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Self-assembly with varying hydrophobic centers: Synthesis of red blood cell-like basic magnesium carbonate microspheres
Particuology 22 (2015) 145–150
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Self-assembly with varying hydrophobic centers: Synthesis of red blood cell-like basic magnesium carbonate microspheres Xinlong Ma, Guoqing Ning ∗ , Bing Chen, Chuanlei Qi, Xingying Lan, Jinsen Gao State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 27 August 2014 Accepted 5 September 2014 Keywords: Self-assembly Red blood cell-like Basic magnesium carbonate Microspheres
a b s t r a c t Basic magnesium carbonate microspheres with a red blood cell (RBC)-like appearance and diameters of ∼3 m were synthesized by amphiphilic molecule-participated self-assembly under hydrothermal conditions. In the self-assembly, sodium dodecyl benzene sulfonate served as a template for the formation of Mg(OH)2 spherical micelles and also as a reactant precursor that releases CO2 to react with Mg(OH)2 . The growth of the microspheres is driven by the continuous generation of new hydrophobic centers because of the consumption of hydrophilic poles ( SO3 − ). The surfactant-directed self-assembly can be applied to the synthesis of other carbonate or metallic oxide self-assemblies, indicating that it is a universal self-assembly method for amphiphilic molecules. © 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Amphiphilic surfactant molecules with both hydrophilic and hydrophobic groups tend to self-assemble into aggregates in aqueous solutions containing micelles, vesicles, lamellae or nanofibers when the surfactant concentration is higher than the critical micelle concentration (Discher & Kamien, 2004; Gu, Shi, Cheng, Lu, & Zheng, 2013; Hao & Hoffmann, 2004; Hartgerink, Beniash, & Stupp, 2001; Scanlon & Aggeli, 2008; Svenson, 2004). As a result, the microstructures, shapes, and properties of the aggregates in the aqueous surfactant solutions mainly depend on the composition and types of surfactant molecules (Liu et al., 2010; Wang et al., 2012). Therefore, amphiphilic molecules are widely used as templates for the preparation of anisotropic nanostructures (Mann, 2009). Biological tissues have been synthesized in aqueous environments under mild physiological conditions with the self- and co-assembly of biomacromolecules. These were primarily proteins, but also carbohydrates and lipids (Lakshmanan, Zhang, & Hauser, 2012; Rajagopal & Schneider, 2004; Sarikaya, Tamerler, Jen, Schulten, & Baneyx, 2003; Zhang, Marini, Hwang, & Santoso, 2002). In these processes, peptides or proteins are building blocks for the ordered assembly of materials and they are also structural templates characterized by significant
∗ Corresponding author. Tel.: +86 10 89733085; fax: +86 10 69724721. E-mail address: [email protected] (G. Ning).
surface activity (Dickinson, 1997; Liang, Patel, Matia-Merino, Ye, & Golding, 2013). The surface-active functional groups in these macromolecules may be transformed or may be removed during self- or co-assembly. This significantly affects the surface forces and the assembly process. Therefore, an investigation into a selfassembly process incorporating the transformation or removal of surface-active functional groups is of great importance. Herein, we report a typical amphiphilic molecule-participated self-assembly process, which leads to the topological growth of RBC-like basic magnesium carbonate microspheres. In the selfassembly process, sodium dodecyl benzene sulfonate (SDBS) serves as a template for the formation of Mg(OH)2 spherical micelles and also as a reactant precursor that releases CO2 , which reacts with Mg(OH)2 . Continuously renewed hydrophobic centers formed because of the decomposition of hydrophilic groups in SDBS, which allows the growth of RBC-like microspheres. The as-obtained microsphere structure has good structural stability and no agglomeration was observed. Its structure was retained after ultrasonic dispersion. Although several reports have dealt with the synthesis of basic magnesium carbonate microtubes (Mitsuhashi et al., 2005), petaloid-like microspheres (Ohkubo et al., 2007) and honeycomb-like microspheres (Gao & Xiang, 2010), RBC-like basic magnesium carbonate microspheres have not been reported. Olive-like copperoxide microspheres have also been successfully synthesized using the same approach, indicating that the suggested surfactant-consuming self-assembly process exists widely in nature.
http://dx.doi.org/10.1016/j.partic.2014.09.002 1674-2001/© 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Fig. 1. (a)–(d) SEM images, (e) TEM image, and (f) SAED pattern of the RBC-like microspheres. The inset of (f) shows a TEM image of the edge of a RBC-like magnesium carbonate microsphere.
