Nonphotochemical pulsed-laser-induced nucleation in a cw-laser-induced phase-separated solution droplet of aqueous glycine formed by optical gradient forces

Omar Gowayed, Tasfia Tasnim, José J. Fuentes-Rivera, Janice E. Aber, Bruce A. Garetz

A centimeter-sized laser-induced phase-separated (LIPS) solution droplet, which was formed by tightly focusing a continuous-wave, near-infrared laser beam at the glass/solution interface of a millimeter-thick layer of glycine in D2O with a supersaturation ratio, S, of 1.36 was irradiated with a single unfocused nanosecond near-infrared laser pulse in order to study the effect of nonphotochemical laser-induced nucleation (NPLIN) on the droplet, as well as to help characterize the behavior of the LIPS droplet. Additionally, a control NPLIN experiment was conducted on an S=1.50 supersaturated solution of glycine/D2O in the same cell to better understand the differences between NPLIN in a LIPS droplet and an ordinary supersaturated solution. These experiments revealed that NPLIN could nucleate crystals within a LIPS droplet, although the growth of these crystals was inhibited during the first 5 minutes of the droplet’s relaxation. For the first 40 minutes of its relaxation, the LIPS droplet was observed to be more labile to spontaneous nucleation than the control S=1.50 solution, although the growth of spontaneously nucleated crystals was also inhibited during the first 5 minutes of the droplet’s relaxation. This suggests that although the macroscopic phase boundary between the LIPS droplet and the surrounding solution disappeared after approximately 5 minutes, the full microscopic relaxation of the LIPS droplet took at least 40 minutes. The resulting crystals were analyzed using powder X-ray diffraction (PXRD), and 100% of crystals formed within the LIPS droplet induced by NPLIN with linearly polarized light and by spontaneous nucleation were α-glycine, while crystals formed outside of the LIPS droplet, or in the periphery, were mixtures of α- and γ-glycine. The results indicate that the LIPS droplet and the surrounding solution are not equilibrium phases of aqueous glycine, but phases in which gradient optical forces have induced a partitioning of large and small solute clusters.

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