Study On The Metastable State Of Crystallization Of Aqueous Solution Of Some Inorganic Salts And Organic Compounds

There is a wide existence of the metastable state of the crystallization of the aqueous solutions of the substances in the nature. And what is different from the solubility curve, the supersaturated solubility curves of the metastable state are a group of curves. Because the metastable state is affected by many factors, the supersaturated solubility curves of the metastable state are neither definite nor single. According to different type of nucleation, the matastable region is divided into three type of metastable region: the first matastable region, the second matastable region and the third matastable region. The matastable region of homogeneous nucleation is the third matastable region. In this thesis the droplet technique was used to measure the matastable state of homogeneous nucleation.The droplet technique is a method to disperse the aqueous solutions into many small droplets in oil which can not catalyze the nucleation. In order to measure the metastable sate in the absence of solid phase, two experiment methods, i.e. dynamic method and static method, were designed in this paper.In the dynamic method, the solution is dispersed into the little droplet by a capillary. The temperature of the nucleation is measured during the falling down of the droplets. In the static method, the droplet is still and drops into a tube full of oil, and the temperature is reduced by the cycled water.By these two experiment methods, the maximum widths of the metastable region of the aqueous solutions of potassium nitrate, sodium nitrate, potassium dichromate, boric acid, potassium chloride and urea, were measured and compared. The widths of the metastable region of the aqueous solutions of potassium nitrate, sodium nitrate, potassium dichromate, boric acid and potassium chloride are in the range of 15~30°C ; and the width of the metastable region of the aqueous solution of urea is in the range of 40~50°C .By using the classical theory, the interfacial tension, the radius of the critical nucleus and the number of molecules of the critical nucleus of the above six aqueous solutions are calculated. The number of the molecules in the critical nucleus is in the range of 30~300. And the calculated interfacial tensions are consistent with the values which were reported in the literatures.