Crystallization of Sodium Percarbonate from Aqueous Solution: Basic

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General Research

The Crystallization of Sodium Percarbonate from Aqueous Solution: Basic Principles of Spherulite Product Design Wenchao Yang, Lixuan Xiong, Meijing Zhang, Chuang Xie, Hongxun Hao, Baohong Hou, Yongfan Yang, Ling Zhou, and Qiuxiang Yin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00506 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Industrial & Engineering Chemistry Research

The Crystallization of Sodium Percarbonate from Aqueous Solution: Basic Principles of Spherulite Product Design Wenchao Yanga, Lixuan Xionga, Meijing Zhanga,b, Chuang Xiea,b, Hongxun Haoa,b, Baohong Houa,b, Yongfan Yanga, Ling Zhoua,*, Qiuxiang Yina,b,* a School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China. b The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Tianjin University, Tianjin 300072, People’s Republic of China.

ABSTRACT Recently, there has been an increasing interest in spherulites because of their excellent physicochemical properties and remarkable radial morphologies. However, their formation mechanisms remain largely unresolved and are often overlooked in industrial crystallization. In this work, it was demonstrated that the sodium percarbonate (SPC) spherical particle primarily results from the spherulitic growth effect rather than agglomeration effect. And then, in-situ nucleation-controlled experiments in petri dish were established to study the growth kinetics and morphologies of SPC spherulite. During the process, a critical growth rate was 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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firstly discovered, which divides the spherulite formation into branching and non-branching stage. A kinetic model was established to study the growth regime for SPC spherulite formation. Moreover, it is found that sodium hexametaphosphate (SHMP) is not necessary for SPC spherulite, but it can promote the branching of SPC which makes it possible to produce compact SPC spherulite product. Based on the experimental results, it is proposed that inducing branching and reducing the kinetic coefficient of the interface reaction are the sufficient conditions for spherulite product, which can be applied to other needle-like crystals to form spherulites. 1. INTRODUCTION Low bulk density, poor flowability and bad compressibility especially in terms of some needle-like microcrystals often cause many problems in their downstream processing. Formation of large and spherical particles directly through the crystallization rather than granulation technique is an effective way to solve these problems. There are three crystallization approaches for the preparation of large spherical particles: spherical agglomeration strategy, interfacial growth strategy and spherulitic growth strategy. The spherical agglomeration strategy, pioneered by Kawashima,

1

offers a general method to

produce large spherical particles by regulating the proportion of good solvent, poor solvent and bridging liquid. The particles obtained by this method often have disorganized inner structure. The basic principle behind the interfacial growth strategy,

2

often called quasi-

emulsion solvent diffusion strategy, is to create quasi-emulsion droplets where the crystals precipitate. However, to the author’s knowledge, the spherulitic growth strategy, which owes the formation of spherical particles much to the radial growth, has not been commonly recognized in industrial manufacture. Some spherical particles with radial growth morphology

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were often attributed to agglomeration effect and no research has been found to offer general principles of spherulite product design.

Spherulite products obtained from spherulitic growth strategy are formed by radial polycrystalline growth with spherical envelope. These spherical particles often exhibit excellent physical and chemical properties. For example, the flowability and particle size distribution have been greatly improved for L-tryptophan3 and L-malic acid. 4 The mechanical and safety properties can also be promoted for 2,6-diamino-3,5-dinitropyrazine -1-oxide.

5

Moreover, as a common natural phenomenon, the spherulite can be widely observed in melt, mineral and polymer. Different spherulitic patterns have been used to discover new polymorphs of DDT6 and aspirin. 7 The research on the spherulites of calcium carbonate, 8,9 barium carbonate10,11 and apatite12,13 can provide some information concerning the mechanisms of biomineralization. The spherulite of human interferon has been demonstrated to have better pharmacokinetics.

14

Therefore, this work may not only be adaptable to

spherulite product design but also give some insights into other spherulite formation mechanism.

