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Effects of surfactant micelles and surfactant-coated nanospheres on methane hydrate growth pattern
Chemical Engineering Science 144 (2016) 108–115
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Effects of surfactant micelles and surfactant-coated nanospheres on methane hydrate growth pattern Fei Wang a,b, Gang Guo a, Guo-Qiang Liu c, Sheng-Jun Luo a,n, Rong-Bo Guo a,nn a Shandong Industrial Engineering Laboratory of Biogas Production & Utilization, Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, Shandong, China b University of Chinese Academy of Science, Beijing 100049, China c Qingdao University of Science & Technology, Qingdao 266042, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
SDS led to the upward methane hydrate growth on the reactor sidewall. SDBS micelles were conducive to the hydrate formation in the bulk of solution. SDS adsorbed on polystyrene nanospheres acts like micelles in hydrate formation.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 28 December 2015 Accepted 16 January 2016 Available online 22 January 2016
Surfactants have been reported to promote the gas hydrate formation by changing the hydrate formation pattern. In this work, methane hydrate formation was carried out in glass tubes to study the effects of the existence of surfactant micelles on methane hydrate growth pattern. For comparison, surfactants could and could not form micelles at hydrate formation temperature were used, which were sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS), respectively. SDS led to obvious upward hydrate growth on the reactor sidewall (cover factors 2.8–3.7), while SDBS resulted in obviously less extent of upward hydrate growth (cover factors 1.8–2.6) due to the micelle effect. When SDS-coated polystyrene nanospheres were used, SDS existed in the reaction solution in the form of mimic micelles and consequently much less extent of upward hydrate growth was achieved (cover factors 1.8–2.3). When SDBS was used together with SDS at non-micelle forming condition, prominent upward hydrate growth was obtained (cover factors 2.7–3.3). & 2016 Elsevier Ltd. All rights reserved.
Keywords: Methane hydrates Surfactants Micelle Mimic micelle Hydrate growth pattern Hydrate growth rate
1. Introduction Natural gas hydrates, formed by water and natural gas molecules (Sloan and Koh, 2007a), have aroused great attention during the past decades due to the significant potential in natural gas storage and transportation (Gudmundsson et al., 1994, 1995; Mori, n
Corresponding author. Tel./fax: þ 86 532 80662750. Corresponding author. Tel./fax: þ 86 532 80662708. E-mail addresses: [email protected] (S.-J. Luo), [email protected] (R.-B. Guo).
nn
http://dx.doi.org/10.1016/j.ces.2016.01.022 0009-2509/& 2016 Elsevier Ltd. All rights reserved.
2003). How to shorten the induction period and accelerate the hydrate growth have been the research focus, because the long induction period and slow hydrate growth are not conducive to the application of natural gas hydrates (Zhong and Rogers, 2000). Surfactants, such as SDS, have been reported with effective promotion to gas hydrate formation (Ganji et al., 2007; Ando et al., 2012; Lo et al., 2012; Du et al., 2014; Veluswamy et al., 2015a, 2015b) because surfactants could change the hydrate formation pattern (Zhong and Rogers, 2000; Gayet et al., 2005; Watanabe et al., 2005a; Okutani et al., 2008; Zhang et al., 2007a; Yoslim et al.,
F. Wang et al. / Chemical Engineering Science 144 (2016) 108–115
2010). In the unstirred gas/liquid system without surfactants, gas hydrates initially formed as a thin hydrate film at the gas/liquid interface (Sloan and Koh, 2007b; Kashchiev and Firoozabadi, 2002) and then grew downwards as dendrites (Lee et al., 2006). However, the hydrate film isolated the liquid phase from the gas phase and therefore led to slow hydrate formation (Mori and Mochizuki, 1996). While in the SDS solution system, according to Zhong and Rogers (2000), hydrates initially formed as particles by the gas dissolved in the SDS micelles and the water associated on the micelle surface. Afterwards, the hydrate particles would be adsorbed on the reactor sidewall, resulting in the formation of concentric hydrate layers held by the reactor sidewall. As a result, the hydrate film that isolated the gas phase from the liquid phase was removed and therefore rapid hydrate formation was achieved. However, this hypothesis was also disputed by several researchers (Watanabe et al., 2005a, 2005b; Zhang et al., 2007a; Di Profio et al., 2007; Zhang et al., 2007b), because no micelles were detected at the SDS concentration reported by Zhong and Rogers (2000) and no efficient promotion was observed although with the existence of surfactant micelles. Another phenomenon about the hydrate formation with surfactants was that hydrates initially formed on the reactor sidewall and then grew upwards under capillary effects (Gayet et al., 2005; Watanabe et al., 2005a, 2005b; Okutani et al., 2008; Zhang et al., 2007a; Yoslim et al., 2010). Gayet et al. (2005) reported that surfactants could prevent the aggregation of hydrate particles at gas/ liquid interface and hydrates primarily formed on reactor sidewall as porous structures. Then the surfactant solution could be sucked up to the hydrate surface through the porous structure under capillary force. Watanabe et al. (2005a, 2005b) reported that the reactor sidewall would be splashed with surfactant solution when charging the solution into reactor, leading to solution films adhered on the sidewall. Therefore, hydrate formation initially took place on the reactor sidewall with the solution films, resulting in the formation of hydrate films on reactor sidewall. Although many researchers have reported the upward hydrate growth on the reactor sidewall, the reasons that cause the upward hydrate growth still remain poorly understood. In our previous study (Wang et al., 2015a), the wettability of reactor sidewall with surfactant solution (revealed by the contact angle) was found to affect methane hydrate growth pattern. SDS solution, with good wettability, led to obvious upward hydrate growth on the reactor sidewall, while SDBS solution resulted in the hydrates formed mainly in the reactor bottom due to the poor wettability. However, when a glass layer, which showed good wettability with surfactant solution, was installed in the reactor, a certain extent of upward hydrate growth was also obtained in methane hydrate formation with SDBS solution. Then in our other previous study (Wang et al., 2015b), direction controlled methane hydrate growth on the reactor sidewall was achieved through changing the hydrophilicity and hydrophobicity of hydrate growth surface. In this work, to further study the methane hydrate growth pattern in the presence of surfactants, methane hydrate formation was carried out in glass reactors, which showed very good wettability with reaction solutions, therefore, the effects of wettability could be avoided. To understand the effects of surfactant micelles on methane hydrate growth, both SDS and SDBS were used, because SDS and SDBS showed similar molecular structures and SDBS could form micelles at hydrate formation conditions while SDS could not.
