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An experimental study of interaction between surfactant and particle hydrogels
Polymer 52 (2011) 452e460
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Polymer journal homepage: www.elsevier.com/locate/polymer
An experimental study of interaction between surfactant and particle hydrogels Yongfu Wu a, Tingji Tang a, Baojun Bai a, *, Xiaofen Tang b, Jialu Wang b, Yuzhang Liu b a b
Department of Geological Science and Engineering, Missouri University of Science and Technology, 129 McNutt Hall, 1400N Bishop Avenue, Rolla, MO 65409, USA Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2010 Received in revised form 29 November 2010 Accepted 1 December 2010 Available online 9 December 2010
Polyacrylamide gel was synthesized to study interaction between surfactant and particle hydrogel. Surfactants used in this study include cationic surfactants, n-dodecylpyridinium chloride, (1-hexadecyl) pyridinium bromide; anionic surfactants, sodium salt of dodecylbenzene sulfonic acid, sodium 4-n-octyl benzene sulfonate and sodium branched alcohol propoxylate sulfate; and nonionic surfactants, IgepalÒ CO-530, TergitolÒ NP-10 and NeodolÒ 25-12. It has been found that, after swelling of the dry particles, surfactant concentration shows a substantial increase. Meanwhile, dynamic modulus (G0 and G") of the gels shows a significant decrease in the surfactant solutions. Based on the experimental results, a mechanism has been proposed to elucidate the reduction of the gel dynamic modulus. Furthermore, this mechanism was discussed through surfactant critical packing parameters (CPP) at the interface of the hydrogel particle and surfactant aqueous solution, and confirmed by recovery of the gel dynamic modulus after removal of the surfactant from the hydrogels. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Hydrogel Surfactant Dynamic modulus
1. Introduction Interaction between surfactant and polymer hydrogel has been a subject of considerable theoretical and practical interest, and has been extensively studied [1e9]. Philippova and co-workers studied interaction of gels with ionic surfactants n-cetylpyridinium chloride and sodium dodecylbenzenesulfonate. They reported that absorption of anionic surfactant is governed primarily by hydrophobic interactions. Due to conditions of electro-neutrality, anionic surfactant penetrates the gel together with corresponding co-ions. Therefore, the uptake of cationic surfactant ions results in gel shrinkage, while the uptake of anionic surfactant induces gel swelling. In the anionic gel/anionic surfactant system, a significant interaction is observed only for the most hydrophobic gels when hydrophobic interactions overcome the electrostatic repulsion between similarly charged groups [10]. Ashbaugh and co-workers studied interactions of mixed dodecyl trimethylammonium bromide (C12TAB) and octaethylene glycol monododecyl ether (C12E8) micelles with a lightly cross-linked Na polyacrylate gel. They found that gel preferentially absorbs C12TAþ under most conditions owing to electrostatic attraction to the oppositely charged gel. For low initial C12E8 surfactant fractions and moderate C12TAþ concentrations, however, the situation is reversed and the nonionic surfactant is preferentially absorbed [11]. Nichifor and coworkers studied interaction of hydrophobically modified cationic * Corresponding author. Tel.: þ1 573 341 4016; fax: þ1 573 341 6935. E-mail address: [email protected] (B. Bai). 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.12.003
dextran hydrogels with biological surfactants. They found that an increase in the length of the alkyl substituent of the hydrogel strongly increases the binding constants K0 and K, but decreases the cooperativity parameter m. This was explained by the formation of mixed micelles between pendant groups of the gel and surfactant molecules [12]. It is well recognized that surfactants play a critical roles in tribology [13]. One of the results caused by addition of surfactants to hydrogel particles is the influence on the frictional behavior of gels. Friction of hydrogels on solid surface or on gels has been extensively studied during the last decade, especially in Gong’s group, and considerable number of papers have been published [14e24]. It has been found that the frictional behaviors of the hydrogels do not conform to Amonton’s law F¼mW, which well describes the friction of solids [15]. Based on the complex friction behavior of polymer gels, Gong and co-workers proposed a repulsioneadsorption mechanism to describe the friction of hydrogels on a smooth substrate. If the interfacial interaction between hydrogel and solid surface is repulsive, then friction is due to lubrication of the hydrated water layer of the polymer network at the interface. In this case, friction is proportional to sliding velocity. If the interaction is attractive, then friction is from two contributions: (1) elastic deformation of the adsorbed polymer chain; (2) lubrication of the hydrated layer of the polymer network [16]. They reported that the friction behavior of hydrogels sliding on smooth substrates strongly depends on the adhesive strength and hydrophobicity of the substrate [17]. Du and co-workers studied friction behaviors of PVA gel sliding against a glass surface in dilute poly
Y. Wu et al. / Polymer 52 (2011) 452e460
453
Table 1 Surfactant Molecular Structure and Gel Swelling Ratio in Surfactant Solutions. Surfactant
Molecular structure
Swelling ratio
+
H3C (CH2) 10 CH2 N
n-Dodecylpyridinium chloride
Cl
+
H3C (CH2) 14 CH2 N
(1-Hexadecyl)pyridinium bromide
Br
H3C (CH2)2 Alfoterra 23 Sodium branched alcohol propoxylate sulfate
22.3e23.3
CH3 + CH2 N CnH2n+1 Cl CH3 (n=8 ~18)
Benzalkonium chloride
Ò
22.2e23.2
H3C (CH2)8~10 CH2 CH CH2 O
22.1e23.1
O
CH3
+ CH2 CH O 3 S O Na
22.3e23.3
O
O Sodium 4-n-octyl benzene sulfonate
H3C (CH2) 6 CH 2
+ S O Na
22.8e23.8
O
O Sodium salt, dodecylbenzene sulfonic acid
+
H 3C (CH2) 10 CH 2
Igepal CO-530 Nonylphenoxypoly(ethyleneoxy) alcohol
H3C C
CH2 C CH2
Tergitol NP-10 Nonylphenol ethoxylated alcohol
NeodolÒ25e12 Linear primary alcohol ethoxylate
H 3C C
(OCH2CH2)6 OH
CH3
CH3
CH3 Ò
22.1e23.1
CH3
CH2 C CH2
CH3
(OCH2CH2)10 OH
CH3
H3C (CH2) 12 CH2 (OCH2CH2)12 OH
In distilled water In 1.00 wt.% NaCl brine
(ethylene oxide) (PEO) aqueous solution, and they found that PEO concentration has a significant influence on the friction of a waterswollen PVA gel on glass surface [20]. Osada and co-workers studied kinetics of surfactant binding into polymer gel and effect of hydrophobic side chain on poly(carboxyl acid) dissociation. They reported that the driving force of surfactant diffusion into the gel is the concentration gradient of the surfactant. The binding of surfactant with the polymer network sustains a high concentration gradient that facilitates the subsequent surfactant diffusion [21]. They also analyzed the structure of poly(ADA-co-AA) polymer and surfactant-poly(ADA-co-AA) complexes by SAXD and WAXD and found that the complexes change their structures from micelle-like to lamellar-like with an increase of mole fraction of acryloyl dodecanoic acid (ADA) in the complexes [24]. To date, interaction between surfactant and particle hydrogel has not been studied systematically. In this paper, we study the
22.6e23.6
O
CH3
CH3 Ò
S O Na
22.0e23.0
22.6e23.6 22.8e23.8 22.4e23.4
influence of surfactant in aqueous solution on dynamic modulus of water-swollen gel in 1.0 wt% NaCl. Surfactants used in this study include nonionic, anionic and cationic surfactants. Gel frictions were measured in terms of storage modulus G0 and loss modulus G" under the same conditions of stress, gap, oscillation frequency and temperature for all surfactants. Gel used in this study was synthesized from acrylamide monomer with ethylene-bis-acrylamide cross-linker. 2. Experimental 2.1. Materials Monomer acrylamide (98.5%) and cross-linker methylene-bisacrylamide (97þ%) were purchased from Alfa Aesar Company (Ward Hill, MA) and used without further purification. Ammonium
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2.3. Measurement of swelling ratio and pore size of PPGs
Fig. 1. DSC curve of the swollen PPGs in the distilled water.
persulfate was used as initiator for polymer gel synthesis. Cationic surfactants, n-dodecylpyridinium chloride (98%), (1-hexadecyl) pyridinium bromide monohydrate (98%), benzalkonium chloride and anionic surfactant, sodium 4-n-octyl benzene sulfonate were also purchased from Alfa Aesar (Ward Hill, MA) and used without further purification. Sodium dodecylbenzene sulfonic acid was purchased from SigmaeAldrich (St. Louis, MO) and used without further purification. Other commercial surfactants were requested from their manufacturers, IgepalÒ CO-530 from Rhodia, Inc. (Bristol, PA), TergitolÒ NP-10 from Dow Chemical (Midland, MI), AlfoterraÒ 23 from Sasol North America Inc. (Houston, TX), NeodolÒ 25-12 from Shell Chemical Company (Houston, TX), and used without further purification. NaCl (99.8%) was purchased from Fisher Scientific Inc. Water used in all experiments was distilled water produced in this lab. Molecular structures of the surfactants are listed in Table 1.
0.30 g of PPGs (120e150 mm size) was swollen in 14.70 g of distilled water contained in a clean test tube. After the resulting suspension was left overnight to reach the equilibrium, the test tube was centrifuged at 5 000 rpm for 15 min to separate the particle gel and distilled water. Volume of the swollen PPG was calculated by measuring the height of the PPG in the tube and diameter of the tube. The same procedures were used for the swelling ratio measurement in 200 ppm surfactant solutions prepared in 1.00 wt.% NaCl solution. The swelling ratio is calculated by the total volume of the swollen particle gel measured in the test tube over the volume of 0.300 g of the dry particles and the results are listed in Table 1. Pore size of the synthesized hydrogel was measured by a TA Instrument DSC 2920 equipped with low temperature cooling accessory (60 C). Swollen hydrogel sample (10e20 mg) was sealed in an aluminum DSC pan with an added excess of bulk water (1e2 mg). During experiment, the temperature was first decreased to 40 C, and then held for 15 min for freezing, after which the sample was heated to 15 C at a rate of 1 C min1 using an empty pan as reference. The DSC spectrum is shown in Fig. 1. The pore size is related to the temperature shift (DT) of the two peaks originated from bulk and confined water, respectively and it can be calculated based on the following equation, where DT is a negative value and quantified as the difference between the temperature at the confined water peak and the onset of the bulk water peak [25].
