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[MIS07-07] “Freezing” of NaClO3 Metastable Crystalline State by Optical Trapping in Unsaturated Microdroplet
Keywords:Metastable phase , Optical trapping, Microdroplet
We used sodium chlorate (NaClO3) as a target compound. Crystallization of NaClO3 from an aqueous solution exhibits two kinds of polymorphs: monoclinic metastable phase and cubic stable. The solubility of the metastable phase is about 1.6 times higher than that of the stable phase.3 An aqueous solution of NaClO3 saturated at 22°C was sprayed to the cover glass using a commercially available spray container. This procedure allowed us to produce hemispherical microdroplets of the NaClO3 solution on the cover glass. The hemispherical microdroplets were confined in an enclosed crystal growth cell [Figure 1 (a) right]. The mother solution is unsaturated state with respect to the metastable phase. A system for optical trapping-induced crystallization was constructed by introducing circularly polarized green laser (532 nm) to an inverted polarized light microscope equipped with a 60× objective lens [Figure 1(a) left]. A circularly polarized light focused by the objective lens was irradiated to the air/NaClO3 microdroplet interface to induce nucleation. We observed crystallization dynamics induced in-situ using the polarized light inverted microscope and a CCD camera.
We have achieved to crystallize NaClO3 metastable single crystal by optical trapping-induced crystallization despite the microdroplet is unsaturated state after the onset of the laser irradiation to the air/solution interface [Figure 1 (b) (left) (i)-(vii)]. On the other hand, the crystal started to dissolve when the laser irradiation was stopped [Figure 1 (b) (left) (viii)-(x)]. The dissolution turned to growth when we restarted the laser irradiation again, namely, we achieved reversible metastable phase control without polymorphic transformation. This crystal growth dynamics shows that laser irradiation causes the change of the magnitude relationship between chemical potentials of the solution phase and the crystalline phase in the microdroplet. Our method achieved to “freeze” a kinetic pathway of crystal formation intermediated by a metastable phase, being useful for precise analysis a short-lived unprecedented phase. Our method shed a light on complete polymorph control.
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 Niinomi et al., Cryst. Growth Des., 13(12), 5188-5192 (2013)