chitosan chloride

Powder Technology 299 (2016) 51–61

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Facile preparation of CaCO3 with diversified patterns modulated by N-[(2-hydroxyl)-propyl-3-trimethylammonium] chitosan chloride Beibei Wang, Xiaodeng Yang ⁎, Ling Wang, Guohong Li, Yan Li Key Laboratory of Fine Chemicals (Shandong Province), Qilu University of Technology, Ji'nan 250353, PR China

a r t i c l e

i n f o

Article history: Received 10 January 2016 Received in revised form 2 May 2016 Accepted 20 May 2016 Available online 21 May 2016 Keywords: N-[(2-hydroxyl)-propyl-3trimethylammonium] chitosan chloride (HTCC) Calcium carbonate Crystallization Interaction

a b s t r a c t Disk-like, rod-like, spindle and bundle-like CaCO3 particles were modulated in N-[(2-hydroxyl)-propyl-3trimethylammonium] chitosan chloride (HTCC) solutions using a slow vapor diffusion method. The CaCO3 particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and thermogravimetry analysis (TGA) methods. The influences of various concentrations of HTCC, Ca2+ and Mg2 +, temperature and aging time, on the properties of CaCO3 particles were investigated. It was found that the electrostatic interaction between HTCC and Ca2+, the intramolecular and intermolecular interactions between HTCC molecules played a key role in the crystallization of CaCO3 with different morphologies and polymorphs. Based on the analysis of CaCO3 particles obtained under different aging time, a mechanism of HTCC modulating CaCO3 crystallization was proposed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction CaCO3 is a key component of various biomaterials, such as in mollusks, coccolithophores, and crustaceans shells, with superior mechanical properties and hierarchical structures [1]. CaCO3 includes six polymorphs, namely, calcite, aragonite, vaterite, calcium carbonate monohydrate (CaCO3·H2O), calcium carbonate hexahydrate (CaCO3·6H2O) and amorphous calcium carbonate (ACC), in descending order of thermodynamic stability [2]. In different biomineralization systems, either organic additives (including biomacromolecules, synthetic macromolecules and small organic molecules) [2–8] or inorganic additives [9–13] was indicated to play a key role in CaCO3 crystallization, such as oriented nucleation, morphology, polymorph and nanostructure [14]. Many efforts have been done to reveal the effects of additives on the crystallization process and properties of CaCO3. For instance, ovalbumin, a major constituent of egg white proteins, favored the formation and stabilization of spherical vaterite [15]. Contrarily, lysozyme, another constituent of egg white proteins, favored the formation of calcite expressing (110) and (104) faces [16]. Hemocyte, obtained from diseased shells of the oyster favored aragonite crystallization and growth, and the growth rate was 67 times faster than that of calcite as a contrast [17]. Phosphoprotein-casein [18] controlled mineralization of dumbbell-like vaterite with spear-like branches through the interaction between phosphate groups and Ca2 + ions, as well as the adsorption of casein molecules onto certain crystal faces. Phosphorylated Stm (where Stm ⁎ Corresponding author. E-mail address: [email protected] (X. Yang).

http://dx.doi.org/10.1016/j.powtec.2016.05.036 0032-5910/© 2016 Elsevier B.V. All rights reserved.

was an intrinsically disordered protein extracted from otoliths of Danio rerio fish) promoted the nucleation of CaCO3 via gathering Ca2+ by negatively charged amino acid residues and phosphate groups [19]. However, it inhibited CaCO3 crystal growth through surrounding the CaCO3 edges. Otolith matrix macromolecule-64, extracted from teleost fish otolith induced vaterite crystallization, and otolin-1, another kind of extracted proteins induced calcite crystallization [20]. It was the intrinsic characteristic of the proteins resided in organisms that determine the specific polymorphisms and the morphologies of CaCO3. Inspired by the effect of biopolymers on CaCO3 crystallization and growth, scientists have synthesized large amount of polyelectrolytes and used them to modulate CaCO3 crystallization with various polymorphisms and morphologies. Negatively charged polymers including carboxylated linear-dendritic block copolymers [21] and polystyrene sulfonate [22] could modulate spherical vaterite and ACC, respectively. It ascribed to the interaction between anion and Ca2+, the aggregation of polymer/Ca2 + complexes and the adsorption of the two polymers on CaCO3 surfaces. Positively charged poly(allylamine hydrochloride) induced the formation of CaCO3-based thin films and fibers through electrostatic interaction between the protonated amine groups and the negatively charged carbonate ions [23,24]. Linear poly(ethylene imine) with a hydrophilic block (i.e. polyethylene glycol) could modulate lamellar vaterite crystals at the air/water interface under acidic conditions [25]. The amine group substituent, molecular weight, side chain length, pKa value, as well as the nature of the amine group exerted a strong effect on the final CaCO3 polymorphs and morphologies [26]. Chitosan, fully or partially deacetylated product of chitin, is the second most abundant polysaccharide after cellulose, and the second most abundant natural nitrogen-containing biopolymer after protein.