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2. Experimental section 2.1. Materials Sodium dodecyl sulfate (SDS, AR) and sodium dodecyl benzene sulfonate (SDBS, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Styrene (St, AR) was provided by Tianjin Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Ammonium persulfate (APS, AR) was purchased from Hengxing Chemical Reagent Co., Ltd. (Tianjin, China). Methane and nitrogen with purities of 99.99% were provided by Heli Gas Co., Ltd. (Qingdao, China). The deioinzed water used in this work was laboratory-made and the conductivity was 1.1 70.1 μs/cm (298.15 K). The molecular structures of the surfactants are shown in Fig. 1. 2.2. Preparation of the mimic micelles of SDS In this work, to clear the effects of surfactant micelles on methane hydrate growth pattern, SDS-coated nanospheres were prepared and used in methane hydrate formation. With SDS molecules fixed on the surface of polystyrene nanospheres, SDScoated nanospheres showed similar structures as SDS micelles, as shown in Fig. 2. Therefore, the SDS-coated nanospheres were also called mimic micelles of SDS (M-micelle-SDS). The mimic micelles of SDS were prepared by fixing the surfactant molecules on the surface of polystyrene nanospheres, which was achieved through emulsion polymerization with SDS as emulsifier, styrene as monomer and ammonium persulfate as initiator. The recipes and conditions of emulsion polymerization are shown in Table 1. The details of emulsion polymerization and the characterizations of M-micelle-SDS were provided in our previous study (Wang and Fang, 2014).
Fig. 1. Molecular structures of the surfactants used in this work.
Fig. 2. Schematic diagram of the structures of SDS micelle and M-micelle-SDS.
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2.3. Contact angles of the reaction solutions on the reactor sidewall To reveal the wettability of the reactor sidewall with reaction solutions more intuitively, contact angles of the reaction solutions on reactor sidewall were measured through hypsometry (Du and Wu, 2007). The details of the measurement and the calculation of the contact angle were shown in our previous study (Wang et al., 2015a). 2.4. Methane hydrate formation Fig. 3 shows the schematic diagram of the methane hydrate formation apparatus in this work, which was modified from the apparatus in our previous work (Wang et al., 2015a). The main part of the apparatus was consisted of a piston container with the volume of 1 L and a reactor with the volume of 200 mL. Both the piston container and the reactor were made of 316 L stainless steel (roughnessr0.2 μm). During the hydrate formation process, methane was charged into the piston container in advance to reach the reaction temperature (275.15 K) and then 50 mL methanol solution (20 vol%) was added into the reactor. Afterwards, glass tubes containing 1 mL reaction solution were placed into the reactor with the tube mouths above the liquid level. Given the stochasticity of hydrate formation, three tubes and fresh surfactant solutions were used in each experiment. The methanol solution was used to promote the heat transfer and the hydrate formation with methanol solution could be avoided because methanol was inhibitor of hydrate formation (Anderson and Prausnitz, 1986). After the temperature of the reaction solution reached 275.15 K, the reactor was vacuumed and cleaned with methane three times and then pressurized with
Product
SDS/g
St/g
APS/g
DI watera/g
sb/rpm
Tc/K
M-micelle-SDS
1
5
0.1
95
250
343.15
b c
DI water-deionized water. s-stirring rate. T-polymerization temperature.
2.5. Calculation of cover factor To reveal the methane hydrate growth in the reactor quantitatively, cover factor (f) was proposed, which was defined as the ratio of the practical and theoretical area of the reactor inner surface covered by the hydrates formed with 1 mL reaction solution. Cover factor was calculated by the following formulas, which were derived in our previous study (Wang et al., 2015b). f¼
4π r 2 12 þ 2π rhp 4π r 2 12 þ2π rht
¼
1 þ hp 1 þht
4 3 1 m π r þ 2π rht ¼ V h ¼ V w þΔV w 3 2 Mw
ð1Þ
ð2Þ
where, r is the inner radius of the hemispherical part of the glass tube; hp and ht are the practical and theoretical heights of hydrates on the sidewall of the tube respectively; Vh is the theoretical volume of hydrates; Vw is the volume of surfactant solution; ΔV is the molar volume difference between methane hydrates and water, which has been reported as 4.6 cm3 per mol water (Makogon, 1997); mw is the weight of water; Mw is the molar mass of water.
3. Results and discussion 3.1. Properties of the surfactants
Table 1 The recipes and conditions of emulsion polymerization.
a
methane to 6 MPa. The temperature and pressure of the reactor were recorded by the computer during the whole process. After the hydrate formation, the glass tubes were taken out and frozen under 193.15 K immediately to preserve the hydrate morphology and then photographs were taken for the hydrates.
The properties of the surfactants, such as the critical micelle concentrations (CMC) and solubilities, have been measured through conductivity method in our previous study (Wang et al., 2015a) and Table 2 shows the results from our previous study and K other references. For SDS, the CMC at 298.15 K ( CMC298:15 ) and SDS 275:15 K ) were 8.2 and 4.4 mmol/L, the solubility at 275.15 K ( CMCSDS respectively, which indicated that SDS could not form micelles
Fig. 3. Schematic diagram of the methane hydrate formation apparatus.
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under hydrate formation temperature (275.15 K) because SDS precipitated before reaching the CMC. For SDBS, the CMC at K K 298.15 K ( CMC298:15 ) and 275.15 K (CMC275:15 ) were 2.1 and SDS SDS 1.8 mmol/L respectively and no precipitation of SDBS took place in the range of 1–10 mmol/L at both 275.15 and 298.15 K (Wang et al., 2015a). In this work, both SDS and SDBS were used at 1, 2, 4 and 8 mmol/L, therefore, SDS could not form micelles while SDBS could at 2, 4 and 8 mmol/L. Table 2 The properties of the surfactants used in this work. Surfactant SDS SDBS
CMC (mmol/L) a
8.2 (298.15 K) 7.8 (298.15 K)c 2.1 (298.15 K)a 1.7 (298.15 K)c 1.8 (275.15 K)a 1.6 (275.15 K)c
Solubility (mmol/L) a
Krafft point (K) 281.15b 289.15e 300.75f
4.4 (275.15 K) 6.1 (275.15 K)d
a
Wang et al. (2015a). Watanabe et al. (2005a). Di Profio et al. (2005). d Zhang et al. (2007b). e Tehrani-Bagha et al. (2013). f Šegota et al. (2006). b c
Table 3 Contact angles of the reaction solutions on the reactor sidewalla. Reaction solution
Deionized water SDS (aq.) SDBS (aq.) M-micelle-SDS (aq.) a
Contact angle (°) 1 mmol/L
2 mmol/L
4 mmol/L
8 mmol/L
387 2.0 29.0 7 0.5 317 0.8 327 1.3
32.0 7 0.4 32 71.1 29 71.0
317 1.6 307 1.4 247 1.3
297 2.2 317 1.3 227 1.7
The measurement was carried out at 275.15 K.
