PHASE-TRANSFORMATION AND METASTABILITY OF HYGROSCOPIC MICROPARTICLES

The hydration and crystallization of inorganic salt particles are inve stigated in an electrodynamic balance, in which a levitated single mic roparticle is undergoing phase transformation and growth under control led humidity conditions. Laser Raman and Mie scattering techniques are used to probe the chemical and physical state of the microparticle be fore and after phase transformation. Here, we report first spectroscop ic evidence that new metastable solid states form in hygroscopic aeros ol particles. Because of the high degree of supersaturation at which a solution droplet solidifies, a metastable crystalline or amorphous st ate often results. The formation of such state is not predicted from b ulk-phase thermodynamics and, in some cases, the resulting metastable state is entirely unknown heretofore. We also document new solid-solut ion and solid-solid phase transitions which occur exclusively in micro particles. Results are presented for particles composed of (NH4)(2)SO4 , Na2SO4, LiClO4, Sr(NO3)(2), KHSO4, RbHSO4 or NH4HSO4, illustrating t he common occurrence of metastable states in hygroscopic aerosol parti cles under ambient conditions. It is also shown that certain thermodyn amic properties can be determined with good precision from the vapor s olid equilibrium during particle growth.