The model compound of this work, SPC (Na2CO3·1.5H2O2, CAS registry No. 15630-89-4), is an environmentally safe bleaching agent. Designed to be an effective oxidizing bleach agent, SPC serves as an environment-friendly alternative to chlorine bleach. 15 Its crystals obtained by the normal crystallization method generally exhibit small needle-like morphologies which can lead to problems such as low bulk density, poor flowability, and poor compressibility. Some additives, such as polyphosphates or polyacrylates, 16 are commonly used in the SPC 3 ACS Paragon Plus Environment


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production in order to deal with the problems, because these additives can convert the acicular crystals into spherical shapes which have higher bulk density and better flowability. However, previous study has not investigated these mechanisms. In addition, SPC is an ideal model compound to study the growth kinetics and morphology evolution of spherulites due to its large particle size which can make it easier to be observed for in-situ experiments.

Our initial motivation was to investigate the formation mechanism of SPC spherulite and then provide a new point of view for other spherulite product design. In this paper, the spherulitic growth strategy was demonstrated in the SPC production. Particle vision measurement (PVM) and focused beam reflectance measurement (FBRM) were used to observe the evolution process in stirred glass crystallizer. Also, in-situ nucleation-controlled experiments were established to study the spherulite formation. Time-lapse optical microscopy was applied to investigate the growth kinetics and morphology of the SPC spherulite. During the process, a critical growth rate was discovered to divide the spherulite formation into branching and nonbranching stage which is so far never reported in the existing research on spherulite formation mechanism and has great significance for compact spherulite product design. A kinetic model was established to investigate the growth regime of SPC spherulite. Based on these findings, the critical growth rate of SPC was studied under different initial supersaturations and concentrations of SHMP and NaCl. Finally, the comparison of spherulite morphologies in the presence and absence of SHMP leads to the conclusion that SHMP can enhance the branching of SPC which makes it possible to form compact spherulite product in crystallizer.


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2. EXPERIMENTAL SECTION 2.1

Materials

For the experiments, sodium carbonate (99% reagent grade), the hydrogen peroxide (30.0 w%, not containing stabilizers), sodium chloride (99% reagent grade) and SHMP (99% reagent grade) and Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were purchased from Kewei (Tianjin, China) and were used as received. Distilled, deionized water was used wherever applicable.

2.2

Crystallization in crystallizer

Sodium carbonate solution was made by adding 84.08 g sodium carbonate, 0.600 g SHMP and 20.00 g sodium chloride into 207.45 g distilled and deionized water in crystallizer at 40 ℃ . Hydrogen peroxide solution was obtained by adding 0.2 g EDTA-2Na and 15.00 g sodium chloride into 114 g hydrogen peroxide in another crystallizer at 20 ℃ . And then, sodium carbonate solution was added to hydrogen peroxide solution by a peristaltic pump at a rate of 3.88 ml/min. When the crystallization process was stopped, the suspension was filtered. The obtained crystalline products were washed with water and dried in a vacuum oven at room temperature. Moreover, an experiment with the same conditions except for SHMP was conducted to study the effect of SHMP.



2.3

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Crystallization in petri dish

The solution SC was prepared by dissolving 16.4 g sodium carbonate into 100 g distilled and deionized water. The SHMP solution was made by dissolving 2 g SHMP into 100 g distilled and deionized water. Different amount of sodium chloride and SHMP was dissolved in 14.33 ml solution SC. 0.01 g EDTA-2Na and different amount of sodium chloride was dissolved in 3.5 ml hydrogen peroxide. And then, the sodium carbonate solution with sodium chloride and SHMP was poured into hydrogen peroxide solution with EDTA-2Na and sodium chloride in petri dish at ambient temperature (20 ± 0.5 ℃ ). Different initial supersaturations were achieved by adding water into different amount of sodium carbonate solution and hydrogen peroxide before adding additives and mixing process. Molar ratio of hydrogen peroxide to sodium carbonate was maintained at 1.575 in all experiments (1.575 is slightly higher than the molar ratio in SPC molecule in consideration of the decomposition of hydrogen peroxide during the process). Moreover, all experiments were repeated for three times. All the experimental conditions in this section are shown in Table 1. During the processes, the timelapse images were acquired using a sensitive Kodak digital camera, installed in a stereo microscope. The obtained spherulites were washed with water and dried in a vacuum oven at room temperature.

