Interaction between Surfactants and SiO2 Nanoparticles in

Yira HurtadoCristian BeltránRichard D. ZabalaSergio H. LoperaCamilo A. FrancoNashaat N. NassarFarid B. Cortés. Energy & Fuels 2018 32 (5), 5824-5833...
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Interaction between Surfactants and SiO2 Nanoparticles in Multiphase Foam and Its Plugging Ability Jiqian Wang,*,† Guobin Xue,† Baoxue Tian,† Songyan Li,† Kai Chen,‡ Dong Wang,† Yawei Sun,† Hai Xu,† Jordan T. Petkov,† and Zhaomin Li*,† †

State Key Laboratory of Heavy Oil Processing, Centre for Bioengineering & Biotechnology, China University of Petroleum (East China), Qingdao 266580, China ‡ Research Institute of Production Engineering, Shengli Oilfield, Sinopec, Dongying 257000, China S Supporting Information *

ABSTRACT: To improve the stability of foam fluids, SiO2 nanoparticles and trace amount of Gemini cationic surfactant were combined with the main foaming agent, nonionic surfactant, to form a tricomponent multiphase foam. The stability of the multiphase foam was assessed through two parameters of half-life time and dilational modulus. The interaction between surfactants and nanoparticles were studied though surface tension, adsorption amount, and ζ potential measurement. The effects of saline ions and temperature on foam stability were also investigated. The plugging ability of the tricomponent multiphase foam was assessed using a sandpack model. The optimized tricomponent multiphase foam was 10 times more stable than corresponding foam without nanoparticles in terms of half-life time and also resisted to saline and temperature to a certain degree because the adsorption of nanoparticles at the interface improved the mechanic strength of foam film. The tricomponent multiphase foam showed more excellent plugging ability in porous media than foam without nanoparticles during flooding. The adsorption of cationic surfactant not only changed the surface hydrophobicity of the SiO2 nanoparticles, but also promoted the adsorption of APG molecules. Combined the results of Gemini C12C3C12Br2 replaced by CTAB or SDS, and C12C3C12Br2/SiO2 replaced by pretreated partially hydrophobic SiO2 nanoparticle (H15), it is deduced that the in situ surface modification by cationic adsorption to a suitable hydrophobicity was a key step in multiphase stability. Compared with the pretreated partially hydrophobic SiO2 nanoparticle, more SiO2 nanoparticles were distributed at the air/liquid interface and utilized effectively in the tricomponent multiphase foam.

1. INTRODUCTION Aqueous foams are a kind of special fluids with gas trapped in bubbles and liquid as the continuous phase, and have been applied in many industrial fields, such as food,1,2 detergent,3 cosmetic,4 pharmacy, and medicine,5,6 mineral flotation.7 It is noted that foam has been used in oil field in various technologies, including drilling, fracturing.8 And foam flooding also has been proved to be a successful enhanced oil recovery (EOR) technique since 1950s.9,10 Since the apparent viscosity of foam is much more greater than water or gas, and furthermore, gas is enclosed in the liquid film, its viscous fingering can be controlled through pore throats blocking selectively, and thus the sweep efficiency would be improved.11,12 Because aqueous foams are unstable thermodynamically in nature, the improvement of their stability is a prerequisite in most of the applications. Polymers,4,13 proteins,1,14,15 and nanoparticles16−18 can stabilize the foam bubbles with or without surfactants. The foam bubbles are stabilized through slowing down of bubble growth or coalescence and the drainage of liquid film. These two mechanisms are often coupled together. The increase of viscosity and mechanic strength of liquid film will improve the foam stability directly whatever which additive is added. Two main parameters are usually adopted to characterize foam properties. Foamability is the ability of aqueous solution to generate foam, and foam stability describes how the foam volume changes over time after © XXXX American Chemical Society

the generation. These two parameters are not necessarily related, in some cases they are even contradictory. For example, lower solution viscosity favors foam generation, however, high viscosity help to stabilize foam bubbles. Stabilizing effects of solid particles in aqueous foams and oil/ water emulsion have been well-known for a long time. The particles could be organic or inorganic, including polymers,19,20 latex,21 silica, alumina, and calcium phosphate.22−24 The nanoparticles could be spherical or rod-like shapes. And they could work alone25 or with surfactants.17 The nanoparticles form dense shells around the bubbles through adsorption at the air/liquid interface, or form three-dimensional network inside the liquid film through gelation. The stability of so-called “armored” bubbles is improved through the increase of bubble film mechanic strength and decrease of the surface energy and Laplace pressure. The surface hydrophobicity of particles is a crucial factor in foam stabilizing although the optimum value of contact angle of particles is controversial in different studies. The inconsistence might be due to the different type of particles or the presence or absence of surfactants in the foam system. In particle stabilized foam, the liquid film might be stabilized through both a monolayer of bridging particles and a bilayer of close-packed particles. Kaptay’s study on the Received: October 7, 2016 Revised: December 3, 2016 Published: December 7, 2016 A

