band at 1631 due to asymmetric C=O −1 represented 1475 C–H cm−1In and 1397 cm thecm C–H scissoring vibration of –CH 2–, and asymmetrical due to asymmetric C=O which was from the [HCO ]−. The weak bands about − stretching −1 3and −1 at −1 range which waswas from the [HCO weakvibration, bands at about 1397 cm represented deformation vibration of –CH respectively. The other bands1475 in thecm 1500–1000cm were due to the the 3 ] . 3,The 1475 cm−1 and 1397 cm−1 represented the C–H scissoring vibration of –CH2–, and asymmetrical stretching vibration. C–H –C–N scissoring vibration ofof–CH and asymmetrical deformation vibration of –CH respectively. −1 range were deformation vibration –CH32, –, respectively. The other bands in the 1500–1000cm due 3to, the −1 range were due to the –C–N stretching vibration. The other bands in the 1500–1000cm –C–N stretching vibration.

Figure The Fourier transform–infrared (FT–IR) spectra of Et3of N and SHT. Figure 3. The3.Fourier transform–infrared (FT–IR) spectra Et3 N and SHT.

Figure 3. The Fourier transform–infrared (FT–IR) spectra of Et3N and SHT.

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Nuclear Magnetic Resonance Materials 2018, 11, x FOR PEER REVIEW (NMR) Analysis

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3.1.2. 13 C Nuclear Magnetic Resonance Analysis Et3 N and SHT were characterized by 13(NMR) C-NMR spectra, as shown in Figure 4. The chemical shifts 3N and ppm SHT were characterized by 13 spectra, as shown in2Figure 4. The chemical shifts peak is found of Et3 N at 11.78 andEt46.40 were assigned toC-NMR the –CH –CH – groups, the solvent 3 and of Et3N at 11.78 and 46.40 ppm were assigned to the –CH3 and –CH2– groups, the solvent peak is at 77.33 ppm (Figure 4a). The chemical shifts of SHT at 8.13 and 46.47 ppm were assigned, respectively, found at 77.33 ppm (Figure 4a). The chemical shifts of SHT at 8.13 and 46.47 ppm were assigned, + while that to the –CH3 andrespectively, –CH2 – groups of3 and [(CH at 160.15 ppm ppm waswas attributed to the to the –CH –CH 2– groups of [(CH 3CH2)3NH]+ while that at 160.15 3 CH 2 )3 NH] attributed to the carbon atom of [HCO3]− (Figure 4b). These indicated that SHT has formed − carbon atom of [HCO3 ] (Figure 4b). These indicated that SHT has formed successfully. successfully.

4. The C nuclear magnetic resonance (NMR) spectra of (a) Et3N in CDCl3, (b) SHT in D2O. Figure 4. The 13Figure C nuclear magnetic resonance (NMR) spectra of (a) Et3 N in CDCl3 , (b) SHT in D2 O. 13

3.2. Optimization of the Formation Process

3.2. Optimization of the Formation Process

Equation (2) describes the percentage (P) of protonated tertiary amines [3]. The protonation is dependent on solution pH and the acid dissociation constant (KaH) of Et3N. It is only related to the Equation (2) +describes the percentage (P) of protonated tertiary amines [3]. The protonation is [H] concentration at the specific temperature for Et3N; therefore, pH is the indicator of reaction dependent on solution pH and the acid dissociation constant (KaH ) of Et3 N. It is only related to the [H]+ equilibrium. + concentration at the specific temperature for Et3 N;Htherefore, pH is the indicator of reaction equilibrium.