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B. Wang et al. / Powder Technology 299 (2016) 51–61

Although chitosan or chitin does not take part in the biomineralization directly, its presence is crucial in biominerals such as crustacean and mollusks shells [4]. Two kinds of negatively charged chitosan derivatives have been used to modulate petunia-shaped vaterite [27] or spherical ACC [28] crystallization. Liang et al. suggested an ion-diffusioncontrolled mechanism for the formation of the petunia-shaped vaterite [27]. Zhong and Chou considered that ACC thin films were firstly crystallized. Amorphous nanoparticles were then deposited and stabilized by acid polysaccharide onto a predefined center area of the film to form a stable ACC core, which acted as nucleus for the radial growth of needlelike calcite subunits [28]. In our previous studies, two kinds of negatively charged chitosan derivatives, (2-hydroxypropyl-3-butoxy) propyl-succinyl chitosan [29] and O-carboxymethylchitosan [30], and a kind of nonionic chitosan derivative, O-(hydroxyl isopropyl) chitosan [31] as well as its complex with cetyltrimethylammonium bromide [32] were employed to control CaCO3 crystallization. We found that in alkaline medium, carboxyl chitosan derivatives [29,30] interacted with Ca2+ ions and formed chitosan derivative/Ca2+ complex, in which the Ca2+ ions acted as both crystallization points and reactants to participate in the crystallization. Subsequently, the carboxyl groups adsorbed on CaCO3 surface, modulating their grow process and polymorphs. Nonionic chitosan derivative influenced the morphologies of CaCO3 through the adsorption of nitrogen atoms in CS on CaCO3 surfaces [31]. However, to our knowledge, the positively charged chitosan derivatives were scarcely applied to control CaCO3 crystallization. In this paper, N-[(2-hydroxyl)-propyl-3trimethylammonium] chitosan chloride (HTCC) was synthesized and applied as a control agent of CaCO3 crystallization. The effects of concentrations of HTCC and Ca2+, temperature, aging time and nMg2+/nCa2+ ratios on CaCO3 crystallization were investigated. Based on the analysis of the CaCO3 particles, a mechanism of HTCC modulating CaCO3 crystallization and growth was proposed. The work is expected to provide good insight into understanding CaCO3 crystallization in positively charged polysaccharides.

2. Experimental section 2.1. Materials Chitosan was purchased from Weifang Sea Source Biological Products Co., Ltd. (Shandong Province, China) and dried in vacuum at 80 °C for 24 h before experiments. The degree of deacetylation (DD) and viscometric average molecular weight (Mv) were 78.5% and 2.0 × 106, determined by elemental analysis and Ubbelohde viscosity methods, respectively. 2,3-Epoxypropyltrimethyl ammonium chloride (EPTAC) was purchased from Adamas Regent Co., Ltd. (Shanghai, China). The content of epoxy was 95.0%, determined by hydrogen bromide-acetic acid non-aqueous titration method. 1-Allyl-3-methylimidazole chloride was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China) and used without further purification. Ethanol and acetone were both A.R. grades and used after being dehydrated. Ammonium carbonate ((NH4)2CO3, Tianjin White Chemical Co., China), calcium chloride anhydrous (Tianjin Kermel Reagents Development Centre, China) and magnesium chloride hexahydrate (Shantou Xilong Chemical Factory, China) were all A.R. agents and used without further purification. Triply distilled water was used in the preparation of solutions.

2.2. Synthesis of HTCC N-[(2-hydroxyl)-propyl-3-trimethylammonium] chitosan chloride (Scheme 1) was synthesized according to Ref. [33], whose structure was demonstrated by FTIR and 1H NMR spectroscopies (Fig. S1 in Supplementary information). The substitution degree of EPTAC was calculated to be 24.1% according to Ref. [34].

Scheme 1. Schematic diagram for HTCC molecular structure.