Experimental Synthesis of RBC-like basic magnesium carbonate microspheres Commercial NH3 ·H2 O (Sinopharm Chemical Reagent Beijing Co. Ltd., China), MgSO4 ·7H2 O and SDBS (Tianjin Fuchen Chemical Reagents Factory, China) of analytic purity were used to synthesize the RBC-like basic magnesium carbonate microspheres. The alkyl chain in SDBS is a straight-chain. The alkyl chain is present at the para-position of SO3 Na. In a typical synthesis, SDBS was added to 50 mL of a MgSO4 aqueous solution (3.0 mol/L) at a mole ratio of SDBS/MgSO4 = 0.12. Then, 20 mL NH3 ·H2 O solution (5.0 mol/L) was added to the prepared solution in a drop by drop manner with agitation. The obtained slurry was sealed in a Teflon autoclave and heated in an oven equipped with a rotary arm. Hydrothermal
treatment at 150 ◦ C for 10 h was carried out at a rotation rate of 30 rpm. After cooling to room temperature the products were filtered, rinsed with deionized water and dried in a vacuum oven at 100 ◦ C for 24 h. Characterization The as-prepared RBC-like basic magnesium carbonate microspheres were characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Holand), transmission electron microscopy (TEM, JEM 2010, JEOL, Japan), X-ray diffraction (XRD, D8 Advance, Bruker, Germany), and thermogravimetric analysis (TGA, STA409PC, Netzsch, Germany) on a Q500 under nitrogen flow at a temperature ramp rate of 10 ◦ C/min. SDBS before and after calcination was characterized by Fourier transform infrared spectroscopy (FT-IR,
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Fig. 2. (a) A SEM image of the sample prepared without SDBS; XRD patterns (b) of whiskers and microspheres; and TGA curves of the whiskers (c) and the RBC-like microspheres (d).
Magna-IR 560E.S.P, Nicolet, American). The reaction’s supernatant was characterized by high performance liquid chromatography combined with ion-trap time of flight–mass spectrometry (HPLCIT-TOF-MS, Shimadzu, Japan). Results and discussion The surfactant-consumed self-assembly synthesis was performed by the hydrothermal treatment of a slurry containing SDBS, NH3 ·H2 O, and MgSO4 . Fig. 1 shows SEM images of the asprepared material. RBC-like microspheres without agglomeration were observed in the wide field of view (Fig. 1(a)). The RBC-like microspheres have a diameter of 2.8–3.5 m and a thickness of 0.8–1.5 m (Fig. 1(b)). Close examination of the RBC-like microspheres (Fig. 1(c) and 1(d)) reveals that they are composed of nanosheets with a thickness of 20–40 nm. The TEM observation (Fig. 1(e)) confirms that the RBC-like structure has a thin core and a thick periphery. The RBC-like microspheres were retained after ultrasonic dispersion during the preparation of the TEM sample, indicating that the as-obtained microspheres have excellent structural stability. The selected area electron diffraction (SAED) pattern of the edge of a microsphere (Fig. 1(f)) was found to consist of intermittent rings, and it was indexed to the interplanar spacings of 4MgCO3 ·Mg(OH)2 ·4H2 O. The intermittent rings also indicate that each of the nanosheets is a single crystal. A control experiment was carried out using almost the same process but without the addition of SDBS. Interestingly, only whiskers with a length of 40–120 m were obtained (Fig. 2(a)), and this result is consistent with that of previous reports (Liu, Xiang, & Jin, 2004; Sun & Xiang, 2008). XRD analysis (Fig. 2(b)) shows that the RBC-like microspheres and the whiskers were composed of 4MgCO3 ·Mg(OH)2 ·4H2 O (PDF No. 25-0513) and 5Mg(OH)2 ·MgSO4 ·3H2 O (PDF No. 07-0415), respectively. No
impurity peaks were found in the XRD patterns indicating high purity products. The TGA curve of the whiskers (Fig. 2(c)) exhibits a weight-loss peak at 1029 ◦ C because of MgSO4 decomposition. The residual mass (51.73%) is consistent with the theoretical MgO yield after the decomposition of 5Mg(OH)2 ·MgSO4 ·3H2 O (51.72%). In the TGA curve of the RBC-like microspheres (Fig. 2(d)), no weight loss was observed at temperatures higher than 800 ◦ C, and the residual mass (42.66%) is almost equal to the theoretical MgO yield because of the decomposition of 4MgCO3 ·Mg(OH)2 ·4H2 O (42.92%). All the analyses indicate that the addition of SDBS leads to the formation of basic magnesium carbonate microspheres. This is in sharp contrast to the basic magnesium sulfate whiskers obtained without SDBS. The SDBS to MgSO4 ratio was found to be an important factor for the formation of RBC-like microspheres (Fig. 3). At a mole ratio less than 0.067, RBC-like morphologies were not in the majority and some whiskers or sheets were present in the as-prepared materials. The amount of RBC-like microspheres increased and the amount of surface impurities decreased with an increase in the SDBS to MgSO4 ratio. Perfect RBC-like microspheres were obtained at a mole ratio of SDBS/MgSO4 = 0.12. A further increase in the SDBS to MgSO4 ratio resulted in the formation of smaller microspheres. This may be attributed to an increase in the number of nucleation centers when using more SDBS. The effect of the SDBS to MgSO4 ratio indicates that the presence of SDBS is crucial for the nucleation of microspheres. The mechanism for the formation of basic magnesium sulfate whiskers is shown in Eq. (1): 6MgSO4 + 10NH4 OH + 3H2 O → 5Mg(OH)2 ·MgSO4 ·3H2 O + 5(NH4 )2 SO4
(1)
However, the mechanism for the formation of basic magnesium carbonate microspheres is complicated and is summarized in
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Fig. 3. SEM images of the samples prepared at SDBS to MgSO4 mole ratios of 0.05 (a), 0.067 (b), 0.083 (c), 0.1 (d), 0.12 (e), and 0.13 (f).
