SALTING-OUT EFFECTS OF INORGANIC ELECTROLYTES ON AZOBENZENE AND ITS DERIVATIVE IN AQUEOUS UREA SOLUTIONS

Abstract

The contribution of structural salting-out effect to the total salting-out phenomena was studied in this work.The solubilities of azobenzene and p-hydroxyazobenzene in water and various inorganic electrolytes-NaCl, LiCl, NaNO3, NaClO4, and NaSCN-aqueous solutions (1 mole/l) were determined at 5°C intervals from 5° to 40°C.From the results the thermodynamic parameters for the process (A) which consists of a transfer of one mole of the solute from water to the aqueous inorganic electrolyte solutions were calculated.The solubilities of the solutes in an aqueous 6 mole/l urea solution and in aqueous 6 mole/l urea-1 mole/l electrolyte solutions were determined at the same condition as that for water. Using the data of solubilities the thermodynamic parameters for the process (B) which consists of a transfer of one mole of the solute from the aqueous 6 mole/l urea solution to the aqueous 6 mole/l urea-1 mole/l inorganic electrolyte solution were calculated.By comparing the free energy changes for the process (A) with those for (B) it was found that an addition of urea into aqueous inorganic electrolyte solutions causes a decrease in salting-out phenomena.Structural salting-out comes from a structural repulsion between different kinds of the water structure around non-polar solutes and that around ions. It has already been established that urea breaks icebergs around hydrophobic parts of solutes in an aqueous environment.Various influences of urea on the salting-out effect of inorganic electrolyte were discussed and the decrease in salting-out phenomena in an aqueous urea solution was explained in terms of iceberg breaking effect of urea.In an aqueous urea solution the iceberg around the solute should be disrupted by the action of urea. This disruption of the iceberg should cause disappearance of structural salting-out effect.The difference between free energy changes for the processes (A) and (B) are attributed to the contribution of the structural salting-out effect.