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3.2. Methane hydrate growth pattern in the presence of surfactants Fig. 4A shows the morphologies of the methane hydrates formed in the presence of SDS, which has been discussed in our previous study (Wang et al., 2015b). SDS led to obvious upward hydrate growth on the reactor sidewall and the SDS concentration did not produce obvious effect on the upward hydrate growth, as shown by the similar cover factors. Table 3 shows that SDS solutions with different concentrations resulted in very narrow and similar contact angles on the glass surface, which indicated very good wettability of the glass surface with SDS solutions. As proposed in our previous study (Wang et al., 2015a), hydrates initially formed at the gas/liquid/solid interline and the good wettability was conducive to the hydrates growing upward on the reactor sidewall. As a result, even with different concentrations, the SDS solutions led to obvious and similar upward hydrate growth on the reactor sidewall. Fig. 4A shows the pressure evolutions of methane hydrate formation with SDS solutions and the pressure decrease indicated the hydrate formation. SDS led to the hydrate nucleation take place within several hours and the hydrate growth completed within 30–40 min, which was consistent with our previous study carried out in stainless steel reactor (Wang et al., 2015a). In addition, it should be noted that SDS precipitated at 8 mmol/L, therefore, it could be proposed that the precipitation of SDS did not produced obvious effects on the methane hydrate formation rate and hydrate growth pattern. Fig. 4B shows the morphologies of methane hydrates formed with SDBS solutions, which led to obviously less extent of upward hydrate growth compared to SDS solutions. In our previous study carried out in stainless steel reactor (Wang et al., 2015a), SDBS also led to less extent of upward methane hydrate growth on the reactor and we proposed that it was the different wettability resulted in the different hydrate growth patterns. However, in this work, SDBS solutions produced similar contact angles on the glass surface as SDS solutions (Table 3), which meant that the reactor surface showed similar wettability with SDS and SDBS solutions,
Fig. 4. Morphologies of the methane hydrates formed in the presence of SDS (A) and SDBS (B). (The initial conditions were 6 MPa and 275.15 K; the red numbers were cover factors and “-” indicated that no hydrate formation was observed). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Pressure evolutions of methane hydrate formation with SDS (A) and SDBS (B). (The initial conditions were 6 MPa and 275.15 K)
therefore, the effects of the wettability on the different methane hydrate growth pattern could be avoided. The possible reason caused the different hydrate growth patterns between SDS and SDBS solutions might be the micelle formation in SDBS solutions at hydrate formation condition. As discussed above, SDS could not form micelles at hydrate formation conditions, while SDBS could when the concentration over 1.8 mmol/L. In consequence, hydrate formation only took place in the SDBS solutions at micelle-forming concentrations (2, 4 and 8 mmol/L) (Fig. 4B) and higher SDBS concentration led to shorter induction period and rapider hydrate growth (Fig. 5B). This verified the assumption made by Zhang et al. (2007a) that the surfactant concentration should affect the hydrate formation significantly if micelles existed in the hydrate formation system and therefore confirmed the micelle effects of SDBS on promoting methane hydrate formation. In the micelle-forming surfactant solutions, as assumed by Zhong and Rogers (2000), gas molecules could dissolve in the surfactant micelles and hydrates initially formed with the gas molecules in the micelles and the water molecules associated on the micelle surface, leading to the formation of hydrate particles below the gas/liquid interface. Therefore, hydrates formed mainly in the bulk of the SDBS solution and less extent of upward hydrate growth compared to SDS was observed. Moreover, Figs. 4 and 5 also show that the upward hydrate growth on the sidewall led to higher growth rate compared with the hydrate growth in the bulk of the reaction solution, which was consistent with our previous study (Wang et al., 2015a). 3.3. Methane hydrate growth with mimic micelles of SDS To confirm the effects of surfactant micelles on methane hydrate growth pattern, SDS should also be used in the form of micelles in methane hydrate formation. However, as discussed above, the CMC of SDS at 298.15 K was 8.2 mmol/L and the solubility of SDS at 275.15 K was 4.4 mmol/L, which indicated that SDS could not form micelles at 275.15 K at 1, 2, 4 and 8 mmol/L.
Fig. 6. Characterizations of M-micelle-SDS (photos-TEM, d-particle size, PDI-particle size distribution index).
Therefore, SDS-coated nanospheres (mimic micelles of SDS) were applied in methane hydrate formation. Fig. 6 shows the TEM photos and particle size of the SDS-coated polystyrene nanospheres, which appeared as uniform spheres at nanoscale with high monodispersity. In our previous study (Wang et al., 2015c), the surfactant fixation ratio was measured as 81.96%, which denoted that about 20% of SDS existed as free molecules in the SDS-coated nanospheres. The concentrations of free SDS in the SDS-coated nanospheres solutions at 1, 2, 4 and 8 mmol/L were about 0.2, 0.4, 0.8 and 1.6 mmol/L respectively, which showing that no SDS micelles existed in the SDS-coated nanospheres solutions. As SDS-coated nanospheres exhibited similar structures as SDS micelles, about 80% of SDS existed in the form of mimic micelles in methane hydrate formation with M-micelle-SDS. Fig. 7A shows the morphologies of methane hydrates formed with M-micelle-SDS solutions, which resulted in obviously less extent of upward hydrate growth compared with SDS (Figs. 4A and
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Fig. 7. Hydrate growth morphology (A) and pressure evolutions (B) of hydrate formation with M-micelle-SDS. (The initial conditions were 6 MPa and 275.15 K)
7A). As shown in Table 3, M-micelle-SDS solutions led to similar contact angles on the reactor sidewall as SDS solutions, therefore, the effects of wettability of the reactor wall could be avoided. One possible reason was that the depletion of free SDS molecules in the M-micelle-SDS solutions caused the less extent of upward hydrate growth. Another possible reason was that in the hydrate formation with M-micelle-SDS solutions hydrates might initially form on the surface of SDS-coated nanospheres. As a result, hydrates mainly formed in the bulk of the M-micelle-SDS solutions, which led to obviously less extent of upward hydrate growth compared with SDS solutions. This confirmed that the surfactant micelles were conducive to the hydrate formation in the bulk of the reaction solution. Moreover, SDS also led to higher hydrate growth rate compared with M-micelle-SDS due to the different hydrate growth patterns (Figs. 5A and 7B). 3.4. Methane hydrate growth with SDBS at non-micelle forming condition To further study the effect of surfactants on methane hydrate growth pattern, SDBS was used at non-micelle forming condition. As SDBS solutions at non-micelle forming condition did not produce effective promotion to methane hydrate formation (Fig. 5), the compounds of SDS and SDBS (named as SDS-co-SDBS) with different SDS molar fractions were used. Moreover, to ensure the non-micelle forming condition, SDS-co-SDBS was used at 1 mmol/ L (total surfactant concentration) and the pyrene fluorescence spectra of SDS-co-SDBS solutions was measured, which has been successfully applied to detect the formation of surfactant micelles (Lucas et al., 2015). The intensity ratio of the third peak (I3, λ ¼384 nm) and the first peak (I1, λ ¼ 374 nm) of the pyrene fluorescence emission spectra could denote the polarities of the microdomains (Zhang et al., 2008) and the formation of micelles
Table 4 I3/I1 values of SDS-co-SDBS solutions and contact angles of SDS-co-SDBS solutions with different SDS molar fractions (mf) on the glass surfacea. SDS-co-SDBS (SDS mf)
0.1
0.3
0.5
0.7
I3/I1 Contact angle (°)
0.92 39 71.6
0.94 297 1.3
0.93 277 1.6
0.93 367 0.9
a The concentration of SDS-co-SDBS was 1 mmol/L; both the I3/I1 values and contact angles were measured at 275.15 K.
