Water Uptake by NaCl Particles Prior to Deliquescence and the Phase Rule

Abstract

Using an environmental transmission electron microscope (ETEM), we show that a significant amount of water, far exceeding the multilayers caused by surface adsorption, is reversibly associated prior to deliquescence with substrate-supported NaCl particles (dry diameters of ∼ 40 nm to 1.5 μ m; ∼ 18°C). We hypothesize that the water is present as an aqueous solution containing dissolved Na and Cl ions. Water uptake occurs at relative humidities (RH) as low as 70%, and the resulting liquid layer coating the particles is stable over extended times if the RH is held constant. We exposed CaSO ₄ and CaSO ₄ · 2H ₂ O particles to elevated RH values in the ETEM to show that chemically nonspecific condensation of gas-phase water on the TEM substrate does not explain our observations. Furthermore, damage to the NaCl surface induced by the electron beam and small fluctuations in RH do not seem to contribute to or otherwise affect water uptake. We have similar observations of water association for other alkali halide particles, including NaBr and CsCl, prior to deliquescence. To explain the observations, we derive the phase rule for this geometry and show that it allows for the coexistence of liquid, solid, and vapor for the binary NaCl/H ₂ O system across a range of RH values. The derivation includes the effects of heterogeneous pressure because of the Laplace-Young relations for the subsystems. Furthermore, in view of the lever rule and the absence of similar observations for free-floating pure NaCl aerosol particles, we hypothesize that the surface energy necessary to support these effects is provided by sample-substrate interactions. Thus, the results of this study may be relevant to atmospheric systems in which soluble compounds are associated with insoluble materials.

Acknowledgments

This work was supported by the National Science Foundation under Grant No. 0304213 from the Division of Atmospheric Chemistry. Any opinions, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the National Science Foundation. We gratefully acknowledge the use of the facilities at the John M. Cowley Center for High Resolution Electron Microscopy within the LeRoy Eyring Center for Solid State Science at Arizona State University. In particular, we thank Karl Weiss, John Wheatley, Grant Baumgardner, Renu Sharma, and Peter Crozier for their assistance with developing our ETEM technique at ASU. We also thank Jonathan Abbatt, Daniel Cziczo, Margaret Tolbert, Melinda Beaver, George Biskos, and John Armstrong for their help in the development of this article. The anonymous reviewers and the editor provided important guidance that improved the article.

Notes

^a MPS-3 microanalysis particle sampler (California Instruments, Inc.).

^b Vaporization-condensation aersosol generation.

^c TSI Nanometer Aerosol Sampler (model #3089).

^d CaSO₄ used as received from the manufacturer.