Eq (2): 5MgSO4 + 10NH4 ·OH + 4CO2 → 4MgCO3 ·Mg(OH)2 ·4H2 O + 5(NH4 )2 SO4
(2)
CO2 is required for the formation of basic magnesium carbonate. However, a question arises about the source of CO2 . We found that decomposition of SDBS (C12 H25 –C6 H4 –SO3 Na) can release CO2 . In an auxiliary experiment, 0.01 mol SDBS was added to a suspension containing 0.01 mol Ca(OH)2 and this suspension was subjected to hydrothermal treatment at 150 ◦ C. XRD analysis shows that CaCO3 formed (Fig. S1). This indicates that SDBS may release CO2 during hydrothermal treatment. The XRD patterns of the 150 and 200 ◦ C calcined SDBS samples are quite different to that of the original SDBS (Fig. S2), indicating that the decomposition of SDBS occurs at
150 ◦ C. In the TGA curve of SDBS (Fig. S3), the weight loss of SDBS at 200 ◦ C is 17.88%. This is consistent with the theoretical value of 17.79%, which corresponds to the release of one H2 O and one CO2 molecule. According to Eq. (2), the yield of 4MgCO3 ·Mg(OH)2 ·4H2 O should be no more than 0.25 mol for 1 mol SDBS. In a typical run, 0.018 mol SDBS was added and this amount corresponds to a theoretical yield of 0.0045 mol (2.19 g) 4MgCO3 ·Mg(OH)2 ·4H2 O. The practical yield of 4MgCO3 ·Mg(OH)2 ·4H2 O was 1.89 g, indicating a yield of 86.5%. The mass balance thus provides powerful evidence for the suggested basic magnesium carbonate microsphere formation reaction. The suggested formation mechanism for the RBC-like microspheres is shown in Fig. 4. SDBS is an anionic surfactant that dissolves easily in aqueous solutions and forms spherical micelles with outer exposed hydrophilic poles ( SO3 − ) (Ma, Boyd, & Drummond, 2006). The Mg(OH)2 colloidal particles adsorb onto
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Fig. 4. Suggested mechanism for the formation of RBC-like microspheres.
the surface of the spherical micelles (Fig. 4(a)). Under the hydrothermal condition used (150 ◦ C), CO2 is released by SDBS (from the hydrophilic poles) and this reacts with the Mg(OH)2 particles leading to 4MgCO3 ·Mg(OH)2 ·4H2 O micro-flakes around the spherical micelle (Fig. 4(b)). FT-IR analysis (Fig. 5) shows that the absorption peaks that correspond to the benzene ring (1627 cm−1 ) and the sulfonic group (690 and 1320 cm−1 ) of SDBS decreased significantly or disappeared after the 150 ◦ C calcination. The peak corresponding to C S C (1452 cm−1 ) increased and the hydrophobic group peak (alkyl side chain, 3453 cm−1 ) did not change after the calcination. According to the high resolution mass spectrum of the reaction supernatant (Fig. 6), SDBS (C18 H29 SO3 Na) decomposed into SO3 Na and C18 H27 (m/z = 243.0547) under the hydrothermal condition used. C18 H27 then further decomposed into a benzene ring and an alkyl chain (m/z = 164.9205). SDBS also released CO2 to form C17 H31 SONa (m/z = 306.8518). C17 H31 SONa combined with one or two MgSO4 molecules in solution to form C17 H31 SONa·MgSO4 (m/z = 426.7972) or C17 H31 SONa·2MgSO4 (m/z = 546.7452). The peak at m/z = 674.2513 can be attributed to Mg(C18 H29 SO3 )2 , indicating that SDBS reacted with MgSO4 to form Na2 SO4 and Mg(C18 H29 SO3 )2 . Additionally, C17 H31 SONa·2MgSO4 combined with Na2 SO4 to form C17 H31 SONa·2MgSO4 ·Na2 SO4 (m/z = 688.6750). Therefore, the decomposed residue of SDBS is likely to be hydrophobic because the hydrophilic poles (R SO3 − ) were converted into C S C hydrophobic structures after the release of CO2 . Therefore,
the micro-flakes can act as new hydrophobic centers (Fig. 4(c)) that attract SDBS to its surface (Fig. 4(d)). This permits the continuous growth of micro-flakes or the attachment of other flakes (Fig. 4(e)). Because of the attractive force between the flakes, more flakes will stack into a RBC-like structure rather than form a perfect sphere. Because the formation of basic magnesium carbonate occurs on the surface of the SDBS micelle, the micro-flakes grow from the core to the surface. RBC-like morphology is thus obtained with a thin core and a thick periphery. The growth of the microspheres from a tiny SDBS micelle to a sphere of several microns is driven by the continuous generation of new hydrophobic centers because of the consumption of hydrophilic poles ( SO3 − ). Such a surfactant-templated and consumed assembly process has also been used to synthesize olive-like copperoxide microspheres (Fig. S4), indicating that it is a universal assembly mode for amphiphilic molecules. According to the mass spectrometry data, SDBS can react with Mg2+ to form the Mg(SO3 –C6 H4 –C12 H25 )2 intermediate, which plays an important role in inhibiting the formation of the magnesium oxysulfate phase. A previous study (Sun & Xiang, 2008) also showed that SDBS can react with Mg2+ (Eq. (3)). Sun and
Fig. 5. FT-IR of SDBS before and after calcination at 150 ◦ C.
