Synthesis of Calcium Carbonate Crystals Using Hydroxypropyl Methyl Cellulose as a Gel Template

Abstract: Calcium carbonate crystals were synthesized in hydroxypropylmethylcellulose (HPMC) hydrogel. The morphology and structure of the obtained composite calcium carbonate were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The results showed that the Polyhydroxy groups are beneficial to the formation of aragonite crystals. However, no aragonite is produced in the methylcellulose hydrogel lacking polyhydroxy groups and the blank system without gel. Calcium crystals, further research infers that this special crystal morphology is closely related to the special combination of macromolecules in the HPMC hydrogel network.

Key words:hydroxypropyl methylcellulose; hydrogel; template; hydrophobic association; calcium carbonate; crystallization

In nature, organic macromolecules in living organisms form a gel-like matrix network through supramolecular assembly, and this network matrix exerts a special influence on the mineralization process, thereby forming biominerals. People's research on biomineralization is very active , and tried to clarify the principle of biomineralization. Many studies have shown that protein biomacromolecules and some macromolecules rich in acidic functional groups play a role in the formation of inorganic crystals such as calcium carbonate. Specific groups on these molecules and molecules The further assembly form of the crystal also affects the crystal growth from many aspects such as electrostatic interaction, spatial orientation, lattice matching, and stereochemistry. Since the organism is a typical colloidal system, many research groups are currently focusing on the Synthesis of calcium carbonate. Gel is a special dispersion system. In the gel, molecules combine to form a spatial network structure and intervene in the nucleation and growth of minerals. Therefore, the study of using gel as a template to synthesize crystals is of great importance to in-depth It is of great significance to understand the mineralization principle of the biological world and theoretically guide the synthesis of functional calcium carbonate materials.

Grassmann et al. prepared CaCO3 crystals in polyacrylamide hydrogels modified by -SO3H and polyacrylamide gels containing -COOH functional groups. Calcite with different shapes was generated. They thought The porous region in the gel is the starting point for the rapid growth of crystals, and the electrostatic interaction and stereochemical interaction between the network of -COOH and -SO3H functional groups and ions are important factors that lead to different shapes. In addition, they are also synthesized in collagen and gelatin matrices. Calcite-type calcium carbonate. Yang et al. also used hydroxyl-rich agarose gel to synthesize eight-lobed star-shaped calcite crystals at 30 °C, and obtained long rod-shaped aragonite at 65 °C. The molecular chains that make up the gel can be combined by chemical bonds (such as poly acrylamide), or it may be combined by physical interaction (such as agarose). Different combinations of molecules change the network structure of the gel, which leads to different effects on the growth of mineral crystals.

In this paper, hydroxypropyl methylcellulose hydrogel was used as a new type of organic template to synthesize calcium carbonate. This gel has polyhydroxyl functional groups. Yu et al. A mixture of calcite and aragonitic calcium carbonate crystals was prepared at 80°C, and their relative contents were calculated. However, there are relatively few studies on weak polar groups such as polyhydroxy groups in the gel network. In addition, hydroxypropyl The arrangement of molecules in HPMC gel is also different from that of gel templates that have appeared. This paper studies the influence of such hydrogel templates on the crystal form and morphology of calcium carbonate crystals.

1. Materials and methods

(1) Experimental materials and instruments

Hydroxypropyl methylcellulose and methylcellulose (MC) were purchased from KIMA CHEMICAL CO.,LTD, both of which were of medical and food grade. The mass fraction of hydroxypropyl group in the solution ranges from 7% to 12%. The viscosities of 2% HPMC and MC solutions at 20°C are 17000 and 12000 mPa s, respectively (the intrinsic viscosities of HPMC and MC measured by Ubbelohde viscosity method are 910 and 780 mL/g). Anhydrous Na2CO3 and anhydrous CaCl2 were analytically pure; experimental water was deionized water (Nankai University, Tianjin). D/MAX-2500 X-ray diffractometer (XRD) from Rigaku Company, Japan; Philips XL30 scanning electron microscope (SEM) from Philips Company in the Netherlands; ZRY-2P TG-DTA comprehensive thermal analyzer from Shanghai Zufa Industrial Co., Ltd.; FTS3000 Fourier transform infrared spectrometer from Bio-rad Company in the United States.