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transferred to a 500 mL glass cylinder rapidly and the initial foam volume was recorded to characterize the foamability. The cylinder was covered with a piece of parafilm and put in an oven with preset temperature. The time that half volume (50 mL) of liquid was separated from the foam was defined as the half-life time to assess the foam stability. The turbidity of the separated liquid was determined with a UV−vis spectroscopy at 400 nm after the foam perished totally. The foam generating experiment was repeated three times and the values of foam volume and half-life time were averaged. The bubble morphology and bubble size distribution were observed with a commercial Foamscan apparatus (Teclis, France). Sixty milliliters of solution was put into the Foamscan. Foam was generated by gas passing through a porous glass disc at a flow rate of 200 mL/min. When foam volume reached 220 mL, the gas injection was stopped. The bubble size and morphology began to be monitored by a CCD camera. The images were collected and analyzed every 20 s. The foam volume was also monitored through conductivity measurements using the electrodes placed along the foam column. Details of Foamscan measurement can be found in our previous works.31,32 2.4. Adsorption of Surfactants at SiO2 Surface. The surfactant adsorption amount on SiO2 was measured using a Jobin−Yvon spectroscopic ellipsometer (France) with the wavelength ranging from 300 to 600 nm at room temperature. A silicon wafer was adopted instead of SiO2 nanoparticle to provide the spectroscopic ellipsometer with a smooth surface. The silicon wafer was first treated with piranha solution to form a thin layer of silica at the wafer surface through oxidation. And then the wafer was washed with 5% Decon 90 and rinsed with water several times. The partially hydrophobic wafer was modified with hexadecyltrimethoxysilane after the wafer was treated and cleaned. A specially designed liquid cell used the ellipsometry measurement. The wafer was placed at the bottom of the cell and 5 mL surfactant solution was injected into the cell. The incident angle of light was 70° with respect to the wafer surface. The obtained data were fitted using the DeltaPsi2 software supplied by Jobin−Yvon to give the thickness and the refractive index of the adsorbed surfactant layer. The amount of surfactant adsorbed was calculated using the equation as below.35−38

maximum capillary pressure proposed that the optimum contact angles was ∼70° and ∼86° for the monolayer and bilayer stabilization, respectively.26 The particle size is another factor in foam stability. It can be tens of nanometers to several micrometers. Smaller particles work better through the point of the maximum capillary pressure but their adsorption to the air/ liquid interface is weaker, and vice versa.27 The partially hydrophobic particles, such as silica nanoparticle, are different to be dispersed in water. Ethanol was adopted to help the dispersion,25,28 which is inconvenient for industrial applications, especially in oilfield. Moreover, although the ultrastable foams have already been prepared with particles only,22,25,29,30 the foam volume were rather small due to the weak foamability of particles. Therefore, additional foaming agents, usually anionic surfactants, were also used in the foam formula in oilfield applications.31−34 Herein, we present a novel multiphase foam with three components with both good stability and foamability. Nonionic surfactant was used as the main foaming agent, and SiO2 nanoparticle was adopted to stabilize the foam. The SiO2 nanoparticle surface was modified in situ by trace amount of cationic surfactant adsorption to proper hydrophobicity. The sandpack plugging ability of the tricomponent multiphase foam was also assessed. We hope that this kind of foam system could be applied in oilfield production.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. SiO2 nanoparticles with hydrophilic (product code N20) and partially hydrophobic surface (product code H15, the surface was modified with silane through gas phase reaction by the producer.) with the purity of >99.8% were purchased from Wacker Chemical Co., Ltd. (Germany). The nanoparticles are spherical or ellipsoidal with the diameter of 15−20 nm (Figure S1). The contact angle of water on H15 SiO2 pressed disc was about 80° (Figure S2A), indicating that its surface is partially hydrophobic. Octylβ-D-glucopyranoside (APG), cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfonate (SDS) with the purity of >95% were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Gemini cationic surfactant C12C3C12Br2 was labsynthesized. Its purity was >95% according to the NMR result (Figure S3). The molecular structure of APG and C12C3C12Br2 are shown in Figure S4. NaCl, CaCl2, and MgCl2·6H2O of analytical purity were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Hexadecyltrimethoxysilane was purchased from SigmaAldrich with the purity of ≥85%. Water was processed through a Millipore purification (Milli-Q) system with a resistivity of ≥18.2 MΩ· cm. 2.2. Surface Tension and Interfacial Dilational Modulus. Surface tensions of surfactant solutions and surfactant/nanoparticle solutions were measured with a Krüss EasyDyne tensiometer (Krüss, Germany) through du Noüy ring method. The sample solution temperature was controlled at 25 °C by a circulating water bath. All the samples were measured at least three times and the results were averaged. The interfacial dilational modulus was measured to characterize the surface viscoelasticity using a pendant drop tensiometer (Tracker H, Teclis, France). The dynamic surface tension was analyzed through axisymmetric drop shape analysis. A periodical oscillation at 0.1 Hz and a volume amplitude of 1 μm3 were given to the pendant drop during measurement. The interfacial dilational modulus was calculated through eq 1.

