anionic surfactants

anionic surfactants

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Powder Technology 192 (2009) 58–64

Contents lists available at ScienceDirect

Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c

The crystallization behavior of calcium carbonate in ethanol/water solution containing mixed nonionic/anionic surfactants 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 30 January 2008 Received in revised form 2 November 2008 Accepted 21 November 2008 Available online 3 December 2008 Keywords: Crystal morphology X-ray diffraction Calcium carbonate Ethanol Surfactants

a b s t r a c t Crystallization of CaCO3 was performed in an ethanol/water solution containing Pluronic F127 (EO97PO68EO97) and sodium dodecyl sulfate (SDS). The effects of the ethanol/water volume ratio, concentration of surfactants, and aging temperature (30–90 °C) on the morphology and polymorphs of CaCO3 were investigated using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The presence of both F127 and SDS in solution favored the vaterite phase of CaCO3. A binary calcite–vaterite mixture and a ternary calcite–vaterite–aragonite mixture were produced at low (b 60 °C) and high temperatures, respectively. Spherical, plate-like, flower-like, and rod-like crystals were obtained at different ethanol/water volume ratios. Controllable synthesis of nearly pure flower-like vaterite and rod-like aragonite can be realized by adjusting the ethanol/water volume ratio. The formation mechanism of flower-like CaCO3 crystals is discussed. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In the past decades, calcium carbonate (CaCO3) has attracted much attention due to its abundance in nature, and as an attractive model mineral for laboratory studies [1–3]. It is well known that there are three anhydrous crystalline phases of CaCO3: calcite, aragonite, and vaterite. Calcite is thermodynamically the most stable, while vaterite is the least stable and easily transforms into one of the other two phases [4]. In practice, the stability of these polymorphic species depends mainly on temperature [5,6] and the properties of additives [7,8]. Recently, various self-assemblies of organic compounds, such as surfactants and polymer–surfactant mixtures, were used to control the morphology and polymorphs of CaCO3 crystals [1,9–11]. However, these experiments were performed mainly at ambient temperature. Alcohols have been proven to be effective in preventing the transformation from vaterite to calcite [12–15]. A vaterite cake-like superstructure was synthesized at 90 °C by simply changing the ethanol/water volume ratio [4]. Vaterite crystals with various morphologies were synthesized in water/ethylene glycol/sodium dodecyl sulfate (SDS) system by microwave heating at 100 °C [2]. Stable vaterite crystals were also formed in the presence of ethanol, propanol and diethylene glycol [16]. There are, however, few reports on the effect of temperature on crystallization of CaCO3. In addition, in the studies on precipitation of

CaCO3 crystals in different media, most experiments were performed in alcohol/water solution without additives. We investigated the crystallization of CaCO3 in ethanol/water system containing Pluronic F127 and SDS on the basis of the following considerations. Amphiphilic non-ionic polymers, such as poly(ethylene oxide)–poly (propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers (Pluronics), are widely used in industrial applications and in pharmaceutical formulations. A mixture of Pluronic P123 and cetyltrimethylammonium bromide (CTAB) has been successfully used to control the synthesis of gold nano- and microplates [17]. Hollow silica spheres with mesostructured shells were prepared with the vesicle template of CTAB–SDS–Pluronic P123 [18]. However, mixtures of Pluronic block copolymer and ionic surfactants have rarely been applied to controlling crystallization of CaCO3. Accordingly, in the present work crystallization of CaCO3 in ethanol/water in the presence of Pluronic F127 and SDS was investigated. Plate-like calcite, flower-like vaterite, and rod-like aragonite were produced under various conditions. The effects of the ethanol/water volume ratio, the surfactant concentrations, and the aging temperature (30–90 °C) on the morphology and polymorphs of CaCO3 were investigated in detail. This study will provide new understanding of the crystallization of CaCO3 in complex systems. 2. Experimental section 2.1. Materials

⁎ 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). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.11.013

All chemicals were analytical grade. Calcium chloride and sodium carbonate were purified before use. Anhydrous ethanol, Pluronic F127