P=

P=

[ ] ×100% + [[HH] ]+K + aH

× 100%

(2)

(2)

The temperature and the concentration of reactants influence the chemical equilibrium and KaH [H] + + reaction rate. The temperature, amine/water volume ratio and CO2 injection rate are optimized (Figure 5). When the temperature increases from 15 °C to 35 °C, the reaction rate increases sharply, The temperature and theslope concentration influence thethechemical equilibrium and then pH decreasing is increasing with of the reactants temperature increase, so when reaction did not reach temperature, equilibrium, the pHamine/water (35 °C) is lower thanvolume the pH (15 ratio °C) when the reaction time is less than reaction rate. The and CO2 injection rate45 are optimized min. Because the reaction is exothermic, when the temperature increases the conversion decreases, ◦ C, the (Figure 5). When the temperature increases from 15 ◦ CWhen to 35 reaction rate increases sharply, therefore the equilibrium pH of the solution increases. the reaction reaches equilibrium, the pH wasslope shown that black line (15 °C) < redthe line temperature (25 °C) < blue linesincrease, (35 °C). The optimal temperature then pH decreasing is increasing with so when the reaction did not is determined to be 25 °C. With increasing Et3N/water volume ratio, the reaction rate sharply ◦ C) ◦ C) reach equilibrium, the pH (35 is lower than the pH (15 when the reaction time is less than increases, allowing the ultimate pH to be decreased. This suggests that the conversion increases. 45 min. BecauseHowever, the reaction isEtexothermic, when increases conversion decreases, when the 3N/water volume ratio isthe less temperature than 1:2, the pH changes little. the Therefore, the optimal Et3N/water volume ratio is 1:2. The reaction rate also increases with the increase of the CO2 therefore the equilibrium pH of the solution increases. When the reaction reaches equilibrium, injection rate, but CO2 conversion decreases. When 10 mL Et3N is used and the reaction equilibrium ◦ C) < red line (25 ◦ C) < blue lines (35 ◦ C). The optimal the pH was shown that line (15 is reached, 1.6 Lblack CO2 is in theory needed. The equilibrium time for 100 mL/min was 110 min, but for 300 mL/min it wasto 55 be min.25 The◦total CO2 supply at equilibrium for 100 mL/min and 300 mL/min temperature is determined C. With increasing Et3time N/water volume ratio, the reaction rate was 11 L and 16.5 L, respectively; the CO2 availability was 15% and 10%, respectively. The reaction sharply increases, allowing the ultimate pH to be decreased. This suggests that conversion rate accelerates with the higher CO2 injecting rate, and then the equilibrium time decreases. The COthe 2

increases. However, when the Et3 N/water volume ratio is less than 1:2, the pH changes little. Therefore, the optimal Et3 N/water volume ratio is 1:2. The reaction rate also increases with the increase of the CO2 injection rate, but CO2 conversion decreases. When 10 mL Et3 N is used and the reaction equilibrium is reached, 1.6 L CO2 is in theory needed. The equilibrium time for 100 mL/min was 110 min, but for 300 mL/min it was 55 min. The total CO2 supply at equilibrium time for 100 mL/min and 300 mL/min was 11 L and 16.5 L, respectively; the CO2 availability was 15% and 10%, respectively. The reaction rate accelerates with the higher CO2 injecting rate, and then the equilibrium time decreases. The CO2 availability decreases when the CO2 injection rate increases. When the CO2 is 500 mL/min, the line (was not shown in Figure 5) was similar to the red line (300 mL/min), so increasing CO2 injection rate to 500 mL/min was meaningless. The optimal conditions are as follows: temperature of 25 ◦ C; tertiary amine to water volume ratio of 1:2; CO2 gas injecting rate of 300 mL/min.

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availability decreases when the CO2 injection rate increases. When the CO2 is 500 mL/min, the line (was not shown in Figure 5) was similar to the red line (300 mL/min), so increasing CO2 injection rate Materials 2018, 11, 2431 to 500 mL/min was meaningless. The optimal conditions are as follows: temperature of 25 °C; tertiary amine to water volume ratio of 1:2; CO2 gas injecting rate of 300 mL/min.

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availability decreases when the CO2 injection rate increases. When the CO2 is 500 mL/min, the line (was not shown in Figure 5) was similar to the red line (300 mL/min), so increasing CO2 injection rate to 500 mL/min was meaningless. The optimal conditions are as follows: temperature of 25 °C; tertiary amine to water volume ratio of 1:2; CO2 gas injecting rate of 300 mL/min.