ε=

dγ d ln A

Γ=

τ(n − nb) a

(2)

Where n is the refractive index of the adsorbed surfactant layer, and τ is the thickness of the adsorbed layer. nb is the refractive index of the surfactant solution, and a = dnb/dc is the change of the refractive index of solution with concentration. Its value is close to 0.13 cm3 g−1 for surfactants.39,40 2.5. Contact Angle Measurement. The static contact angle of water on silicon wafer and SiO2 nanoparticle pressed disc was conducted on a contact angle apparatus (Shuolun, Shanghai). Two microliters of water was dropped on the surface, then the image was collected with a CCD, and the image was analyzed to obtain the contact angles. The measurement was repeated for at least three times and the results were averaged. The SiO2 nanoparticle was pressed into a disc to make a flat surface with a tablet press. The hydrophilic wafer and partially hydrophobic wafer were processed as in the ellipsometry measurement. For the cationic surfactant adsorbed wafer, clean hydrophilic wafer was immersed in C12C3C12Br2 or CTAB solution for 40 min then washed with water gently, and dried under nitrogen blowing. 2.6. ζ Potential Measurement. The SiO2 nanoparticle dispersion was filtered with a 0.22 μm membrane first, and then mixed by pipetting up and down several times before being injected into a clear disposable ζ potential cell (DTS1061, Malvern). The ζ potential was determined at 25 °C using a zetasizer (Malvern Nano-S, UK). The measurement was conducted for 5 times and averaged. 2.7. Sandpack Plugging Experiment. Silica sands with different sizes were loaded into a stainless steel cylinder with the inner diameter of 2.5 cm and the length of 30 cm. The sandpack model was connected into the core flooding apparatus at the downstream of a foam generator. Details of the core flooding apparatus can be found in

(1)

Where ε is the interfacial dilational modulus, γ is the surface tension, and A is the surface area of the pendant drop. 2.3. Foam Properties Analysis. Foam was generated using a Waring blender. Typically, 100 mL solution was added into the blender and stirred for 3 min at 8000 rpm. The prepared foam was B

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Figure 1. Initial foam volume and half-life time of foams produced by APG (A), APG with 1.5 wt% SiO2 nanoparticle (B). our previous papers.28,31,32 The back pressure was set at 1.0 MPa through a back pressure regulator with an accuracy of 0.001 MPa during experiment. The sandpack was first vacuumed and saturated with water to measure its pore volume and permeability. The pore volume was calculated according to the weight increment after water saturation, and the permeability was calculated using the following eq 3.

κ=

QμL AΔP

Since SiO2 nanoparticle could improve the foam stability, its amount on foam stability was also assessed. As shown in Figure S6, the foam half-life time first increased and then decreased with SiO2 nanoparticle concentration. The maximum half-life time appeared at 1.5 wt% SiO2 nanoparticle concentration. Based on the primary results above, we tried a foam formula with three components, APG, C12C3C12Br2, and SiO2 nanoparticles. The concentration of APG and SiO2 nanoparticle were fixed at 0.175 and 1.5 wt%, respectively. The concentration of C12C3C12Br2 varied from 0 to 0.02 wt%. As shown in Figure 2, the initial volume of the tricomponent foam

(3)

Where Q is the fluid injection rate (mL/min), μ is the fluid viscosity (mPa·s), L is the sandpack length (cm), A is the sandpack sectional area (cm2), and ΔP is the differential pressure (MPa) between the inlet and outlet of the sandpack model. The permeability of sandpack model was 3.95 μm2, and the porosity was 42.1%. For the flooding experiment, 1.0 PV (pore volume) of water was first injected into the sandpack model at 1.0 mL/min to build a stable system pressure. Then 2.0 PV of foam was injected at the same rate with water. Finally, water was injected again for 4.0 PV at 1.0 mL/min. The sandpack weight and differential pressure were monitored during the injection process. The gas volume in sandpack model was the ratio of weight difference between sandpack and water saturated sandpack to water density, and the gas phase saturation was defined as the ratio of the gas volume to pore volume of the sandpack. The resistance factor was defined as the ratio of differential pressure of foam injection to that of water injection.

