AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM8
Scanning Electron Microscopy Studies of the Hydration of Alkali Halides

Tuesday, October 3, 2000, 10:40 am, Room 208

Session: Reactions on Oxides and Environmental Chemistry
Presenter: S.A. Joyce, Pacific Northwest National Laboratory
Authors: S.A. Joyce, Pacific Northwest National Laboratory
J.P. Cowin, Pacific Northwest National Laboratory
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Morphological and phase changes of several salts which are important constituents of sea-salt aerosols were examined in an environmental scanning electron microcospe as function of both water vapor pressure (from ~2 to 18 Torr) and temperature. For highly soluble materials, dissolution can occur by incorporation of water from the gas phase. The dissolution of NaCl and NaBr samples upon exposure to water vapor have been imaged and the observed temperature dependence of the deliquescence pressures agree well with previous studies. The deliquescence is initially observed as the surface of a crystal becomes featureless, followed by a rounding and spreading of the solution droplet on the sample holder. NaBr converts to the dihydrate (NaBr*2H@sub 2@O) at water pressures roughly half the deliquescence pressure. The formation of the hydrate can be readily observed in the SEM. Unlike the case of deliquescence, no significant changes in the microtopography are initially observed, rather hydration results in a change in the contrast. Dark regions on the surface nucleate and grow as the hydration proceeds. The rate of hydration/dehydration is a strong function of the pressure (i.e. the saturation ratio). Prolonged hydration results in an increase in the roughness of the surface presumably due to microcrystallite formation as a stress-relief mechanism. Cycling hydration/dehydration results in higher roughness. Delamination and spalling have been observed upon rapid dehydration. Both effects increase the net surface area. Such cycling is very common in the atmosphere versus changes in altitude, time, and the day/night cycle, and should have a strong impact on water-soluble aerosol morphology. * Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.