Freeze Concentration of Dyes

Concentration of water soluble direct, acid, basic, and reactive dyes occurs when dilute solutions are frozen at a temperature below the melting point and above the eutectic point of the solution. When freezing is done in still solution, a concentration of up to 500% is achieved in one step, with the dye solution collecting in an oblately spheroidal liquid pocket surrounded by clear ice containing voids, formed from air pockets, radiating outward. Three repetitive freezing cycles concentrate the dye 13,000%. The higher the water solubility of the dye, the lower the ability to concentrate during freezing. The freezing rate of four acid and direct dyes had no close relationship with the size of the dyes studied. Over a larger molecular weight range, an effect was observed for other kinds of molecules. Freeze concentration of salt solution (MW = 58.5 ) gives almost a 700% concentration, detergent solution (sodium dodecyl sulfate, MW = 288.4) gives 400%, and bovine serum albumin, a large water soluble macro molecule (MW = 66,200), gives 160%. A theory is presented suggesting that the concentration effect and the formation of the central sphere are consistent with min imizing of the free energy of the overall system. This simple technique may find application in the concentration of heat sensitive, labile dyes for analytical purposes, as well as in the recovery of dyes and other chemicals on a bench scale.