could lead to the obvious increase in the I3/I1 values, as shown in Fig. S1 in the supporting information. Table 4 shows that the I3/I1 values of SDS-co-SDBS solutions at 1 mmol/L were 0.92–0.94, which were similar as those of the solutions of dodecyl alcohol ethoxylates at non-micelle forming conditions (shown in Fig. S1 in the supporting information), confirming that no micelles were formed in the SDS-co-SDBS solutions at 1 mmol/L. In addition, Table 4 also shows that SDS-co-SDBS solutions (1 mmol/L) led to similar contact angles on the glass surface compared to SDS and SDBS solutions (Table 1). Fig. 8A shows the pressure evolutions of methane hydrate formation with 1 mmol/L SDS-co-SDBS solutions, which produced efficient promotion to hydrate formation. In this work, SDS solutions at 0.1, 0.3 and 0.5 mmol/L did not produce promotion to methane hydrate formation (shown in Fig. S2 in the supporting information), therefore, it might be proposed that the recombination action of SDS and SDBS led to efficient promotion to methane hydrate formation, especially at low molar fraction of SDS. Fig. 8B shows the morphologies of methane hydrates formed with SDS-co-SDBS solutions, which led to much higher extent of upward hydrate growth compared with SDBS solutions at micelle forming condition. One possible reason was that SDS was much more conducive to the upward methane hydrate growth compared
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Fig. 8. Pressure evolutions (A) and hydrate growth morphologies (B) of hydrate formation with SDS-co-SDBS solutions. (The concentration of SDS-co-SDBS was 1 mmol/L, the initial conditions were 6 MPa and 275.15 K)
with SDBS, therefore, when SDS-co-SDBS was used the existence of SDS could promote the hydrates to grow upwards. However, it should be noted that even at very low molar fraction of SDS (e.g. 0.1) SDS-co-SDBS resulted in obvious upward hydrate growth. Therefore, another more possible reason was that surfactants at non-micelle forming condition were more conducive to the upward hydrate growth, while surfactants at micelle forming condition were more conducive for the hydrate formation in the bulk of the reaction solution. As the promotion mechanism of surfactants to gas hydrate formation still remains poorly understood, the results in this work could provide better understanding about the effects of surfactant micelles during methane hydrate formation. In addition, in the hydrate-based gas storage and transportation, the hydrates are usually separated and compacted for storage and transportation. Therefore, it is very necessary to obtain gas hydrates with high apparent density. As shown in this work, the SDS-coated nanospheres led to the hydrates formed at the bottom of the reactor with higher apparent density, therefore, the results of this work was of great significance to the hydrate-based gas storage and transportation.
was conducive to the hydrate formation in the bulk of solution. When SDS-co-SDBS at non-micelle forming condition was used, prominent upward hydrate growth was obtained, indicating that non-micelle forming surfactants were conducive to the upward hydrate growth on the reactor sidewall.
Nomenclature f r hp ht Vh Vw ΔV mw Mw
cover factor inner radius of the glass tube practical heights of the hydrates on the reactor sidewall theoretical heights of the hydrates on the reactor sidewall theoretical volume of the hydrates volume of surfactant solution molar volume difference between hydrates and water weight of water molar mass of water
4. Conclusions The effects of different surfactants (SDS and SDBS) on methane hydrate growth pattern were studied. Non-micelle forming SDS led to obvious upward hydrate growth on the reactor sidewall, while SDBS led to much less extent of upward hydrate growth due to the micelle effect. When M-micelle-SDS, with similar structure as SDS micelle, was used, obviously less extent of upward hydrate growth was observed, confirming that the existence of micelles
Acknowledgment This work was supported by the National Natural Science Foundation of China (31101918), Key Projects in the National Science and Technology Pillar Program (20140015) and Qingdao Science and Technology and People's Livelihood Project (14-2-369-nsh).
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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2016.01.022.
