EFFECTS OF UREAS AND FORMAMIDES ON THE AQUEOUS SOLUBILITY OF DISPERSE DYES

Abstract

The solubilities of disperse dyes such as azobenzene and p-hydroxyazobenzene in water and aqueous hydrotropic agent solutions were determined at 5°C intervals from 5°C to 40°C. The hydrotropic agents used in this work are urea, N-monomethylurea, N, N′-dimethylurea, formamide, and N, N-dimethylformamide.From the results the thermodynamic parameters for the process (A) which consists of the transfer of one mole of the solute from a water environment to an aqueous hydrotropic agent solution have been calculated from the equations (1), (2), and (3). Process (A): disperse dye in water (mole fraction N0)→ disperse dye in aqueous hydrotropic agent solution (mole fraction N)In any case the solubilities of disperse dyes in aqueous hydrotropic agent solutions increase regularly with the concentration of the hydrotropic agents. The effects of hydrotropic agents on the solubilities are in the following order, urea<formamide<monomethylurea<dimethylurea<dimethylformamide.The resulting ΔSu•trans• are all positive and ΔHtrans• are zero or positive, so it is clear that the favorable transfer of the disperse dyes from a water environment to an aqueous hydrotropic agent solution comes from the contribution of entropy term.From a consideration of the above data and the other physicochemical properties, it seems most reasonable to conclude that (1) urea and formamide disrupt directly the ordered water structure around the nonpolar parts of the disperse dye, and (2) in aqueous monomethylurea, dimethylurea and dimethylformamide solutions there exist hydrophobic interactions between nonpolar parts of the dye and alkyl chains of these hydrotropic agents.