2.3. Crystallization of calcium carbonate A CaCl2 solution of 100.0 mmol/L and a HTCC solution of 4.0 g/L were prepared as stock solutions. The crystallization of CaCO3 followed the procedure in Ref. [29]. In a typical experiment, 8.0 mL of 100.0 mmol/L CaCl2 and 4.0 mL of 4.0 g/L HTCC solutions were transferred into a 100 mL glass beaker. The mixed solution was diluted to 40 mL and stirred uniformly for 5–10 min. The beaker was then placed at the bottom of the desiccator. A culture dish containing (NH4)2CO3 powder in stoichiometric proportion of CaCO3 was placed side by side with the beaker. The culture dish was sealed with parafilm punched with three needle holes for the diffusion of NH3 and CO2 gases. The slow diffusion and the subsequent dissolution of NH3 and CO2 gases into HTCC solutions caused the simultaneous crystallization of CaCO3. The desiccator was laid into a thermostat. The aging time and temperature were 8 h and 40 °C, respectively, unless special explanation. The CaCO3 particles were collected by centrifugation, washed for three times with absolute ethanol/triply distilled water (volume ratio of 1/4) and then dried under vacuum desiccator at 80 °C for 24 h. The influencing factors, including concentrations of HTCC and Ca2+, temperature, aging time, and nMg2+/nCa2+ ratios on the crystallization of CaCO3 were investigated.

2.4. Characterization of calcium carbonate The size and morphology of CaCO3 particles were characterized by a Quanta 200 environmental scanning electron microscope (SEM) at an accelerating voltage of 20 kV after being sputtered with gold. Transmission electron microscopy (TEM) images were obtained from a Hitachi H 800 transmission electron microscopy operating at 200 kV. The upper clear liquid of the reaction systems were collected with a carbon film supported by a copper grid from CaCO3 crystallization systems. The powder X-ray diffraction (XRD) patterns were recorded on a θ-θ Bruker AXS D8 advance diffractometer with graphitemonochromatized high-intensity CuKα radiation (λ = 1.5406 Å). The 0.02° steps/(25 s) and the 2θ range from 20° to 60° were selected to analyze crystal structure and orientation. Fourier transform infrared (FTIR) spectroscopic measurements were performed on a Shimadzu IRPrestige-21 FTIR spectrometer with KBr pellets in the wave number range of 400–4000 cm−1. The thermogravimetric analysis (TGA) of CaCO3 was carried out on a TA instrument SDT Q600 thermoanalyzer from room temperature to 800 °C under a nitrogen atmosphere flow of 100 mL/min. The temperature was increased linearly at a rate of 10 °C/min.


3. Results and discussion 3.1. Effect of HTCC concentration on CaCO3 At a fixed concentration of Ca2+ (20 mmol/L), temperature (40 °C) and aging time (8 h), the rhombohedral CaCO3 particles with smooth edges and sharp angles were obtained from aqueous solutions without HTCC (Fig. 1A). Rhombohedral particles and small amount of disk-like particles are obtained from 100 mg/L HTCC aqueous solution (Fig. 1B). However, rod-like aggregates composed of smaller CaCO3 rods (ca. 30 μm in length and 2 μm in width) (inset of Fig. 1C) and small amount of rhombohedral particles were obtained from both 200 mg/L (Fig. 1C and Fig. S2) and 400 mg/L (Fig. 1D) HTCC aqueous solutions. The average sizes (long side) of the rhombohedral particles were 7.5 (A), 22.6 (B), 16.2 (C) and 4.4 μm (D), i.e., which decreased with the increase in HTCC concentration. The XRD patterns (Fig. 2(a)) show that all samples obtained from aqueous solutions without (control) or with HTCC were calcite (JCPDS PDF: 47–1743). However, the FTIR spectra (Fig. 2(b)) show that the CaCO3 samples are mixtures of calcite, vaterite and aragonite. The peaks at 708, 874 and 1419 cm− 1 are characteristic of calcite [35], which were assigned to in-plane bending (708 cm− 1, v4), out-plane bending (874 cm−1, v2) and asymmertrical stretching vibration peaks of O\\C\\O (1419 cm−1, v3), respectively. The ratios of v2/v3, namely 0.51 (A), 0.74 (B), 0.80 (C) and 0.81 (D), increase with the increase in HTCC concentrations, implying there are defects in CaCO3 particles, or different exists in the arrangement of particles, which leads to the

