We present an instrument that addresses two critical requirements for quantitative measurements of the homogeneous crystal nucleation rate in supersaturated aqueous solution. First, the need to perform repeated measurements of nucleation incubation times is met by automating experiments to enable programmable cycling of thermodynamic conditions. Second, the need for precise and robust control of the chemical potential in supersaturated aqueous solution is met by implementing a novel technique for regulating relative humidity. The apparatus levitates and weighs micron-sized samples in an electric field, providing access to highly supersaturated states. We report repeated observations of the crystal nucleation incubation time in a supersaturated aqueous sodium chloride droplet, from which we infer the nucleation rate.

Supporting InformationThe relative humidity, or activity of water (a w ), inside the particle levitation chamber is controlled by modulating the pressure of a nitrogen stream that contains a fixed mole fraction of water vapor such that:where a w,o is the activity of water in the nitrogen stream recorded at atmospheric pressure (p o ) by a humidity sensor, and p is the absolute pressure inside the chamber recorded by a pressure transducer. Accuracy of eq S1 requires that the mole fraction of water vapor inside the chamber is identical to that at the humidity sensor. As humidity increases, however, there is an increasing likelihood that water is sorbed by surfaces within the apparatus, introducing capacitance that prevents very rapid changes in a w . The actual instrument response to imposed changes in a w was measured during continuous ( Figure S1) and stepwise ( Figure S2) scans with an aqueous sodium chloride droplet, which maintains equilibrium with the surrounding atmosphere. Discrepancy in the measured mass between continuous scans up and down in a w was used to quantify the lag and derive a correction that was applied to the PEO data in the main text. The difference in water activity between that recorded during the up scan (a w,up ) and down scan (a w,dn ) at a given m/m o for 4 < m/m o < 11 was determined from the data in Figure S1. Since the equilibration time exhibited in Figure S2 is symmetric with respect to a steps up and down in activity, we assume that the actual activity is a w = (a w,up + a w,dn )/2, and, therefore, the data requires a lag correction of ∆a w = (a w,up -a w,dn )/2; ∆a w increases with increasing a w for a w > 0.79 ( Figure S3). A linear fit to the data in Figure S3 was used to correct for the hysteresis exhibited in the continuous scan at high activity ( Figure S4). The corrected data displays no hysteresis and agrees well with the 4 equilibrium data points determined from the stepwise scan.

We show that polymer solutions exhibit memory effects as they undergo repeated dissolution (deliquescence) and crystallization (efflorescence) transitions, similar to those previously observed in polymer melts. These memory effects have been probed using scanning activity gravimetric analysis (SAGA) on aqueous poly(ethylene oxide). SAGA is a new method that enables manipulation of the thermodynamic state of a polymer/ solvent system under isothermal conditions. The measurements have been performed on individual micron-sized (∼10 ng) polymer particles that are levitated electrostatically and subjected to programmed changes in the activity of solvent vapor. The particle mass rapidly changes due to sorption or evaporation of solvent in response to changes in vapor activity, providing insights into the changing state of the polymer/solvent system. Repeated cycling of solvent activity within a levitated particle reveals systematic shifts up and down in the deliquescence and efflorescence activities depending upon the extreme activities reached. Melt memory effects have previously been attributed to remnants of lamellar crystals that persist beyond the nominal melting temperature. The present observations suggest that the thickness of these remnants evolves in solution, changing the supersaturation at which they may serve as athermal nuclei. An extension to the classical thermodynamic model of lamellae that includes a reduced interfacial tension at the perimeter of the high energy fold surface predicts the existence of a metastable radius that allows remnants to slowly increase or decrease in thickness under certain conditions, thereby rationalizing the ability to manipulate efflorescence behavior. We also present an analysis that suggests that SAGA can be extended to other polymer/solvent systems with the appropriate choice of temperature.