Table 1. Experimental conditions for crystallization in petri dish with different initial supersaturations and concentrations of SHMP and NaCl Sodium carbonate solution Solution

Sodium

SHMP

Hydrogen peroxide solution Water

Hydrogen

Sodium

EDTA-2Na

Water 6

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SC /ml

chloride /g

/g

/g

peroxide /ml

chloride /g

/g

/g

14.33

2.75

1.00

0

3.50

1.00

0.01

0

14.33

2.75

0.75

0.25

3.50

1.00

0.01

0

14.33

2.75

0.50

0.50

3.50

1.00

0.01

0

14.33

1.75

0.50

0

3.50

1.00

0.01

0

14.33

2.00

0.50

0

3.50

1.00

0.01

0

14.33

2.25

0.50

0

3.50

1.00

0.01

0

14.33

2.50

0.50

0

3.50

1.00

0.01

0

13.72

2.75

0.50

0.57

3.35

1.00

0.01

0.15

13.11

2.75

0.50

1.14

3.20

1.00

0.01

0.30

12.49

2.75

0.50

1.71

3.05

1.00

0.01

0.45

11.88

2.75

0.50

2.28

2.90

1.00

0.01

0.60

2.4

Characterization

In crystallizer, the particle size distribution was continuously monitored by FBRM (Mettler Toledo, Redmond, WA) and the evolution process of sodium percarbonate spherulite was observed by PVM (Mettler Toledo, Redmond, WA). Scanning electron microscopy (SEM, TM3000, Hitachi, Japan) was used to observe the morphologies of SPC spherulites. A stereo7 ACS Paragon Plus Environment


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microscope (Stemi 508, Zeiss, Germany) was used to observe the spherulite formation in petri dish.

3. RESULTS AND DISCUSSION 3.1

Evolution of the Spherulite product

According to the industrial manufacture of SPC, at least one salting-out agent and one stabilizer need to be used in the process. The salting-out agent is understood to denote an agent which decreases the solubility of SPC in aqueous solution to obtain a higher yield of SPC production. Stabilizer is understood to denote any compound capable of protecting hydrogen peroxide against decomposition and, consequently, avoid interference from the oxygen bubbles during on-line and in-situ observation. In this work, sodium chloride and EDTA-2Na were used as salting-out agent and stabilizer respectively. The FBRM data (Figure 1) shows that the SPC crystals formed about 7 min after the addition of sodium carbonate solution. The number of large particles (chord length from 50 to 300 μm) surged within a minute, while the number of smaller ones increased more slowly. In this process, spherulitic growth strategy can be verified, because only SPC spherulites with radial growth structures can be recognized and no needle-like microcrystals can be seen during the early stage after nucleation (Figure S1 and 1a). With the growth of SPC spherulites, more and more needle-like microcrystals can be observed (Figure 1b), which can be interpreted as subunits peeled away from the outer surfaces which are not as compact as the cores. Needle-like crystals of SPC were obtained in the absence of SHMP (Figure 2a, c), while spherulitic particles of SPC were obtained in the presence of SHMP (Figure 2b, d).


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Figure 1. a: FBRM data for the experiment in crystallizer: on-line particle chord length distribution of SPC; b and c: PVM images at 9 and 40 min respectively.



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Figure 2. SEM images of SPC. a and c: sodium percarbonate crystals obtained in the absence of SHMP; b and d: sodium percarbonate spherulites in the presence of SHMP

3.2

Crystal Growth Kinetics of the Spherulite

As mentioned above, it has been demonstrated that radial polycrystalline growth, rather than agglomeration, accounts for the outer spherical envelope of SPC. Agglomeration effect is inevitable during the process in crystallizer. Therefore, crystallization experiments in petri


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dish were performed to investigate the growth kinetics and mechanism of radial polycrystalline growth of SPC to avoid the influence of agglomeration.