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variation of vibration of CO2– 3 [29]. In control experiment, the absorption peaks at 856 and 1475 cm−1 are characteristic ones of aragonite, which belong to the v'2 (856 cm−1) and v'3 (1475 cm−1) vibration of CO2– 3 [36]. The absorption peaks at 747 cm−1 and 1080 cm−1 are the characteristic peaks of vaterite [37]. The contents of vaterite in the mixture calculated according to the method Vagenas and coworkers [38] proposed are 10.13 (A), 11.35 (B), 13.38 (C) and 14.25 (D) %, respectively. And those of aragonite are 1.58 (A) and 5.6 (C) %, which approximates to the detection limit of XRD method [39]. Therefore, diffraction peaks of vaterite and aragonite are absent from the XRD patterns. For samples B and D, the contents of vaterite cannot be calculated for the weak absorption peaks at 747 cm−1. The absorption peaks at 1475 cm− 1 for samples B, C and D are partially ascribe to the residence of HTCC in CaCO3 particles [32]. Compared to the control sample, the first weight loss step begins at ca. 240 °C in TGA curves (Fig. 3) for samples obtained from HTCC aqueous solutions, corresponding to the decomposition temperature of HTCC [33]. This verified the residence of HTCC in CaCO3 particles. The content of HTCC in CaCO3 particles increases from 1.3% (B) to 3.1% (C), and then 4.1% (D) with the increase of HTCC concentration. The initiating (and terminating) decomposition temperature of CaCO3 obtained from HTCC solutions are 704 (782) (B), 681(769) (C) and 645 (751) (D) oC, respectively. Namely, the initiating and terminating decomposition temperature of CaCO3 decrease with the increase in HTCC concentration. However, the decomposition temperature window (the difference between terminating and initiating decomposition temperature) increases from 78 to 88 and then to 106 °C. It is caused by the

Fig. 1. Typical SEM images of CaCO3 particles obtained from HTCC aqueous solutions with different concentratins; cHTCC = 0 (A), 100 (B), 200 (C) and 400 (D) mg/L.

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Fig. 2. The XRD patterns (a) and FTIR spectra (b) of CaCO3 particles obtained from HTCC solutions with different concentrations. The samples correspond to those in Fig.1.

distortion of crystal lattice for the residence of HTCC in CaCO3 crystals. The result is consistent with that Schenk and coworkers [26] obtained. Schenk and coworkers [26] demonstrated polymer-induced liquid precursor (PILP) mechanism, by which positively charged additive poly(allylamine hydrochloride) modulated CaCO3 crystallization by subsequently adsorbing onto the specific surfaces of CaCO3 particles during their growth. The larger the HTCC concentration is, the larger the amount it reside in CaCO3 particles, and the more severe the crystal lattice is distorted. 3.2. Effect of Ca2+ concentration on CaCO3 At 40 °C, the mixtures of rhombohedral-, hexagonal plate- and spindle-like particles composed of nano-rods are obtained from 400 mg/L HTCC solutions with Ca2 + concentrations of 40 and 60 mmol/L, as shown in Fig. 4. The average side lengths (long side) of particles obtained from 40 mmol/L Ca2+ solutions are 11.2 μm (rhombohedral particles, Fig. 4A), 8.3 μm (spindle-like particles, and the average diameter is 2.5 μm, Fig. 4A), and 2.8 μm (hexagonal plate-like particles, Fig. 4B). Those for CaCO3 particles obtained from 60 mmol/L Ca2 + solutions are 3.8 μm (rhombohedral particles, Fig. 4C), and 7.1 μm (spindle-like particles, and its average diameter is 2.6 μm, Fig. 4C). The spindle-like particles are aggregates of rod-like calcium carbonate, which magnified image is shown in Fig. 4D. Taking into account that the CaCO3 particles obtained from Ca2+ aqueous solutions (Fig. 1B), it can be found that a smaller Ca2+ concentration favors the growth of CaCO3 particles, while a higher concentration favors the

Fig. 3. The TGA curves of CaCO3 particles obtained from HTCC solutions with different concentrations. The samples correspond to those in Fig. 1.