Dp ¼
39:604
DTðKÞ þ 0:1207
þ 2:24
(1)
where, DT is temperature shift in K; Dp is pore size of the swollen hydrogel particle in nm. 2.4. Measurement of the NaCl concentration change upon gel swelling
2.2. Synthesis and fabrication of preformed particle gels (PPGs) 150 g of acrylamide were added to 498.7 g of distilled water. The solution was purged with nitrogen gas for 60 min and stirred at 270 RPM until all the solid dissolved. 0.650 g of methylene-bis-acrylamide was added to the solution and stirred until completely dissolved. 0.650 g of (NH4)2S2O8 was then added with stirring to the solution prepared above. The mixture solution was placed in an oven at 60 C for 14 h. A strong bulk gel was formed and it was cut into small pieces. The formed hydrogel was then purified by soaking in large amount of distilled water for one week and followed by drying at 60 C for 4 days to yield 168.390 g of a slightly yellow gel. The yellow color may be due to oxidation of acrylamide. The dried gel solids were crushed into small particle powder in a blender machine (Black & Decker). PPGs with the particle size between 100 and 120 mesh (150 mme120 mm) were selected through the standard testing sieves (Fisher Scientific Company).
All the tested surfactant solutions were made in 1.0 wt % NaCl solution which is dependent on the salinity of the reservoir formation water and concentration change of the NaCl solution after complete swelling was measured by the anion metathesis of NaCl with AgNO3. By measuring the weight of precipitation of AgCl, the concentration NaCl can be calculated. The data and results are listed in Table 2. 2.5. Measurement of the concentration change of surfactants after gel swelling Initial concentration for all surfactants was prepared at relatively low concentration of 200.00 ppm for higher accuracy from the UVeVis spectroscopy measurement. Dry PPGs at 2.00 wt% were swollen in different surfactant solutions. This equilibrium solution over the top of the swollen gels contains NaCl, surfactant and un-
Table 2 Analysis Results of NaCl Concentration after Particle Gel Swelling. Parallel Tests
For 1.00 wt.% NaCl solution (g)
Empty Test Tube NaCl Solution NaCl Contained Total weight after drying Mass of AgCl (MW: 143.5) Mass of NaCl (MW: 58.5) Test Results
13.9754 13.9021 5.0100 5.0081 0.0501 To be measured 14.0986 14.0259 0.1232 0.1238 0.0502 0.0505 Equili. NaCl concentration after gel swelling is 1.01 wt.%, which is almost the same as the initial NaCl concentration.
For Equilibrium NaCl solution after swelling (g)
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Table 3 Measurement of Concentration Change of Surfactant after Gel Swelling. Surfactant Initial Concentration: 200 ppm Surfactant n-Dodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide Benzalkonium chloride Sodium 4-n-octyl benzene sulfonate Sodium salt, dodecylbenzene sulfonic acid IgepalÒ CO-530 TergitolÒ NP-10
At Equilibrium of Swelling
lmax (nm)
Initial ABS 2.451 2.106 0.179 0.232 0.227 0.806 0.429
257 259 261 260 260 276 275
crosslinked polymer dissolved from the particle gel and the absorbance of this solution was used to calculate the equilibrium surfactant concentration after swelling. The background absorbance of the excess solution from the PPGs in 1.00 wt% NaCl solution is measured as the background absorbance, which will be deducted from the above measured total UV absorbance. For each surfactant solution, the UVeVis spectrum was scanned from 1100 nm to 190 nm to identify the absorption peak position (lmax). The equilibrium concentrations for each surfactant were calculated through its absorbance at lmax by use of a UVeVis spectrophotometer (UVmini-1240V, Shimadzu). Concentration changes for the surfactants investigated are listed in Table 3. 2.6. Measurement of particle gel dynamic modulus To investigate influence of surfactant on the particle gel strength, a rheometer, HAAKE RheoScope 1(Thermo Scientific) was employed to measure storage modulus G0 and loss modulus G" for the swollen gels. Measurement was set oscillation mode with frequency f ¼ 1.000 Hz, controlled stress (CS) and the stress applied to the gel was set s ¼ 1.0 Pa to ensure that the gel’s strain and stress have a linear relationship during the measurement. The sensor used for measurement is PP35 Ti Po LO2 016 with a gap of 0.200 mm; Temperature: 25.0 C. For each sample, measurements of G0 and G" were taken every 30 s for 5 min. The results are shown in Table 4 and Fig. 2. 3. Results and discussion 3.1. Particle gel swelling ratio The swelling ratios in different surfactant solutions at 200.00 ppm concentration as well as in 1% NaCl brine and distilled water are listed in Table 1. It can be clearly seen from Table 1 that the swelling ratios in surfactant solutions, distilled water and 1.0 wt.% NaCl are between 22 and 24. The difference in the swelling ratios is in the range of experimental error and thus the swelling ratio is independent on the swelling media. This implies the neutral charge balance of the synthesized polyacrylamide gels. Therefore, the pore size and distributions of the synthesized polyacrylamide gels do not change when the surfactants and NaCl were added. Also,
Net ABS 2.326 2.432 0.187 0.247 0.262 1.137 0.608
Ceq. (ppm) 190 231 209 213 231 282 283
Conc. Change 5.0% 15.5% 4.5% 6.5% 15.5% 41.0% 41.5%
DSC was used to measure the pore size of the swollen PPGs as shown in Fig. 1. Based on the Eq. (1), it can be derived that the average pore size of the PPGs is around 4.1 nm, which is much less than the size of micelles formed from our tested surfactants [26]. 3.2. Concentration change of NaCl after equilibrium of swelling Accuracy of the experimental results was evaluated by analysis results of a standard NaCl aqueous solution. The expected NaCl for the analyzed solution is 0.0501 g. The mass of NaCl calculated through the precipitation of AgCl after anion metathesis is 0.0502 g, leading to a relative experimental error of 0.2%. When PPGs were swollen in the brine solution, the equilibrium concentration of NaCl was found to be at 1.01 wt % changing from 1.00 wt %. The difference of 0.01 wt.% is attributed to the experimental error. The analysis results are listed in Table 2, from which it can be concluded that the NaCl concentration in the excess brine is the same as the initial concentrations. After swelling equilibrium, particle gel doesn’t change the concentration of NaCl in the excess solution since the sizes of sodium and chloride ions are much smaller than the pore sizes of the swollen PPGs. 3.3. Concentration change of surfactants after equilibrium swelling of PPGs The initial concentration for all surfactants used in this experiment is 200.00 ppm. After PPGs were completely swollen, equilibrium concentration of surfactant in the excess solution was measured by the UV absorbance. The equilibrium concentrations for different surfactant solutions at PPG swelling equilibrium are listed in Table 3. In Table 3, it was found that the concentration of the most of the surfactant solutions increased after the swelling equilibrium of PPGs. For cationic surfactant, (1-hexadecyl) pyridinium bromide, the concentration was changed to 231 ppm from 200 ppm corresponding to an increase of 15.5% from its initial concentration. However, for another cationic surfactant of n-dodecylpyridinium chloride and non-surfactant chemical of benzalkonium chloride, their concentrations were changed as 5% and 4.5%, respectively, which were considered in the experimental error of 5% originated from the UV absorbance measurement and thus the surfactant
Table 4 Results of Storage Modulus G’ and Loss Modulus G" for Gel Particles in Surfactant Solution (1000 ppm) and in 1.0 wt.% NaCl Brine. Surfactant
Conc. in 1.0% NaCl
G’ (Pa)
Change
G" (Pa)
Change
In 1.0 wt.% NaCl AlfoterraÒ 23 Sodium salt, dodecylbenzene sulfonic acid neDodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide IgepalÒ CO-530 (HLB ¼ 10.8) NeodolÒ 25e12 (HLB ¼ 14.4)
brine 1.93 2.87 3.52 2.45 2.16 1.35
2582 689 1042 1133 1739 824 946
0% 73% 60% 56% 33% 68% 63%
112 45 64 66 87 51 56
0% 59% 43% 41% 22% 54% 50%
103 103 103 103 103 103
M M M M M M
456
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Fig. 2. Results of G0 and G" for particle gel in surfactant Alfoterra 23 solution (1000 ppm) prepared with 1.0wt.% NaCl brine and in 1.0wt.% NaCl brine only (blank test).
molecules (not forming micelles, see Table 5) can migrate freely through the swollen PPG porous structure. For the two anionic surfactants tested, sodium 4-n-octyl benzene sulfonate and sodium dodecylbenzene sulfonic acid, the latter shows an obvious increase of its equilibrium concentration after gel swelling, but sodium 4-noctyl benzene sulfonate shows little change in the final equilibrium concentration due to its higher CMC than 200 ppm (see Table 5). For the commercial nonionic surfactants of IgepalÒ CO-530 and TergitolÒ NP-10, their equilibrium concentrations increased to 282 and 283 ppm, respectively, from the initial concentration of 200 ppm, corresponding to the change of 41% and 41.5%. The reason that contributes to the dramatic increase of surfactant concentration after gel particle swelling is the formation of surfactant micelles in the solution, which has a much larger size than that of opening of the gel network. Average area per surfactant molecule adsorbed at the air/liquid interface is 60 Å2 [26], therefore, one surfactant molecule has a dimension of 9 Å in diameter of hydrophilic head and about 20 Å in length of a hydrophobic tail. This size is much smaller than the average pore size of the hydrogel, 4.1 nm or 41 Å. On the other hand, approximate size for a rod-like surfactant micelle is about 54 Å in the rod diameter and 140 Å in the rod length [27], which is much larger than the average pore size of the hydrogel. When the dry PPGs contact with the aqueous surfactant solution, the particles absorb water first, other molecules and ions will diffuse into the network structure because of the concentration gradient and their much smaller size. However, surfactant micelles cannot go through the network due to their much larger size. Only unassociated single surfactant molecules can go through the opening and diffuse into the network of the swollen gel, which forms the dynamic equilibrium with the formed micelles
absorbed outside the swollen gel network. After the swelling reaches equilibrium, more water has been absorbed by the gel and less surfactant molecules can get into gel network. As a result, the concentration of surfactants remained in the excess solution increases. Therefore, it is expected that the lower the surfactant critical micelle concentration (CMC) is, the greater the increase of concentration in the excess solution will be after swelling since the initial concentration of surfactants was all set at 200 ppm. For a surfactant, if its CMC is high or its initial concentration is about or lower than CMC, the concentration will change little after the gel swelling reaches equilibrium such as n-dodecylpyridinium chloride and sodium 4-n-octyl benezene sulfonate from Table 5. From Table 5, it can be found that the change of surfactant concentration is related to the ratio of the initial concentration (Cinit) to surfactant CMC. If the ratio is less than 1.0, that means the surfactant initial concentration is lower than its CMC, the concentration change is little and in the range of experimental error. If the ratio is greater than 1.0, the equilibrium concentration increased dramatically. For example, this ratio for nonionic surfactants, IgepalÒ CO-530 and TergitolÒ NP-10, is greater than 10, their equilibrium concentration increased from their initial concentration by 41.0% and 41.5%, respectively. This suggests that most of their molecules exist in the solution in the form of micelles for these nonionic surfactants with very low CMC. The micelles cannot go through network of the swollen particle gel due to their bigger size than the average pore size of the PPGs, resulting in the dramatic increase of their equilibrium concentration since PPGs absorbs a large amount of water during the swelling.