Fig. 6. High resolution mass spectrum of the reaction supernatant.
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Xiang (2008) reported that small and poorly crystallized Mg(OH)2 precursors formed in the presence of SDBS resulting in the homogeneous precipitation of 513MOS whiskers without intersections. However, the mass ratio of SDBS used in the experiment (0.2 wt%) was far lower than that used in this work (25.31 wt%). At a low SDBS content (12.66–22.48 wt%), the partial formation of 513MOS whiskers was observed (Fig. 3(a)–(d)), and this is consistent with Xiang’s report. However, at a SDBS content of 25.31 wt% no 513MOS whiskers were observed, indicating that the higher amount of SDBS contributes to inhibiting the formation of 513MOS. Because large amounts of SDBS can combine with MgSO4 , as shown in Eq. (3), MgSO4 does not easily combine with Mg(OH)2 to form 513MOS, as shown in Eq. (4). As a result, RBC-like basic magnesium carbonate microspheres of high purity without 513MOS whiskers form. MgSO4 + 2SDBS → Mg(SO3 –C6 H4 –C12 H25 )2 + Na2 SO4
(3)
5Mg(OH)2 + MgSO4 + 3H2 O → 5Mg(OH)2 ·MgSO4 ·3H2 O
(4)
Conclusions In summary, RBC-like basic magnesium carbonate microspheres were synthesized by a hydrothermal process in the presence of SDBS. The formation of RBC-like microspheres is attributed to surfactant-participated self-assembly during which the Mg(OH)2 particles that are attached to the SDBS micelles react with the CO2 released by SDBS and are finally converted to basic magnesium carbonate micro-flakes. Because of the consumption of hydrophilic poles ( SO3 − ), the resultant flakes transform into new hydrophobic centers and this permits the continuous growth of micro-flakes or the attachment of other flakes. Surfactant-directed self-assembly can be applied to the synthesis of other carbonate or metallic oxide self-assemblies. Therefore, the reported method can be used for the universal self-assembly of amphiphilic molecules. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21206191) and the Science Foundation of China University of Petroleum, Beijing (No. 2462013YXBS007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.partic. 2014.09.002.
References Dickinson, E. (1997). Properties of emulsions stabilized with milk proteins: Overview of some recent developments. Journal of Dairy Science, 80, 2607–2619. Discher, D. E., & Kamien, R. D. (2004). Self-assembly: Towards precision micelles. Nature, 430, 519–520. Gao, G., & Xiang, L. (2010). Emulsion-phase synthesis of honeycomb-like Mg5 (OH)2 (CO3 )4 ·4H2 O micro-spheres and subsequent decomposition to MgO. Journal of Alloys and Compounds, 495, 242–246. Gu, Y., Shi, L., Cheng, X., Lu, F., & Zheng, L. (2013). Aggregation behavior of 1-dodecyl3-methylimidazolium bromide in aqueous solution: Effect of ionic liquids with aromatic anions. Langmuir, 29, 6213–6220. Hao, J., & Hoffmann, H. (2004). Self-assembled structures in excess and salt-free cat anionic surfactant solutions. Current Opinion in Colloid and Interface Science, 9, 279–293. Hartgerink, J. D., Beniash, E., & Stupp, S. I. (2001). Self-assembly and mineralization of peptide-amphiphile nanofibers. Science, 294, 1684–1688. Lakshmanan, A., Zhang, S., & Hauser, C. A. (2012). Short self-assembling peptides as building blocks for modern nanodevices. Trends in Biotechnology, 30, 155–165. Liang, Y., Patel, H., Matia-Merino, L., Ye, A., & Golding, M. (2013). Structure and stability of heat-treated concentrated dairy-protein-stabilised oil-in-water emulsions: A stability map characterisation approach. Food Hydrocolloids, 33, 297–308. Liu, F., Xiang, L., & Jin, Y. (2004). Hydrothermal synthesis process of magnesium oxysulfate whiskers. Journal of Inorganic Materials, 19, 784–788 (in Chinese) Liu, T., Li, D., Zou, Y., Yang, D., Li, H., Wu, Y., et al. (2010). Preparation of metal@silica core-shell particle films by interfacial self-assembly. Journal of Colloid and Interface Science, 350, 58–62. Ma, J.