Ultrasonic Field Induced Chiral Symmetry Breaking of NaClO3 Crystallization Youting Song, Wanchun Chen, and Xiaolong Chen* Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1448–1450
ReceiVed October 30, 2007; ReVised Manuscript ReceiVed March 2, 2008
ABSTRACT: It is found that chiral symmetry breaking can be achieved in NaClO3 crystallization under an ultrasonic field, with a large
crystal enantiomeric excess (mean values > 0.967) in almost every crystallization, and complete chiral symmetry breaking can be realized by seed inducing under the ultrasonic field. The chirality propagating process through secondary nucleation originating from an initial nucleus or a seed can be directly observed, and this large enantiomeric excess can be explained through secondary nucleation induced by the cavitation of the ultrasonic field and a recycling process. The phenomenon of chiral symmetry breaking exists extensively from elementary particles to large biological molecules. It is wellknown that nature selects L-amino acids and D-sugars as the constituting ingredients of proteins and DNA in living organisms, respectively, but the reason for this remains up to now a puzzle.1 However, spontaneous chiral symmetry breaking occurs rarely in an abiotic chemical reaction. In 1990, Kondepuli et al.2 demonstrated that crystals that crystallized from a supersaturation solution of NaClO3 were dominated by a single chiral enantiomer when the solution was stirred, and similar results could also be obtained from a stirring 1,1′-binaphthyl melt.3 Besides the stirring, the chiral symmetry breaking in NaClO3 crystallization can also be reached by seeding,4,5 introducing impurities,4 beta particles,6 and other asymmetry factors. For the chiral symmetry breaking in stirring crystallization, three mechanisms have been put forward. The secondary nucleation caused by the stirring was suggested to be a possible mechanism.7–12 However, the experimental evidence of chiral symmetry breaking from primary nucleation in crystallization has been shown.13 Some people thought that a recycling process by which a less abundant enantiomer can be transformed into a more abundant one is also indispensable in order to reach complete chiral symmetry breaking.14–17 Therefore, the mechanisms to explain the chiral symmetry breaking in crystallization are not yet fully understood. The ultrasonic field can affect the nucleation process of crystals; for example, the cavitations generated by an ultrasonic field can shorten induction time of primary nucleation18,19 and cause secondary nucleation of crystals.20,21 Thus, we expect that the chiral symmetry breaking in NaClO3 crystallization can be realized under an ultrasonic field. In this communication, we have investigated the effect of an ultrasonic field on chiral distribution of NaClO3 crystallization and found that crystal chiral symmetry breaking can certainly be achieved under an ultrasonic field, and we demonstrate that secondary nucleation caused by cavitation of an ultrasonic field and a recycling process are responsible for this chiral symmetry breaking. Our experiments comprise two parts: first, the nucleation of NaClO3 was directly performed under an ultrasonic field; second, a seed was used to induce the nucleation of NaClO3 under the ultrasonic field. A solution with 55 wt% NaClO3 (corresponding to a saturation solution of 50 °C) was prepared by dissolving NaClO3 (99.0% purity) in deionized water and was heated to 75 °C in order to make NaClO3 dissolve completely and then was filtered through a 4 µm filter. The approximately 100 mL NaClO3 solution at 75 °C was poured into a beaker with a lid on it, and then the beaker was put in the bath of an ultrasonic generator at 40 kHz and 50 W. The solution in the beaker was naturally cooled * To whom correspondence
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Figure 1. Chirality propagating process in NaClO3 crystallization under an ultrasonic field. (1a-1) Catastrophic secondary nucleation originated from a primary nucleus. (1a-2) rapidly diffused. (1a-3) penetrated throughout the whole solution; (1b-1) catastrophic secondary nucleation started from a seed. (1b-2) followed still it with the movement of the seed. (1b-3) finally diffused throughout the whole solution.