5. The variation of solution pH as a function of reaction time at different temperatures, Figure 5. The Figure variation of solution pH as a function of reaction time at different temperatures, Et3N/H2O volume ratios, and CO2 injecting rates. Et3 N/H2 O volume ratios, and CO2 injecting rates.

3.3. The Role of SHT in Emulsion Stabilization

3.3. The Role of SHTThe inrole Emulsion Stabilization of asphaltenes in stabilizing maltenes-in-water emulsions can also be ascertained by comparing the emulsions with different asphaltene contents, as shown in Figures 6a and 7a–c. It can

The role of asphaltenes in stabilizing maltenes-in-water emulsions be seen that the emulsions made by the organic phase containing asphaltenes are muchcan morealso stable. be ascertained The rate of demulsification becomes slower and the amount of emulsions increases when the by comparing the emulsions with different asphaltene contents, as shown in Figures 6 and 7–c. asphaltene content in the oil phase increases. Asphaltenes formed a rigid skin at the oil–water It can be seen that the emulsions made by the organic phase containing asphaltenes are much more interface, allowing the stability of the emulsions to be improved [25,26]. stable. The ate of demulsification becomes slower and the amount of emulsions increases when the asphaltene content in the oil phase increases. Asphaltenes formed a rigid skin at the oil–water interface, allowing the stability of the emulsions to be improved [25,26]. The role of Et3 N or SHT in stabilizing maltenes-in-water emulsions is shown in Figures 6a and Figure 5. The variation of solution pH as a function of reaction time at different temperatures, 7d,e. Since the demulsification of maltene–water emulsions by SHT is too fast to observe, the water Et3N/H2O volume ratios, and CO2 injecting rates. phase is used to quantify the volume of emulsions. Comparing Figure 7d,e, SHT could accelerate demulsification Etin could stabilize emulsions. After SHT dissolves into the aqueous phase, 3.3. The Role ofbut SHT Emulsion Stabilization 3N the anion and cation compress the double electric layer. As a result, the zeta potential decreases, The role of asphaltenes in stabilizing maltenes-in-water emulsions can also be ascertained by leading to a reduction in thewith interaction It is easy for these oil droplets and coalesce comparing the emulsions differentforce. asphaltene contents, as shown in Figuresto 6acollide and 7a–c. It can under van der Waals force and, therefore, the emulsions are broken. Comparing Figure 7a,e, SHT be seen that the emulsions made by the organic phase containing asphaltenes are much more stable.shows the demulsification effect, and the theory slower is similar electrolyte Although SHT The rate of demulsification becomes andtothe amount demulsification. of emulsions increases when thecould individually accelerate thethe demulsification process, the synergistic of skin SHTatand asphaltene content in oil phase increases. Asphaltenes formedeffect a rigid theasphaltenes oil–water on interface, allowing the stability of the emulsions to be stabilizing maltenes-in-water emulsions is obvious (asimproved shown in[25,26]. Figure 6a).

Figure 6. Cont.

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Figure 6. The effect of (a) asphaltene (1.7 g/L) and SHT content SHT (11%); (b) SHT (2.2%) (without Figure 6. The effect of (a) asphaltene (1.7g/L) g/L) and content SHT (11%); (b) (b) SHTSHT (2.2%) (without Figure 6. The effect of (2.2 (a) asphaltene andSHT SHT content SHT (2.2%) (without %) (without(1.7 asphaltenes) on the stability of (11%); maltenes-in-water emulsions. asphaltenes) and Et3N asphaltenes) and Et3N (2.2 %) (without asphaltenes) on the stability of maltenes-in-water emulsions. asphaltenes) and Et3N (2.2 %) (without asphaltenes) on the stability of maltenes-in-water emulsions.