-G., Boyd, B. J., & Drummond, C. J. (2006). Positional isomers of linear sodium dodecyl benzene sulfonate: Solubility, self-assembly, and air/water interfacial activity. Langmuir, 22, 8646–8654. Mann, S. (2009). Self-assembly and transformation of hybrid nano-objects and nanostructures under equilibrium and non-equilibrium conditions. Nature Materials, 8, 781–792. Mitsuhashi, K., Tagami, N., Tanabe, K., Ohkubo, T., Sakai, H., Koishi, M., et al. (2005). Synthesis of microtubes with a surface of house of cards structure via needlelike particles and control of their pore size. Langmuir, 21, 3659–3663. Ohkubo, T., Suzuki, S., Mitsuhashi, K., Ogura, T., Iwanaga, S., Sakai, H., et al. (2007). Preparation of petaloid microspheres of basic magnesium carbonate. Langmuir, 23, 5872–5874. Rajagopal, K., & Schneider, J. P. (2004). Self-assembling peptides and proteins for nanotechnological applications. Current Opinion in Structural Biology, 14, 480–486. Sarikaya, M., Tamerler, C., Jen, A. K.-Y., Schulten, K., & Baneyx, F. (2003). Molecular biomimetics: Nanotechnology through biology. Nature Materials, 2, 577–585. Scanlon, S., & Aggeli, A. (2008). Self-assembling peptide nanotubes. Nano Today, 3, 22–30. Sun, X. T., & Xiang, L. (2008). Synthesis of magnesium oxysulfate whiskers in the presence of sodium dodecyl benzene sulfonate. Crystal Research and Technology, 43, 479–482. Svenson, S. (2004). Controlling surfactant self-assembly. Current Opinion Colloid and Interface Science, 9, 201–212. Wang, T., Zhuang, J., Lynch, J., Chen, O., Wang, Z., Wang, X., et al. (2012). Selfassembled colloidal superparticles from nanorods. Science, 338, 358–363. Zhang, S., Marini, D. M., Hwang, W., & Santoso, S. (2002). Design of nanostructured biological materials through self-assembly of peptides and proteins. Current Opinion Chemical Biology, 6, 865–871.
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Self-assembly with varying hydrophobic centers: Synthesis of red blood cell-like basic magnesium carbonate microspheres Xinlong Ma, Guoqing Ning ∗ , Bing Chen, Chuanlei Qi, Xingying Lan, Jinsen Gao State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 27 August 2014 Accepted 5 September 2014 Keywords: Self-assembly Red blood cell-like Basic magnesium carbonate Microspheres
a b s t r a c t Basic magnesium carbonate microspheres with a red blood cell (RBC)-like appearance and diameters of ∼3 m were synthesized by amphiphilic molecule-participated self-assembly under hydrothermal conditions. In the self-assembly, sodium dodecyl benzene sulfonate served as a template for the formation of Mg(OH)2 spherical micelles and also as a reactant precursor that releases CO2 to react with Mg(OH)2 . The growth of the microspheres is driven by the continuous generation of new hydrophobic centers because of the consumption of hydrophilic poles ( SO3 − ). The surfactant-directed self-assembly can be applied to the synthesis of other carbonate or metallic oxide self-assemblies, indicating that it is a universal self-assembly method for amphiphilic molecules. © 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Amphiphilic surfactant molecules with both hydrophilic and hydrophobic groups tend to self-assemble into aggregates in aqueous solutions containing micelles, vesicles, lamellae or nanofibers when the surfactant concentration is higher than the critical micelle concentration (Discher & Kamien, 2004; Gu, Shi, Cheng, Lu, & Zheng, 2013; Hao & Hoffmann, 2004; Hartgerink, Beniash, & Stupp, 2001; Scanlon & Aggeli, 2008; Svenson, 2004). As a result, the microstructures, shapes, and properties of the aggregates in the aqueous surfactant solutions mainly depend on the composition and types of surfactant molecules (Liu et al., 2010; Wang et al., 2012). Therefore, amphiphilic molecules are widely used as templates for the preparation of anisotropic nanostructures (Mann, 2009). Biological tissues have been synthesized in aqueous environments under mild physiological conditions with the self- and co-assembly of biomacromolecules. These were primarily proteins, but also carbohydrates and lipids (Lakshmanan, Zhang, & Hauser, 2012; Rajagopal & Schneider, 2004; Sarikaya, Tamerler, Jen, Schulten, & Baneyx, 2003; Zhang, Marini, Hwang, & Santoso, 2002). In these processes, peptides or proteins are building blocks for the ordered assembly of materials and they are also structural templates characterized by significant
∗ Corresponding author. Tel.: +86 10 89733085; fax: +86 10 69724721. E-mail address: [email protected] (G. Ning).
