Materials Letters 62 (2008) 4018–4021
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Crystallization of strontium carbonate in alcohol or water solution containing mixed nonionic/anionic surfactants Guolong Guo a, Guowei Yan a, Lina Wang b, Jianhua Huang a,b,⁎ a b
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
A R T I C L E
I N F O
Article history: Received 10 December 2007 Accepted 23 May 2008 Available online 9 June 2008 Keywords: SrCO3 Alcohol Surfactants Electron microscopy Crystal growth
A B S T R A C T The crystallization of strontium carbonate is performed in aqueous solution or alcohol/water solution in the presence of mixed nonionic/anionic surfactants Pluronic F127/sodium dodecyl sulfate. With an increase of the volume ratio of ethanol/water, SrCO3 pancakes transform into ﬂowers, and then to irregular ﬂakes. X-ray diffraction (XRD) pattern reveals that both of pancake-like and ﬂower-like SrCO3 particles are orthorhombic phases. Larger pancakes with porous structure are produced when the content of ethylene glycol reaches a relatively high value. Results indicate that –OH groups of alcohol play an important role in morphological controlling of SrCO3. © 2008 Elsevier B.V. All rights reserved.
In the past decades, calcium carbonate had attracted a great many attentions due to its abundance in nature and also an attractive model mineral for laboratory studies [1–3]. SrCO3, itself not an important biomineral, is nevertheless interesting since its crystallization yields insights into the formation of the isostructural CaCO3 phase, aragonite. Meanwhile, SrCO3 is an important industrial reagent, which can be used in the production of glass for color television tubes and ferrite magnets for small DC motors . It was also proven to be a potential chemical sensor . A variety of processes for the preparation of SrCO3 have been reported [6–8]. So far, SrCO3 particles with different morphologies have been produced, such as nanowires [9,10], ﬂowerlike nanostructures  and hexahedral ellipsoids . Urease was found to play a major role in the transformation of spheroidal SrCO3 particles to needles . Sastry et al  indicated that proteins played a crucial role in modulating the morphology of strontianite crystal. As a kind of additive, surfactants turn out to be very effective in controlling the morphology of inorganic materials . In this work, we studied the crystallization of SrCO3 in alcohol/water system containing Pluronic F127 (EO97PO68EO97) and sodium dodecyl sulfate (SDS). The effects of ethanol and ethylene glycol on the morphology of SrCO3 were investigated.
All reagents used in our experiments were of analytical grade. Stock solutions of SrCl2 (0.1 M), Na2CO3 (0.1 M), SDS (0.2 M) and Pluronic F127 (20 g/L) were freshly prepared before use. In a typical synthesis procedure, 2 mL F127 solution (20.0 g/L), 6 mL SDS solution (0.2 M) and 2 mL SrCl2 solution (0.1 M) were consecutively added into a 50 mL color comparison tube containing 8 mL distilled water. The mixture was then stirred for about 1 min and stood for 10 min. Afterwards, 2 mL Na2CO3 solution (0.1 M) was injected rapidly into it under stirring. The ﬁnal concentrations of SrCO3, SDS and F127 in the mixture are 0.01 M, 0.06 M and 2.0 g/L, respectively. The mixture was then aged at 50 °C for 2 h. Finally, the precipitates were centrifuged, washed with distilled water and anhydrous ethanol for three times, and then dried in a vacuum oven at 60 °C for 6 h. The effect of alcohol, including ethanol and ethylene glycol, on the crystallization of SrCO3 was investigated by introducing an amount of alcohol in the solvent with the concentration of SrCl2 and Na2CO3 both ﬁxed at 0.01 M, and the ﬁnal concentration of F127 and SDS were kept at 2.0 g/L and 0.06 M, respectively. Scanning electron microscope (SEM) images and enlarged SEM pictures were taken on a JEOL JSM-5610LV microscope and a Hitachi S4800 ﬁeld-emission microscope, respectively; X-ray diffraction (XRD) patterns were recorded on a X'TRA powder X-ray diffractometer with Cu–Kα radiation (λ = 1.54178 Å). 3. Results and discussion 3.1. Crystallization of SrCO3 in aqueous solution
⁎ Corresponding author. Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China. Tel.: +86 571 86843233; fax: +86 571 88084419. E-mail address: [email protected]
(J. Huang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.05.052
Fig. 1 shows SEM images of SrCO3 particles prepared from aqueous solution with and without surfactants, respectively. In the absence of surfactant, the as-obtained
G. Guo et al. / Materials Letters 62 (2008) 4018–4021
Fig. 1. SEM images of samples produced in aqueous solution. a) without surfactants; b) in the presence of 2.0 g/L F127; c) in the presence of 0.06 M SDS; d) in the presence of 2.0 g/L F127/0.06 M SDS.