A typical growth curve for SPC in the presence of SHMP is presented in Figure 3. Growth/Sigmoidal function was used to fit the growth curves for SPC. 𝐵

(1)

𝑅 𝑖 = 𝐴 + 𝐶 + 𝑒𝑡

where A, B, and C are the parameters of this equation. Ri is the radius of each spherulite i. The coefficient of correlation, R2 is used to evaluate the applicability of this fitting model. According to the value of R2, the calculated data by eq 1 present perfect agreement with experiment values. As shown in Figure 4 (data related are shown in Table S1), during the spherulite formation, there were some changes in terms of the roughness of the growth front of this spherulite. At first, the circular growth front was coarse, which can be ascribed to insufficient noncrystallographic branching. And then, gradually sufficient branching resulted in a smooth growth front. However, the smooth growth front started to become coarse again at a particular moment. The growth rate can be obtained by taking a derivative with respect to time as shown in Figure 3. In this work, the growth rate which corresponds to the moment when the smooth growth front of spherulite starts to become coarse is defined as a critical growth rate. There won’t be any branching when growth rate is lower than the critical growth rate, which divides the spherulite formation into branching and non-branching stage. The spherulite experiences a sustained decrease in growth rate, which challenges the claim from Yang that the growth rate remains constant during the early stage of spherulite formation.3 Experimental conditions for Figure 3 and Figure 4 are shown in Table 2. 11 ACS Paragon Plus Environment


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Table 2. Experimental conditions for Figure 3 and Figure 4 Sodium carbonate solution

Hydrogen peroxide solution

Solution SC Sodium

SHMP

/ml

chloride /g

14.33

1.75

Hydrogen

Sodium

EDTA-2Na

solution /ml peroxide /ml

chloride /g

/g

0.5

1.00

0.01

3.5

Figure 3. Growth curve and growth rate curve for SPC spherulite


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Figure 4. Optical microscopy images of spherulite formation

The existence of critical growth rate has not been discussed directly, but the importance of critical supersaturation has been stated before. For stoichiometric solutions of ions, the driving force for crystallization can be roughly approximated by Δμ/RT ≈ vln(c/ceq), where Δμ is the difference in chemical potentials, R is the universal gas constant, T is the crystal growth temperature, v is the number of ions in the neutral complex, c is the salt concentration in the solution, ceq is the saturation concentration. Spherulitic growth often requires a high crystallization driving force. In this case,

it has been reported that the thermodynamic

supersaturation divided by the number of ions in a neutral complex, Δμ/RTv, is exceeding 2.3 for calcium carbonate, 2.5 for calcium oxalate dehydrate, 9 for scheelite, CaWO4, 0.6 for sodium bicarbonate.17 However, the threshold driving force for spherulite formation is ambiguous. In this work, it is difficult to obtain the critical supersaturation which corresponds to the critical growth rate due to the lack of a proper in-situ monitor of the supersaturation during the process.

Moreover, there are some possible reasons accounting for this neglect of the critical growth rate. Firstly, many experiments concerning spherulite products were performed in crystallizers, 13 ACS Paragon Plus Environment


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which will inevitably lose some details of the spherulite morphology due to the destruction of the stir, as shown in the experiment above in this work. Secondly, the critical growth rate can be easily recognized by in-situ experiment, especially when the growth front can be clearly observed. However, in-situ experiment is not a common method to study the spherulite formation mechanism due to the small size of some spherulites and special experimental conditions which will cause some difficulties to conduct in-situ experiments. Therefore, timeresolved morphological evolution of spherulites was often studied by taking out spherulites at a set interval, which makes it impossible to compare the roughness of the growth front during the whole process and inevitably destroys the spherulite morphology. Finally, in terms of morphology evolution of spherulites, main attention has been paid to the relative size of the core and the radial corona according to traditional categories.18 However, little attention has been paid to the morphology evolution of category 1 spherulites which grow from the central precursor and branch multidirectionally without double-leaf morphology, because it was often considered that there would not be any morphology changes worth studying for them.