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(EO97PO68EO97, MW = 12,600 g mol− 1; BASF Corporation, Mount Olive, New Jersey), and anionic surfactant sodium dodecyl sulfate (SDS) (ACROS, N99%) were used as received. Double-distilled water was used for the preparation of all solutions. D2O (N99.9 at.% 2H) and CD3CD2OD (N99 at.% 2H, b5% D2O) were purchased from Cambridge Isotope Laboratories, Inc. 2.2. Synthesis In a typical synthesis, 0.100 g F127 and 0.289 g SDS were consecutively added to a 100 ml conical flask containing 25 mL distilled water and 15 mL anhydrous ethanol. 5 mL CaCl2 aqueous solution (0.1 M) was introduced after complete dissolution of F127 and SDS. The conical flask was then heated with stirring for 2 min in a 60 °C water bath. 5 mL of Na2CO3 aqueous solution (0.1 M) was then rapidly injected into the solution with continuous stirring for another 2 min, and the resulting mixture was aged for 3 h. Finally, the

Fig. 2. XRD patterns of products prepared at various ethanol/water ratios (r, v/v). (a) 0; (b) 0.25; (c) 0.35; (d) 0.43; (e) 1.0; (f) 2.33. C, calcite phase (JCPDS Card No. 47-1743); V, vaterite phase (JCPDS Card No. 33-0268); A, aragonite phase (JCPDS Card No. 41-1475). Similar symbols are used in the following XRD patterns.

Fig. 1. SEM images of products prepared at various ethanol/water ratios (r, v/v). (a) 0; (b) 0.25; (c) 0.35; (d) 0.43; (e) 1.0; (f) 2.33. CF127 = 2 g L− 1, CSDS = 20 mM, and the aging temperature T = 60 °C.

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Table 1 Phase composition of products prepared at various ethanol/water volume ratios r (CF127 = 2 g L− 1, CSDS = 20 mM, T = 60 °C) r

Calcite (%)

Aragonite (%)

Vaterite (%)

0 0.25 0.35 0.43 0.67 0.85 0.92 1.0 1.08 1.33 1.63 2.03 2.33

17.1 66.0 64.3 4.8 12.8 17.1 9.8 1.5 16.8 24.1 43.3 48.1 21.2

0 6.4 7.9 2.8 16.1 20.9 46.2 90.7 32.5 21.7 12.8 11.8 11.8

82.9 27.6 27.8 92.4 71.1 62.0 44.0 7.8 50.7 54.2 43.9 40.1 67.0

precipitates were centrifuged, washed with distilled water and anhydrous ethanol three times, then dried in vacuum at 60 °C for 12 h for further characterization. The effects of the solvent, surfactant

and aging temperature on the crystallization of CaCO3 were investigated by adjusting the ethanol/water volume ratio, the concentration of surfactants and the aging temperature, respectively. The concentrations of CaCl2 and Na2CO3 in the system were both fixed at 10 mM, and the aging time was 3 h. 2.3. Characterization Scanning electron microscope (SEM) images were obtained using a JEOL JSM-5610 microscope at an applied voltage of 10 kV. Enlarged SEM images were recorded using a Hitachi S-4800 fieldemission microscope at 5 kV. X-ray diffraction (XRD) patterns were recorded using an X'TRA powder X-ray diffractometer with CuKα radiation (λ = 1.54178 Å) at 0.02° s− 1 scanning rate in the 2θ range 20–60°. XRD data were used to analyze the calcium carbonate phases. Calculation of the relative proportions of polymorphs was performed using the 104, 110 and 221 reflections for calcite, vaterite and aragonite, respectively. Calcite–vaterite and calcite–aragonite mixtures were

Fig. 3. 1H NMR spectra of F127(2 g L− 1)/SDS (20 mM) mixture in CD3CD2OD/ D2O solution at various CD3CD2OD/D2O volume ratios r. (a) SDS–CH2–α, (a) SDS–CH2–β, (c) SDS–(CH2)9–, (d) SDS–CH3, (e) EO–CH2–, (f) PO–CH2–, and (g) PO–CH3 regions.

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Fig. 4. SEM image (a) and XRD pattern (b) of product obtained at r = 0.43 in the absence of surfactants.

quantified by Eqs. (1) and (2), respectively, and calcite–vaterite– aragonite mixtures were quantified using Eqs. (2)–(4) [19]. I110 fV = 7:691× V104 fC IC

ð1Þ

I221 fA = 3:157× A104 fC IC

ð2Þ

1 H NMR experiments were carried out using a Brüker Advance DMX-400 NMR spectrometer. 1H NMR spectra of SDS and F127 in D2O were recorded at concentrations 20 mM and 2 g L− 1, respectively, then mixtures of F127 (2 g L− 1) and SDS (20 mM) were investigated at various volume ratios of CD3CD2OD/D2O.