3.4. Influence of surfactant on friction of the particle gel surface To study influence of surfactant on the particle gel viscoelasticity, the dry PPGs at 2 wt.% were mixed with 1000 ppm surfactant solutions prepared with 1.0 wt.% NaCl brine in a centrifuge tube, and the PPGs and surfactant solution mixture was shaken well to ensure the particles get completely swollen. The reason to prepare surfactant solution at 1000 ppm that is much higher than that in previous study is to ensure all surfactants investigated in this study get aggregated in the solution. The gel viscoelasticity was measured by the HAAKE RheoScope as described before. During the process of measurement, both G0 and G" show a slow and steady increase because of the evaporation of water contained in gel sample, data taken at the very beginning of measurement are used as G0 and G" results. A blank test of the gel strength in 1.0 wt.% NaCl brine without surfactant was also conducted for comparison. Measurement results for the particle gel strength (both G0 and G") in 1000 ppm and 1.0 wt % NaCl brine solution are listed in Table 4. A typical measurement of the gel strength as a function of scan time was illustrated in Fig. 2 for PPGs in 1.0 wt% NaCl and 1.0 wt % NaCl/ 1000 ppm AlfoterraÒ 23, respectively. It can be clearly seen from Table 4 and Fig. 2 that the introduction of AlfoterraÒ 23 into the swelling media significantly decreased the swollen gel strength. For
Table 5 Surfactant CMC, ratio of Cinit./CMC and Concentration Change after Gel Swelling. Surfactant
M.W.
CMC* in 1.0% NaCl
Cinit.(200 ppm)
n-Dodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide Benzalkonium chloride Sodium 4-n-octyl benzene sulfonate Sodium salt, dodecylbenzene sulfonic acid IgepalÒ CO-530 TergitolÒ NP-10
284 402 w410 292 348 464 660
2.8 103 M 4.5 104 M Not a surfactant 1.3 103 M 4.6 104 M 4.1 105 M 2.8 105 M
7.0 5.0 4.9 6.8 5.7 4.3 3.0
* Rosen, M. J.: “Surfactants and Interfacial Phenomena”, Wiley-Interscience, 3rd Ed., 2004, 185e188.
104 104 104 104 104 104 104
M M M M M M M
Cinit./CMC
Conc. Change
0.3 1.1 n/a 0.5 1.2 10.5 10.7
5.0% 15.5% 4.5% 6.5% 15.5% 41.0% 41.5%
Y. Wu et al. / Polymer 52 (2011) 452e460
example, in the blank test, G0 and G" for the particle gel are 2582 Pa and 112 Pa, respectively. However, when AlfoterraÒ 23 was used in the swelling solution, G0 and G" dramatically decrease to 689 Pa and 45 Pa, corresponding to the significant reduction of 73% and 59%, respectively. From the data listed in Table 4, it can also be observed that all the tested surfactants have strong influence on viscoelasticity of the particle gel. Addition of surfactant to NaCl brine used for gel swelling can substantially reduce the gel dynamic modulus G0 and G". Another anionic surfactant of sodium dodecylbenzene sulfonic acid reduced the gel’s storage modulus G0 by 60%, along with the reduction of the loss modulus G" by 43%. For the cationic surfactants such as n-dodecylpyridinium chloride and (1-hexadecyl) pyridinium bromide, they can decrease the gel’s storage modulus by 56% and 33%, respectively, and loss modulus by 41% and 22%, respectively. For nonionic surfactants such as IgepalÒ CO530 and NeodolÒ 25-12, their presence in the swelling solution reduced the gel’s storage modulus by 68% and 63%, respectively, along with the loss modulus decrease by 54% and 50%, respectively. In order to further elucidate the reason of the dramatic decrease of the dynamic modulus (G0 and G") of the particle gel by the introduced surfactant in the solution, one has to understand that the measurement of the dynamic modulus (G0 and G") by a plate rheometer is accomplished by measuring the torque exerted on the tested and swollen PPGs. The swollen particle gel sample is placed on the horizontal glass plate and another metal sensor plate is placed on top of the gel samples. Typically the top sensor plate is rotated and the torque exerted on it is measured. However, the movement of this plate is resisted by the frictional force, which is proportional to the frictional coefficient and the stress applied on it. Equations (2), (3) and (4) below describe quantitatively the relationship for torque (T), stress (N) and frictional coefficient (m).
T ¼ rF
(2)
F ¼ m,N
(3)
T ¼ r F ¼ r ðm,NÞ
(4)
where, T is the torque exerted on the sample; r is typically the length of the lever arm and here it is related to geometry of the rheometer sensor. For a given sensor, it is the same for all gel samples measured; F is the frictional force, it is the product of frictional coefficient (m) between two surfaces and the force applied on the surfaces (N). In this experiment, a model of controlled stress is employed for the measurement. Therefore, the torque exerted on the sample is directly proportional to the frictional force between the particle gel and the surfaces of the plates. Furthermore, for a given cap of 0.200 mm between the plate and sensor, measurement results of the dynamic modulus (G0 and G") are directly
457
Table 6 Critical Packing Parameters (CPP) of the Surfactants Investigated. Surfactant
l a0(nm)2 CPP v (nm)3 (nm)
AlfoterraÒ 23 Sodium salt, dodecylbenzene sulfonic acid neDodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide IgepalÒ CO-530 NeodolÒ 2512
0.538 2.690 0.600 0.486 2.245 0.600
0.333 Spherical 0.361 Rod-like
0.324 0.532 0.405 0.378
0.369 0.423 0.344 0.353
1.505 2.155 1.420 1.801
0.584 0.584 0.829 0.595
Micelle structure
Rod-like Rod-like Rod-like Rod-like
proportional to the frictional force coefficient between the particle gel and the plate surfaces. Surface friction of polymer bulk gels on solid surfaces has been intensively investigated through the use of commercial tribometer or rheometer. It has been reported by Gong and co-worker that the friction behaviors of the bulk hydrogels on glass or other slid surfaces do not conform to Amonton’s law as shown in Eq. (3) which describes the friction of solid materials [16]. However, to our best knowledge, the friction investigation between the PPGs and surfactants has not been reported. Therefore, we proposed a simple mechanism, shown in Fig. 3, as a qualitative discussion of friction reduction by surfactant between the surfaces of particle gels, stainless steel sensor plate and glass plate. As shown in Fig. 3 (a) without addition of surfactant to NaCl brine for the particle gel swelling, the stainless steel sensor plate presses the particle gel and rotates on it at a constant stress mode when the dynamic modulus is measured. The original dried particle size ranges between 0.125 and 0.150 mm. Based on the volume swelling ratio of 23, the swollen particle size is between 0.355 and 0.427 mm in diameter. The gap between the sensor plate and bottom glass plate for G0 and G" measurement is at 0.200 mm, which is only about half of the particle size. Therefore, the swollen gel particles experienced a remarkable deformation during the measurement along with the friction between the surfaces of particle gels, the sensor plate and the glass plate which is dominated by the sliding or translational motion, very similar to the case of bulk hydrogel on the solid surfaces. In Fig. 3(b), however, with the addition of surfactant, most of surfactant molecules aggregate to form the micelles, which have a much larger size than that of the opening of the gel network and are adsorbed onto the swollen particle gel surface as discussed in the previous section of this paper. These micelles may act as many small and flexible balls between the surfaces of particle gels, the sensor plate and the glass plate in the similar way of a lubricant. In this manner, the friction behaviors between these surfaces may be dominated by the rolling motion of the micelles. Hence, this will dramatically reduce the frictional coefficient between the surfaces of particle gel, the sensor plate and the glass plate. Consequently,
Rod-like micelles
Stainless steel sensor Particle
Particle gel Particle gel Particle gel
Glass plate A
• • •
Particle gel
• • •
Stainless steel sensor • • •
Particle gel
•• •• ••
Particle gel
• • •
•• •• ••
•• • •
Particle gel
•• • •
Glass plate B
Fig. 3. Schematic illustration of the mechanism for friction reduction between the surfaces of particle gels, stainless steel sensor and glass plate. (A) Without addition of surfactant; (B) With addition of surfactant to NaCl brine for the particle gel swelling.
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Y. Wu et al. / Polymer 52 (2011) 452e460
Fig. 4. Plots of G0 w CPP and G" w CPP for the surfactants investigated at 1000 ppm.
motion resistance of the sensor and the torque exerted on the instrument during the measurement will be dramatically decreased as well. As a result, the dynamic modulus of G0 and G" are decreased comparing with those measured without surfactant added. Based on this above mentioned mechanism, the ability of surfactants to reduce hydrogels dynamic modulus may be related to geometry of the surfactant micelles such as shape and size. For each surfactant, it has a critical packing parameter in different medium. Israelachvili and co-workers [28] have defined this packing parameter, CPP, which is related to the shape of a surfactant molecule.