3. RESULTS AND DISCUSSION 3.1. Foam Volume and Foam Stability. The foamability and foam stability of nonionic surfactant APG, Gemini type cationic surfactant C12 C3C 12Br2, APG, and C 12C 3C12Br2 mixture, and APG with hydrophilic SiO2 nanoparticle (1.5 wt %) were first assessed using Waring blending method. As shown in Figure 1A, both the initial foam volume and half-life time reached their maximum at the APG concentration of about 0.3 wt%. The foam volume was about 500 mL, and the half-life time was 450 s. When 1.5 wt% hydrophilic SiO2 nanoparticles were added in the solution, the half-life time increased from about 450 s to about 600 s, and a maximum value appeared at the 0.175 wt% APG concentration (Figure 1B). The initial foam volume decreased slightly after the addition of SiO2 nanoparticles. We supposed that there was a positive synergistic effect between APG surfactant and SiO2 nanoparticles. The cationic Gemini surfactant C12C3C12Br2 showed a weak foamability and the foam generated with C12C3C12Br2 was unstable (Figure S5A). According to Figure S5B, there might be weak synergistic interaction between APG and C12C3C12Br2 since the half-life time of foam generated by APG/C12C3C12Br2 mixture was longer than APG foam and the half-life time also had a peak with C12C3C12Br2 concentration.

Figure 2. Initial foam volume and half-life time with C12C3C12Br2 concentration. The foam was produced with APG, C12C3C12Br2, and SiO2 nanoparticle, and APG and SiO2 nanoparticle centration was fixed as 0.175 and 1.5 wt%, respectively. G means Gemini surfactant C12C3C12Br2 in the data label.

was well-matched to that APG foam or APG/SiO2 foam. However, its stability was significantly enhanced at proper C12C3C12Br2 concentration. The half-life time first increased and then decreased with C12C3C12Br2 concentration. It was nearly 5000 s at C12C3C12Br2 concentration of 0.003 wt%, which was much more higher than those of APG foam and APG/SiO2 foam. The formula with C12C3C12Br2 and SiO2 nanoparticle was also studied for comparison. It hardly generated foams when C12C3C12Br2 concentration was less than 0.01 wt%. Although 100−200 mL foam could be generated when C12C3C12Br2 concentration was larger than 0.01 wt%, the foam was very unstable. The changes of foam bubble morphology, bubble size, and foam volume along time were monitored by Foamscan, and the results are shown in Figure 3 to Figure 5. As shown in Figure 3 and Figure 4, the bubble size of both tricomponent foam and C

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Figure 3. Foam morphology changes with time. The images in the upper row are foam produced with APG/C12C3C12Br2, and the images in the bottom row are foam produced with APG/C12C3C12Br2/SiO2. The concentrations of APG, C12C3C12Br2, and SiO2 are 0.175, 0.003, and 1.5 wt%, respectively.

Figure 4. Bubble size distribution of foams with and without SiO2 nanoparticles. (A) APG/C12C3C12Br2 foam at 100 s, (a) APG/C12C3C12Br2 foam at 600 s, (B) APG/C12C3C12Br2/SiO2 foam at 100 s, (b) APG/C12C3C12Br2/SiO2 foam at 600 s.

foam without nanoparticles (APG/C12C3C12Br2 foam) was almost the same at the beginning stage (100 s). The bubbles grew and coalesced with time because of the pressure difference in bubbles of different size (Young−Laplace effect). The bubbles of tricomponent foam also increased with time, but the size was much smaller than that of foam without nanoparticles. After 600 s, although some bubbles also enlarged in the tricomponent foam, the ratio of smaller bubbles was much more bigger than that of APG/C12C3C12Br2 foam. After 2100 s, most of bubbles coalesced in APG/C12C3C12Br2 foam, while the bubbles in tricomponent foam still maintained their morphologies. According to Figure 5, the volume of tricomponent foam attenuated more slowly than other foams. Combined the above experimental results, we can preliminary deduce that APG worked as the main agent to generate foam, and SiO2 nanoparticle enhanced the foam stability, while

Figure 5. Foam volumes of different foam systems with time. G means Gemini surfactant C12C3C12Br2 in the data label. D