from 75 °C and finally became supersaturated. Usually, one or two NaClO3 nuclei first appeared in the solution, and then catastrophic secondary nuclei were observed, and these nuclei always originated from a primary nucleus, and rapidly diffused throughout the whole solution in 1–2 min (Figure 1a). The supercooling of the solution was measured to be at a temperature range from 10 to 16 °C when catastrophic secondary nucleation occurred. After another 20 min, these nuclei settled completely to the bottom of the beaker. After ultrasonic vibration was stopped, the solution was spontaneously
10.1021/cg701072r CCC: $40.75 2008 American Chemical Society Published on Web 03/20/2008
Communications
Crystal Growth & Design, Vol. 8, No. 5, 2008 1449
Table 1. The Chiral Distribution of NaClO3 Crystallization under an Ultrasonic Field L-dominated
D-dominated
nL
nD
cee
nL
nD
cee
193 1347 1151 1201 1352 435 346 151 113 402 315 155 284 245 132 156 102 234
18 30 1 46 11 46 0 0 0 0 0 9 0 0 0 0 0 0
0.829 0.956 0.998 0.926 0.984 0.809 1 1 1 1 1 0.890 1 1 1a 1a 1a 1a
0 7 10 68 2 34 23 2 7 0 0 0 0 6 0 0 0 0 0
340 1059 1355 622 1345 2200 208 500 303 402 454 726 596 101 113 420 189 135 214
–1 –0.987 –0.985 –0.803 –0.997 –0.97 –0.801 –0.992 –0.955 –1 –1 –1 –1 –0.888 –1 –1 –1a –1a –1a
8314
152
159
11282
a
Represents results obtained by stopping immediately ultrasonic vibration just after secondary nucleation diffuse throughout the whole solution.
cooled to room temperature. After about 12 h of growth, the crystals with millimeter sizes could be obtained. To track crystal chirality propagating process the ultrasonic field was immediately removed just after catastrophic secondary nucleation diffused throughout whole solution (usually 1–2 min), the following operation is same as that described above. For the nucleation experiment induced by a seed, a left-handed or right-handed seed with the sizes of 0.5–1 mm was washed by deionized water to remove the opposite chiral particles adhered on the surface of the seed, and then introduced into the unsaturated NaClO3 solution at 75 °C just after the ultrasonic field was applied. The catastrophic secondary nucleation first appear around the seed, and still followed it as the seed moved, finally diffused to the whole solution (Figure 1b). Using a polarizing microscopy, the chirality of NaClO3 crystals was determined. Using the method described by the above first part we performed 37 different crystallizations with a total of over 19907 crystals (Table 1). 8473 were found to be left-handed and 11434 were righthanded. The difference between numbers of two enantiomers is 15%. Of the 37 crystallizations, 18 produced L-crystals and 19 was almost total D-crystals. As that reported previously, in the absence of outside perturbations, statistically equal numbers of L- and D-crystals were obtained in each crystallization. Usually, crystal chiral symmetry breaking is quantified by crystal enantiomeric excess (cee), expressed as cee ) (nL - nD)/(nL + nD),11 where nL and nD are the numbers of levorotatory and dextrorotatory crystals. Figure 2 has given the cees of the ultrasonic nucleation of NaClO3. These data, centered near +1 and -1, distributed in a two-peak pattern and indicated the realization of the crystal chiral symmetry breaking. Although immediately stopped ultrasonic irradiation just after catastrophic secondary nucleation diffused throughout the whole solution, the chiral purity of one of two enantiomers remains 100% (see Table 1). For the 10 crystallizations induced by a seed under the ultrasonic field, a left–handed seed always induced to produce all levorotatory crystals, a right-handed seed resulted in total dextrorotatory crystals (Figure 2), which is consistent with the results induced by a seed under the condition of the stirring. What causes this chiral symmetry breaking? In the stirring crystallization of NaClO3, Kondepudi3 suggested that secondary nucleation plays a key role for crystal chiral symmetry breaking for the rapid production of secondary nucleation compared to the primary nucleation makes it possible for one crystal of L or D
Figure 2. Chiral symmetry breaking of NaClO3 crystallization under ultrasonic field. Upper panel represents the result crystallized directly under ultrasonic field, and the lower panel is the result induced by a seed.