nucleation of CaCO3 particles. This result agrees with that obtained by our previous work [29,30]. The XRD patterns (Fig. 4E) show that the as-made particles are mixtures of calcite (JCPDS PDF: 47-1743) and aragonite (JCPDS PDF: 760606). The intensity of (104) plane decreases obviously when the concentration of Ca2 + increases to 60 mmol/L, whereas that of (200) plane for aragonite increases notably. Unfortunately, the content of aragonite cannot be calculated for the weak reflection peak of (221) according to the equation proposed by Kontoyannis et al. [39]. The absorption peaks at 700 and 856 cm−1 in FTIR spectra (Fig. 4F) demonstrate the presence of aragonite. Their corresponding contents were calculated to be 3.2% (A) and 8.1% (B) according to the method proposed by Vagenas and coworkers [38]. Wang et al. [40] revealed that alginate molecules modulated the formation of needle-like and shuttle-like aragonite particles through the chelation interaction of alginate molecules and Ca2+ and continual aggregation of alginate/Ca2+ chelates. The type of aragonite particles was dependent on the concentration of alginate molecules and Ca2+. Cooper and de Leeuw confirmed the adsorption of methanoic acid on the surfaces of calcite and aragonite through atomistic simulation techniques [41]. In the present study, the content of aragonite increases with the increase in Ca2+ concentrations ascribes to the selectively adsorption of HTCC on aragonite planes, preventing their transition to thermodynamic stable calcite. 3.3. Effect of temperature on CaCO3 The morphologies of CaCO3 particles obtained from 400 mg/L HTCC and 20 mmol/L Ca2+ solutions with different temperatures are shown in Fig. 5. Compared with the particles obtained at 40 °C, those from 25 °C consist of rhombohedral particles with smooth edges and sharp angles, and disk-like aggregates of nanoparticles (Fig. 5A). The average side length (long side) of the rhombohedral particles is 12.5 μm, and the average diameter of disk-like aggregates is 3.5 μm. The spindlelike particles with an average length of 30.7 μm and a diameter of 5.6 μm are obtained at 60 °C (Fig. 5B), which are larger than those obtained at 40 °C (Fig. 4A and B). The bundle-like CaCO3 particles consisting of nanorods with length of ca. 100 μm and average diameter of 3.4 μm are obtained at 80 °C (Fig. 5C). According to Ref. [42], polyelectrolyte molecular chains are stretched and associated with each other at a high temperature, favoring the formation of polyelectrolyte/Ca2 +/ linear complexes [23]. These linear complexes play a key role in CO2− 3 the resulting morphology of CaCO3 particles. The XRD patterns (Fig. 5E) and FTIR spectra (Fig. 5E) show that the CaCO3 particles are all calcite, except for a small amount of vaterite (29.2%) and aragonite (2.76%) that are obtained at 60 °C. It seems that HTCC suppress the formation of aragonite at a high temperature. The result is consistent with that reported by Altay et al. [43], but contrary to that Li et al. [44] obtained. Altay et al. [43] considered that the


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Fig. 4. (A–D) Typical SEM imagesof CaCO3 particles obtained from 400 mg/L HTCC aqueous solutions with Ca2+ concentration of 40 (A and B) and 60 (C and D) mmol/L. Image D is a magnified image of a spindle-like particle. (E) XRD patterns and (F) FTIR spectra of CaCO3 particles obtained from 400 mg/L HTCC aqueous solutions with [Ca2+] of 40 (a) and 60 (b) mmol/L.

suppressing extent was concentration-dependent. Li et al. [44] synthesized coral- and dendrite-shaped CaCO3 particles that consist of calcite and aragonite in 12.5 mmol/L dodecyl trimethyl ammonium bromide aqueous solutions at 60 and 80 °C, respectively. The content of aragonite increased from 47.8% to 93.2% with the increase of temperature, suggesting that a high temperature favored the formation of aragonite [45]. In this section, the concentration of HTCC is only 400 mg/L, which is somewhat low. In addition, high temperature promotes the decomposition of (NH4)2CO3, increasing the concentration of CO2– 3 and supersaturation, which favors the formation of calcite [40]. Therefore, the content of aragonite decreases with the raised temperature. The

absorption peaks at 1660, 1590, 1482, 1155, 1068 and 1028 cm−1 demonstrate the residence of HTCC in CaCO3 [33]. 3.4. Effect of Mg2+ concentration on CaCO3 The effect of Mg2+ ions on CaCO3 crystallization in HTCC solution has also been investigated when the concentrations of HTCC and Ca2+ are fixed at 400 mg/L and 20 mmol/L, respectively. The concentrations of Mg2 + vary from 5 to 80 mmol/L (that is, the [Mg2 +]/[Ca2 +] ratio ranges from 1/4 to 4/1). The rhombohedral particles with passivated edges and spindle-like CaCO3 aggregates are obtained in HTCC solution