Fig. 2. SEM images of samples collected in aqueous solution containing both F127 and SDS. a) 2.0 g/L F127/0.02 M SDS; b) 2.0 g/L F127/0.04 M SDS; c) 2.0 g/L F127/0.08 M SDS. products appeared as bundle-like aggregates consisting of many small SrCO3 needles (Fig. 1a). When F127 (2.0 g/L) was added into the reaction system, bigger particles with the same morphology were obtained, indicating that F127 has little effect on the crystallization of SrCO3 (Fig. 1b). However, spherical particles were precipitated in aqueous solution in the presence of SDS (0.06 M). The particle size broadly disperses with a few particles about 4 μm and many small particles around 1 μm (Fig. 1c). F127 is one kind of nonionic polymers, there is no electrostatic interaction between F127 and crystal face. Therefore, the interaction between F127 and crystal face is weak so that F127 alone has no effect on SrCO3 crystallization. But anionic surfactant SDS can strongly interact with the crystal face of SrCO3 after nuclei formation, and inevitably inﬂuence the ﬁnal morphology of SrCO3 crystal. When F127 (2.0 g/L) and SDS (0.06 M) were simultaneously added into the system, pancake-like particles of about 3 μm in diameter were obtained (Fig. 1d), which are homogeneous whatever in shape and in size. The enlarged SEM image inserted in Fig. 1d shows that pancake-like SrCO3 crystals consist of nano-sized rods. This result indicates that the presence of SDS and F127 is necessary for the formation of pancakelike SrCO3 particles. Then, we investigated the effect of the concentration of SDS on the morphology of SrCO3 crystal by keeping the concentration of F127 to be 2.0 g/L. With the increase of the concentration of SDS, SrCO3 crystals changed from bundle-like aggregates, to spheres and then to pancakes, see Fig. 2. Needle-aggregated loose spheres were formed in the presence of 2.0 g/L F127/0.02 M SDS (Fig. 2a). When the concentration of SDS reached 0.04 M, spheres with smooth surface were formed as shown in Fig. 2b. With a further increase of the concentration of SDS to 0.08 M, pancake-like particles, similar to those shown in Fig. 1d, appeared (Fig. 2c). F127 is well known for its surface activity and its relatively low critical micelle concentration. It was reported that addition of SDS results in the complete breakdown of F127 micelles and the formation of SDS-F127 complex micelles with SDS micelles binding on F127 monomer . Dynamic light scattering (DLS) measurements was unable to investigate the aggregation properties of this binary-additive in
aqueous solution at 50 °C due to the bubbles generated during the heating process. Instead of, our DLS measurements performed at 30 °C gave a micellar peak around 28 nm in the pure 2.0 g/L F127 aqueous solution. Whereas this micellar peak disappeared in F127/SDS mixed solution, conﬁrming that the addition of SDS resulted in the complete breakdown of F127 micelles and the formation of SDS-F127 complex
Fig. 3. XRD patterns of SrCO3 particles formed in aqueous solution (a) and in ethanol/ water solution with the volume ratio of ethanol/water r = 0.54 (b). The concentration of F127 and SDS is 2.0 g/L and 0.06 M, respectively.