3. Results and discussion 3.1. Effect of the ethanol/water ratio (r, v/v) on crystallization of CaCO3

3:157×IA221 fA = 104 IC + 3:157×IA221 + 7:691×IV110

ð3Þ

fV = 1:0 − fA − fC

ð4Þ

Here the subscripts C, V and A denote calcite, vaterite and aragonite, respectively. I is the intensity of the corresponding reflection peak, and f is the fraction of phase component in the product.

We investigated the effect of the solvent composition on crystallization of CaCO3 by varying the ethanol/water volume ratio (r), at fixed concentrations of F127 (2 g L− 1) and SDS (20 mM). Fig. 1 shows SEM images of the final products obtained at various ethanol/water volume ratios. Fig. 1a shows the spherical particles with diameter 1–2 μm that were precipitated in F127 + SDS aqueous solution. At r = 0.25, plate-like

Fig. 5. SEM images of products prepared at various SDS concentrations with other experimental parameters as per the ‘standard condition’. CSDS = (a) 0; (b) 4; (c) 12; (d) 20 mM.

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crystals with size about 2 μm appeared, most of them exhibiting hexagonal structures (Fig. 1b). Similar plate-like crystals but with larger size (8–10 μm) were formed in the PVP + SDS aqueous system due to the co-effects of PVP and SDS on the crystal/solution interface [7]. In the present case, however, the formation process of plate-like crystals may be more complicated since the presence of ethanol introduces an additional solvent effect. Flower-like particles with a large central protuberance were produced at r = 0.35 (Fig. 1c). Similar products were obtained at r = 0.43 as shown in Fig. 1d: this condition is designated hereinafter as the ‘standard condition’. Interestingly, some particles show a well-defined symmetrical structure consisting of six petals arranged around the orbicular center (Fig. 1(d1)). Each petal has a long central backbone with roughly symmetrical secondary branches, similar to previously reported observations [20,21]. The enlarged SEM image (Fig. 1(d2)) shows that the flower-like particle is an aggregate of nanoparticles with size 20–30 nm. It should be noted that a small amount of rhombohedral crystals was also observed in Fig. 1d. However, an increasing proportion of rod-like crystals was obtained with increase of the ethanol content. When r reached 1.0, the product consisted mainly of the rod-like crystals shown in Fig. 1e, and most rods were aggregates of small particles. With further increase in the ethanol content, the proportion of rod-like crystals decreased and fluffy aggregates appeared (Fig. 1f). The results showed that a high content of ethanol favored the formation of aggregated product. The corresponding XRD patterns and phase compositions (calculated by Eqs. (1)–(4)) are shown in Fig. 2 and Table 1, respectively. Vaterite and calcite were the dominant components at low ethanol content. Nearly pure vaterite appeared at r = 0.43. The fraction of aragonite increased with r, and reached almost 100% at r = 1.0. However, the aragonite content rapidly decreased with further increase of r, so that, for example, there was only 11.8% aragonite at r = 2.33. It is known that the energy of a crystal consists of surface energy and bulk lattice energy. A theoretical study [22] showed that the bulk energies of the three polymorphs are very close to each other: the values reported for calcite, aragonite, and vaterite were −1.6867 × 108, −1.7851 × 108 and −1.6577 × 108 kJ mol− 3, respectively. Consequently, the surface energies for these three polymorphs should play a key role in the total energy of a crystal, which is directly related to the solvent properties and the adsorption behavior of F127 + SDS mixtures. Thus, the three polymorphs can be selectively controlled by altering the value of r. Previous studies reported that ethanol acted as co-surfactant at low concentration, and co-solvent at high concentration [23–25]. However, it was difficult to obtain an absolutely pure phase in our system. To obtain information on the aggregation properties of F127 + SDS in ethanol/water solution, we studied the 1H NMR chemical shifts of F127 (2 g L− 1) + SDS (20 mM) mixture at various volume ratios of CD3CD2OD/D2O. All experiments were carried out at 60 °C. Fig. 3(a)–(d)

Fig. 6. XRD patterns of products prepared at various SDS concentrations with other experimental parameters as per the ‘standard condition’. CSDS = (a) 0; (b) 4 (c) 12 mM.