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Fig. 5. Typical SEM images (A–C), XRD patterns (D) and FTIR spectra (E) of CaCO3 particles obtained from 400 mg/L HTCC and 20 mmol/L Ca2+ aqueous solutions at 25 (A), 60 (B) and 80 (C) ○C. The lines A, B and C in figures (D) and (E) correspond to the sample of SEM images.

with a [Mg2+]/[Ca2+] ratio of 1/4 (Fig. 6A). The average side length of rhombohedral particles is 4.7 μm, and spindle-like aggregates are 11.7 μm in length and 2.9 μm in diameter (Fig. 6A). The edges of rhombohedral particles become more severely passivated with increasing [Mg2 +]/[Ca2 +] ratio. In addition, cylindrical-like particles appear in HTCC solutions with a [Mg2+]/[Ca2+] ratio larger than 1/4, and the larger the [Mg2+]/[Ca2+] ratio, the more the amount of cylindrical-like particles are obtained. The magnified SEM image shows that the cylindrical-like particle is composed of smaller edge passivated rhombohedral ones (inset in Fig. 6B). The TEM image (Fig. 6B’) demonstrates the presence of nanosized crystals. The result is consistent with that obtained in Ref. [30]. The average lengths (and diameters) are 12.7 μm (7.9 μm) (B), 12.5 μm (7.6 μm) (C), 10.0 μm (3.7 μm) (D) and 11.7

(3.4 μm) (E), respectively. In short, the average length and diameter of cylindrical-like particles obtained at various [Mg2+]/[Ca2+] ratios are almost constant. It might be the synergistic effect of HTCC and Mg2+ on CaCO3 crystallization. The Mg2+ ions show notable effect on CaCO3 crystallization and growth due to their smaller dimensions, higher charge density, and greater hydration energy [46,47]. XRD patterns and FTIR spectra (Fig. 7) show that the CaCO3 are mixtures of calcite and aragonite (JCPDS PDF: 41-1475). During the crystallization process, Mg2+ ions plays a key role in inducing the precipitation of aragonite [47]. With the increase in [Mg2+]/[Ca2+] ratio, the spacing values (d) of calcite (104) lattice planes decrease. On the contrary, the full width at half maximum (FWHM) of those peaks decrease (Table 1), indicating a decrease and disordering in the lattice constant


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Fig. 6. Typical SEM and TEM images of CaCO3 particles obtained from 400 mg/L HTCC solutions with [Mg2+]/[Ca2+] ratios of 1/4 (A), 1/2 (B), 1/1 (C and C′), 2/1 (D) and 4/1 (E). Insets show the detailed structure of cylindrical-like particles.

of calcite [48]. The average sizes of CaCO3 particles (La in Table 1) calculated from the Scherrer equation [49] show that the particle sizes decrease with the increase in [Mg2 +]/[Ca2 +] ratio firstly, and then increase. It ascribes to the incorporation of Mg2+ ions into calcite lattice [9]. The result is consistent with those obtained in our previous works [30,50]. The contents of aragonite calculated based on XRD [39] and

FTIR [38] methods are listed in Table 1. The content of aragonite increases with the increase of [Mg2+]/[Ca2+] ratio from 1/4 to 2/1, whereas, it decreases at the [Mg2+]/[Ca2+] ratio of 4/1. According to Liu et al. [47], the presence of Mg2+ ions could induce the transition from calcite to aragonite during the crystallization, and a higher [Mg2+]/[Ca2+] ratio prolonged the transition time dramatically. The ratios of v2/v3, namely

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Fig. 7. XRD patterns (a) and FTIR spectra (b) of CaCO3 particles obtained from HTCC solutions with different [Mg2+]/[Ca2+] ratios. The samples correspond to those in Fig. 6.

Table 1 The position (d) and full width at half maximum (FWHM) of diffraction peaks, molar content of aragonite calculated based on XRD and FTIR methods and average particle size (L) of calcium carbonate. Mg2þ Ca2þ

d

FWHMa

Caa (%)

Cab (%)

La (nm)

1/4 1/2 1/1 2/1 4/1

3.026 3.020 3.014 3.011 2.985

0.188 0.199 0.237 0.181 0.250

17.03 21.14 29.73 63.83 32.95

20.91 21.50 29.12 36.47 18.13

86.54 81.69 68.61 64.97 89.83

Note: a Obtained from XRD patterns b Obtained from FTIR spectra.