enantiomer, created randomly, to become the parent crystal that generates a large number of secondary crystals with the same enantoimeric form resulting in a large cee. A single nucleation event can give rise to most or all of the hundreds or thousands of crystals formed.10,22 Secondary nucleation may come from contact nucleation (the collision between a stirring bar and a crystal, or between a crystal and container wall, as well as between a crystal and another8,11) and shear flow or embryos coagulation.9 However Viedma16 experimentally confirmed that total symmetry breaking can be achieved from a system that initially includes both enantiomers, he explained this result through nonlinear autocatalysis and recycling (a dissolution-recrystallization process) by which one of enantiomers is converted to another. Cartwright12 et al. demonstrated with simulations of the dynamics of the system that secondary nucleation is actually a nonlinear autocatalytic phenomenon, and in their simulation primary nuclei are allowed to appear randomly at any point with a probability depending upon the supersaturation or supercooling, when secondary nucleation is introduced, a large enantiomer excess can be acquired, but not reaching complete chirality purity. When they included a recycling process and secondary nucleation in their theoretical model and the result of simulation is the realization of complete chiral symmetry breaking,17 similar results concerning a recycling process can be found in refs 14 and 15. For the chiral symmetry breaking under an ultrasonic field, according to ultrasonic principle, when the acoustic waves pass NaClO3 solution, the solution is driven to flow and multiple transient cavitation bubbles are formed at the negative pressure region of the solution; when the bubbles collapse in the positive pressure region and the local pressures in excess of 1000 atm can be produced, and give a big hit to the local surface of an object, forming as-called cavitation effect of the ultrasonic field. For a NaClO3 solution, as the temperature of the solution was decreased, primary heterogeneous nucleation first occurred for it needed lower supersaturation than homogeneous nucleation. After a nucleus was formed, it became large gradually. Since the rate of secondary nucleation depends greatly on the supersaturation of the solution,23 when the supersaturation of the solution was further increased and
1450 Crystal Growth & Design, Vol. 8, No. 5, 2008
Figure 3. High shear flow in the vicinity of cavitation in NaClO3 solution under an ultrasonic field.
reached a critical value, a primary nucleus (as a self-seed) induced to produce the catastrophic secondary nucleation (which has the same chirality as an initial nucleus) through the cavitation effect of an ultrasonic field. As that described above, the catastrophic secondary nucleation always appeared in the immediate vicinity of a primary nucleus, and then diffused throughout the whole solution only in 1–2 min (Figure 1). The rapid production of secondary nucleation soon lowers the concentration of the solution below the threshold for the formation of primary nuclei,7,10 and the liquid flow induced by ultrasonic irradiation could disperse these nuclei throughout the whole solution, and thus the primary nucleation of the crystals with opposite chirality was suppressed, finally leading to crystal chiral symmetry breaking. Thousands of the nuclei occurred in so short a time (usually 1–2 min) and the chirality purity of an enantiomer sometimes could reach 100%, which evidenced that catastrophic secondary nucleation must be a nonlinear autocatalytic process.12 Frank24 first put forward a form of autocatalysis by which each enantiomer catalyzes its own production, suppress that opposite to its chirality, as a result small initial fluctuation in the concentrations of the enantiomers was amplified through nonlinear dynamics. But only secondary nucleation with chiral autocatalytic process cannot completely explain this chiral symmetry breaking in the ultrasonic nucleation, according to the results of previous experiment,16 theoretical simulations,15,17 and recent thermodynamic analysis,25–28 as soon as the nuclei with opposite chirality appear in primary nucleation, a recycling process must transform them into another enantiomer form (the same chirality as first crystal in primary nucleation), therefore besides secondary nucleation with nonlinear autocatalytic process, a dissolution-recrystallization process should also contribute to this chiral symmetry breaking. The catastrophic secondary nucleation always appeared in the immediate vicinity of a primary nucleus and it could also be observed that the crystals with bigger sizes formed previously were broken into smaller particles with longer time of ultrasonic irradiation, thus we inferred that at least partly secondary nucleation directly came from particles detached from the surface of a primary nucleus since the continuous high pressures produced by ultrasonic field could give a big stroke to a primary nucleus. In addition, the collision between particles and high shear flow in the solution were also observed in the NaClO3 ultrasonic nucleation (Figure 3), and these could also cause secondary nucleation. Through the microscopic visualization of the sonocrystallization of ice, Chow20,21
Communications found that the collapse of cavitation bubbles generated by ultrasound could destroy the surface of ice and melt any ice in their paths and result in the fragmentation of the dendrite ice, producing many new secondary nucleation crystal sites. In summary, the crystal chiral symmetry breaking in NaClO3 crystallization can be achieved under ultrasonic field and 100% chiral pure of one enantiomer can be acquired by the inducing of a seed under ultrasonic field. In these experiments the chiral propagating process through secondary nucleation originating from a primary nucleus or a seed can be directly observed. This chiral symmetry breaking in NaClO3 ultrasonic nucleation can be explained by the secondary nucleation produced by the cavitation of ultrasonic field and a dissolution-recrystallization process.
Acknowledgment. This work was supported by Space Science Research Fund, administered by Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant No. 111077416701).
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