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APG concentration, which indicated the adsorbed nanoparticles participated and affected the interfacial tension changes with surfactants during the periodical oscillation. 3.2. Effects of Salts and Temperature on Initial Foam Volume and Stability. The effects of sodium chloride, calcium chloride, and magnesium chloride on the initial foam volume and stability of the tricomponent foam were studied because Na+, Ca2+, and Mg2+ are the main salt ions in saline water in reservoir. As shown in Figure 7, the initial foam volume was slightly increased, while the half-life time decreased obviously with the salt concentration. Divalent ions Ca2+ and Mg2+ did not show more deteriorating effects on foam stability than monovalent ion Na+. The surfactant critical micelle concentration (cmc) would decrease and the surface activity would increase due to the compression of the electric double layer of the headgroups (ionic surfactant) and salt-out effects of the hydrophobic chains (nonionic surfactant). The initial foam volume increased due to the increased surface activity. Because the cmc decreased and thus less amount of surfactant adsorbed at the air/liquid interface, the foam stability decreased. Both the initial foam volume and half-life time of the tricomponent foam decreased with temperature increasing (Figure 8). At 20 °C, the initial foam volume and half-life time

cationic Gemini surfactant C12C3C12Br2 also played a crucial role in the tricomponent foam system. The interfacial dilational moduli of the four foaming formula solutions were measured with the increasing of APG concentration and the results are shown in Figure 6. The

Figure 6. Interfacial dilational moduli of different systems with the concentration of APG. The SiO2 and C12C3C12Br2 concentrations were fixed at 1.5 and 0.003 wt%, respectively. The oscillation frequency was 0.1 Hz. G means Gemini surfactant C12C3C12Br2 in the data label.

solutions with SiO2 nanoparticles had much bigger dilational modulus than those without SiO2 nanoparticles. It was indicated that the adsorption of nanoparticles on the air/liquid surface increased the interfacial mechanical strength. And the tricomponent solution had the biggest interfacial dilational modulus among the four solutions. It was supposed that more nanoparticles might adsorb on surface in the tricomponent solution than the APG/SiO2 solution. For all the four solutions, the interfacial dilational moduli first increased and then decreased with APG concentration. At low APG concentration, the amount of surfactants adsorbed at the air/liquid interface increased with the APG concentration increasing, and thus led to the interfacial dilational moduli increasing. When the APG concentration was high enough, the amount of interfacial adsorbed surfactants would not increase with the APG concentration increasing. On the other hand, the high bulk concentration of surfactants would lead to a more rapid compensation of surfactant amount at the interface during dilational oscillation due to the higher concentration gradient. Thus, the interfacial dilational modulus would decrease. The diffusion of APG molecules between bulk solution and interface would dominate the surface tension change during the oscillation, and thus the interfacial moduli decreased to almost the same. Comparing with those without nanoparticles, the peak modulus value of foam with nanoparticles arose at low

Figure 8. Influences of temperature on foam volume and half-life time.

were 460 mL and 4800 s. When the temperature increased to 75 °C, the initial foam volume decreased to about 400 mL, and the half-life time decreased to about 2000 s. At low temperature, the foam rupture was driven by the gas diffusion and liquid drainage. While at high temperature, water evaporation in the bubble film and relatively high pressure in the bubble would accelerate the coalescence and rupture of bubbles. However, the half-life time of tricomponent multiphase foam was still longer than that of foams without nanoparticles, because the participation of nanoparticles

Figure 7. Effects of salt ions on initial foam volume and half-life time. (A), Na+, (B), Ca2+, (C), Mg2+. E

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Figure 9. Adsorption of surfactants on SiO2 surface. (A) Adsorption amounts determined by ellipsometry. The concentration of APG was 0.175 wt %, and the concentration of Gemini C12C3C12Br2 was 0.003 wt%. (B) Surface tension changes after surfactants adsorbed on SiO2 nanoparticle surface. The concentration of C12C3C12Br2 was 0.003 wt%, and the concentration of SiO2 nanoparticle was 1.5 wt%. G means Gemini surfactant C12C3C12Br2 in the data label.