Table 2 Phase composition of products prepared at various F127 and SDS concentrations (r = 0.43, T = 60 °C) CF127 (g L− 1)

CSDS (mM)

Calcite (%)

Aragonite (%)

Vaterite (%)

0 2 2 2 2

0 0 4 12 20

2.5 5.0 3.1 1.6 4.8

88.6 54.0 13.5 5.6 2.8

8.9 41.0 83.4 92.8 92.4

present 1H NMR spectra of α methylene protons, β methylene protons, the nine methylene protons and the terminal methyl protons of SDS, respectively. The spectral profiles of the methylene protons of PEO, the methylene protons and the methyl protons of PPO are shown in Fig. 3 (e)–(g), respectively. All resonance peaks clearly showed upfield shifts at low ethanol content compared with aqueous solution, such as the r = 0.25 volume ratio of CD3CD2OD to D2O. All chemical shifts gradually increased with ethanol content, and became larger than those of aqueous solution for r N 1.0. We also observed a new resonance signal when the volume ratio r was ≥1.0. It is sufficient to assume that the above variations of the chemical shifts reflect changes in the local environment of the observed nuclei, suggesting change of the aggregation properties of F127 + SDS in solution. However, it is still difficult to fully understand the mechanism of the control by F127 + SDS over the crystallization of CaCO3, and further work needs to be carried out. 3.2. Effect of surfactants on crystallization of CaCO3 It has been shown that surfactants influence crystallization steps by selective adsorption on certain crystal planes, and finally control the formation of the crystal phase [10,11,26]. In our experiments, the effects of F127 + SDS on the crystallization of CaCO3 were investigated by changing the concentration of F127 + SDS, with other experimental parameters as per the standard condition. Rod-like CaCO3 was precipitated at r = 0.43 in the absence of surfactants (Fig. 4a). The rods had diameter 300–400 nm with aspect ratio about 8 as shown in the inserted magnified image. The corresponding XRD pattern is plotted in Fig. 4b, which shows that the rod-like crystals contained 88.6% aragonite, 2.5% calcite and 8.9% vaterite. Flower-like and dendrite-like crystals co-existed (Fig. 5a) when only F127 (2 g L− 1) was present, and the proportion of dendrite-like crystals decreased with increase of the concentration of SDS. Fig. 5b shows the image of product obtained in the presence of F127 (2 g L− 1) + SDS (4 mM). With increase of SDS concentration to 12 mM, the proportion of dendrite-like crystals further decreased and the size of the flower-like crystals decreased (Fig. 5c). Ultimately, nearly pure flower-like crystals were obtained when the concentration of SDS reached 20 mM (Figs. 1d and 5d). The related XRD patterns (Fig. 6) provide further evidence of the transformation of aragonite to vaterite. Based on Eqs. (2)–(4), it was found that calcite remained at 2–5% while the proportion of aragonite decreased remarkably with increase of SDS concentration, and vaterite showed the contrary trend. Detailed results are presented in Table 2. Except for dendrite-like crystals, no flower-like crystals were obtained when only 20 mM SDS was present (Fig. 7a), indicating that the interaction of F127 and SDS plays a dominant role in the crystallization of CaCO3 in the system. SDS affects the adsorption behavior of Ca2+ ions and affects the morphology of the product by interacting with the F127 monomer or micelles. The corresponding XRD data are plotted in Fig. 7b. A recent paper reported the formation of PbS branch-like crystals through linking of short rod-like monomeric Pb2+–F127 aggregates [27]. The formation of flower-like crystals in our experiment may also occur via such a linking process. Previous study confirmed complexation of Ca2+ with EO units of poly (ethylene oxide) matrix [28], and it is

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Fig. 7. SEM image (a) and XRD pattern (b) of product obtained at r = 0.43 in the presence of 20 mM SDS.