Figure 7. Visual appearance of emulsions at room temperature (a) without asphaltenes, SHT, and Et3N; (b) asphaltenes (1.7 g/L); (c) SHT (11%) and asphaltenes (1.7 g/L); (d) Et3N (2.2%); (e) SHT (2.2%).

Figure 7. Visual appearance of emulsions at room temperature (a) without asphaltenes, SHT, and Et3 N; Figure 7. Visual appearance of emulsions at room temperature (a) without asphaltenes, SHT, and (b) asphaltenes g/L); (c) SHT (11%) andmaltenes-in-water asphaltenes (1.7 g/L); (d) Et3N (2.2%);in (e)Figures SHT (2.2%). The role of(1.7 Et3N or SHT in stabilizing emulsions is shown 6b and Et3N; (b) asphaltenes (1.7 g/L); (c) SHT (11%) and asphaltenes (1.7 g/L); (d) Et3N (2.2%); (e) SHT (2.2%). 7d,e. Since the demulsification of maltene–water emulsions by SHT is too fast to observe, the water 3.4. IFT of the Oil and Water phase is used to quantify the volume of emulsions. Comparing Figure 7d and Figure 7e, SHT could The role of Et3N or SHT in stabilizing maltenes-in-water emulsions is shown in Figures 6b and accelerate demulsification butinterfacial Et3N couldtension stabilize emulsions. After SHT dissolves7.4 intomN/m, the aqueous shown in Figure 8, the decreases from 33.7 when the 7d,e. As Since the demulsification of maltene–water emulsions by SHT ismN/m too fasttoto observe, the water phase, the anion and cation compress the double electric layer. As a result, the zeta potential asphaltene content increases from 0 to 17 g/L. Asphaltenes can adsorb at the oil-water interface; phasedecreases, is used to quantify the volume of interaction emulsions.force. Comparing Figure 7e, SHT leading to a reduction in the It is easy Figure for these7d oiland droplets to collide andcould they formdemulsification a protective layer; the oil-water interfacial tension decreases, and the layer could stabilize accelerate but Et 3 N could stabilize emulsions. After SHT dissolves into the aqueous coalesce under van der Waals force and, therefore, the emulsions are broken. Comparing Figure 7a

emulsions [25,27].and Thecation interfacial tension from 33.7 mN/m 13.2 the mN/m, the phase, the anion compress the decreases double electric layer. As a to result, zeta when potential SHT content increases from 0 to 11%. SHT works as a surfactant because it can decrease the decreases, leading to a reduction in the interaction force. It is easy for these oil droplets to collide and oil–water interfacial tension, this phenomenon canemulsions be found in literature [6]. SHTFigure enhances coalesce under van der Waalsand force and, therefore, the arethe broken. Comparing 7a the adsorption of asphaltenes at the o/w interface, because the interfacial tension decreases from

As shown in Figure 8, the interfacial tension decreases from 33.7 mN/m to 7.4 mN/m, when the asphaltene content increases from 0 to 17 g/L. Asphaltenes can adsorb at the oil-water interface; they form a protective layer; the oil-water interfacial tension decreases, and the layer could stabilize emulsions [25,27]. The interfacial tension decreases from 33.7 mN/m to 13.2 mN/m, when the SHT Materials 2018, 11, 2431 content increases from 0 to 11%. SHT works as a surfactant because it can decrease the oil–water8 of 10 interfacial tension, and this phenomenon can be found in the literature [6]. SHT enhances the adsorption of asphaltenes at the o/w interface, because the interfacial tension decreases from 23.9 23.9 mN/m 21.6mN/m mN/m when is added toasphaltene the asphaltene solution. Et3 Ndecrease could decrease mN/m toto21.6 when SHTSHT is added to the solution. Et3N could the o/w the o/w interfacial interfacialtension tensionfrom from 33.7 mN/m to 23.6 mN/m. SHT easily resolves Et N under N and both 33.7 mN/m to 23.6 mN/m. SHT easily resolves Et3N under 3 N2, and both 2 , SHT and Et the oil–water interfacial tension. The combination of asphaltenes and SHT is SHT SHT and Et33NNcan candecrease decrease the oil–water interfacial tension. The combination of asphaltenes and observed helpfulfor for the the reduction interfacial tension. is observed totobebehelpful reductionofofthe theoil–water oil–water interfacial tension.