enhanced the mechanic strength of the bubble liquid film and might prevent the water evaporation in liquid film. 3.3. Coadsorption of Surfactants at SiO2 Surface. The adsorption amount of surfactants was measured through ellipsometry. A surface oxidized silicon wafer was used instead of the SiO2 nanoparticles. As shown in Figure 9A, the adsorption of APG, C12C3C12Br2, and APG/C12C3C12Br2 mixture reached equilibrium very quickly (less than 1 min). The equilibrium adsorption amount of APG was only 0.6 mg/ m2 at the concentration of 0.175 wt%, and that of C12C3C12Br2 was 0.9 mg/m2 at the concentration of 0.003 wt%. For surfactant mixture at the same concentration (0.175 wt% APG and 0.003 wt% C12C3C12Br2), the adsorption amount was 3.2 mg/m2. The adsorption amount of the mixture was much larger than either APG or C12C3C12Br2 separately, which indicated that the cationic C12C3C12Br2 improved the adsorption of nonionic APG at a rather low concentration. The surface tension of supernatants of the four formulas with different APG concentration was also measured after the nanoparticles were separated through centrifugation. As shown in Figure 9B, at lower APG concentration, the surface tension of solutions with nanoparticles was higher than that of solutions without nanoparticles, which also proved that the surfactants adsorbed on the nanoparticles and were separated from the solution with the nanoparticles. When the APG concentration was large enough, the surface tension decrease was mainly contributed by APG, thus the surface tensions of four supernatants were almost the same. The contact angle of water on the surface oxidized silicon wafer was measured after the wafer had been treated with C12C3C12Br2 solutions of different concentration. As shown in Figure 10, the oxidized silicon wafer was hydrophilic with the water contact of about 50°. With the increasing of C12C3C12Br2 concentration, the contact angle also increased and reached a maximum value of about 65° at the concentration of 0.04 wt%. After that, the contact angle decreased with the increasing of C12C3C12Br2 and could be as low as 37° at C12C3C12Br2 concentration of 0.5 wt%. It is well-known that the cationic surfactants adsorbed on the negatively charged hydrophilic SiO2 surface with the positively charged headgroup.41,42 At low concentration, the adsorbed cationic surfactant molecules formed a single layer on the hydrophilic silica surface with the hydrocarbon tails pointing to the solution, and thus the surface was partially hydrophobic. While at high cationic surfactant concentration, the adsorbed surfactant molecules

Figure 10. Static contact angles of SiO2 substrates treated with different concentration C12C3C12Br2 solutions.

formed a double-layer structure with the hydrophobic tails of the two layers interfingered and the headgroups of the second layer pointing to the solution, and thus the surface became hydrophilic again. The change of half-life time with C12C3C12Br2 concentration was consistent with the change contact angle. It was indicated that the partially hydrophobic SiO2 nanoparticles could adsorb at the air/liquid interface with the nonionic surfactant and improved the foam stability. When the SiO2 nanoparticles became hydrophilic due to the doublelayer adsorption of C12C3C12Br2, the nanoparticles moved to the bulk solution and foam stability decreased again. The ζ potential of SiO2 nanoparticles in C12C3C12Br2 and C12C3C12Br2/APG solutions with the C12C3C12Br2 concentration ranged from 0.0 wt% to 0.5 wt% was also measured (Figure 11). The ζ potential of SiO2 nanoparticles without cationic surfactant was about −10 mV indicating they were negatively charged in water. For both solutions, the absolute ζ potential values decreased with the C12C3C12Br2 concentration and became positively charged when the C 12 C 3 C 12 Br 2 concentration was above 0.003 wt%. It was also proved that the adsorption of cationic surfactants on the SiO2 nanoparticles. It can be deduced that the in situ surface modified SiO2 nanoparticles through cationic C12C3C12Br2 adsorption with suitable hydrophobicity could stabilize the bubbles in tricomponent foam system. 3.4. Foams with Other Cationic Surfactant or Partially Hydrophobic SiO2 Nanoparticle. It was supposed that the in situ modification of SiO2 nanoparticles by cationic Gemini F