CPP ¼
n
(5)
l,a0
where, v is the volume of surfactant hydrocarbon core in (nm)3; l is the length of surfactant hydrocarbon chain in nm; a0 is effective area of surfactant hydrophilic group in (nm)2. v, l and a0 can be calculated using the following equations [29]:
n ¼ 0:027 nc þ nmethyl
(6)
l ¼ 0:15 þ 0:127nc
(7)
a0 ¼ 0:016m þ 0:333
(8)
where, nc is the number of carbon atoms of hydrocarbon chain without methyl groups; nmethyl is the number of methyl groups in the hydrocarbon chain; m is the number of ethylene oxide groups. The critical packing parameters for the surfactants investigated in this study have been calculated using Eqs. (5)e(8). The results are listed in Table 6. From the table, it can be found that the packing parameters for all the surfactants investigated are between 0.333 and 0.423. It was known that geometry of surfactant micelle depends upon the value of CPP. If CPP is less than 1/3, the surfactant will form spherical micelles in solution; if CPP is between 1/3 and 1/ 2, it will form rod-like micelles; if CPP is close to 1, it will form the lamellar structure; if CPP is greater than 1, it will form bi-continuous phase; if CPP is much greater than 1, it will form the reversed micelles or reversed rod-like micelles in solution. It is expected that surfactants with a CPP less than 0.333 will be the most efficient agent to reduce the particle hydrogels dynamic modulus because they form spherical micelles in solution. For surfactants with the CPP between 0.333 and 0.500, the smaller the CPP is, the more
Fig. 5. Storage modulus G0 of particle gels with and without surfactant. After surfactant molecules have been washed off, the modulus G0 increases to the value of the particle gel swollen in 1.0 wt.% NaCl.
effective on reduction it will be. The gel strength in terms of G0 and G" in different surfactant solutions are plotted against CPP and shown in Fig. 4. In the figure, it can be found that the particle gel strength decreases with decrease of the surfactant CPP and there is a linear relationship for G0 wCPP and G"wCPP. The linear correlation coefficients for the two plots are 0.9989 and 0.9974, respectively. This indicates that surfactant micelles absorbed on the particle gel surface play a key role to reduce friction of the gel on the surface and the measured dynamic modules G0 and G". The surfactant forming spherical micelles like AlfoterraÒ 23 is the most effective agent to reduce the gel friction. The other surfactants with CPP between 0.333 and 0.500 should have rod-like micelles in the solution and listed in Table 6. For rod-like micelles, it is expected that both rod diameter and rod length should have effect on the hydrogel G0 and G", and the micelles with greater rod diameter and less rod length should be more effective on reducing the G0 and G". But this assumption of relation between micelle size and performance needs more work to get confirmed. In order to confirm our proposed mechanism discussed above, some parallel tests were also conducted for all the surfactants listed in Table 6. In the parallel test, the dry gel particle samples were mixed with surfactant solutions following the exact procedures described previously. After the particles were completely swollen, the swollen PPGs were washed by 1.0 wt% NaCl brine solution to remove all the surfactants with the aid of centrifuge. For the samples prepared in 1.0 wt% NaCl solution, it was also conducted at the similar procedure in order to maintain all the particle gel samples treated under the same conditions of shearing and agitation which is believed to have significant impact on the rheological properties of the gels. The gel strength after removing the surfactants is shown in Fig. 5. Surprisingly, it has been found that the values of G0 for all the particle gels after removing surfactants increase significantly back to the values of the particle gel mixed with 1.0 wt. NaCl solution, which indicates that the friction behaviors between the surfaces of the particle gel, the sensor plate and the glass plate has been switched from rolling motion back to the sliding or translational motion because the surfactant micelles have been removed from the gel particle surface, leading to the recovery of G0 and G". To provide further support of the proposed possible mechanism responsible for the dynamic modulus reduction of the particle
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Fig. 6. Storage modulus G0 of particle gels measured with various surfactants at different concentrations shows a substantial decrease at the concentration of their critical micelle concentration (CMC).
hydrogel by surfactant, the elastic modulus G0 of the synthesized particle hydrogel swollen in the surfactant solution prepared with 1.0 wt.% NaCl at the concentrations below and above their critical micelle concentration (CMC) has been systematically measured. The results are shown in Fig. 6. In the figure, one can see that for the six surfactants used in this study, the G0 shows a substantial decrease at their CMC for each surfactant. AlfoterraÒ 23 with a CMC of 10 ppm, G0 decreases from 2660 Pa to 1600 Pa around the CMC; IgepalÒ CO-530 with a CMC of 20 ppm, G0 decreases from 2580 Pa to 1700 Pa around the CMC; NeodolÒ 25-12 with a CMC of 70 ppm, G0 decreases from 2650 Pa to 1680 Pa around the CMC; Sodium salt of dodecylbenzene sulfonic acid (C12 Benzenesulfonic) with a CMC of 160 ppm, G0 decreases from 2650 Pa to 1880 Pa around the CMC; (1-Hexadecyl)pyridinium bromide (C16 Pyridinium) with a CMC of 181 ppm, G0 decreases from 2600 Pa to 2040 Pa around the CMC; n-Dodecylpyridinium chloride (C12 Pyridinium) with a CMC of 795 ppm, G0 decreases from 2500 Pa to 1200 Pa around the CMC. The very slow decrease of G0 for all surfactants at the concentrations below CMC may be due to the experimental errors. However, the noticeable decrease of G0 at the concentrations above CMC is due to the increase of micelle concentration for the all surfactants investigated. Because as a surfactant solution is continuously increased to a concentration above its CMC, the monomer concentration in the solution will no longer increase while the micelle concentration in the solution will continue increasing with increase of the surfactant concentration. This results in a continuous decrease in the dynamic modulus (G0 ) of the hydrogel. Furthermore, it can also be found that among these surfactants investigated, AlfoterraÒ 23 has the lowest CMC. At 10 ppm, it can effectively reduce elastic modulus because it is the only one that can form spherical aggregates in the solution and most effective on reducing gel surface friction.
459
particles enter and stay in the fracture and large size porous media while the surfactant solutions enter into the small pores in the formation where most hydrocarbon oil is trapped by the capillary force. In this way, the surfactant solution will reduce interfacial tension at oil/brine interface and change wettability of rock surface in the formation. Based on our results of interaction between the particle gel and surfactant, it has been found that surfactants have strong influence on particle gel friction. Therefore, injectivity of the particle gels can be greatly improved by the proper screening of surfactant. In other words, the gel resistance can be modified by the selected surfactants for the best applications. It is also worth mentioning that the results of this investigation of the interaction between the surfactant and particle gels demonstrate a new idea of forced imbibitions through combination of particle gel injection and surfactant imbibition. Development of forced imbibition technology will enable oil producers to increase oil recovery while reduce water production. This study benefits the oil industry with development of a new technology combining gel treatment and surfactant flooding to improve oil/gas recovery while reduce water production. 5. Conclusions Equilibrium concentration of NaCl in excess brine remains the same after the swelling of particle gel. It is also expected that concentration of water-soluble salts, e.g. KCl, MgCl2, CaCl2, Na2CO3, Na2SO4, will not change as well after the swelling of particle gels. This means that gel swelling will not increase the salinity of the underground formation water. Equilibrium surfactant concentration in excess brine increases after swelling of gel particles, which is due to formation of surfactant micelles in the brine. Comparing with the open pore size (4.1 nm) of the synthesized gels, the larger size of the formed micelles could not diffuse into the network of particle gel. Surfactants with larger size of molecules and lower critical micelle concentration (CMC) show greater increase in equilibrium. It is expected that nonionic surfactants will show greater increase. Gel friction in terms of dynamic modulus G0 and G" can be reduced dramatically which might be due to the fact that the surfactant micelles adsorbed on the surface of particle gel change the friction between the surfaces of particle gel and solid from sliding/translational motion to the rolling motion. The latter has a much smaller friction coefficient. However, the gel resistance can be recovered after the surfactants have been removed indicating the physical absorption of the formed micelles over the surfaces of the swollen PPGs. Injectivity of particle gels can be significantly improved by use of proper surfactants. Moreover, a new technology of forced surfactant imbibition can be developed by combination of the PPGs and surfactant. The new technology will greatly benefit to oil industry by the way to improve oil recovery while reduce water production. Acknowledgements
4. Potential applications Gel treatment has been proved to be an effective method to reduce water production in oil industry. A new trend in gel treatments is application of preformed particle gels (PPG) that are formed at surface facilities before injection. In order to improve the gel treatment efficacy, gel particles are placed in the surfactant brine solution. When the particle gel and surfactant solution are injected into the underground reservoir, the filtrated solution can be squeezed into the matrix during the injection. As a result, the gel
The authors are grateful for financial support from Research Partnership to Secure Energy for America (RPSEA) and PetroChina Research Institute of Petroleum Exploration & Development (RIPED). Funding for this project provided by RPSEA is through the “Ultra-Deepwater and Unconventional Natural Gas and Other Petroleum Resources” program authorized by the U.S. Energy Policy Act of 2005. RPSEA (www.rpsea.org) is a nonprofit corporation whose mission is to provide a stewardship role in ensuring the focused research, development and deployment of safe and
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environmentally responsible technology that can effectively deliver hydrocarbons from domestic resources to the citizens of the United States. RPSEA, operating as a consortium of premier U.S. energy research universities, industry, and independent research organizations, manages the program under a contract with the U.S. Department of Energy’s National Energy Technology Laboratory. References [1] Kokufuta E, Suzuki H, Yoshida R, Yamada K, Hirata M, Kaneko F. Langmuir 1998;14(4):788. [2] Starodoubtsev SG, Churochkina NA, Khokhlov AR. Langmuir 2000;16(4):1529. [3] Shinde VS, Badiger MV, Lele AK, Mashelkar RA. Langmuir 2001;17(9):2585. [4] Lynch I, Sjostrom J, Piculell L. J Phys Chem B 2005;109(9):4258. [5] Caykara T, Demiray M, Gueven O. Colloid Polym Sci 2005;284(3):258. [6] Chen L, Yu X, Li Q. J App Polym Sci 2006;102(4):3791. [7] Mohan YM, Joseph DK, Geckeler KE. J App Polym Sci 2007;103(5):3423. [8] Mangiapia G, Ricciardi R, Auriemma F, De Rosa C, Lo Celso F, Triolo R, et al. J Phys Chem B 2007;111(9):2166. [9] Yamazaki Y, Matsunaga T, Ichinokawa A, Fujishiro Y, Saito E, Sato T. J App Polym Sci 2009;114(5):2764. [10] Philippova OE, Hourdet D, Audebert R, Khokhlov AR. Macromolecules 1996;29 (8):2822. [11] Ashbaugh HS, Piculell L, Lindman B. Langmuir 2000;16(6):2529.