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Mori, Y.H., 2003. Recent advances in hydrate-based technologies for natural gas storage – a review. J. Chem. Ind. Eng. 54 (Suppl.), 1–17. Okutani, K., Kuwabara, Y., Mori, Y.H., 2008. Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using methane and sodium alkyl sulfates. Chem. Eng. Sci. 63, 183–194. Šegota, S., Heimer, S., Težak, Đ., 2006. New catanionic mixtures of dodecyl dimethyl ammonium bromide/sodium dodecyl benzene sulphonate/water: I. Surface properties of dispersed particles. Colloids Surf. A 274, 91–99. Sloan, E.D., Koh, C.A., 2007a. Heinemann, H., Speight, J.G. (Eds.), Clathrate Hydrates of Natural Gases. CRC press, Boca Raton, p. 1. Sloan, E.D., Koh, C.A., 2007b. Heinemann, H., Speight, J.G. (Eds.), Clathrate Hydrates of Natural Gases. CRC press, Boca Raton, p. 130. Tehrani-Bagha, A.R., Singh, R.G., Holmberg, K., 2013. Solubilization of two organic dyes by anionic, cationic and nonionic surfactants. Colloids Surf. A 417, 133–139. Veluswamy, H.P., Chen, J.Y., Linga, P., 2015a. Surfactant effect on the kinetics of mixed hydrogen/propane hydrate formation for hydrogen storage as clathrate. Chem. Eng. Sci. 126, 488–499. Veluswamy, H.P., Ang, W.J., Zhao, D., Linga, P., 2015b. Influence of cationic and nonionic surfactant on the kinetics of mixed hydrogen/tetrahydrofuran hydrates. Chem. Eng. Sci. 132, 186–199. Wang, F., Fang, K.J., 2014. Effect of molecular structures of Gemini and polymerizable emulsifiers on cationic emulsion copolymerization of styrene and butyl acrylate. Colloid Polym. Sci. 292, 1449–1455. Wang, F., Jia, Z.Z., Luo, S.J., Fu, S.F., Wang, L., Shi, X.S., Wang, C.S., Guo, R.B., 2015a. Effects of different anionic surfactants on methane hydrate formation. Chem. Eng. Sci. 137, 896–903. Wang, F., Wang, L., Wang, C.S., Guo, G., Liu, G.Q., Luo, S.J., Guo, R.B., 2015b. Cryst. Growth Des. 15, 5112–5117. Wang, F., Luo, S.J., Fu, S.F., Jia, Z.Z., Dai, M., Wang, C.S., Guo, R.B., 2015c. Methane hydrate formation with surfactants fixed on the surface of polystyrene nanospheres. J. Mater. Chem. A 3, 8316–8323. Watanabe, K., Imai, S., Mori, Y.H., 2005a. Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using HFC-32 and sodium dodecyl sulfate. Chem. Eng. Sci. 60, 4846–4857. Watanabe, K., Niwa, S., Mori, Y.H., 2005b. Surface tensions of aqueous solutions of sodium alkyl sulfates in contact with methane under hydrate-forming conditions. J. Chem. Eng. Data 50, 1672–1676. Yoslim, J., Linga, P., Englezos, P., 2010. Enhanced growth of methane–propane clathrate hydrate crystals with sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium hexadecyl sulfate surfactants. J. Cryst. Growth 31, 68–80. Zhang, J.S., Lee, S., Lee, J.W., 2007a. Kinetics of methane hydrate formation from SDS solution. Ind. Eng. Chem. Res. 46, 6353–6359. Zhang, J.S., Lee, S., Lee, J.W., 2007b. Does SDS micellize under methane hydrateforming conditions below the normal Krafft point? J. Colloid Interface Sci. 31, 313–318. Zhong, Y., Rogers, R.E., 2000. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 55, 4175–4187. Zhang, J.S., Lo, C., Somasundaran, P., Lu, S., Couzis, A., Lee, J.W., 2008. Adsorption of sodium dodecyl sulfate at THF hydrate/liquid interface. J. Phys. Chem. C 112, 12381–12385.
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Effects of surfactant micelles and surfactant-coated nanospheres on methane hydrate growth pattern Fei Wang a,b, Gang Guo a, Guo-Qiang Liu c, Sheng-Jun Luo a,n, Rong-Bo Guo a,nn a Shandong Industrial Engineering Laboratory of Biogas Production & Utilization, Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, Shandong, China b University of Chinese Academy of Science, Beijing 100049, China c Qingdao University of Science & Technology, Qingdao 266042, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
SDS led to the upward methane hydrate growth on the reactor sidewall. SDBS micelles were conducive to the hydrate formation in the bulk of solution. SDS adsorbed on polystyrene nanospheres acts like micelles in hydrate formation.
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 28 December 2015 Accepted 16 January 2016 Available online 22 January 2016
Surfactants have been reported to promote the gas hydrate formation by changing the hydrate formation pattern. In this work, methane hydrate formation was carried out in glass tubes to study the effects of the existence of surfactant micelles on methane hydrate growth pattern. For comparison, surfactants could and could not form micelles at hydrate formation temperature were used, which were sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS), respectively. SDS led to obvious upward hydrate growth on the reactor sidewall (cover factors 2.8–3.7), while SDBS resulted in obviously less extent of upward hydrate growth (cover factors 1.8–2.6) due to the micelle effect. When SDS-coated polystyrene nanospheres were used, SDS existed in the reaction solution in the form of mimic micelles and consequently much less extent of upward hydrate growth was achieved (cover factors 1.8–2.3). When SDBS was used together with SDS at non-micelle forming condition, prominent upward hydrate growth was obtained (cover factors 2.7–3.3). & 2016 Elsevier Ltd. All rights reserved.
Keywords: Methane hydrates Surfactants Micelle Mimic micelle Hydrate growth pattern Hydrate growth rate
1. Introduction Natural gas hydrates, formed by water and natural gas molecules (Sloan and Koh, 2007a), have aroused great attention during the past decades due to the significant potential in natural gas storage and transportation (Gudmundsson et al., 1994, 1995; Mori, n
Corresponding author. Tel./fax: þ 86 532 80662750. Corresponding author. Tel./fax: þ 86 532 80662708. E-mail addresses: [email protected] (S.-J. Luo), [email protected] (R.-B. Guo).
nn
http://dx.doi.org/10.1016/j.ces.2016.01.022 0009-2509/& 2016 Elsevier Ltd. All rights reserved.
2003). How to shorten the induction period and accelerate the hydrate growth have been the research focus, because the long induction period and slow hydrate growth are not conducive to the application of natural gas hydrates (Zhong and Rogers, 2000). Surfactants, such as SDS, have been reported with effective promotion to gas hydrate formation (Ganji et al., 2007; Ando et al., 2012; Lo et al., 2012; Du et al., 2014; Veluswamy et al., 2015a, 2015b) because surfactants could change the hydrate formation pattern (Zhong and Rogers, 2000; Gayet et al., 2005; Watanabe et al., 2005a; Okutani et al., 2008; Zhang et al., 2007a; Yoslim et al.,
F. Wang et al. / Chemical Engineering Science 144 (2016) 108–115
2010). In the unstirred gas/liquid system without surfactants, gas hydrates initially formed as a thin hydrate film at the gas/liquid interface (Sloan and Koh, 2007b; Kashchiev and Firoozabadi, 2002) and then grew downwards as dendrites (Lee et al., 2006). However, the hydrate film isolated the liquid phase from the gas phase and therefore led to slow hydrate formation (Mori and Mochizuki, 1996). While in the SDS solution system, according to Zhong and Rogers (2000), hydrates initially formed as particles by the gas dissolved in the SDS micelles and the water associated on the micelle surface. Afterwards, the hydrate particles would be adsorbed on the reactor sidewall, resulting in the formation of concentric hydrate layers held by the reactor sidewall. As a result, the hydrate film that isolated the gas phase from the liquid phase was removed and therefore rapid hydrate formation was achieved. However, this hypothesis was also disputed by several researchers (Watanabe et al., 2005a, 2005b; Zhang et al., 2007a; Di Profio et al., 2007; Zhang et al., 2007b), because no micelles were detected at the SDS concentration reported by Zhong and Rogers (2000) and no efficient promotion was observed although with the existence of surfactant micelles. Another phenomenon about the hydrate formation with surfactants was that hydrates initially formed on the reactor sidewall and then grew upwards under capillary effects (Gayet et al., 2005; Watanabe et al., 2005a, 2005b; Okutani et al., 2008; Zhang et al., 2007a; Yoslim et al., 2010). Gayet et al. (2005) reported that surfactants could prevent the aggregation of hydrate particles at gas/ liquid interface and hydrates primarily formed on reactor sidewall as porous structures. Then the surfactant solution could be sucked up to the hydrate surface through the porous structure under capillary force. Watanabe et al. (2005a, 2005b) reported that the reactor sidewall would be splashed with surfactant solution when charging the solution into reactor, leading to solution films adhered on the sidewall. Therefore, hydrate formation initially took place on the reactor sidewall with the solution films, resulting in the formation of hydrate films on reactor sidewall. Although many researchers have reported the upward hydrate growth on the reactor sidewall, the reasons that cause the upward hydrate growth still remain poorly understood. In our previous study (Wang et al., 2015a), the wettability of reactor sidewall with surfactant solution (revealed by the contact angle) was found to affect methane hydrate growth pattern. SDS solution, with good wettability, led to obvious upward hydrate growth on the reactor sidewall, while SDBS solution resulted in the hydrates formed mainly in the reactor bottom due to the poor wettability. However, when a glass layer, which showed good wettability with surfactant solution, was installed in the reactor, a certain extent of upward hydrate growth was also obtained in methane hydrate formation with SDBS solution. Then in our other previous study (Wang et al., 2015b), direction controlled methane hydrate growth on the reactor sidewall was achieved through changing the hydrophilicity and hydrophobicity of hydrate growth surface. In this work, to further study the methane hydrate growth pattern in the presence of surfactants, methane hydrate formation was carried out in glass reactors, which showed very good wettability with reaction solutions, therefore, the effects of wettability could be avoided. To understand the effects of surfactant micelles on methane hydrate growth, both SDS and SDBS were used, because SDS and SDBS showed similar molecular structures and SDBS could form micelles at hydrate formation conditions while SDS could not.