0.60 (A), 0.63 (B), 0.69 (C), 0.73 (D) and 0.71 (E), increase with the increase in [Mg2 +]/[Ca2 +] ratio, implying the variation of vibration of CO2– 3 caused by the defects or difference in calcite particles during their crystallization process [29]. 3.5. The mechanism of CaCO3 crystallization in HTCC aqueous solution To explore the mechanism of CaCO3 crystallization in HTCC aqueous solutions by the slow vapor diffusion method, the CaCO3 particles precipitated in different aging time are characterized while the concentrations of HTCC and Ca2 + are fixed at 100 mg/L and 20 mmol/L, and temperature at 40 °C. Fig. 8A shows that random particles (inset in Fig. 8A) with only a small amount of rhombohedra are precipitated at aging time of 0.5 h. The TEM image (Fig. 8A’) shows that the random particles are arranged along with the molecular configuration of HTCC, and the selected area electron diffraction (SEAD, inset in Fig. 8A’) results show that the random particles are ACC. When the aging time is longer than 1 h, the mixtures of disk-like and rhombohedral CaCO3 particles are obtained. The average side lengths (or diameters of the disk-like particles) are 5.0 (A), 4.6 (B, 3.6 μm for disk-like particles), 4.3 (C, 4.3 μm for disk-like particles) and 10.5 μm (D, 7.6 μm for disk-like particles). The surfaces of both rhombohedral and disk-like particles become rough, and an amplified image (Fig. 8C’) shows that the surface is composed of layerarranged nanosized particles. The XRD patterns (Fig. S3(a)) show that the particles are all calcite, although only the diffraction peak of (104) plane is obvious, which is different from the result shown by the FTIR spectra (Fig. S3(b)). The absorption peaks at 711 and 874 cm− 1 are very weak, indicating that the CaCO3 particles obtained at aging time of 0.5 and 1 h are mainly ACC. The result is consistent with that observed

from magnified TEM image (Fig. 8A”). The absorption peaks at 874, 711 and 856 cm−1 indicate that the particles obtained at aging time of 2 and 4 h are mixtures of calcite and aragonite. Unfortunately, the contents of aragonite cannot be calculated for the absence of its characteristic absorption peak at 700 cm−1 [38]. Two crystallization mechanisms for crystal growth in solution are widely accepted. One is the Ostwald ripening, which is characterized by the initial formation of smaller crystals and the smaller ones are consumed as “nutrients” for the growth of larger ones. The other is oriented aggregation or oriented attachment, which is characterized by special aggregation of nanocrystals into unique and symmetry-defying crystal aggregates. Commonly, the oriented aggregation of nanocrystals into ordered aggregates is mainly driven by a special binding interaction of bridging ligands capped on the surface of nanocrystals. Occasionally, specific assembled aggregates of elementary nanocrystals including twins and (or) intergrowths are formed [51]. Fig. 8 shows that the crystallization and growth of disk-like and spindle CaCO3 modulated in HTCC solutions are driven by oriented aggregation mechanism. Although polyelectrolytes with tertiary and quaternary amines are inactive in affecting CaCO3 crystallization and growth [26], there are lots of \\NH2 and \\OH groups on HTCC molecules that could interact with positively charged Ca2+ ions through lone electron pair and intramolecular interaction, forming HTCC/Ca2+ complexes. Fig. 8A’ and the magnified TEM image (Fig. 8A”) show that at the initial stage of crystallization, the nano-sized ACC particles are arranged along the molecular configuration of HTCC, indicating that the Ca2+ ions in HTCC/Ca2+ complexes act as nucleating points, and also sources of CaCO3 particles. Within the initial 1 h, the nano-sized ACC particles are rearranged into disklike aggregates (Fig. 8B’) under the bridging effect of HTCC molecules, inducing the precipitation of disk-like particles (Fig. 8B). With aging time prolonging, the average diameter of disk-like particles increases from 3.6 (1 h) to 4.3 (2 h), 7.6 (4 h) and 12 μm (8 h) through oriented aggregating process. At a large HTCC concentration or high temperature, HTCC molecules interact with Ca2+ ions through intermolecular interaction, forming linear HTCC/Ca2+ complexes. The linear HTCC/Ca2+ complexes modulate the formation of rod-like CaCO3 particles, which aggregate into spindle or bundle-like CaCO3 particles through bridging effect of HTCC. During the aggregating process, the thermodynamically instable ACC particles transform into metastable and thermodynamically stable calcite. Meanwhile, HTCC molecules wrapped on the CaCO3 surfaces are encapsulated into the disk-like aggregates. The interaction between HTCC and Ca2+ (or CaCO3) slows the dissolution rate of CaCO3 particles, and decreases the transition rate from ACC to calcite [29]. Therefore, mixtures of calcite, vaterite and aragonite are obtained even at aging time of 8 h. The