surface activity (Dickinson, 1997; Liang, Patel, Matia-Merino, Ye, & Golding, 2013). The surface-active functional groups in these macromolecules may be transformed or may be removed during self- or co-assembly. This significantly affects the surface forces and the assembly process. Therefore, an investigation into a selfassembly process incorporating the transformation or removal of surface-active functional groups is of great importance. Herein, we report a typical amphiphilic molecule-participated self-assembly process, which leads to the topological growth of RBC-like basic magnesium carbonate microspheres. In the selfassembly process, sodium dodecyl benzene sulfonate (SDBS) serves as a template for the formation of Mg(OH)2 spherical micelles and also as a reactant precursor that releases CO2 , which reacts with Mg(OH)2 . Continuously renewed hydrophobic centers formed because of the decomposition of hydrophilic groups in SDBS, which allows the growth of RBC-like microspheres. The as-obtained microsphere structure has good structural stability and no agglomeration was observed. Its structure was retained after ultrasonic dispersion. Although several reports have dealt with the synthesis of basic magnesium carbonate microtubes (Mitsuhashi et al., 2005), petaloid-like microspheres (Ohkubo et al., 2007) and honeycomb-like microspheres (Gao & Xiang, 2010), RBC-like basic magnesium carbonate microspheres have not been reported. Olive-like copperoxide microspheres have also been successfully synthesized using the same approach, indicating that the suggested surfactant-consuming self-assembly process exists widely in nature.
http://dx.doi.org/10.1016/j.partic.2014.09.002 1674-2001/© 2014 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Fig. 1. (a)–(d) SEM images, (e) TEM image, and (f) SAED pattern of the RBC-like microspheres. The inset of (f) shows a TEM image of the edge of a RBC-like magnesium carbonate microsphere.
Experimental Synthesis of RBC-like basic magnesium carbonate microspheres Commercial NH3 ·H2 O (Sinopharm Chemical Reagent Beijing Co. Ltd., China), MgSO4 ·7H2 O and SDBS (Tianjin Fuchen Chemical Reagents Factory, China) of analytic purity were used to synthesize the RBC-like basic magnesium carbonate microspheres. The alkyl chain in SDBS is a straight-chain. The alkyl chain is present at the para-position of SO3 Na. In a typical synthesis, SDBS was added to 50 mL of a MgSO4 aqueous solution (3.0 mol/L) at a mole ratio of SDBS/MgSO4 = 0.12. Then, 20 mL NH3 ·H2 O solution (5.0 mol/L) was added to the prepared solution in a drop by drop manner with agitation. The obtained slurry was sealed in a Teflon autoclave and heated in an oven equipped with a rotary arm. Hydrothermal
treatment at 150 ◦ C for 10 h was carried out at a rotation rate of 30 rpm. After cooling to room temperature the products were filtered, rinsed with deionized water and dried in a vacuum oven at 100 ◦ C for 24 h. Characterization The as-prepared RBC-like basic magnesium carbonate microspheres were characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Holand), transmission electron microscopy (TEM, JEM 2010, JEOL, Japan), X-ray diffraction (XRD, D8 Advance, Bruker, Germany), and thermogravimetric analysis (TGA, STA409PC, Netzsch, Germany) on a Q500 under nitrogen flow at a temperature ramp rate of 10 ◦ C/min. SDBS before and after calcination was characterized by Fourier transform infrared spectroscopy (FT-IR,
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Fig. 2. (a) A SEM image of the sample prepared without SDBS; XRD patterns (b) of whiskers and microspheres; and TGA curves of the whiskers (c) and the RBC-like microspheres (d).
Magna-IR 560E.S.P, Nicolet, American). The reaction’s supernatant was characterized by high performance liquid chromatography combined with ion-trap time of flight–mass spectrometry (HPLCIT-TOF-MS, Shimadzu, Japan). Results and discussion The surfactant-consumed self-assembly synthesis was performed by the hydrothermal treatment of a slurry containing SDBS, NH3 ·H2 O, and MgSO4 . Fig. 1 shows SEM images of the asprepared material. RBC-like microspheres without agglomeration were observed in the wide field of view (Fig. 1(a)). The RBC-like microspheres have a diameter of 2.8–3.5 m and a thickness of 0.8–1.5 m (Fig. 1(b)). Close examination of the RBC-like microspheres (Fig. 1(c) and 1(d)) reveals that they are composed of nanosheets with a thickness of 20–40 nm. The TEM observation (Fig. 1(e)) confirms that the RBC-like structure has a thin core and a thick periphery. The RBC-like microspheres were retained after ultrasonic dispersion during the preparation of the TEM sample, indicating that the as-obtained microspheres