[12] Nichifor M, Zhu XX, Cristea D, Carpov A. J Phys Chem B 2001;105(12):2314. [13] Barreiro-Iglesias R, Alvarez-Lorenzo C, Concheiro A. J Controlled Release 2001;77:59. [14] Osada Y, Gong J-P. Adv Mater 1998;10:827. [15] Gong J-P, Osada Y. Prog Polym Sci 2002;27:3. [16] Du M, Gong JP. In: Biresaw G, Mittal KL, editors. Surface friction and lubrication of polymer gels. Boca Raton, Florida: CRC Press; May 2008. p. 223e46. ch. 11. [17] Tominaga T, Takedomi N, Biederman H, Furukawa H, Osada Y, Gong J-P. Soft Matter 2008;4:1033. [18] Oogaki S, Kagata G, Kurokawa T, Kuroda S, Osada Y, Gong J-P. Soft Matter 2009;5:1879. [19] Tominaga T, Takayuki T, Furukawa H, Osada Y, Gong J-P. Soft Matter 2008;4:1645. [20] Du M, Maki Y, Tominaga T, Furukawa H, Gong J-P, Osada Y, et al. Macromolecules 2007;40:4313. [21] Narita T, Gong JP, Osada Y. J Phys Chem B 1998;102:4566. [22] Kagata G, Gong JP, Osada Y. J Phys Chem B 2003;107:10221. [23] Kagata G, Gong JP, Osada Y. J Phys Chem B 2002;106:4596. [24] Chen YM, Matsumoto S, Gong JP, Osada Y. Macromolecules 2003;36:8830. [25] Wang J, Gonzalez AD, Ugaz VM. Adv Mater 2008;20:4482. [26] Rosen MJ. Surfactants and interfacial phenomena. 3rd ed. Wiley-Interscience; 2004. [27] Robson RJ, Dennis EA. J Phys Chem 1997;81(11):1075. [28] Israelachvili J, Mitchell DJ, Ninham BW. J Chem Soc. Faraday Trans 1976;72:1525. [29] Chevalier Y, Zemb T. Rep Prog Phys 1990;53:279.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
An experimental study of interaction between surfactant and particle hydrogels Yongfu Wu a, Tingji Tang a, Baojun Bai a, *, Xiaofen Tang b, Jialu Wang b, Yuzhang Liu b a b
Department of Geological Science and Engineering, Missouri University of Science and Technology, 129 McNutt Hall, 1400N Bishop Avenue, Rolla, MO 65409, USA Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2010 Received in revised form 29 November 2010 Accepted 1 December 2010 Available online 9 December 2010
Polyacrylamide gel was synthesized to study interaction between surfactant and particle hydrogel. Surfactants used in this study include cationic surfactants, n-dodecylpyridinium chloride, (1-hexadecyl) pyridinium bromide; anionic surfactants, sodium salt of dodecylbenzene sulfonic acid, sodium 4-n-octyl benzene sulfonate and sodium branched alcohol propoxylate sulfate; and nonionic surfactants, IgepalÒ CO-530, TergitolÒ NP-10 and NeodolÒ 25-12. It has been found that, after swelling of the dry particles, surfactant concentration shows a substantial increase. Meanwhile, dynamic modulus (G0 and G") of the gels shows a significant decrease in the surfactant solutions. Based on the experimental results, a mechanism has been proposed to elucidate the reduction of the gel dynamic modulus. Furthermore, this mechanism was discussed through surfactant critical packing parameters (CPP) at the interface of the hydrogel particle and surfactant aqueous solution, and confirmed by recovery of the gel dynamic modulus after removal of the surfactant from the hydrogels. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Hydrogel Surfactant Dynamic modulus
1. Introduction Interaction between surfactant and polymer hydrogel has been a subject of considerable theoretical and practical interest, and has been extensively studied [1e9]. Philippova and co-workers studied interaction of gels with ionic surfactants n-cetylpyridinium chloride and sodium dodecylbenzenesulfonate. They reported that absorption of anionic surfactant is governed primarily by hydrophobic interactions. Due to conditions of electro-neutrality, anionic surfactant penetrates the gel together with corresponding co-ions. Therefore, the uptake of cationic surfactant ions results in gel shrinkage, while the uptake of anionic surfactant induces gel swelling. In the anionic gel/anionic surfactant system, a significant interaction is observed only for the most hydrophobic gels when hydrophobic interactions overcome the electrostatic repulsion between similarly charged groups [10]. Ashbaugh and co-workers studied interactions of mixed dodecyl trimethylammonium bromide (C12TAB) and octaethylene glycol monododecyl ether (C12E8) micelles with a lightly cross-linked Na polyacrylate gel. They found that gel preferentially absorbs C12TAþ under most conditions owing to electrostatic attraction to the oppositely charged gel. For low initial C12E8 surfactant fractions and moderate C12TAþ concentrations, however, the situation is reversed and the nonionic surfactant is preferentially absorbed [11]. Nichifor and coworkers studied interaction of hydrophobically modified cationic * Corresponding author. Tel.: þ1 573 341 4016; fax: þ1 573 341 6935. E-mail address: [email protected] (B. Bai). 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.12.003
dextran hydrogels with biological surfactants. They found that an increase in the length of the alkyl substituent of the hydrogel strongly increases the binding constants K0 and K, but decreases the cooperativity parameter m. This was explained by the formation of mixed micelles between pendant groups of the gel and surfactant molecules [12]. It is well recognized that surfactants play a critical roles in tribology [13]. One of the results caused by addition of surfactants to hydrogel particles is the influence on the frictional behavior of gels. Friction of hydrogels on solid surface or on gels has been extensively studied during the last decade, especially in Gong’s group, and considerable number of papers have been published [14e24]. It has been found that the frictional behaviors of the hydrogels do not conform to Amonton’s law F¼mW, which well describes the friction of solids [15]. Based on the complex friction behavior of polymer gels, Gong and co-workers proposed a repulsioneadsorption mechanism to describe the friction of hydrogels on a smooth substrate. If the interfacial interaction between hydrogel and solid surface is repulsive, then friction is due to lubrication of the hydrated water layer of the polymer network at the interface. In this case, friction is proportional to sliding velocity. If the interaction is attractive, then friction is from two contributions: (1) elastic deformation of the adsorbed polymer chain; (2) lubrication of the hydrated layer of the polymer network [16]. They reported that the friction behavior of hydrogels sliding on smooth substrates strongly depends on the adhesive strength and hydrophobicity of the substrate [17]. Du and co-workers studied friction behaviors of PVA gel sliding against a glass surface in dilute poly
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453
Table 1 Surfactant Molecular Structure and Gel Swelling Ratio in Surfactant Solutions. Surfactant
Molecular structure
Swelling ratio
+
H3C (CH2) 10 CH2 N
n-Dodecylpyridinium chloride
Cl
+
H3C (CH2) 14 CH2 N
(1-Hexadecyl)pyridinium bromide
Br
H3C (CH2)2 Alfoterra 23 Sodium branched alcohol propoxylate sulfate
22.3e23.3
CH3 + CH2 N CnH2n+1 Cl CH3 (n=8 ~18)
Benzalkonium chloride
Ò
22.2e23.2
H3C (CH2)8~10 CH2 CH CH2 O
22.1e23.1
O
CH3
+ CH2 CH O 3 S O Na
22.3e23.3
O
O Sodium 4-n-octyl benzene sulfonate
H3C (CH2) 6 CH 2
+ S O Na
22.8e23.8
O
O Sodium salt, dodecylbenzene sulfonic acid
+
H 3C (CH2) 10 CH 2
Igepal CO-530 Nonylphenoxypoly(ethyleneoxy) alcohol
H3C C
CH2 C CH2
Tergitol NP-10 Nonylphenol ethoxylated alcohol
NeodolÒ25e12 Linear primary alcohol ethoxylate
H 3C C
(OCH2CH2)6 OH
CH3
CH3
CH3 Ò
22.1e23.1
CH3
CH2 C CH2
CH3
(OCH2CH2)10 OH
CH3
H3C (CH2) 12 CH2 (OCH2CH2)12 OH
In distilled water In 1.00 wt.% NaCl brine
(ethylene oxide) (PEO) aqueous solution, and they found that PEO concentration has a significant influence on the friction of a waterswollen PVA gel on glass surface [20]. Osada and co-workers studied kinetics of surfactant binding into polymer gel and effect of hydrophobic side chain on poly(carboxyl acid) dissociation. They reported that the driving force of surfactant diffusion into the gel is the concentration gradient of the surfactant. The binding of surfactant with the polymer network sustains a high concentration gradient that facilitates the subsequent surfactant diffusion [21]. They also analyzed the structure of poly(ADA-co-AA) polymer and surfactant-poly(ADA-co-AA) complexes by SAXD and WAXD and found that the complexes change their structures from micelle-like to lamellar-like with an increase of mole fraction of acryloyl dodecanoic acid (ADA) in the complexes [24]. To date, interaction between surfactant and particle hydrogel has not been studied systematically. In this paper, we study the
22.6e23.6
O
CH3
CH3 Ò
S O Na
22.0e23.0
22.6e23.6 22.8e23.8 22.4e23.4
influence of surfactant in aqueous solution on dynamic modulus of water-swollen gel in 1.0 wt% NaCl. Surfactants used in this study include nonionic, anionic and cationic surfactants. Gel frictions were measured in terms of storage modulus G0 and loss modulus G" under the same conditions of stress, gap, oscillation frequency and temperature for all surfactants. Gel used in this study was synthesized from acrylamide monomer with ethylene-bis-acrylamide cross-linker. 2. Experimental 2.1. Materials Monomer acrylamide (98.5%) and cross-linker methylene-bisacrylamide (97þ%) were purchased from Alfa Aesar Company (Ward Hill, MA) and used without further purification. Ammonium
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2.3. Measurement of swelling ratio and pore size of PPGs
Fig. 1. DSC curve of the swollen PPGs in the distilled water.
persulfate was used as initiator for polymer gel synthesis. Cationic surfactants, n-dodecylpyridinium chloride (98%), (1-hexadecyl) pyridinium bromide monohydrate (98%), benzalkonium chloride and anionic surfactant, sodium 4-n-octyl benzene sulfonate were also purchased from Alfa Aesar (Ward Hill, MA) and used without further purification. Sodium dodecylbenzene sulfonic acid was purchased from SigmaeAldrich (St. Louis, MO) and used without further purification. Other commercial surfactants were requested from their manufacturers, IgepalÒ CO-530 from Rhodia, Inc. (Bristol, PA), TergitolÒ NP-10 from Dow Chemical (Midland, MI), AlfoterraÒ 23 from Sasol North America Inc. (Houston, TX), NeodolÒ 25-12 from Shell Chemical Company (Houston, TX), and used without further purification. NaCl (99.8%) was purchased from Fisher Scientific Inc. Water used in all experiments was distilled water produced in this lab. Molecular structures of the surfactants are listed in Table 1.