According to previous research in the modeling of spherulite growth,3 a simple empirical power-law relationship can be used to determine whether the growth of SPC spherulite is interface controlled or diffusion controlled: 𝑑𝑅𝑖 𝑑𝑡

= 𝑘[𝑆𝑖(𝑡) ― 1]𝑔

(2)

where, k is the growth rate coefficient. g is the growth rate order, which can be used to determine the growth regime. If the growth is diffusion controlled, then g is 1. If the growth is


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interface controlled, then g should be larger than 1. Si(t) is the local supersaturation ratio of the supersaturated solution:

𝑆𝑖(𝑡) =

𝑐𝑖(𝑡)

(3)

𝑐𝑒𝑞

where ci(t) is the local concentration of the supersaturated solution and ceq is the equilibrium solubility of SPC at ambient temperature and pressure. To evaluate Si(t) of the SPC spherulite, the following model was used to calculate the local concentration of SPC as a function of Ri

Figure 5. A crystallization experiment in petri dish 𝑉0(𝑐0 ― 𝑐𝑖(𝑡)) = 𝑛𝑉𝑖(𝑡)ρε

(4)

where c0 is the initial concentration of SPC. ε is the porosity of the spherulite. ρ is the solid density of SPC. V0 is the volume of the solution. n is the number of the spherulites in a petri dish. It can be assumed that the number of the spherulites remains the same. Moreover, the same growth rate among these spherulites ensures that they have the same size at any time during the spherulite formation in petri dish, i.e. the growth rates of these spherulites decrease at the same rate. This assumption is almost consistent with the experimental phenomena, as



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shown in Figure 5. Vi(t) is the volume of the individual spherulite which can be expressed as follows

𝑉𝑖(𝑡) = 𝜋𝑅𝑖ℎ𝑖(𝑡)2 ― 𝑅𝑖

𝜋ℎ𝑖(𝑡)3

(5)

3

𝑅𝑖

(6)

ℎ𝑖(𝑡) = 𝑠𝑖𝑛𝛼 ― 𝑡𝑎𝑛𝛼

Figure 6. A spherulite obtained from the crystallization experiment in petri dish

where α is regarded as contact angle of SPC spherulite to glass base. hi(t) is the height of the spherulite as show in Figure 6. By inserting eq 5 and eq 6 into eq 4, the local concentration of SPC can be expressed as follows

𝑐𝑖(𝑡) = 𝑐0 ―

𝑛𝜋𝜌𝜀(1 ― 𝑐𝑜𝑠𝛼)2(2 + 𝑐𝑜𝑠𝛼)𝑅3𝑖 3𝑉0𝑠𝑖𝑛3𝛼

(7)

Based on the ci(t) from eq 7, we can obtain Si(t) by eq 3. And then, the growth rate coefficient k and the growth rate order g can be obtained by the eq 8 expressed as follows

ln

𝑑𝑅𝑖

( ) = 𝑔𝑙𝑛[𝑆𝑖(𝑡) ― 1] + ln (𝑘) 𝑑𝑡

(8)


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Figure 7. Relationship between ln(dRi/dt) and ln(Si(t)-1) and the corresponding linear regression line obtained according to the red data points

As shown in Figure 7 (data related are shown in Table S2), there is a good linear relationship between ln(dRi/dt) and ln(Si(t)-1), except the initial and final stage during the process. Insufficient noncrystallographic branching and inhomogeneous concentration distribution can account for the nonlinear regime at the initial stage. At the final stage, these spherulites grew together, which casts a doubt on the assumption about n according to eq 4, and makes it hard to evaluate the total mass of the spherulites in petri dish. Moreover, during the non-branching stage, ε could experience a great decrease, which will result in the nonlinear regime at the final stage. Therefore, the data before the critical growth point and after the nonlinear regime at the initial stage can be more persuasive to evaluate g according to eq 8. The data selected to evaluate g are marked in red in Figure 7. The coefficient of correlation, R2, of the linear



