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Issue 8, 2011
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Ultra-slow water diffusion in aqueous sucrose glasses

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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, DH2O, from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for DH2O-values as low as 10−24 m2 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 DH2O. We present a new parameterization of DH2O 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.

Graphical abstract: Ultra-slow water diffusion in aqueous sucrose glasses

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Article information

22 Jul 2010
13 Dec 2010
First published
13 Jan 2011

Phys. Chem. Chem. Phys., 2011,13, 3514-3526
Article type

Ultra-slow water diffusion in aqueous sucrose glasses

B. Zobrist, V. Soonsin, B. P. Luo, U. K. Krieger, C. Marcolli, T. Peter and T. Koop, Phys. Chem. Chem. Phys., 2011, 13, 3514
DOI: 10.1039/C0CP01273D

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