0.30 g of PPGs (120e150 mm size) was swollen in 14.70 g of distilled water contained in a clean test tube. After the resulting suspension was left overnight to reach the equilibrium, the test tube was centrifuged at 5 000 rpm for 15 min to separate the particle gel and distilled water. Volume of the swollen PPG was calculated by measuring the height of the PPG in the tube and diameter of the tube. The same procedures were used for the swelling ratio measurement in 200 ppm surfactant solutions prepared in 1.00 wt.% NaCl solution. The swelling ratio is calculated by the total volume of the swollen particle gel measured in the test tube over the volume of 0.300 g of the dry particles and the results are listed in Table 1. Pore size of the synthesized hydrogel was measured by a TA Instrument DSC 2920 equipped with low temperature cooling accessory (60 C). Swollen hydrogel sample (10e20 mg) was sealed in an aluminum DSC pan with an added excess of bulk water (1e2 mg). During experiment, the temperature was first decreased to 40 C, and then held for 15 min for freezing, after which the sample was heated to 15 C at a rate of 1 C min1 using an empty pan as reference. The DSC spectrum is shown in Fig. 1. The pore size is related to the temperature shift (DT) of the two peaks originated from bulk and confined water, respectively and it can be calculated based on the following equation, where DT is a negative value and quantified as the difference between the temperature at the confined water peak and the onset of the bulk water peak [25].
Dp ¼
39:604
DTðKÞ þ 0:1207
þ 2:24
(1)
where, DT is temperature shift in K; Dp is pore size of the swollen hydrogel particle in nm. 2.4. Measurement of the NaCl concentration change upon gel swelling
2.2. Synthesis and fabrication of preformed particle gels (PPGs) 150 g of acrylamide were added to 498.7 g of distilled water. The solution was purged with nitrogen gas for 60 min and stirred at 270 RPM until all the solid dissolved. 0.650 g of methylene-bis-acrylamide was added to the solution and stirred until completely dissolved. 0.650 g of (NH4)2S2O8 was then added with stirring to the solution prepared above. The mixture solution was placed in an oven at 60 C for 14 h. A strong bulk gel was formed and it was cut into small pieces. The formed hydrogel was then purified by soaking in large amount of distilled water for one week and followed by drying at 60 C for 4 days to yield 168.390 g of a slightly yellow gel. The yellow color may be due to oxidation of acrylamide. The dried gel solids were crushed into small particle powder in a blender machine (Black & Decker). PPGs with the particle size between 100 and 120 mesh (150 mme120 mm) were selected through the standard testing sieves (Fisher Scientific Company).
All the tested surfactant solutions were made in 1.0 wt % NaCl solution which is dependent on the salinity of the reservoir formation water and concentration change of the NaCl solution after complete swelling was measured by the anion metathesis of NaCl with AgNO3. By measuring the weight of precipitation of AgCl, the concentration NaCl can be calculated. The data and results are listed in Table 2. 2.5. Measurement of the concentration change of surfactants after gel swelling Initial concentration for all surfactants was prepared at relatively low concentration of 200.00 ppm for higher accuracy from the UVeVis spectroscopy measurement. Dry PPGs at 2.00 wt% were swollen in different surfactant solutions. This equilibrium solution over the top of the swollen gels contains NaCl, surfactant and un-
Table 2 Analysis Results of NaCl Concentration after Particle Gel Swelling. Parallel Tests
For 1.00 wt.% NaCl solution (g)
Empty Test Tube NaCl Solution NaCl Contained Total weight after drying Mass of AgCl (MW: 143.5) Mass of NaCl (MW: 58.5) Test Results
13.9754 13.9021 5.0100 5.0081 0.0501 To be measured 14.0986 14.0259 0.1232 0.1238 0.0502 0.0505 Equili. NaCl concentration after gel swelling is 1.01 wt.%, which is almost the same as the initial NaCl concentration.
For Equilibrium NaCl solution after swelling (g)
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Table 3 Measurement of Concentration Change of Surfactant after Gel Swelling. Surfactant Initial Concentration: 200 ppm Surfactant n-Dodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide Benzalkonium chloride Sodium 4-n-octyl benzene sulfonate Sodium salt, dodecylbenzene sulfonic acid IgepalÒ CO-530 TergitolÒ NP-10
At Equilibrium of Swelling
lmax (nm)
Initial ABS 2.451 2.106 0.179 0.232 0.227 0.806 0.429
257 259 261 260 260 276 275
crosslinked polymer dissolved from the particle gel and the absorbance of this solution was used to calculate the equilibrium surfactant concentration after swelling. The background absorbance of the excess solution from the PPGs in 1.00 wt% NaCl solution is measured as the background absorbance, which will be deducted from the above measured total UV absorbance. For each surfactant solution, the UVeVis spectrum was scanned from 1100 nm to 190 nm to identify the absorption peak position (lmax). The equilibrium concentrations for each surfactant were calculated through its absorbance at lmax by use of a UVeVis spectrophotometer (UVmini-1240V, Shimadzu). Concentration changes for the surfactants investigated are listed in Table 3. 2.6. Measurement of particle gel dynamic modulus To investigate influence of surfactant on the particle gel strength, a rheometer, HAAKE RheoScope 1(Thermo Scientific) was employed to measure storage modulus G0 and loss modulus G" for the swollen gels. Measurement was set oscillation mode with frequency f ¼ 1.000 Hz, controlled stress (CS) and the stress applied to the gel was set s ¼ 1.0 Pa to ensure that the gel’s strain and stress have a linear relationship during the measurement. The sensor used for measurement is PP35 Ti Po LO2 016 with a gap of 0.200 mm; Temperature: 25.0 C. For each sample, measurements of G0 and G" were taken every 30 s for 5 min. The results are shown in Table 4 and Fig. 2. 3. Results and discussion 3.1. Particle gel swelling ratio The swelling ratios in different surfactant solutions at 200.00 ppm concentration as well as in 1% NaCl brine and distilled water are listed in Table 1. It can be clearly seen from Table 1 that the swelling ratios in surfactant solutions, distilled water and 1.0 wt.% NaCl are between 22 and 24. The difference in the swelling ratios is in the range of experimental error and thus the swelling ratio is independent on the swelling media. This implies the neutral charge balance of the synthesized polyacrylamide gels. Therefore, the pore size and distributions of the synthesized polyacrylamide gels do not change when the surfactants and NaCl were added. Also,
Net ABS 2.326 2.432 0.187 0.247 0.262 1.137 0.608
Ceq. (ppm) 190 231 209 213 231 282 283
Conc. Change 5.0% 15.5% 4.5% 6.5% 15.5% 41.0% 41.5%
DSC was used to measure the pore size of the swollen PPGs as shown in Fig. 1. Based on the Eq. (1), it can be derived that the average pore size of the PPGs is around 4.1 nm, which is much less than the size of micelles formed from our tested surfactants [26]. 3.2. Concentration change of NaCl after equilibrium of swelling Accuracy of the experimental results was evaluated by analysis results of a standard NaCl aqueous solution. The expected NaCl for the analyzed solution is 0.0501 g. The mass of NaCl calculated through the precipitation of AgCl after anion metathesis is 0.0502 g, leading to a relative experimental error of 0.2%. When PPGs were swollen in the brine solution, the equilibrium concentration of NaCl was found to be at 1.01 wt % changing from 1.00 wt %. The difference of 0.01 wt.% is attributed to the experimental error. The analysis results are listed in Table 2, from which it can be concluded that the NaCl concentration in the excess brine is the same as the initial concentrations. After swelling equilibrium, particle gel doesn’t change the concentration of NaCl in the excess solution since the sizes of sodium and chloride ions are much smaller than the pore sizes of the swollen PPGs. 3.3. Concentration change of surfactants after equilibrium swelling of PPGs The initial concentration for all surfactants used in this experiment is 200.00 ppm. After PPGs were completely swollen, equilibrium concentration of surfactant in the excess solution was measured by the UV absorbance. The equilibrium concentrations for different surfactant solutions at PPG swelling equilibrium are listed in Table 3. In Table 3, it was found that the concentration of the most of the surfactant solutions increased after the swelling equilibrium of PPGs. For cationic surfactant, (1-hexadecyl) pyridinium bromide, the concentration was changed to 231 ppm from 200 ppm corresponding to an increase of 15.5% from its initial concentration. However, for another cationic surfactant of n-dodecylpyridinium chloride and non-surfactant chemical of benzalkonium chloride, their concentrations were changed as 5% and 4.5%, respectively, which were considered in the experimental error of 5% originated from the UV absorbance measurement and thus the surfactant
Table 4 Results of Storage Modulus G’ and Loss Modulus G" for Gel Particles in Surfactant Solution (1000 ppm) and in 1.0 wt.% NaCl Brine. Surfactant
Conc. in 1.0% NaCl
G’ (Pa)
Change
G" (Pa)
Change
In 1.0 wt.% NaCl AlfoterraÒ 23 Sodium salt, dodecylbenzene sulfonic acid neDodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide IgepalÒ CO-530 (HLB ¼ 10.8) NeodolÒ 25e12 (HLB ¼ 14.4)
brine 1.93 2.87 3.52 2.45 2.16 1.35
2582 689 1042 1133 1739 824 946
0% 73% 60% 56% 33% 68% 63%
112 45 64 66 87 51 56
0% 59% 43% 41% 22% 54% 50%
103 103 103 103 103 103
M M M M M M
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Fig. 2. Results of G0 and G" for particle gel in surfactant Alfoterra 23 solution (1000 ppm) prepared with 1.0wt.% NaCl brine and in 1.0wt.% NaCl brine only (blank test).
molecules (not forming micelles, see Table 5) can migrate freely through the swollen PPG porous structure. For the two anionic surfactants tested, sodium 4-n-octyl benzene sulfonate and sodium dodecylbenzene sulfonic acid, the latter shows an obvious increase of its equilibrium concentration after gel swelling, but sodium 4-noctyl benzene sulfonate shows little change in the final equilibrium concentration due to its higher CMC than 200 ppm (see Table 5). For the commercial nonionic surfactants of IgepalÒ CO-530 and TergitolÒ NP-10, their equilibrium concentrations increased to 282 and 283 ppm, respectively, from the initial concentration of 200 ppm, corresponding to the change of 41% and 41.5%. The reason that contributes to the dramatic increase of surfactant concentration after gel particle swelling is the formation of surfactant micelles in the solution, which has a much larger size than that of opening of the gel network. Average area per surfactant molecule adsorbed at the air/liquid interface is 60 Å2 [26], therefore, one surfactant molecule has a dimension of 9 Å in diameter of hydrophilic head and about 20 Å in length of a hydrophobic tail. This size is much smaller than the average pore size of the hydrogel, 4.1 nm or 41 Å. On the other hand, approximate size for a rod-like surfactant micelle is about 54 Å in the rod diameter and 140 Å in the rod length [27], which is much larger than the average pore size of the hydrogel. When the dry PPGs contact with the aqueous surfactant solution, the particles absorb water first, other molecules and ions will diffuse into the network structure because of the concentration gradient and their much smaller size. However, surfactant micelles cannot go through the network due to their much larger size. Only unassociated single surfactant molecules can go through the opening and diffuse into the network of the swollen gel, which forms the dynamic equilibrium with the formed micelles
absorbed outside the swollen gel network. After the swelling reaches equilibrium, more water has been absorbed by the gel and less surfactant molecules can get into gel network. As a result, the concentration of surfactants remained in the excess solution increases. Therefore, it is expected that the lower the surfactant critical micelle concentration (CMC) is, the greater the increase of concentration in the excess solution will be after swelling since the initial concentration of surfactants was all set at 200 ppm. For a surfactant, if its CMC is high or its initial concentration is about or lower than CMC, the concentration will change little after the gel swelling reaches equilibrium such as n-dodecylpyridinium chloride and sodium 4-n-octyl benezene sulfonate from Table 5. From Table 5, it can be found that the change of surfactant concentration is related to the ratio of the initial concentration (Cinit) to surfactant CMC. If the ratio is less than 1.0, that means the surfactant initial concentration is lower than its CMC, the concentration change is little and in the range of experimental error. If the ratio is greater than 1.0, the equilibrium concentration increased dramatically. For example, this ratio for nonionic surfactants, IgepalÒ CO-530 and TergitolÒ NP-10, is greater than 10, their equilibrium concentration increased from their initial concentration by 41.0% and 41.5%, respectively. This suggests that most of their molecules exist in the solution in the form of micelles for these nonionic surfactants with very low CMC. The micelles cannot go through network of the swollen particle gel due to their bigger size than the average pore size of the PPGs, resulting in the dramatic increase of their equilibrium concentration since PPGs absorbs a large amount of water during the swelling.