(2) Synthesis of calcium carbonate crystals

A certain amount of powdered anhydrous CaCl2 was added to 0.5% (mass fraction, the same below) and 1% HPMC solution under rapid stirring, so that the mass fraction of CaCl2 was 3%. Take about 10 mL of the mixed solution and put it in In the test tube, the test tube was placed in a constant temperature water bath at 65°C to form a gel. After the gel was formed, 3% sodium carbonate solution preheated to 65°C was slowly added to the upper layer of the gel. The above system was placed in the water bath for 3 days Finally, the gel was cooled back to the solution state, the product was centrifuged and washed three times with water, and then dried in an oven for 48 h in vacuum. Using the same method, calcium carbonate was synthesized in 0.5% MC hydrogel. Deionized water was used instead of HPMC solution in 65 for a blank comparison experiment.

(3) Characterization of calcium carbonate crystals

 The morphology of the product was observed under a scanning electron microscope. The crystal form of the product was analyzed by X-ray diffractometer. Cu Ka radiation, wavelength 1.54056 Å, tube voltage 40.0kV, tube current 100 mA, scanning range 3° ~ 80° , with a scan step of 0.02°. A certain amount of samples were analyzed using a TG-DTA comprehensive thermal analyzer in the range of 21-600 °C with a heating rate of 15/min to characterize the organic matter and In addition, the crystallized products were taken for 1, 2, and 3 days respectively, and the taken out gels were returned to solution, and dried by film coating. The three samples were respectively pressed with KBr, and analyzed by infrared spectroscopy with a resolution of 4 cm- 1. Scan 20 times, and the scanning range is 4000 ~ 400 cm-1. Also make 0.5% HPMC solution and CaCl2-HPMC solution form a gel at 65 °C, and then return to a solution, and use the same method for infrared spectrum analysis.

2. Results and Discussion

2.1 Formation of CaCO3 crystals

As we all know, HPMC gel is formed by the hydrophobic association of methoxy groups on the molecules. At lower temperatures, the cellulose molecules are hydrated, and when the temperature rises, the macromolecules lose bound water and make the methoxy groups on them Hydrophobic association occurs when the base is exposed to the surrounding environment. The 0.5% and 1% hydroxypropyl methylcellulose solutions are heated to form a gel, and the gel is used as a matrix template to synthesize calcium carbonate. The XRD pattern of the product after 3 days It was shown that the crystal forms of the product were aragonite and calcite. However, using deionized water instead of cellulose solution, the product was found to be only calcite after culturing calcium carbonate at 65 for 3 days under the same experimental conditions. This indicated that a small amount of HPMC with weakly polar groups such as ect. is conducive to the formation of the unstable calcium carbonate crystal form in nature - aragonite crystal form. This result is similar to the results of some other system studies. In these studies, also with polyhydroxy groups The polyvinyl alcohol (PVA) solution also leads to the formation of aragonite. Considering that methyl cellulose and hydroxypropyl methyl cellulose can form gel under heating conditions, and the gel mechanism is the same. We are still At the same temperature, CaCO3 was synthesized in MC gel by the same method. XRD analysis results showed that only one crystal form of calcite was produced. Comparing MC and HPMC molecules, except that the MC molecule lacks the polyhydroxyl groups contained in the HPMC molecule Besides, the structures and properties of the two are almost the same. It can be seen that the polyhydroxyl groups play a crucial role in the formation of aragonitic CaCO3, and they are likely to be the nucleation sites of aragonite.

