Phase Transition in Monodisperse Gold Sol. Microscopic Observation of Gas, Liquid and Solid States

A gold sol of narrow size distribution with particles 120 nm in average diameter undergoes phase transition from gaseous (dispersed) state to solid (crystalline) state via liquid state when the concentration of the counterions in the medium is increased. The phenomenon takes place in a very thin layer of the settled sol, as observed with a microscope at 2000 magnification. The transition from the gaseous to the liquid state occurs at the ion concentration of 0.02 mol dm–3 in a deposit of a number of small drops of a few micrometers in diameter. These »drops« start to crystallize at 0.03 mol dm–3. After ten hours, the sol is converted into a state in which the solid and its vapor (dispersed sol) coexist. The phenomenon is quite reversible. On dilution with water, the solid phase redisperses into the original gaseous state.