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2. Experimental section 2.1. Materials Sodium dodecyl sulfate (SDS, AR) and sodium dodecyl benzene sulfonate (SDBS, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Styrene (St, AR) was provided by Tianjin Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Ammonium persulfate (APS, AR) was purchased from Hengxing Chemical Reagent Co., Ltd. (Tianjin, China). Methane and nitrogen with purities of 99.99% were provided by Heli Gas Co., Ltd. (Qingdao, China). The deioinzed water used in this work was laboratory-made and the conductivity was 1.1 70.1 μs/cm (298.15 K). The molecular structures of the surfactants are shown in Fig. 1. 2.2. Preparation of the mimic micelles of SDS In this work, to clear the effects of surfactant micelles on methane hydrate growth pattern, SDS-coated nanospheres were prepared and used in methane hydrate formation. With SDS molecules fixed on the surface of polystyrene nanospheres, SDScoated nanospheres showed similar structures as SDS micelles, as shown in Fig. 2. Therefore, the SDS-coated nanospheres were also called mimic micelles of SDS (M-micelle-SDS). The mimic micelles of SDS were prepared by fixing the surfactant molecules on the surface of polystyrene nanospheres, which was achieved through emulsion polymerization with SDS as emulsifier, styrene as monomer and ammonium persulfate as initiator. The recipes and conditions of emulsion polymerization are shown in Table 1. The details of emulsion polymerization and the characterizations of M-micelle-SDS were provided in our previous study (Wang and Fang, 2014).
Fig. 1. Molecular structures of the surfactants used in this work.
Fig. 2. Schematic diagram of the structures of SDS micelle and M-micelle-SDS.
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2.3. Contact angles of the reaction solutions on the reactor sidewall To reveal the wettability of the reactor sidewall with reaction solutions more intuitively, contact angles of the reaction solutions on reactor sidewall were measured through hypsometry (Du and Wu, 2007). The details of the measurement and the calculation of the contact angle were shown in our previous study (Wang et al., 2015a). 2.4. Methane hydrate formation Fig. 3 shows the schematic diagram of the methane hydrate formation apparatus in this work, which was modified from the apparatus in our previous work (Wang et al., 2015a). The main part of the apparatus was consisted of a piston container with the volume of 1 L and a reactor with the volume of 200 mL. Both the piston container and the reactor were made of 316 L stainless steel (roughnessr0.2 μm). During the hydrate formation process, methane was charged into the piston container in advance to reach the reaction temperature (275.15 K) and then 50 mL methanol solution (20 vol%) was added into the reactor. Afterwards, glass tubes containing 1 mL reaction solution were placed into the reactor with the tube mouths above the liquid level. Given the stochasticity of hydrate formation, three tubes and fresh surfactant solutions were used in each experiment. The methanol solution was used to promote the heat transfer and the hydrate formation with methanol solution could be avoided because methanol was inhibitor of hydrate formation (Anderson and Prausnitz, 1986). After the temperature of the reaction solution reached 275.15 K, the reactor was vacuumed and cleaned with methane three times and then pressurized with
Product
SDS/g
St/g
APS/g
DI watera/g
sb/rpm
Tc/K
M-micelle-SDS
1
5
0.1
95
250
343.15
b c
DI water-deionized water. s-stirring rate. T-polymerization temperature.
2.5. Calculation of cover factor To reveal the methane hydrate growth in the reactor quantitatively, cover factor (f) was proposed, which was defined as the ratio of the practical and theoretical area of the reactor inner surface covered by the hydrates formed with 1 mL reaction solution. Cover factor was calculated by the following formulas, which were derived in our previous study (Wang et al., 2015b). f¼
4π r 2 12 þ 2π rhp 4π r 2 12 þ2π rht
¼
1 þ hp 1 þht
4 3 1 m π r þ 2π rht ¼ V h ¼ V w þΔV w 3 2 Mw
ð1Þ
ð2Þ
where, r is the inner radius of the hemispherical part of the glass tube; hp and ht are the practical and theoretical heights of hydrates on the sidewall of the tube respectively; Vh is the theoretical volume of hydrates; Vw is the volume of surfactant solution; ΔV is the molar volume difference between methane hydrates and water, which has been reported as 4.6 cm3 per mol water (Makogon, 1997); mw is the weight of water; Mw is the molar mass of water.
3. Results and discussion 3.1. Properties of the surfactants
Table 1 The recipes and conditions of emulsion polymerization.
a
methane to 6 MPa. The temperature and pressure of the reactor were recorded by the computer during the whole process. After the hydrate formation, the glass tubes were taken out and frozen under 193.15 K immediately to preserve the hydrate morphology and then photographs were taken for the hydrates.
The properties of the surfactants, such as the critical micelle concentrations (CMC) and solubilities, have been measured through conductivity method in our previous study (Wang et al., 2015a) and Table 2 shows the results from our previous study and K other references. For SDS, the CMC at 298.15 K ( CMC298:15 ) and SDS 275:15 K ) were 8.2 and 4.4 mmol/L, the solubility at 275.15 K ( CMCSDS respectively, which indicated that SDS could not form micelles
Fig. 3. Schematic diagram of the methane hydrate formation apparatus.