reasonable to assume that crosslinks of Ca2+–F127 aggregates are formed due to weak interaction between EO units of F127 and Ca2+ ion, and flower-like crystals with larger size appear when only F127 is present (Fig. 5a). When a small amount of SDS is introduced, it will adsorb onto F127 chains to form SDS–F127 complexes [29] which make crosslinking and the ability of F127 chains to adsorb Ca2+ ions more efficient. Consequently, more flower-like particles are obtained, and the size of the final product gradually decreases due to the strong ability of SDS–F127 complex to adsorb Ca2+ ion (see Fig. 5(b–d)). By contrast, the chains of SDS are too short to link together, so it cannot act as a structure-directing material for the formation of flower-like structures when only SDS is present. These results clearly show that the mixed surfactants in ethanol/ water solution play a crucial role in mediating the morphology and polymorphs of CaCO3 crystals. 3.3. Influence of the aging temperature on the morphology and phase discrimination of CaCO3 Temperature influences the nucleation and growth rate of calcium carbonate, and subsequently affects the polymorphism and morphology [30]. In aqueous systems, CaCO3 exists as aragonite and calcite at high temperatures (60–80 °C), whereas at intermediate temperatures (40–50 °C) the formation of vaterite is also possible [31]. We investigated the effect of the temperature on the polymorph discrimination of CaCO3, while other parameters were kept as per the ‘standard condition’. XRD patterns showed that the products were binary mixtures of calcite and vaterite at low temperatures (Fig. 8a(a,b)) and ternary calcite–vaterite– aragonite mixtures at high temperatures (Fig. 8a(c,d)). The relative proportions of calcite, aragonite and vaterite are shown in Fig. 8b. The

proportions of calcite and vaterite showed different behavior with increase of temperature. The former decreased with increasing temperature, while the latter reached a maximum value at about 60 °C. As a general trend, higher temperature favored the formation of aragonite, and aragonite did not form at temperatures below 60 °C. At temperatures above 60 °C, the aragonite content increased slowly with increasing temperature and reached about 19% at 90 °C. However, pure aragonite phase formed at 90 °C in the absence of surfactant [4]. These results further demonstrate the influence that F127 and SDS exert on the formation of vaterite. The maximum at 60 °C in the vaterite content originates from the competitive effects of temperature and the mixed surfactants. The corresponding SEM images (Fig. 9) reflect the XRD results. The spherical particles in Fig. 9a (indicated by arrows) could be identified as vaterite phase. With increased aging temperature, flower-like vaterite crystals appeared (Figs. 5d and 9b–d). When the temperature exceeded 60 °C, dendrite-like aragonite and flower-like vaterite coexisted, and the fraction of dendritic aragonite increased with increasing temperature. The above transformation process agrees well with the curves plotted in Fig. 8b. 3.4. Proposed mechanism of formation of flower-like crystals Our results show that flower-like crystals can be synthesized by adjusting the ethanol/water volume ratio, the concentrations of the surfactants, and the aging temperature. The enlarged SEM micrographs (inserts in Fig. 1d) clearly show that the petals of the flowerlike particles are aggregates of 20–30 nm nano-crystals. That is to say, the formation of flower-like crystals does not occur by an oriented attachment process [21], but through an assembly process. Based on

Fig. 8. (a) XRD patterns of samples prepared at various temperatures with other experimental parameters as per the ‘standard condition’. (a) 30; (b) 45; (c) 75; (d) 90 °C. (b) Temperature dependence of the proportions of the components in the as-prepared samples.

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Fig. 9. SEM images of samples obtained at various temperatures with other experimental parameters as per the ‘standard condition’. (a) 30; (b) 45; (c) 75; (d) 90 °C.

the above observations, a mechanism of formation of the flower-like crystals is proposed as follows. SDS–F127 complexes first form in solution, and then transform into Ca2+–DS–F127 aggregates when Ca2+ is present. The interaction between microscopic interfacial dynamics (surface tension, surface kinetics, and anisotropy) and external macroscopic forces (diffusion kinetics, solvents, and surfactants) [16,32] leads to crosslinking of Ca2+–DS–F127 aggregates, accelerated by appropriate ethanol/water volume ratio and temperature. As a result, flower-like CaCO3 crystals are formed upon addition of sodium carbonate. However, the real mechanism involved in the system is rather complicated and needs more dedicated work in the future. 4. Conclusion The crystallization behavior of CaCO3 was studied in ethanol/water solution containing F127 and SDS. The effects of the solvent composition, relative concentrations of surfactants and the aging temperature on the morphology and polymorph of CaCO3 were investigated in detail. F127/SDS favors the formation of vaterite. A binary calcite–vaterite mixture and a ternary calcite–vaterite–aragonite mixture are produced at different temperatures. Controllable synthesis of nearly pure flower-like vaterite and rod-like aragonite crystal phases can be realized by adjusting the ethanol/water ratio. The formation of flower-like crystals is an assembly process rather than an oriented attachment process, and may originate from crosslinking of Ca2+–DS–F127 aggregates. Acknowledgements This work was supported by the Natural Science Foundation of China (No. 20771092), Zhejiang Provincial Natural Science Foundation of China (No. Y406295) and PCSIRT (No. 0654).

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