3.4. Influence of surfactant on friction of the particle gel surface To study influence of surfactant on the particle gel viscoelasticity, the dry PPGs at 2 wt.% were mixed with 1000 ppm surfactant solutions prepared with 1.0 wt.% NaCl brine in a centrifuge tube, and the PPGs and surfactant solution mixture was shaken well to ensure the particles get completely swollen. The reason to prepare surfactant solution at 1000 ppm that is much higher than that in previous study is to ensure all surfactants investigated in this study get aggregated in the solution. The gel viscoelasticity was measured by the HAAKE RheoScope as described before. During the process of measurement, both G0 and G" show a slow and steady increase because of the evaporation of water contained in gel sample, data taken at the very beginning of measurement are used as G0 and G" results. A blank test of the gel strength in 1.0 wt.% NaCl brine without surfactant was also conducted for comparison. Measurement results for the particle gel strength (both G0 and G") in 1000 ppm and 1.0 wt % NaCl brine solution are listed in Table 4. A typical measurement of the gel strength as a function of scan time was illustrated in Fig. 2 for PPGs in 1.0 wt% NaCl and 1.0 wt % NaCl/ 1000 ppm AlfoterraÒ 23, respectively. It can be clearly seen from Table 4 and Fig. 2 that the introduction of AlfoterraÒ 23 into the swelling media significantly decreased the swollen gel strength. For
Table 5 Surfactant CMC, ratio of Cinit./CMC and Concentration Change after Gel Swelling. Surfactant
M.W.
CMC* in 1.0% NaCl
Cinit.(200 ppm)
n-Dodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide Benzalkonium chloride Sodium 4-n-octyl benzene sulfonate Sodium salt, dodecylbenzene sulfonic acid IgepalÒ CO-530 TergitolÒ NP-10
284 402 w410 292 348 464 660
2.8 103 M 4.5 104 M Not a surfactant 1.3 103 M 4.6 104 M 4.1 105 M 2.8 105 M
7.0 5.0 4.9 6.8 5.7 4.3 3.0
* Rosen, M. J.: “Surfactants and Interfacial Phenomena”, Wiley-Interscience, 3rd Ed., 2004, 185e188.
104 104 104 104 104 104 104
M M M M M M M
Cinit./CMC
Conc. Change
0.3 1.1 n/a 0.5 1.2 10.5 10.7
5.0% 15.5% 4.5% 6.5% 15.5% 41.0% 41.5%
Y. Wu et al. / Polymer 52 (2011) 452e460
example, in the blank test, G0 and G" for the particle gel are 2582 Pa and 112 Pa, respectively. However, when AlfoterraÒ 23 was used in the swelling solution, G0 and G" dramatically decrease to 689 Pa and 45 Pa, corresponding to the significant reduction of 73% and 59%, respectively. From the data listed in Table 4, it can also be observed that all the tested surfactants have strong influence on viscoelasticity of the particle gel. Addition of surfactant to NaCl brine used for gel swelling can substantially reduce the gel dynamic modulus G0 and G". Another anionic surfactant of sodium dodecylbenzene sulfonic acid reduced the gel’s storage modulus G0 by 60%, along with the reduction of the loss modulus G" by 43%. For the cationic surfactants such as n-dodecylpyridinium chloride and (1-hexadecyl) pyridinium bromide, they can decrease the gel’s storage modulus by 56% and 33%, respectively, and loss modulus by 41% and 22%, respectively. For nonionic surfactants such as IgepalÒ CO530 and NeodolÒ 25-12, their presence in the swelling solution reduced the gel’s storage modulus by 68% and 63%, respectively, along with the loss modulus decrease by 54% and 50%, respectively. In order to further elucidate the reason of the dramatic decrease of the dynamic modulus (G0 and G") of the particle gel by the introduced surfactant in the solution, one has to understand that the measurement of the dynamic modulus (G0 and G") by a plate rheometer is accomplished by measuring the torque exerted on the tested and swollen PPGs. The swollen particle gel sample is placed on the horizontal glass plate and another metal sensor plate is placed on top of the gel samples. Typically the top sensor plate is rotated and the torque exerted on it is measured. However, the movement of this plate is resisted by the frictional force, which is proportional to the frictional coefficient and the stress applied on it. Equations (2), (3) and (4) below describe quantitatively the relationship for torque (T), stress (N) and frictional coefficient (m).
T ¼ rF
(2)
F ¼ m,N
(3)
T ¼ r F ¼ r ðm,NÞ
(4)
where, T is the torque exerted on the sample; r is typically the length of the lever arm and here it is related to geometry of the rheometer sensor. For a given sensor, it is the same for all gel samples measured; F is the frictional force, it is the product of frictional coefficient (m) between two surfaces and the force applied on the surfaces (N). In this experiment, a model of controlled stress is employed for the measurement. Therefore, the torque exerted on the sample is directly proportional to the frictional force between the particle gel and the surfaces of the plates. Furthermore, for a given cap of 0.200 mm between the plate and sensor, measurement results of the dynamic modulus (G0 and G") are directly
457
Table 6 Critical Packing Parameters (CPP) of the Surfactants Investigated. Surfactant
l a0(nm)2 CPP v (nm)3 (nm)
AlfoterraÒ 23 Sodium salt, dodecylbenzene sulfonic acid neDodecylpyridinium chloride (1-Hexadecyl)pyridinium bromide IgepalÒ CO-530 NeodolÒ 2512
0.538 2.690 0.600 0.486 2.245 0.600
0.333 Spherical 0.361 Rod-like
0.324 0.532 0.405 0.378
0.369 0.423 0.344 0.353
1.505 2.155 1.420 1.801
0.584 0.584 0.829 0.595
Micelle structure
Rod-like Rod-like Rod-like Rod-like
proportional to the frictional force coefficient between the particle gel and the plate surfaces. Surface friction of polymer bulk gels on solid surfaces has been intensively investigated through the use of commercial tribometer or rheometer. It has been reported by Gong and co-worker that the friction behaviors of the bulk hydrogels on glass or other slid surfaces do not conform to Amonton’s law as shown in Eq. (3) which describes the friction of solid materials [16]. However, to our best knowledge, the friction investigation between the PPGs and surfactants has not been reported. Therefore, we proposed a simple mechanism, shown in Fig. 3, as a qualitative discussion of friction reduction by surfactant between the surfaces of particle gels, stainless steel sensor plate and glass plate. As shown in Fig. 3 (a) without addition of surfactant to NaCl brine for the particle gel swelling, the stainless steel sensor plate presses the particle gel and rotates on it at a constant stress mode when the dynamic modulus is measured. The original dried particle size ranges between 0.125 and 0.150 mm. Based on the volume swelling ratio of 23, the swollen particle size is between 0.355 and 0.427 mm in diameter. The gap between the sensor plate and bottom glass plate for G0 and G" measurement is at 0.200 mm, which is only about half of the particle size. Therefore, the swollen gel particles experienced a remarkable deformation during the measurement along with the friction between the surfaces of particle gels, the sensor plate and the glass plate which is dominated by the sliding or translational motion, very similar to the case of bulk hydrogel on the solid surfaces. In Fig. 3(b), however, with the addition of surfactant, most of surfactant molecules aggregate to form the micelles, which have a much larger size than that of the opening of the gel network and are adsorbed onto the swollen particle gel surface as discussed in the previous section of this paper. These micelles may act as many small and flexible balls between the surfaces of particle gels, the sensor plate and the glass plate in the similar way of a lubricant. In this manner, the friction behaviors between these surfaces may be dominated by the rolling motion of the micelles. Hence, this will dramatically reduce the frictional coefficient between the surfaces of particle gel, the sensor plate and the glass plate. Consequently,
Rod-like micelles
Stainless steel sensor Particle
Particle gel Particle gel Particle gel
Glass plate A
• • •
Particle gel
• • •
Stainless steel sensor • • •
Particle gel
•• •• ••
Particle gel
• • •
•• •• ••
•• • •
Particle gel
•• • •
Glass plate B
Fig. 3. Schematic illustration of the mechanism for friction reduction between the surfaces of particle gels, stainless steel sensor and glass plate. (A) Without addition of surfactant; (B) With addition of surfactant to NaCl brine for the particle gel swelling.
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Fig. 4. Plots of G0 w CPP and G" w CPP for the surfactants investigated at 1000 ppm.
motion resistance of the sensor and the torque exerted on the instrument during the measurement will be dramatically decreased as well. As a result, the dynamic modulus of G0 and G" are decreased comparing with those measured without surfactant added. Based on this above mentioned mechanism, the ability of surfactants to reduce hydrogels dynamic modulus may be related to geometry of the surfactant micelles such as shape and size. For each surfactant, it has a critical packing parameter in different medium. Israelachvili and co-workers [28] have defined this packing parameter, CPP, which is related to the shape of a surfactant molecule.