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regression line is 0.9998, which shows a good linear relationship between ln(dRi/dt) and ln(Si(t)-1). The value of the order of the crystal growth for SPC spherulites is 1.3966，which indicates that the growth of SPC spherulites is a mixed growth regime. However, the growth regime of spherulite formation needs more careful examination, because the kinetic model is an empirical power-law relationship and the data selected to evaluate g is just before the critical growth point and after the nonlinear regime at the initial stage. However, to the best of our knowledge, there has not been a proper method to offer an ultimate evidence to put an end to the discussion about the growth regime of spherulite formation, because some coefficients, including the diffusion boundary layer and the salt concentrations in the solution at the growth front, change with time and are difficult to obtain. A general role of chemical diffusion and reaction in shaping particles was proposed by Han without regard to the diffusion boundary layer.19 This method can not be used in our work, but it will give some insight into the further research on the growth regime of spherulite formation.

3.3

Effect of Additives and Supersaturation on Spherulitic Morphology

Compact spherulite particles are more valuable in industrial application than coarse ones in that the compact ones have higher bulk density, better flowability, compressibility and stability. Therefore, the growth rate of spherulites should be higher than the critical growth rate, otherwise the subunits will be peeled away from the coarse growth front which results from insufficient noncrystallographic branching. Moreover, the critical growth rate offers a new method to study the noncrystallographic branching. The in-situ monitor of the supersaturation and the concentration of the additive is an ideal design, which can be used to study the branching conditions quantitatively. However, as for in-situ monitor of the ion 18 ACS Paragon Plus Environment

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supersaturation, ion selective electrode (ISE) is not suitable in this case because the concentration of the sodium ion is beyond the linear range of the sodium ISE. When it comes to the in-situ monitor of the concentration of SHMP, no proper color-developing agent can be used to determine the trace amounts of SHMP by spectrophotometry or fluorimetry. Therefore, the critical growth rate of SPC was studied qualitatively under different initial supersaturations and concentrations of SHMP and NaCl.

As shown in Figure 8a, higher SHMP concentration could result in lower initial growth rate and higher critical growth rate, which means that SHMP can inhibit the growth and induce non-crystallographic branching of SPC. There is little influence of NaCl concentration on the critical growth rate of SPC and higher NaCl concentration could result in higher initial growth rate of SPC according to Figure 8b. As shown in Figure 8c, lower initial concentration of SPC means relatively higher SHMP concentration, which can result in higher critical growth rate. This is consistent with the claim under different SHMP concentrations. At the initial stage of the spherulite formation, the initial growth rate does not have obvious trends under different initial concentrations of SPC and NaCl, which could result from no remarkable change and inhomogeneous concentration distribution. Unpaired two-sample T-tests were used to evaluate the effect of SHMP, NaCl and supersaturation on the critical growth rate of SPC. It is assumed that the variances of any two population are equal. The T-test shows statistical significance that higher SHMP concentration can result in higher critical growth rate (volume of solution SHMP: 0.50 and 1.00 ml; p = 0.042) and lower initial concentration of SPC can lead to higher critical growth rate (volume of hydrogen peroxide solution: 3.35 and 2.90 ml; p



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= 0.002). The difference in NaCl concentration and critical growth rate did not reach statistical significance (mass of sodium chloride: 2.75 and 3.50 g; p = 0.409).