F. Wang et al. / Chemical Engineering Science 144 (2016) 108–115
under hydrate formation temperature (275.15 K) because SDS precipitated before reaching the CMC. For SDBS, the CMC at K K 298.15 K ( CMC298:15 ) and 275.15 K (CMC275:15 ) were 2.1 and SDS SDS 1.8 mmol/L respectively and no precipitation of SDBS took place in the range of 1–10 mmol/L at both 275.15 and 298.15 K (Wang et al., 2015a). In this work, both SDS and SDBS were used at 1, 2, 4 and 8 mmol/L, therefore, SDS could not form micelles while SDBS could at 2, 4 and 8 mmol/L. Table 2 The properties of the surfactants used in this work. Surfactant SDS SDBS
CMC (mmol/L) a
8.2 (298.15 K) 7.8 (298.15 K)c 2.1 (298.15 K)a 1.7 (298.15 K)c 1.8 (275.15 K)a 1.6 (275.15 K)c
Solubility (mmol/L) a
Krafft point (K) 281.15b 289.15e 300.75f
4.4 (275.15 K) 6.1 (275.15 K)d
a
Wang et al. (2015a). Watanabe et al. (2005a). Di Profio et al. (2005). d Zhang et al. (2007b). e Tehrani-Bagha et al. (2013). f Šegota et al. (2006). b c
Table 3 Contact angles of the reaction solutions on the reactor sidewalla. Reaction solution
Deionized water SDS (aq.) SDBS (aq.) M-micelle-SDS (aq.) a
Contact angle (°) 1 mmol/L
2 mmol/L
4 mmol/L
8 mmol/L
387 2.0 29.0 7 0.5 317 0.8 327 1.3
32.0 7 0.4 32 71.1 29 71.0
317 1.6 307 1.4 247 1.3
297 2.2 317 1.3 227 1.7
The measurement was carried out at 275.15 K.
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3.2. Methane hydrate growth pattern in the presence of surfactants Fig. 4A shows the morphologies of the methane hydrates formed in the presence of SDS, which has been discussed in our previous study (Wang et al., 2015b). SDS led to obvious upward hydrate growth on the reactor sidewall and the SDS concentration did not produce obvious effect on the upward hydrate growth, as shown by the similar cover factors. Table 3 shows that SDS solutions with different concentrations resulted in very narrow and similar contact angles on the glass surface, which indicated very good wettability of the glass surface with SDS solutions. As proposed in our previous study (Wang et al., 2015a), hydrates initially formed at the gas/liquid/solid interline and the good wettability was conducive to the hydrates growing upward on the reactor sidewall. As a result, even with different concentrations, the SDS solutions led to obvious and similar upward hydrate growth on the reactor sidewall. Fig. 4A shows the pressure evolutions of methane hydrate formation with SDS solutions and the pressure decrease indicated the hydrate formation. SDS led to the hydrate nucleation take place within several hours and the hydrate growth completed within 30–40 min, which was consistent with our previous study carried out in stainless steel reactor (Wang et al., 2015a). In addition, it should be noted that SDS precipitated at 8 mmol/L, therefore, it could be proposed that the precipitation of SDS did not produced obvious effects on the methane hydrate formation rate and hydrate growth pattern. Fig. 4B shows the morphologies of methane hydrates formed with SDBS solutions, which led to obviously less extent of upward hydrate growth compared to SDS solutions. In our previous study carried out in stainless steel reactor (Wang et al., 2015a), SDBS also led to less extent of upward methane hydrate growth on the reactor and we proposed that it was the different wettability resulted in the different hydrate growth patterns. However, in this work, SDBS solutions produced similar contact angles on the glass surface as SDS solutions (Table 3), which meant that the reactor surface showed similar wettability with SDS and SDBS solutions,
Fig. 4. Morphologies of the methane hydrates formed in the presence of SDS (A) and SDBS (B). (The initial conditions were 6 MPa and 275.15 K; the red numbers were cover factors and “-” indicated that no hydrate formation was observed). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Pressure evolutions of methane hydrate formation with SDS (A) and SDBS (B). (The initial conditions were 6 MPa and 275.15 K)
therefore, the effects of the wettability on the different methane hydrate growth pattern could be avoided. The possible reason caused the different hydrate growth patterns between SDS and SDBS solutions might be the micelle formation in SDBS solutions at hydrate formation condition. As discussed above, SDS could not form micelles at hydrate formation conditions, while SDBS could when the concentration over 1.8 mmol/L. In consequence, hydrate formation only took place in the SDBS solutions at micelle-forming concentrations (2, 4 and 8 mmol/L) (Fig. 4B) and higher SDBS concentration led to shorter induction period and rapider hydrate growth (Fig. 5B). This verified the assumption made by Zhang et al. (2007a) that the surfactant concentration should affect the hydrate formation significantly if micelles existed in the hydrate formation system and therefore confirmed the micelle effects of SDBS on promoting methane hydrate formation. In the micelle-forming surfactant solutions, as assumed by Zhong and Rogers (2000), gas molecules could dissolve in the surfactant micelles and hydrates initially formed with the gas molecules in the micelles and the water molecules associated on the micelle surface, leading to the formation of hydrate particles below the gas/liquid interface. Therefore, hydrates formed mainly in the bulk of the SDBS solution and less extent of upward hydrate growth compared to SDS was observed. Moreover, Figs. 4 and 5 also show that the upward hydrate growth on the sidewall led to higher growth rate compared with the hydrate growth in the bulk of the reaction solution, which was consistent with our previous study (Wang et al., 2015a). 3.3. Methane hydrate growth with mimic micelles of SDS To confirm the effects of surfactant micelles on methane hydrate growth pattern, SDS should also be used in the form of micelles in methane hydrate formation. However, as discussed above, the CMC of SDS at 298.15 K was 8.2 mmol/L and the solubility of SDS at 275.15 K was 4.4 mmol/L, which indicated that SDS could not form micelles at 275.15 K at 1, 2, 4 and 8 mmol/L.
Fig. 6. Characterizations of M-micelle-SDS (photos-TEM, d-particle size, PDI-particle size distribution index).