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decreased after reaching the maximum value of 70°, which was also corresponding to the evolution of monolayer adsorption to double layer adsorption of cationic surfactant on silica surface. The ζ potential of SiO2 nanoparticles also increased from −10 mV to about 15 mV with CTAB concentration increasing. Comparing these two tricomponent foam systems with C12C3C12Br2 and CTAB, the half-life time and maximum contact angle of CTAB system were bigger than those of C12C3C12Br2 system, which might originate from the molecular structure difference. In the third contrast experiment, partially hydrophobic H15 SiO2 nanoparticle with the water contact angle of ∼80° was used to substitute for hydrophilic SiO2 nanoparticle/cationic surfactant. As shown in Figure 13A, the initial foam volume increased with APG concentration, and the foam half-life time first increased and then decreased with APG concentration. The maximum half-life time was at the APG concentration of 0.175 wt%. When the APG concentration was fixed at 0.175 wt%, the half-life time increased while the initial foam volume decreased with H15 SiO2 nanoparticle concentration. At the condition of 0.175 wt% APG and 1.5 wt% SiO2 nanoparticle, the half-life time was 2500 s. Although it was shorter than that of tricomponent foams of APG/C12C3C12Br2/ SiO2 and APG/CTAB/SiO2, it was still much more longer than that of APG/SiO2 foam. The adsorption of APG on partially hydrophobic SiO2 surface was also measured through surface tension determination and ellipsometry (Figure S9). According to Figure S9A, the surface tension of APG/H15 SiO 2 supernatant was higher than that of APG solution at relatively low APG concentration, which indicated parts of APG molecules adsorbed on the nanoparticle surface. APG adsorbed amount determined by ellipsometry also confirmed the adsorbed amount on partially hydrophobic SiO2 surface was much more than that on hydrophilic surface (Figure S9B). The morphology evolution with time of APG/CTAB/SiO2 foam and APG/SiO2 (H15) foam was recorded using a Foamscan apparatus. Although the bubbles in both foams increased with time, their morphology still maintained after 2100 s and the bubble size of APG/CTAB/SiO2 foam was smaller than that of APG/SiO2 (H15) foam (Figure 14). The foam volume attenuation of APG/CTAB/SiO2 foam and APG/ SiO2 (H15) foam was also monitored by Foamscan and compared with that of APG/C12C3C12Br2/SiO2 foam. As shown in Figure S10, all the three kind of foam were rather stable and the foam volume did not decrease obviously after 35 min. The attenuation rates of APG/CTAB/SiO2 foam and APG/C12C3C12Br2/SiO2 foam were a bit slower than that of APG/SiO2 (H15) foam. SiO2 nanoparticles adsorbed on the bubble surface rose with foam bubbles in the cylinder during the Waring blending experiments. After the rupture of bubbles, most of these SiO2 nanoparticles adhered at the cylinder wall. And those dispersed in the bulk solution were still in the aqueous solution. Thus, the turbidity of the separated liquid can be used to estimate the amount of SiO2 nanoparticle adsorbed on the bubble surface roughly. The bigger the turbidity of the separated liquid was, the more nanoparticles in the bulk solution was, thus the less nanoparticles adsorbed on the bubble surface. Among the three multiphase foams, the turbidity of separated liquid ranked as APG/SiO2 (H15) > APG/C12C3C12Br2/SiO2 > APG/CTAB/ SiO2 at the same SiO2 concentration of 1.5 wt% (Figure 15), which was corresponding with the foam stability. It was indicated that the more nanoparticles adsorbed at the bubble surface, the more stable of the foam was; and the in situ

Figure 11. ζ potential of SiO2 nanoparticle in different concentration C12C3C12Br2 and APG/C12C3C12Br2 solutions. The concentration of SiO2 nanoparticle was 1.5 wt%, and the concentration of APG was fixed as 0.175 wt%. G means Gemini surfactant C12C3C12Br2 in the data label.

surfactant to produce a proper hydrophobic surface was the key step in tricomponent multiphase foam stability. To prove the hypothesis, three other experiments were conducted as control. In the first experiment, cationic C12C3C12Br2 was replaced by an anionic surfactant sodium dodecyl sulfonate (SDS). As shown in Figure S7, the initial foam volume and half-life time were only slightly increased compared with APG/SiO2 foam, because the negatively charged SDS could not adsorb and modify the SiO2 nanoparticle surface. In the second experiment, another cationic surfactant, cetyltrimethylammonium bromide (CTAB), was used instead of C12C3C12Br2. Just like APG/C12C3C12Br2/SiO2 tricompoment foam, the half-life time of APG/CTAB/SiO2 tricompoment foam first increased and then decreased with CTAB concentration. At the CTAB concentration of 0.007 wt%, the half-life time was 6000 s, which was much longer than APG/SiO2 foam (Figure 12). The

Figure 12. Foam volume and half-life time of APG/CTAB/SiO2 foam. The concentration of APG was 0.175 wt%, and the concentration of SiO2 was 1.5 wt%.

adsorption of CTAB and APG/CTAB mixture was also studied by ellipsometry, contact angle, and ζ potential measurements (Figure S8). The adsorption performance of CTAB was similar to that of cationic Gemini surfactant C12C3C12Br2. CTAB promoted the adsorption of APG on SiO2 surface, the adsorption amount of APG/CTAB mixture was 3.0 mg/m2, which was similar to that of APG/C12C3C12Br2 mixture. After treated with CTAB solution of different concentration, the water contact angle of silicon wafer first increased, then G

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Figure 13. Foam volume and half-life time of APG/SiO2 (H15) foam. (A) Effects of APG concentration, SiO2 (H15) concentration was fixed as 1.5 wt%. (B) Effects of SiO2 (H15) concentration, APG concentration was fixed as 0.175 wt%.

Figure 14. Foam morphology changes with time of APG/CTAB/SiO2 (upper row) and APG/SiO2(H15) (bottom row) foams.