Figure 8. The oil–water interfacial tension ofasphaltenes asphaltenes content, SHT content, and Figure 8. The oil–water interfacial tensionas asaa function function of content, SHT content, and Et 3N Et3 N content at ambient conditions (“hybrid”means means 0.33 0.33 mL g asphaltenes). content at ambient conditions (“hybrid” mL SHT SHTand and0.05 0.05 g asphaltenes).

3.5. The of SHT andand Asphaltenes’ Effecton onStabilizing Stabilizing Emulsions 3.5.Mechanism The Mechanism of SHT Asphaltenes’Synergistic Synergistic Effect thethe Emulsions The synergistic of SHT and asphaltenes on stabilizing maltenes-in-water emulsions is The synergistic effecteffect of SHT and asphaltenes on stabilizing maltenes-in-water emulsions is shown shown Figure SHTasphaltenes can help asphaltenes toat adsorb at the o/w interface by forming ion pairs. is in Figure 9. in SHT can9.help to adsorb the o/w interface by forming ion pairs. It isIt easy to easy to transfer the oil–water interface and this helps asphaltenes adsorb at the o/w interface. transfer the oil–water interface and this helps asphaltenes adsorb at the o/w interface. Asphaltenes Asphaltenes that accumulate the oil-water hinder demulsification. Asphaltenes that accumulate at the oil-wateratinterface couldinterface hinder could demulsification. Asphaltenes could adsorb could adsorb at the oil–water interface by themselves, and the presence of SHT will facilitate the at the oil–water interface by themselves, and the presence of SHT will facilitate the stability of the stability of the oil–water emulsions. Materials 2018, 11, x FOR PEER REVIEW 9 of 10 oil–water emulsions.

effect between SHT andand asphaltenes in stabilizing the Figure 9. The Themechanism mechanismofofthe thesynergistic synergistic effect between SHT asphaltenes in stabilizing emulsions. the emulsions.

4. Conclusions 4. Conclusions The formation conditions for SHT have been optimized by operational tests. The optimal The formation conditions for SHT have been optimized by operational tests. The optimal conditions are determined at 25 ◦ C with the triethylamine to water volume ratio of 1:2, and the conditions are determined at 25 °C with the triethylamine to water volume ratio of 1:2, and the CO2 injecting rate at 300 mL/min. It is found that SHT and Et3N can work as a surfactant to decrease the oil–water interfacial tension. However, only Et3N can stabilize the emulsions, while SHT can help to break the maltene–water emulsions. When asphaltenes are present in the oil, SHT is found to be able to work together with asphaltenes to stabilize maltenes-in-water emulsions. The improvement in the

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CO2 injecting rate at 300 mL/min. It is found that SHT and Et3 N can work as a surfactant to decrease the oil–water interfacial tension. However, only Et3 N can stabilize the emulsions, while SHT can help to break the maltene–water emulsions. When asphaltenes are present in the oil, SHT is found to be able to work together with asphaltenes to stabilize maltenes-in-water emulsions. The improvement in the stability of emulsions by SHT and asphaltenes can be mainly attributed to the interaction between SHT and asphaltenes by forming [Et3 NH]+ -asphaltene ion pairs at the oil–water interface. Author Contributions: Conceptualization, X.L., J.H. and H.S.; Formal analysis, X.L., J.H., H.S. and L.S.; Investigation, X.L., J.H. and L.X.; Resources, X.L.; Writing—original draft, X.L. and J.H.; Writing—review and editing, H.S., L.S. and L.X. Funding: This research was funded by 973 National Basic Research Program of China, grant number 2015CB251403. Acknowledgments: We thank He Lin for his useful suggestions on the experimental design and he helped us to revise the paper. Conflicts of Interest: The authors declare no conflict of interests.

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