CPP ¼
n
(5)
l,a0
where, v is the volume of surfactant hydrocarbon core in (nm)3; l is the length of surfactant hydrocarbon chain in nm; a0 is effective area of surfactant hydrophilic group in (nm)2. v, l and a0 can be calculated using the following equations [29]:
n ¼ 0:027 nc þ nmethyl
(6)
l ¼ 0:15 þ 0:127nc
(7)
a0 ¼ 0:016m þ 0:333
(8)
where, nc is the number of carbon atoms of hydrocarbon chain without methyl groups; nmethyl is the number of methyl groups in the hydrocarbon chain; m is the number of ethylene oxide groups. The critical packing parameters for the surfactants investigated in this study have been calculated using Eqs. (5)e(8). The results are listed in Table 6. From the table, it can be found that the packing parameters for all the surfactants investigated are between 0.333 and 0.423. It was known that geometry of surfactant micelle depends upon the value of CPP. If CPP is less than 1/3, the surfactant will form spherical micelles in solution; if CPP is between 1/3 and 1/ 2, it will form rod-like micelles; if CPP is close to 1, it will form the lamellar structure; if CPP is greater than 1, it will form bi-continuous phase; if CPP is much greater than 1, it will form the reversed micelles or reversed rod-like micelles in solution. It is expected that surfactants with a CPP less than 0.333 will be the most efficient agent to reduce the particle hydrogels dynamic modulus because they form spherical micelles in solution. For surfactants with the CPP between 0.333 and 0.500, the smaller the CPP is, the more
Fig. 5. Storage modulus G0 of particle gels with and without surfactant. After surfactant molecules have been washed off, the modulus G0 increases to the value of the particle gel swollen in 1.0 wt.% NaCl.
effective on reduction it will be. The gel strength in terms of G0 and G" in different surfactant solutions are plotted against CPP and shown in Fig. 4. In the figure, it can be found that the particle gel strength decreases with decrease of the surfactant CPP and there is a linear relationship for G0 wCPP and G"wCPP. The linear correlation coefficients for the two plots are 0.9989 and 0.9974, respectively. This indicates that surfactant micelles absorbed on the particle gel surface play a key role to reduce friction of the gel on the surface and the measured dynamic modules G0 and G". The surfactant forming spherical micelles like AlfoterraÒ 23 is the most effective agent to reduce the gel friction. The other surfactants with CPP between 0.333 and 0.500 should have rod-like micelles in the solution and listed in Table 6. For rod-like micelles, it is expected that both rod diameter and rod length should have effect on the hydrogel G0 and G", and the micelles with greater rod diameter and less rod length should be more effective on reducing the G0 and G". But this assumption of relation between micelle size and performance needs more work to get confirmed. In order to confirm our proposed mechanism discussed above, some parallel tests were also conducted for all the surfactants listed in Table 6. In the parallel test, the dry gel particle samples were mixed with surfactant solutions following the exact procedures described previously. After the particles were completely swollen, the swollen PPGs were washed by 1.0 wt% NaCl brine solution to remove all the surfactants with the aid of centrifuge. For the samples prepared in 1.0 wt% NaCl solution, it was also conducted at the similar procedure in order to maintain all the particle gel samples treated under the same conditions of shearing and agitation which is believed to have significant impact on the rheological properties of the gels. The gel strength after removing the surfactants is shown in Fig. 5. Surprisingly, it has been found that the values of G0 for all the particle gels after removing surfactants increase significantly back to the values of the particle gel mixed with 1.0 wt. NaCl solution, which indicates that the friction behaviors between the surfaces of the particle gel, the sensor plate and the glass plate has been switched from rolling motion back to the sliding or translational motion because the surfactant micelles have been removed from the gel particle surface, leading to the recovery of G0 and G". To provide further support of the proposed possible mechanism responsible for the dynamic modulus reduction of the particle
Y. Wu et al. / Polymer 52 (2011) 452e460
Fig. 6. Storage modulus G0 of particle gels measured with various surfactants at different concentrations shows a substantial decrease at the concentration of their critical micelle concentration (CMC).
hydrogel by surfactant, the elastic modulus G0 of the synthesized particle hydrogel swollen in the surfactant solution prepared with 1.0 wt.% NaCl at the concentrations below and above their critical micelle concentration (CMC) has been systematically measured. The results are shown in Fig. 6. In the figure, one can see that for the six surfactants used in this study, the G0 shows a substantial decrease at their CMC for each surfactant. AlfoterraÒ 23 with a CMC of 10 ppm, G0 decreases from 2660 Pa to 1600 Pa around the CMC; IgepalÒ CO-530 with a CMC of 20 ppm, G0 decreases from 2580 Pa to 1700 Pa around the CMC; NeodolÒ 25-12 with a CMC of 70 ppm, G0 decreases from 2650 Pa to 1680 Pa around the CMC; Sodium salt of dodecylbenzene sulfonic acid (C12 Benzenesulfonic) with a CMC of 160 ppm, G0 decreases from 2650 Pa to 1880 Pa around the CMC; (1-Hexadecyl)pyridinium bromide (C16 Pyridinium) with a CMC of 181 ppm, G0 decreases from 2600 Pa to 2040 Pa around the CMC; n-Dodecylpyridinium chloride (C12 Pyridinium) with a CMC of 795 ppm, G0 decreases from 2500 Pa to 1200 Pa around the CMC. The very slow decrease of G0 for all surfactants at the concentrations below CMC may be due to the experimental errors. However, the noticeable decrease of G0 at the concentrations above CMC is due to the increase of micelle concentration for the all surfactants investigated. Because as a surfactant solution is continuously increased to a concentration above its CMC, the monomer concentration in the solution will no longer increase while the micelle concentration in the solution will continue increasing with increase of the surfactant concentration. This results in a continuous decrease in the dynamic modulus (G0 ) of the hydrogel. Furthermore, it can also be found that among these surfactants investigated, AlfoterraÒ 23 has the lowest CMC. At 10 ppm, it can effectively reduce elastic modulus because it is the only one that can form spherical aggregates in the solution and most effective on reducing gel surface friction.
459
particles enter and stay in the fracture and large size porous media while the surfactant solutions enter into the small pores in the formation where most hydrocarbon oil is trapped by the capillary force. In this way, the surfactant solution will reduce interfacial tension at oil/brine interface and change wettability of rock surface in the formation. Based on our results of interaction between the particle gel and surfactant, it has been found that surfactants have strong influence on particle gel friction. Therefore, injectivity of the particle gels can be greatly improved by the proper screening of surfactant. In other words, the gel resistance can be modified by the selected surfactants for the best applications. It is also worth mentioning that the results of this investigation of the interaction between the surfactant and particle gels demonstrate a new idea of forced imbibitions through combination of particle gel injection and surfactant imbibition. Development of forced imbibition technology will enable oil producers to increase oil recovery while reduce water production. This study benefits the oil industry with development of a new technology combining gel treatment and surfactant flooding to improve oil/gas recovery while reduce water production. 5. Conclusions Equilibrium concentration of NaCl in excess brine remains the same after the swelling of particle gel. It is also expected that concentration of water-soluble salts, e.g. KCl, MgCl2, CaCl2, Na2CO3, Na2SO4, will not change as well after the swelling of particle gels. This means that gel swelling will not increase the salinity of the underground formation water. Equilibrium surfactant concentration in excess brine increases after swelling of gel particles, which is due to formation of surfactant micelles in the brine. Comparing with the open pore size (4.1 nm) of the synthesized gels, the larger size of the formed micelles could not diffuse into the network of particle gel. Surfactants with larger size of molecules and lower critical micelle concentration (CMC) show greater increase in equilibrium. It is expected that nonionic surfactants will show greater increase. Gel friction in terms of dynamic modulus G0 and G" can be reduced dramatically which might be due to the fact that the surfactant micelles adsorbed on the surface of particle gel change the friction between the surfaces of particle gel and solid from sliding/translational motion to the rolling motion. The latter has a much smaller friction coefficient. However, the gel resistance can be recovered after the surfactants have been removed indicating the physical absorption of the formed micelles over the surfaces of the swollen PPGs. Injectivity of particle gels can be significantly improved by use of proper surfactants. Moreover, a new technology of forced surfactant imbibition can be developed by combination of the PPGs and surfactant. The new technology will greatly benefit to oil industry by the way to improve oil recovery while reduce water production. Acknowledgements
4. Potential applications Gel treatment has been proved to be an effective method to reduce water production in oil industry. A new trend in gel treatments is application of preformed particle gels (PPG) that are formed at surface facilities before injection. In order to improve the gel treatment efficacy, gel particles are placed in the surfactant brine solution. When the particle gel and surfactant solution are injected into the underground reservoir, the filtrated solution can be squeezed into the matrix during the injection. As a result, the gel
The authors are grateful for financial support from Research Partnership to Secure Energy for America (RPSEA) and PetroChina Research Institute of Petroleum Exploration & Development (RIPED). Funding for this project provided by RPSEA is through the “Ultra-Deepwater and Unconventional Natural Gas and Other Petroleum Resources” program authorized by the U.S. Energy Policy Act of 2005. RPSEA (www.rpsea.org) is a nonprofit corporation whose mission is to provide a stewardship role in ensuring the focused research, development and deployment of safe and
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environmentally responsible technology that can effectively deliver hydrocarbons from domestic resources to the citizens of the United States. RPSEA, operating as a consortium of premier U.S. energy research universities, industry, and independent research organizations, manages the program under a contract with the U.S. Department of Energy’s National Energy Technology Laboratory. References [1] Kokufuta E, Suzuki H, Yoshida R, Yamada K, Hirata M, Kaneko F. Langmuir 1998;14(4):788. [2] Starodoubtsev SG, Churochkina NA, Khokhlov AR. Langmuir 2000;16(4):1529. [3] Shinde VS, Badiger MV, Lele AK, Mashelkar RA. Langmuir 2001;17(9):2585. [4] Lynch I, Sjostrom J, Piculell L. J Phys Chem B 2005;109(9):4258. [5] Caykara T, Demiray M, Gueven O. Colloid Polym Sci 2005;284(3):258. [6] Chen L, Yu X, Li Q. J App Polym Sci 2006;102(4):3791. [7] Mohan YM, Joseph DK, Geckeler KE. J App Polym Sci 2007;103(5):3423. [8] Mangiapia G, Ricciardi R, Auriemma F, De Rosa C, Lo Celso F, Triolo R, et al. J Phys Chem B 2007;111(9):2166. [9] Yamazaki Y, Matsunaga T, Ichinokawa A, Fujishiro Y, Saito E, Sato T. J App Polym Sci 2009;114(5):2764. [10] Philippova OE, Hourdet D, Audebert R, Khokhlov AR. Macromolecules 1996;29 (8):2822. [11] Ashbaugh HS, Piculell L, Lindman B. Langmuir 2000;16(6):2529.
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