Investigation of Crystal Nucleation of Highly Supersaturated Aqueous KNO₃ Solution from Single Levitated Droplet Experiments

Abstract

In this work, we use an electrodynamic balance (EDB) to study primary crystal nucleation from single levitated aqueous potassium nitrate (KNO3) solution droplets under isothermal conditions. We investigate crystallization in droplets with volumes less than one nanoliter. From induction time distributions we derive nucleation rates of KNO3. Nucleation processes were found to occur at different mechanisms. Results are interpreted based on the classical nucleation theory (CNT) to gain more information about the prevailing nucleation mechanism. We also investigate the shape and morphology of crystals using scanning electron microscopy (SEM). Two typical morphology types of crystallized particles could be identified, depending on whether nucleation occurred during the evaporation phase or at constant supersaturation of solute.

Crystallization in Confinement

“…[84,116,117] The droplets are maintained under conditions of controlled humidity and temperature, and crystallization can be induced through a change of temperature [118,119] (for example to investigate a freezing transition) or through evaporation. [116,117,[120][121][122] A facile exchange of gases occurs between the droplet and its environment, and evaporation can be controlled leading to a concomitant increase in supersaturation and resultant crystallization. .…”

“…These systems can be used to study nucleation, where techniques such as optical microscopy, [123] and angle-resolved light scattering [118][119][120] allow nucleation rates to be determined. The entire crystallization pathway occurring within individual droplets can also be characterized using in situ analytical techniques including IR, [124] UV-vis spectroscopy, [125] Raman spectroscopy and synchrotron X-ray diffraction [116,122,126] and images of the crystals forming within the droplets can be recorded using a high-speed camera.…”

“…A number of studies have also used levitated droplets to obtain nucleation kinetics of compounds ranging from alkanes [118] and ice, [119] to soluble inorganics, [84,120] and proteins, [123] where data is obtained from hundreds of experiments with identical droplets. These are conducted under isothermal conditions, and with controlled humidity such that a constant supersaturation is maintained during the measurement.…”

“…[119] The nucleation of potassium nitrate, in contrast, showed nonlinearity and the curve was fitted with a two-exponential function that yielded two volume-dependent rates that varied by one to two orders of magnitude. [120] The faster rate undoubtedly corresponds to heterogeneous nucleation on impurities in the droplets. Extraction of the pre-exponential factor for the slower rate yielded a lower value than that expected for homogeneous nucleation, suggesting that it also corresponded to heterogeneous nucleation, possibly governed by the droplet interface rather than impurities.…”

Effect of Ultrasonic Irradiation on the Probability of Primary Nucleation of L‐Arginine Hydrochloride

“…(5) and (6) can be written, and the probability is expected to follow a Poisson distribution as Eqs. (7) and (8) [15].…”