Fig. 8. Typical SEM and TEM images of CaCO3 obtained from 100 mg/L HTCC solutions at aging time of 0.5 (A, A’ and A”), 1 (B and B’), 2 (C and C′) and 4 (D) h. The white dots in A” are caused by the photographic plate.


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intergrowth of disk-like particles (inset in Fig. 8D) confirms the oriented aggregation mechanism of CaCO3 in HTCC. Regretfully, the detailed information on nanocrystals of the disk-like CaCO3 cannot be provided due to the limitation of the Quanta 200 environmental scanning electron microscope. In this paper, the amount of Ca2+ is surplus compared to that of – NH2 and –OH in HTCC molecules, i.e., thus certain amount of Ca2+ is dissociated. The ACC particles formed from free Ca2+ transform into rhombohedral ones with average side length of 5.0 μm in a very short time. The average side length of rhombohedra decreases to 4.6 (1 h) and then 4.3 μm (2 h) with aging time prolonging, but subsequently increases to 10.5 (4 h) and 22.6 (8 h) μm due to the dissolution and recrystallization of the CaCO3 particles (namely, the Ostwald ripening process). To confirm the mechanism we proposed, the effects of HTCC on CaCO3 nucleation and crystallization will be investigated in following studies, including a higher concentration, different average molecular weight, and different substitution degree of EPTAC. 4. Conclusion Disk-like and spindle CaCO3 particles have been obtained in HTCC aqueous solutions with different concentrations at 40 °C. The HTCC affects CaCO3 crystallization and polymorphs through the electrostatic interaction between \\NH2 and –OH groups in HTCC and Ca2 + ions (forming HTCC/Ca2+ complexes), as well as the bridging effect among HTCC molecules and the adsorption on CaCO3 surfaces. The Ca2+ ions in HTCC/Ca2+ ions act as nucleating points and also sources of CaCO3 crystallization. The Ca2+ ions that do not interact with HTCC result in the formation of rhombohedral CaCO3 particles. At a low concentration of HTCC, the intramolecular interaction in HTCC molecules induces the formation of disk-like CaCO3 particles. And at a high concentration, the intermolecular interaction among HTCC molecules induces the formation of rod-like CaCO3 particles, which aggregate into spindle CaCO3 particles. A high temperature stretches the HTCC molecule, inducing the foration of rod-like CaCO3 particles. In a HTCC solution, the Mg2+ ions incorporate into calcite lattice affecting the morphology and polymorph of CaCO3, promoting the formation of aragonite. Acknowledgments The authors acknowledge the financial support from National Natural Science Foundation of China (21306092 and 21376125), Science and Technology Plan Project of Shandong Provincial University (J12LA02), Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, Star Youth Science and Technology Plans Project of Ji'nan City (20110103). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2016.05.036. References [1] J.S. Evans, “Tuning in” to mollusk shell nacre- and prismatic-associated protein terminal sequences. implications for biomineralization and the construction of high performance inorganic–organic composites, Chem. Rev. 108 (2008) 4455–4462. [2] H. Cölfen, Precipitation of carbonates: recent progress in controlled production of complex shapes, Curr. Opin. Colloid Interface Sci. 8 (2003) 23–31. [3] A. George, A. Veis, Phosphorylated proteins and control over apatite nucleation, crystal growth, and inhibition, Chem. Rev. 108 (2008) 4670–4693. [4] J.L. Arias, M.S. Fernández, Polysaccharides and proteoglycans in calcium carbonatebased biomineralization, Chem. Rev. 108 (2008) 4475–4482. [5] P.J. Smeets, K.R. Cho, R.G. Kempen, N.A. Sommerdijk, J.J. De Yoreo, Calcium carbonate nucleation driven by ion binding in a biomimetic matrix revealed by in situ electron microscopy, Nat. Mater. 14 (2015) 394–399.

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