HYDRATION AND RAMAN-SCATTERING STUDIES OF LEVITATED MICROPARTICLES - BA(MO(3))(2), SR(NO3)(2), AND CA(NO3)(2)

The phase transformation and hydration of inorganic salt particles com posed of alkaline earth metal nitrates, Ba(NO3)(2), Sr(NO3)(2), and Ca (NO3)(2) are investigated in a quadrupole cell, in which a levitated s ingle microparticle is in dynamic equilibrium with water vapor under c ontrolled humidity conditions. Laser Raman and Mie scattering techniqu es are used to probe the chemical and physical states of the micropart icle before and after phase transformation. Because of the high degree of supersaturation that only a suspended solution droplet can attain before solidifying, metastable states not predicted from bulk solution thermodynamics often result. Thus it is found that, except for Ba(NO3 )(2) particles which form the stable anhydrous crystalline state upon efflorescence, Sr(NO3)(2) and Ca(NO3)(2) droplets solidify to a metast able amorphous state that contains residual water persisting even in h igh vacuum. Raman spectra of the amorphous particles reveal that the n itrate ions form contact-ion pairs with the bivalent cations and that the residual water molecules possess very little, if any, hydrogen bon d characteristics. The deliquescence properties of the amorphous parti cles, which are found to be different from those expected of the bulk crystalline states, are also reported. (C) 1997 American Institute of Physics.