Phase Transformation and Metastability of Hygroscopic Microparticles

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Abstract
The hydration and crystallization of inorganic salt particles are investigated in an electrodynamic balance, in which a levitated single microparticle is undergoing phase transformation and growth under controlled humidity conditions. Laser Raman and Mie scattering techniques are used to probe the chemical and physical state of the microparticle before and after phase transformation. Here, we report first spectroscopic evidence that new metastable solid states form in hygroscopic aerosol particles. Because of the high degree of supersaturation at which a solution droplet solidifies, a metastable crystalline or amorphous state often results. The formation of such state is not predicted from bulk-phase thermodynamics and, in some cases, the resulting metastable state is entirely unknown heretofore. We also document new solid-solution and solid-solid phase transitions which occur exclusively in microparticles. Results are presented for particles composed of (NH4)2SO4, Na2SO4, LiClO4, Sr(NO3)2, KHSO4, RbHSO4 or NH4HSO4, illustrating the common occurrence of metastable states in hygroscopic aerosol particles under ambient conditions. It is also shown that certain thermodynamic properties can be determined with good precision from the vapor solid equilibrium during particle growth.