have excellent structural stability. The selected area electron diffraction (SAED) pattern of the edge of a microsphere (Fig. 1(f)) was found to consist of intermittent rings, and it was indexed to the interplanar spacings of 4MgCO3 ·Mg(OH)2 ·4H2 O. The intermittent rings also indicate that each of the nanosheets is a single crystal. A control experiment was carried out using almost the same process but without the addition of SDBS. Interestingly, only whiskers with a length of 40–120 m were obtained (Fig. 2(a)), and this result is consistent with that of previous reports (Liu, Xiang, & Jin, 2004; Sun & Xiang, 2008). XRD analysis (Fig. 2(b)) shows that the RBC-like microspheres and the whiskers were composed of 4MgCO3 ·Mg(OH)2 ·4H2 O (PDF No. 25-0513) and 5Mg(OH)2 ·MgSO4 ·3H2 O (PDF No. 07-0415), respectively. No
impurity peaks were found in the XRD patterns indicating high purity products. The TGA curve of the whiskers (Fig. 2(c)) exhibits a weight-loss peak at 1029 ◦ C because of MgSO4 decomposition. The residual mass (51.73%) is consistent with the theoretical MgO yield after the decomposition of 5Mg(OH)2 ·MgSO4 ·3H2 O (51.72%). In the TGA curve of the RBC-like microspheres (Fig. 2(d)), no weight loss was observed at temperatures higher than 800 ◦ C, and the residual mass (42.66%) is almost equal to the theoretical MgO yield because of the decomposition of 4MgCO3 ·Mg(OH)2 ·4H2 O (42.92%). All the analyses indicate that the addition of SDBS leads to the formation of basic magnesium carbonate microspheres. This is in sharp contrast to the basic magnesium sulfate whiskers obtained without SDBS. The SDBS to MgSO4 ratio was found to be an important factor for the formation of RBC-like microspheres (Fig. 3). At a mole ratio less than 0.067, RBC-like morphologies were not in the majority and some whiskers or sheets were present in the as-prepared materials. The amount of RBC-like microspheres increased and the amount of surface impurities decreased with an increase in the SDBS to MgSO4 ratio. Perfect RBC-like microspheres were obtained at a mole ratio of SDBS/MgSO4 = 0.12. A further increase in the SDBS to MgSO4 ratio resulted in the formation of smaller microspheres. This may be attributed to an increase in the number of nucleation centers when using more SDBS. The effect of the SDBS to MgSO4 ratio indicates that the presence of SDBS is crucial for the nucleation of microspheres. The mechanism for the formation of basic magnesium sulfate whiskers is shown in Eq. (1): 6MgSO4 + 10NH4 OH + 3H2 O → 5Mg(OH)2 ·MgSO4 ·3H2 O + 5(NH4 )2 SO4
(1)
However, the mechanism for the formation of basic magnesium carbonate microspheres is complicated and is summarized in
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Fig. 3. SEM images of the samples prepared at SDBS to MgSO4 mole ratios of 0.05 (a), 0.067 (b), 0.083 (c), 0.1 (d), 0.12 (e), and 0.13 (f).
Eq (2): 5MgSO4 + 10NH4 ·OH + 4CO2 → 4MgCO3 ·Mg(OH)2 ·4H2 O + 5(NH4 )2 SO4
(2)
CO2 is required for the formation of basic magnesium carbonate. However, a question arises about the source of CO2 . We found that decomposition of SDBS (C12 H25 –C6 H4 –SO3 Na) can release CO2 . In an auxiliary experiment, 0.01 mol SDBS was added to a suspension containing 0.01 mol Ca(OH)2 and this suspension was subjected to hydrothermal treatment at 150 ◦ C. XRD analysis shows that CaCO3 formed (Fig. S1). This indicates that SDBS may release CO2 during hydrothermal treatment. The XRD patterns of the 150 and 200 ◦ C calcined SDBS samples are quite different to that of the original SDBS (Fig. S2), indicating that the decomposition of SDBS occurs at
150 ◦ C. In the TGA curve of SDBS (Fig. S3), the weight loss of SDBS at 200 ◦ C is 17.88%. This is consistent with the theoretical value of 17.79%, which corresponds to the release of one H2 O and one CO2 molecule. According to Eq. (2), the yield of 4MgCO3 ·Mg(OH)2 ·4H2 O should be no more than 0.25 mol for 1 mol SDBS. In a typical run, 0.018 mol SDBS was added and this amount corresponds to a theoretical yield of 0.0045 mol (2.19 g) 4MgCO3 ·Mg(OH)2 ·4H2 O. The practical yield of 4MgCO3 ·Mg(OH)2 ·4H2 O was 1.89 g, indicating a yield of 86.5%. The mass balance thus provides powerful evidence for the suggested basic magnesium carbonate microsphere formation reaction. The suggested formation mechanism for the RBC-like microspheres is shown in Fig. 4. SDBS is an anionic surfactant that dissolves easily in aqueous solutions and forms spherical micelles with outer exposed hydrophilic poles ( SO3 − ) (Ma, Boyd, & Drummond, 2006). The Mg(OH)2 colloidal particles adsorb onto
X. Ma et al. / Particuology 22 (2015) 145–150
149
Fig. 4. Suggested mechanism for the formation of RBC-like microspheres.
