Thermodynamic Studies on the Solution Processes of Disperse Dyes in Water

Abstract

The solution process of disperse dyes in water was assumed to be divided into two imaginary processes: i the transformation of a dye crystal into a super cooled liquid; ii the dissolu- lion of the super cooled dye in water. The characters of these processes were discussed thermodynamically.
The aqueous solubilities of the disperse dyes shown in Table 1 were deter mined at 10°C intervals from 30°C to 70°C by means of the equilibrium dialysis method. The change of enthalpy and entropy for the solution processes of the dye crystals in water were calculated (Table 2). The heats of fusion at melting point (tall) and the specific heats of the disperse dyes were determined by scanning calorimeter (Fig.1, 2 and Table 3).
The thermodynamic parameters for the processes i and ii were estimated from the value of zi. H, difference in the specific heat between the crystalline and the liquid state of the disperse dye (zicp) and the thermodynamic parameters in Table 2 by Eq. ( 5 )( 9 ) (Table 3). The free energy change in the processe ii were always far greater than that in the processe i .The very low aqueous solubilities of the disperse dyes were ascribed to the large positive heat of solution in processe ii.
The introduction of 4'-nitro group in Dye 1-3 (Dye 4-6) lead to altering of the entropy change for processeii from negative to positive value but to lowering of the aqueous solu- bility to about 20 times that of Dye 1, --3 due to an increase in(Table 3). The introduction of p-nitro group in anilie and Dye 1 increased heat of vaporization (L) and that of sublimation (L5) (Table 4). These results were explained in terms of dipole structure of disperse dyes.
A definite correlation was found between the aqueous solubility and the affinity of the disperse dyes on nylon 6 or secondary acetate (Fig.3).
From these results it was deduced that the very low aqueous solubility and the high affinity on hydrophobic substrate containing polar groups were both attributed to the dipole structure of disperse dyes.