G. Guo et al. / Materials Letters 62 (2008) 4018–4021
Fig. 4. SEM images of products obtained in ethanol/water solution. The volume ratio of ethylene glycol/water r = a) 0.11; b) 0.43; c) 0.54; d) 1.0. micelles. For 2.0 g/L F127/0.02 M SDS solution, besides the strong peak of SDS-F127 complex micelles around at 200 nm, there was a broad weak peak below 6 nm. While for 2.0 g/L F127/0.06 M SDS solution, we noticed the peak of complex micelles shifting to ~ 300 nm and two weak peaks centering at ~ 2 nm and 10 nm, respectively. We considered that F127 chains were saturated by SDS micelles, and the repulsion between SDS micelles forced the polymer chain stretch in the solution, resulting in the increase of the size of SDS-F127 complex micelles. Yu et al.  pointed out that amorphous SrCO3 nanoparticles ﬁrstly formed, then aggregated and evolved into small crystals with different surface energies. In the absence of SDS, the growth rate of the end faces was higher than that of the side faces, resulted in the formation of needle-aggregated bundles. The morphological change of SrCO3 particles with varying SDS concentration may be ascribed to the different adsorptive feature of SDS-F127 complex micelles with different size on the crystal plane of SrCO3. However, it is still difﬁcult to fully
understand the controlling mechanism of F127/SDS over the crystallization of SrCO3, and this will be the subject of our further work. Curve (a) in Fig. 3 shows the XRD pattern of pancake-like SrCO3 crystals. All diffraction peaks can be indexed to a pure orthorhombic phase of strontianite with lattice constants a = 5.107 Å, b = 8.414 Å, and c = 6.029 Å (JCPDS 05-0418). 3.2. Crystallization of SrCO3 in alcohol/water solution The effect of alcohol on the crystallization of SrCO3 was investigated by introducing ethanol or ethylene glycol in solution. Here, the concentration of F127 and SDS were set at 2.0 g/L and 0.06 M, respectively. Fig. 4a shows that pancake-like and nearly spherical particles of 1–3 μm in diameter were precipitated when the volume ratio of ethanol/ water r = 0.11. With the increase of the content of ethanol, ﬂower-like crystals of about
Fig. 5. SEM images of products formed in ethylene glycol/water solution. The volume ratio of glycol/water r = a) 0.11; b) 0.54; c) 1.0; d) 2.15.
G. Guo et al. / Materials Letters 62 (2008) 4018–4021 5 μm were produced, as shown in Fig. 4b and c, where r = 0.43 and 0.54, respectively. Such ﬂower-like SrCO3 crystals have not been reported before. The enlarged SEM image presented in the inset of Fig. 4c shows that the subunit of ﬂowers is smaller than that of pancakes. These ﬂower-like crystals were broken and appeared as irregular ﬂake-like crystals with the further addition of ethanol, Fig. 4d shows SEM image of products obtained at r = 1.0. In order to investigate the inﬂuence of ethanol on the phase structure of SrCO3 particles, ﬂower-like crystals were selected for XRD measurement. The result was plotted as curve (b) in Fig. 3. We observe that curve (b) has the same diffraction peaks as curve (a). This is due to the fact that strontium carbonate is single-phase crystal. Ethylene glycol exerted different effect on the crystallization of SrCO3. As we observe in Fig. 5a and b, spheres were precipitated at r = 0.11 and 0.54, respectively. However, pancake-like crystals with porous structures were formed at r = 1.0, whose diameters were bigger than 5 μm (Fig. 5c). And pancake-like crystals were broken when r reached 2.15 (Fig. 5d). Unlike ﬂower-like crystals was produced in ethanol/water solution, no ﬂower-like crystals were observed in ethylene glycol/water solution even at big values of r. Theoretically, alcohols affect the dieletric constant of the medium, the inter-ionic attraction and the solute-solvent interaction . The –OH groups of polyols (ethylene glycol) were found to play a key role in the vaterite crystal formation by adsorption on the nuclei of vaterite and change the surface energy. While the weak electric ﬁeld of one –OH group in ethanol could not change the surface energy of vaterite effectively . It was also reported that the presence of alcohol accelerated the crystal growth rate . In our case, if the precipitation is performed in aqueous solution, no SrCO3 crystal is detected when the aging temperature is below 50 °C. However, SrCO3 crystals can be easily and quickly precipitated even at 30 °C in alcohol/water system, which suggests that alcohol favors the transformation of strontium dodecyl sulfate to SrCO3 and accelerates the growth rate of SrCO3 crystals. As we discussed in Section 3.1 that addition of SDS results in the complete breakdown of F127 micelles and the formation of SDS-F127 complex micelles with SDS micelles binding on F127 monomer. Alcohol may show amphiphilic behavior and act as cosurfactants locating preferably at the interface between the PEO-rich and the PPO-rich domains . The addition of alcohol is also expected to destroy the water structure and hence to diminish the hydrophobic effect of surfactants[20–23]. These above interactions among F127, SDS and the solvents may induce the formation of SrCO3 crystals with special morphology. The different abilities of ethanol and ethylene glycol to control the morphology of SrCO3 crystals may be due to the fact that they possess different amount of –OH groups. The detailed reasons will be further explored in our future work.
4. Conclusion In summary, our results show that ethanol exerts strong effect on the crystallization of SrCO3 in the presence of F127/SDS. SrCO3 particles
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