the surface of the spherical micelles (Fig. 4(a)). Under the hydrothermal condition used (150 ◦ C), CO2 is released by SDBS (from the hydrophilic poles) and this reacts with the Mg(OH)2 particles leading to 4MgCO3 ·Mg(OH)2 ·4H2 O micro-flakes around the spherical micelle (Fig. 4(b)). FT-IR analysis (Fig. 5) shows that the absorption peaks that correspond to the benzene ring (1627 cm−1 ) and the sulfonic group (690 and 1320 cm−1 ) of SDBS decreased significantly or disappeared after the 150 ◦ C calcination. The peak corresponding to C S C (1452 cm−1 ) increased and the hydrophobic group peak (alkyl side chain, 3453 cm−1 ) did not change after the calcination. According to the high resolution mass spectrum of the reaction supernatant (Fig. 6), SDBS (C18 H29 SO3 Na) decomposed into SO3 Na and C18 H27 (m/z = 243.0547) under the hydrothermal condition used. C18 H27 then further decomposed into a benzene ring and an alkyl chain (m/z = 164.9205). SDBS also released CO2 to form C17 H31 SONa (m/z = 306.8518). C17 H31 SONa combined with one or two MgSO4 molecules in solution to form C17 H31 SONa·MgSO4 (m/z = 426.7972) or C17 H31 SONa·2MgSO4 (m/z = 546.7452). The peak at m/z = 674.2513 can be attributed to Mg(C18 H29 SO3 )2 , indicating that SDBS reacted with MgSO4 to form Na2 SO4 and Mg(C18 H29 SO3 )2 . Additionally, C17 H31 SONa·2MgSO4 combined with Na2 SO4 to form C17 H31 SONa·2MgSO4 ·Na2 SO4 (m/z = 688.6750). Therefore, the decomposed residue of SDBS is likely to be hydrophobic because the hydrophilic poles (R SO3 − ) were converted into C S C hydrophobic structures after the release of CO2 . Therefore,
the micro-flakes can act as new hydrophobic centers (Fig. 4(c)) that attract SDBS to its surface (Fig. 4(d)). This permits the continuous growth of micro-flakes or the attachment of other flakes (Fig. 4(e)). Because of the attractive force between the flakes, more flakes will stack into a RBC-like structure rather than form a perfect sphere. Because the formation of basic magnesium carbonate occurs on the surface of the SDBS micelle, the micro-flakes grow from the core to the surface. RBC-like morphology is thus obtained with a thin core and a thick periphery. The growth of the microspheres from a tiny SDBS micelle to a sphere of several microns is driven by the continuous generation of new hydrophobic centers because of the consumption of hydrophilic poles ( SO3 − ). Such a surfactant-templated and consumed assembly process has also been used to synthesize olive-like copperoxide microspheres (Fig. S4), indicating that it is a universal assembly mode for amphiphilic molecules. According to the mass spectrometry data, SDBS can react with Mg2+ to form the Mg(SO3 –C6 H4 –C12 H25 )2 intermediate, which plays an important role in inhibiting the formation of the magnesium oxysulfate phase. A previous study (Sun & Xiang, 2008) also showed that SDBS can react with Mg2+ (Eq. (3)). Sun and
Fig. 5. FT-IR of SDBS before and after calcination at 150 ◦ C.
Fig. 6. High resolution mass spectrum of the reaction supernatant.
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Xiang (2008) reported that small and poorly crystallized Mg(OH)2 precursors formed in the presence of SDBS resulting in the homogeneous precipitation of 513MOS whiskers without intersections. However, the mass ratio of SDBS used in the experiment (0.2 wt%) was far lower than that used in this work (25.31 wt%). At a low SDBS content (12.66–22.48 wt%), the partial formation of 513MOS whiskers was observed (Fig. 3(a)–(d)), and this is consistent with Xiang’s report. However, at a SDBS content of 25.31 wt% no 513MOS whiskers were observed, indicating that the higher amount of SDBS contributes to inhibiting the formation of 513MOS. Because large amounts of SDBS can combine with MgSO4 , as shown in Eq. (3), MgSO4 does not easily combine with Mg(OH)2 to form 513MOS, as shown in Eq. (4). As a result, RBC-like basic magnesium carbonate microspheres of high purity without 513MOS whiskers form. MgSO4 + 2SDBS → Mg(SO3 –C6 H4 –C12 H25 )2 + Na2 SO4
(3)
5Mg(OH)2 + MgSO4 + 3H2 O → 5Mg(OH)2 ·MgSO4 ·3H2 O
(4)
Conclusions In summary, RBC-like basic magnesium carbonate microspheres were synthesized by a hydrothermal process in the presence of SDBS. The formation of RBC-like microspheres is attributed to surfactant-participated self-assembly during which the Mg(OH)2 particles that are attached to the SDBS micelles react with the CO2 released by SDBS and are finally converted to basic magnesium carbonate micro-flakes. Because of the consumption of hydrophilic poles ( SO3 − ), the resultant flakes transform into new hydrophobic centers and this permits the continuous growth of micro-flakes or the attachment of other flakes. Surfactant-directed self-assembly can be applied to the synthesis of other carbonate or metallic oxide self-assemblies. Therefore, the reported method can be used for the universal self-assembly of amphiphilic molecules. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21206191) and the Science Foundation of China University of Petroleum, Beijing (No. 2462013YXBS007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.partic. 2014.09.002.
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