Therefore, SDS-coated nanospheres (mimic micelles of SDS) were applied in methane hydrate formation. Fig. 6 shows the TEM photos and particle size of the SDS-coated polystyrene nanospheres, which appeared as uniform spheres at nanoscale with high monodispersity. In our previous study (Wang et al., 2015c), the surfactant fixation ratio was measured as 81.96%, which denoted that about 20% of SDS existed as free molecules in the SDS-coated nanospheres. The concentrations of free SDS in the SDS-coated nanospheres solutions at 1, 2, 4 and 8 mmol/L were about 0.2, 0.4, 0.8 and 1.6 mmol/L respectively, which showing that no SDS micelles existed in the SDS-coated nanospheres solutions. As SDS-coated nanospheres exhibited similar structures as SDS micelles, about 80% of SDS existed in the form of mimic micelles in methane hydrate formation with M-micelle-SDS. Fig. 7A shows the morphologies of methane hydrates formed with M-micelle-SDS solutions, which resulted in obviously less extent of upward hydrate growth compared with SDS (Figs. 4A and
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Fig. 7. Hydrate growth morphology (A) and pressure evolutions (B) of hydrate formation with M-micelle-SDS. (The initial conditions were 6 MPa and 275.15 K)
7A). As shown in Table 3, M-micelle-SDS solutions led to similar contact angles on the reactor sidewall as SDS solutions, therefore, the effects of wettability of the reactor wall could be avoided. One possible reason was that the depletion of free SDS molecules in the M-micelle-SDS solutions caused the less extent of upward hydrate growth. Another possible reason was that in the hydrate formation with M-micelle-SDS solutions hydrates might initially form on the surface of SDS-coated nanospheres. As a result, hydrates mainly formed in the bulk of the M-micelle-SDS solutions, which led to obviously less extent of upward hydrate growth compared with SDS solutions. This confirmed that the surfactant micelles were conducive to the hydrate formation in the bulk of the reaction solution. Moreover, SDS also led to higher hydrate growth rate compared with M-micelle-SDS due to the different hydrate growth patterns (Figs. 5A and 7B). 3.4. Methane hydrate growth with SDBS at non-micelle forming condition To further study the effect of surfactants on methane hydrate growth pattern, SDBS was used at non-micelle forming condition. As SDBS solutions at non-micelle forming condition did not produce effective promotion to methane hydrate formation (Fig. 5), the compounds of SDS and SDBS (named as SDS-co-SDBS) with different SDS molar fractions were used. Moreover, to ensure the non-micelle forming condition, SDS-co-SDBS was used at 1 mmol/ L (total surfactant concentration) and the pyrene fluorescence spectra of SDS-co-SDBS solutions was measured, which has been successfully applied to detect the formation of surfactant micelles (Lucas et al., 2015). The intensity ratio of the third peak (I3, λ ¼384 nm) and the first peak (I1, λ ¼ 374 nm) of the pyrene fluorescence emission spectra could denote the polarities of the microdomains (Zhang et al., 2008) and the formation of micelles
Table 4 I3/I1 values of SDS-co-SDBS solutions and contact angles of SDS-co-SDBS solutions with different SDS molar fractions (mf) on the glass surfacea. SDS-co-SDBS (SDS mf)
0.1
0.3
0.5
0.7
I3/I1 Contact angle (°)
0.92 39 71.6
0.94 297 1.3
0.93 277 1.6
0.93 367 0.9
a The concentration of SDS-co-SDBS was 1 mmol/L; both the I3/I1 values and contact angles were measured at 275.15 K.
could lead to the obvious increase in the I3/I1 values, as shown in Fig. S1 in the supporting information. Table 4 shows that the I3/I1 values of SDS-co-SDBS solutions at 1 mmol/L were 0.92–0.94, which were similar as those of the solutions of dodecyl alcohol ethoxylates at non-micelle forming conditions (shown in Fig. S1 in the supporting information), confirming that no micelles were formed in the SDS-co-SDBS solutions at 1 mmol/L. In addition, Table 4 also shows that SDS-co-SDBS solutions (1 mmol/L) led to similar contact angles on the glass surface compared to SDS and SDBS solutions (Table 1). Fig. 8A shows the pressure evolutions of methane hydrate formation with 1 mmol/L SDS-co-SDBS solutions, which produced efficient promotion to hydrate formation. In this work, SDS solutions at 0.1, 0.3 and 0.5 mmol/L did not produce promotion to methane hydrate formation (shown in Fig. S2 in the supporting information), therefore, it might be proposed that the recombination action of SDS and SDBS led to efficient promotion to methane hydrate formation, especially at low molar fraction of SDS. Fig. 8B shows the morphologies of methane hydrates formed with SDS-co-SDBS solutions, which led to much higher extent of upward hydrate growth compared with SDBS solutions at micelle forming condition. One possible reason was that SDS was much more conducive to the upward methane hydrate growth compared
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Fig. 8. Pressure evolutions (A) and hydrate growth morphologies (B) of hydrate formation with SDS-co-SDBS solutions. (The concentration of SDS-co-SDBS was 1 mmol/L, the initial conditions were 6 MPa and 275.15 K)
with SDBS, therefore, when SDS-co-SDBS was used the existence of SDS could promote the hydrates to grow upwards. However, it should be noted that even at very low molar fraction of SDS (e.g. 0.1) SDS-co-SDBS resulted in obvious upward hydrate growth. Therefore, another more possible reason was that surfactants at non-micelle forming condition were more conducive to the upward hydrate growth, while surfactants at micelle forming condition were more conducive for the hydrate formation in the bulk of the reaction solution. As the promotion mechanism of surfactants to gas hydrate formation still remains poorly understood, the results in this work could provide better understanding about the effects of surfactant micelles during methane hydrate formation. In addition, in the hydrate-based gas storage and transportation, the hydrates are usually separated and compacted for storage and transportation. Therefore, it is very necessary to obtain gas hydrates with high apparent density. As shown in this work, the SDS-coated nanospheres led to the hydrates formed at the bottom of the reactor with higher apparent density, therefore, the results of this work was of great significance to the hydrate-based gas storage and transportation.
was conducive to the hydrate formation in the bulk of solution. When SDS-co-SDBS at non-micelle forming condition was used, prominent upward hydrate growth was obtained, indicating that non-micelle forming surfactants were conducive to the upward hydrate growth on the reactor sidewall.
Nomenclature f r hp ht Vh Vw ΔV mw Mw
cover factor inner radius of the glass tube practical heights of the hydrates on the reactor sidewall theoretical heights of the hydrates on the reactor sidewall theoretical volume of the hydrates volume of surfactant solution molar volume difference between hydrates and water weight of water molar mass of water
4. Conclusions The effects of different surfactants (SDS and SDBS) on methane hydrate growth pattern were studied. Non-micelle forming SDS led to obvious upward hydrate growth on the reactor sidewall, while SDBS led to much less extent of upward hydrate growth due to the micelle effect. When M-micelle-SDS, with similar structure as SDS micelle, was used, obviously less extent of upward hydrate growth was observed, confirming that the existence of micelles
Acknowledgment This work was supported by the National Natural Science Foundation of China (31101918), Key Projects in the National Science and Technology Pillar Program (20140015) and Qingdao Science and Technology and People's Livelihood Project (14-2-369-nsh).
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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2016.01.022.
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