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Figure 8. The initial and critical growth rates under (a) different concentrations of SHMP; (b) different concentrations of NaCl; (c) different initial concentrations of SPC 21 ACS Paragon Plus Environment


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The morphologies of SPC spherulites under different conditions are shown in Figure 9. No branching can be seen without additive (Figure 9d) and crystal faces can be clearly recognized (Figure 9g). When the NaCl, as a salting-out agent, was added into this system to increase the supersaturation, the spherulite became more compact than before (Figure 9e) and some slight branching can be recognized (Figure 9h). When the NaCl and SHMP were added at the same time, the of branching was enhanced significantly (Figure 9f) and some subunits stuck to each other to form a fins-like morphology (Figure 9i). Moreover, these subunits under high supersaturation and at the presence of SHMP did not consist of flat faces with specific orientations (Figure 9i). Furthermore, the nucleation of SPC was inhibited significantly by SHMP, which can be explained by the stabilization of SPC pre-nucleation cluster, amorphous or dense liquid states preceding the appearance of crystalline phase. This stabilization can result from the H-bonds between the SHMP and hydrogen peroxide. A study of 65 crystal structures and over 260 hydrogen bonds reveals that H2O2 always forms two H-bonds as proton donors and up to four H-bonds as a proton acceptor.20 As in the case of kaolinite system, the oxygen atoms of SHMP anions may receive many electrons from the Al-OH surface and form H-bonds with the hydrogen atoms of surface hydroxyl groups.21 Prenucleation cluster, amorphous and liquid precursor are common phenomena for inorganic system,22,23 but these species were not discovered for SPC in this work, because the instability of hydrogen peroxide and SPC makes it impossible to find them by transmission electron microscopy or dynamic light scattering.


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Figure 9. Morphologies of SPC spherulites. a, d, and g: SPC spherulites without additive; b, e, and h: SPC spherulites in the presence of NaCl to increase the supersaturation; c, f, and i: SPC spherulites in the presence of NaCl and SHMP

Based on the study above, we propose the basic principles of spherulite product design. Needle-like crystals can be divided into non-branching and branching crystals depending on whether these crystals have undergone noncrystallographic branching, which is different from crystallographic branching and diffusion-limited aggregates and typically varies between 0 and 15 °. Noncrystallographic branching is often considered as a necessary condition to form 23 ACS Paragon Plus Environment


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spherulites. Moreover, noncrystallographic branching is not a sufficient condition for spherulite formation and only the branching crystals with almost the same growth rate for different subunits can form spherulites. In this work, we suggest that the obviously different growth rates among subunits may result from maldistribution of solute molecules especially when the experiments were performed in droplet,3,6 in melt between glass slides4,6 or in petri dish4 where there was no agitation as in crystallizer. Even in crystallizer, a perfectly stirred reactor is an ideal model which can’t be achieved in real production process. Therefore, we make the assumption that the formation of spherulite owes much to the interface controlled growth. Especially when the kinetic coefficient is large for highly soluble salts, diffusion becomes more important. In these cases, spherulites are rare. 17

Based on the analysis of the possibility of spherulite formation, basic principles of spherulite product design can be suggested. As for needle-like crystals, inducing non-crystallographic branching and decreasing the kinetic coefficient of the interface reaction constitute sufficient conditions to form spherulite. As shown in Figure 10, any needle-like crystal has a possibility to form a spherulite and when the frequency of branching is sufficient enough, compact spherulite product can be obtained. In this work, the kinetic coefficient of the interface reaction for SPC is smaller enough to result in interface controlled growth regime. Therefore, SPC spherulite is easy to obtain even without additive. Moreover, SPC is an addition compound of hydrogen peroxide and sodium carbonate in which the hydrogen peroxide molecule is only weakly bound to carbonate ion via hydrogen bonding, similar to water of crystallization. Non-crystallographic branching of SPC is easy to be induced because of the structural instability which may result from the two crystallographically independent


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hydrogen peroxide sites.24 According to the study above, high supersaturation and SHMP can induce branching for SPC spherulite, which can make the SPC spherulite compact enough to form spherulite product in crystallizer. Therefore, it is concluded that SHMP is not a necessary condition for spherulite in petri dish but a necessary one for spherulite product in crystallizer.

Figure 10. Illustration of spherulite design

Additives are often found to be responsible for spherulite morphologies. However, the influence of them is still unknown and many related studies up to now have been descriptive



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in nature. The basic principles of spherulite product design in this work will give an insight into the real role played by additive in spherulite formation.