In addition, combined with the XRD spectrum, according to the definition: when only aragonite and calcite exist in the system, the content of aragonite can be calculated from the ratio of the strongest diffraction peak intensity IA of aragonite to the strongest diffraction peak intensity IC of calcite, 0.5% and 1% The content of aragonite produced by the HPMC gel was 4% and 6.6%, respectively. It can be seen that as the concentration of HPMC increases, the content of aragonite tends to increase. This may be because, as the concentration of HPMC increases , the content of hydroxypropyl groups increases, which can attract more Ca2+, increase the local supersaturation of ions, and decrease the nucleation free energy (△G) of calcium carbonate. As we all know, the critical The nucleation free energy △G* is greater than the critical value of calcite. Therefore, the continuous decrease of △G makes it easier to form aragonite in the system with high concentration, and its content is also higher.

2.2 Morphology of CaCO3 crystals

It can be seen from the SEM image that the CaCO3 produced in the blank system where deionized water replaces HPMC is a rhombohedral hexahedron. However, the crystals produced in the HPMC gel system have many long rod-shaped crystals besides the rhombohedral hexahedron. Someone once used The agarose gel containing polyhydroxyls was used as a template, and long rod-shaped aragonite was also obtained at 65 °C. However, the long rod-shaped crystals synthesized in HPMC gel showed a more special shape. They were composed of some small rod-shaped crystals Corn cob-shaped crystals arranged according to certain rules along the long axis of the ingot. The reason for the formation of such a unique calcium carbonate morphology is mainly due to the special network structure of HPMC gel. Different from other gel structures, HPMC intramolecular and intermolecular through Adjacent methoxy groups undergo hydrophobic interactions to form hydrophobic microdomains. The arrangement of molecular chains in these microdomains is more orderly than in the solution state. These microdomains further serve as physical crosslinking points to form the entire network structure. On the one hand, These hydrophobic regions provide a certain microenvironment for mineralization, which can control the diffusion of ions in the medium. When reactant ions diffuse into the gel, the polyhydroxyl groups in these hydrophobic microregions intercept ions and affect the diffusion rate of reactants. The aggregation of multiple ions makes crystals nucleate rapidly at specific positions in the microdomains, forming corn kernels on long cobs. On the other hand, the growth of crystals is also hindered and restricted by these hydrophobic regions, which eventually leads to the production of corncob-shaped carbonic acid Calcium crystals. In addition, it can also be seen that the size of corn kernels formed in 1% gel is smaller than that of crystal particles in 0.5% gel. This may be because, with the increase of HPMC concentration, the number of methoxyl groups per unit volume increase, the arrangement of the hydrophobic regions in the 1% gel is tighter. Therefore, the corn kernels in the gel with a higher concentration are more and smaller in size. Since such a morphology is observed for the first time, further analysis is still in progress.

2.3 Content of organic matter in CaCO3 crystals

Using TG-DTA comprehensive thermal analyzer to analyze the product crystals can check whether the product crystals contain organic matter and determine its content. Therefore, we carried out thermogravimetric analysis on the product formed in 0.5% HPMC gel, and its thermogravimetric curve It can be seen that thermal weight loss occurs near 380 °C, and the weight loss is 1.4%, which may be caused by the thermal decomposition of HPMC, an organic compound contained in calcium carbonate, indicating that the product contains residual HPMC template.

3. Conclusion

CaCO3 crystals were prepared in the HPMC gel template. In the comparative experiment where deionized water was used instead of HPMC, only calcite was formed, while two crystal forms of aragonite and calcite were produced in the HPMC gel. It was shown that HPMC can induce aragonite The formation of aragonite. And with the increase of HPMC concentration, the aragonite content increases. FTIR analysis shows that there is an interaction between the crystal and the ether bond hydroxyl group. Combined with the results of the MC gel template, it can be speculated that the polyhydroxyl group on the HPMC molecular chain is the key to the formation of aragonite. In addition, corncob-like calcium carbonate crystals were observed in this paper. This special morphology is closely related to the hydrophobic microdomains inside the HPMC gel, that is, the special formation mechanism and structure of the gel. Synthesis of HPMC hydrogel Calcium carbonate provides a new basis and supplement for the study of the modulation of gel templates on CaCO3 crystals.