3.5. Core Plugging Ability of Tricomponent Multiphase Foam. The plugging ability of the tricomponent multiphase foam was assessed through the sandpack experiment and compared with a foam without SiO2 nanoparticles. The formula of the tricomponent foam was 1.75 wt% APG, 0.003 wt % C12C3C12Br2, and 1.5 wt% SiO2 nanoparticle. The formula of foam without SiO2 nanoparticles was 1.75 wt% APG and 0.003 wt% C12C3C12Br2. The permeability and porosity of the sandpack model were 3.95 μm2 and 42.1%, respectively. As shown in Figure 16A, after 2 PV foam was injected, the resistance factor of tricomponent multiphase foam was as high as 410, while it was only 100 for the APG/C12C3C12Br2 foam. It was indicated that the multiphase foam was also more stable in porous media than foam without nanoparticle. The stable bubbles plugged at the throats of porous media and increased the filtrational resistance. After 4 PV water was injected subsequently, the resistance factor of tricomponent multiphase foam still maintained about 100, while it decreased to almost 0 for the APG/C12C3C12Br2 foam. The change of gas phase saturation of the sandpack model with fluids injection is also shown in Figure 16B. It was akin to the change of resistance factor. After 2 PV foam injection, the sandpack gas phase saturation ratio was 0.5 for tricomponent multiphase foam, and only 0.1 for the APG/C12C3C12Br2 foam. After 4 PV water was injected, the residue gas phase saturation ratio was still 0.25 for the tricomponent multiphase foam. It was indicated more gas was trapped in the sandpack model. Based on the above results, it could be concluded that the multiphase foam were rather stable in porous media and had better resisting performance to water erosion during flooding.

Figure 15. Turbidity of solutions separated out of three stable multiphase foams. G means Gemini surfactant C12C3C12Br2 in the data label.

hydrophobic modification of SiO2 surface adsorbed more easily at the air/liquid surface than that of the pretreated partially hydrophobic H15 SiO2 nanoparticles. Based on the above results, we confirmed that the adsorption of cationic surfactant modified the surface hydrophobicity of SiO2 nanoparticles and also promoted the adsorption of nonionic surfactant APG. The adsorption was important to make a suitable SiO2 surface and improved the distribution of nanoparticle at air/liquid interface, which was the key factor in multiphase foam stability. Since the surface hydrophobicity could be easily regulated by adjusting the cationic surfactant concentration, the in situ surface modification in tricomponent multiphase foam is a better methodology than pretreated modification. H

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Figure 16. Resistance factor (A) and gas phase saturation (B) varies with injected pore volume in core experiments. G means Gemini surfactant C12C3C12Br2 in the data label.

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CONCLUSION A kind of multiphase foam with nonionic surfactant APG, cationic Gemini surfactant C12C3C12Br2, and SiO2 nanoparticle was developed. The measurement and observation of initial foam volume, half-life time, bubble morphology, and dilational modulus showed that the tricomponent multiphase foam was much more stable than foams with any two components (i.e., APG/SiO2 foam, APG/C12C3C12Br2 foam, and C12C3C12Br2/ SiO2 foam), or APG foam. The foam generation was mainly contributed by APG, because the formulas with APG had almost the same initial foam volumes of about 550 mL. The half-life time of the tricomponent was nearly 10 times longer than those without nanoparticles. The adsorption of SiO2 nanoparticles at air/liquid interface enhanced the mechanic strength of bubble film and thus improved the foam stability. Although the half-life time decreased with saline ions and temperature increasing, the tricomponent multiphase foam still showed considerable resistance to saline and temperature. The adsorption of cationic surfactant could in situ modify the SiO2 surface, and promote the adsorption of nonionic APG. The proper hydrophobicity of SiO2 nanoparticle surface was a key factor in foam stability. More SiO2 nanoparticles were distributed at air/liquid interface and utilized effectively through in situ modification by cationic surfactant adsorption than through the pretreated hydrophobic modification. The tricomponent multiphase foam showed much better plugging ability than the foam without nanoparticles in sandpack experiment.

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surface, foam stability of different multiphase foams (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected]. *[email protected]. ORCID

Jiqian Wang: 0000-0002-7525-5943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial supporting by the National Natural Science Foundation of China under grant No. U1262102, 51274228, 51304229.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02592. Figures S1−S10 including TEM images of SiO2 nanoparticles, contact angle measurement photos, 1H NMR spectrum of synthesized C12C3C12Br2 and molecular structures of surfactants, initial foam volume and half-life time of foams produced by C 12 C 3 C 12 Br 2 and C12C3C12Br2/APG mixture, effect of SiO2 nanoparticle concentration on foam volume and stability, effect of SDS in tricomponent multiphase foam, adsorption of CTAB and coadsorption of CTAB/APG on SiO2 surface, adsorption of APG on partially hydrophobic SiO2 I

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