title: Ultra-slow water diffusion in aqueous sucrose glasses.
abstract: We present measurements of water uptake and release by single micrometre-sized aqueous sucrose particles. The experiments were performed in an electrodynamic balance where the particles can be stored contact-free in a temperature and humidity controlled chamber for several days. Aqueous sucrose particles react to a change in ambient humidity by absorbing/desorbing water from the gas phase. This water absorption (desorption) results in an increasing (decreasing) droplet size and a decreasing (increasing) solute concentration. Optical techniques were employed to follow minute changes of the droplet's size, with a sensitivity of 0.2 nm, as a result of changes in temperature or humidity. We exposed several particles either to humidity cycles (between ∼2% and 90%) at 291 K or to constant relative humidity and temperature conditions over long periods of time (up to several days) at temperatures ranging from 203 to 291 K. In doing so, a retarded water uptake and release at low relative humidities and/or low temperatures was observed. Under the conditions studied here, the kinetics of this water absorption/desorption process is controlled entirely by liquid-phase diffusion of water molecules. Hence, it is possible to derive the translational diffusion coefficient of water molecules, D(H(2)O,) from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for D(H(2)O)-values as low as 10(-24) m(2) s(-1) are determined using data at temperatures down to 203 K deep in the glassy state. From the experiment and modelling we can infer strong concentration gradients within a single particle including a glassy skin in the outer shells of the particle. Such glassy skins practically isolate the liquid core of a particle from the surrounding gas phase, resulting in extremely long equilibration times for such particles, caused by the strongly non-linear relationship between concentration and D(H(2)O). We present a new parameterization of D(H(2)O) that facilitates describing the stability of aqueous food and pharmaceutical formulations in the glassy state, the processing of amorphous aerosol particles in spray-drying technology, and the suppression of heterogeneous chemical reactions in glassy atmospheric aerosol particles.
authors.affiliation: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland. bernhard.zobrist@env.ethz.ch
authors.forename: Bernhard
authors.initials: B
authors.lastname: Zobrist
authors.forename: Vacharaporn
authors.initials: V
authors.lastname: Soonsin
authors.forename: Bei P
authors.initials: BP
authors.lastname: Luo
authors.forename: Ulrich K
authors.initials: UK
authors.lastname: Krieger
authors.forename: Claudia
authors.initials: C
authors.lastname: Marcolli
authors.forename: Thomas
authors.initials: T
authors.lastname: Peter
authors.forename: Thomas
authors.initials: T
authors.lastname: Koop
mesh_terms: D004058:Diffusion; D006813:Humidity; D008962:Models, Theoretical; D044367:Phase Transition; D013395:Sucrose / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D013395:Sucrose