4. CONCLUSIONS This paper attempts to investigate the general principles of spherulite product design based on the study of SPC spherulites. In this work, it is demonstrated that the primary reason accounting for spherical morphology of SPC product is spherulitic growth rather than agglomeration effect. And then, in-situ nucleation-controlled experiments in petri dish were established to investigate the mechanism. During this in-situ experiment, a critical growth rate was discovered for the first time, which represents a boundary between branching and nonbranching area for well-developed SPC spherulite. The growth regime during the process is proved to be a mixed growth regime by a kinetic model established in this work. Moreover, investigations on the crystallization conditions indicated that addition of SHMP and higher initial concentration contribute to the formation of compact spherulites. Finally, spherulites in the presence and absence of SHMP were compared to investigate the role of SHMP. It is concluded that SHMP can induce the branching of SPC, which is necessary for compact spherulite product in crystallizer but not necessary for spherulite in petri dish.

Based on the study above, we propose the basic principles of spherulite product design. In order to obtain spherulite product for needle-like crystals, the branching needs to be induced and the kinetic coefficient of the interface reaction needs to be reduced. However, how to


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induce branching and how to regulate the growth regime of spherulite need to be further investigated.

Supporting Information Figure S1 PVM images at about 7 min. Table S1 Data on the growth curve and growth rate curve in Figure 4. Table S2 Data on the relationship between ln(dRi/dt) and ln(Si(t)-1) corresponding to Figure 7. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * Ling Zhou: Tel: +86-22-27405754. Fax: +86-22-27374971. E-mail: [email protected] * Qiuxiang Yin: Tel: +86-22-27405754. Fax: +86-22-27374971. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS



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This research is financially supported by the Major National Scientific Instrument Development Project of China (No.21527812) and the Tianjin Municipal Natural Science Foundation (No. 16JCZDJC32700).

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(14) Jiang, Y. B.; Shi, K.; Xia, D. N.; Wang, S. O.; Song, T.; Cui, F. D. Protein Spherulites for Sustained Release of Interferon: Preparation, Characterization and in vivo Evaluation. J. Pharm. Sci-us 2011, 100, 1913-1922. (15) Koukabi, N. Sodium Percarbonate: A Versatile Oxidizing Reagent. Synlett 2010, 2010, 2969-2970. (16) Jakob, H.; Leininger, S.; Lehmann, T.; Jacobi, S.; Gutewort, S. Peroxo Compounds, Inorganic. Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2000. (17) Shtukenberg, A. G.; Punin, Y. O.; Gunn, E.; Kahr, B. Spherulites. Chem. Rev. 2011, 112, 1805-1838. (18) Granasy, L.; Pusztai, T.; Tegze, G.; Warren, J. A.; Douglas, J. F. Growth and Form of Spherulites. Phys. Rev. E 2005, 72, 011605. (19) Yang, T.; Liu, J. M.; Dai, J. H.; Han, Y. S. Shaping Particles by Chemical Diffusion and Reaction. CrystEngComm 2017, 19, 72-79. (20) Chernyshov, I. Y.; Vener, M. V.; Prikhodchenko, P. V.; Medvedev, A. G.; Lev, O.; Churakov, A. V. Peroxosolvates: Formation Criteria, H2O2 Hydrogen Bonding, and Isomorphism with the Corresponding Hydrates. Cryst. Growth Des. 2016, 17, 214-220. (21) Han, Y. H.; Liu, W. L.; Zhou, J.; Chen, J. H. Interactions between Kaolinite Al-OH Surface and Sodium Hexametaphosphate. Appl. Surf. Sci. 2016, 387, 759-765. (22) Gebauer, D.; Colfen, H.; Verch, A.; Antonietti, M. The Multiple Roles of Additives in CaCO3 Crystallization: a Quantitative Case Study. Adv. Mater. 2009, 21, 435-439. 30 ACS Paragon Plus Environment

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