Abstract

The results of conductometric and spectrophotometric investigations suggest that the Cr-Al synthetic tanning materials form micelles at a particular concentration below which they do not normally form. The CMC is determined for Cr-Al syntan in aqueous as well as in different additive environments at different temperatures. With this micellisation concept, a new equation for the determination of the association constant (K_A) for the mixed system is framed obeying the law of mass action principle. The validity of this equation is established in glycine, urea and NaCl environments as model systems. From the temperature dependence of the association constant and with the help of the Gibb’s equation, various thermodynamic parameters were estimated. The above analysis has also been modelled conceptually as a different phase. The thermodynamic parameters supported the transfer of Cr-Al syntan to glycine (at 25-54°C) and urea (at 25°C or below) containing sites, whereas the reverse is not true. The reverse is true for urea at temperatures above 25°C and for NaCl also (at 25-54°C). Evidence was also obtained for the Cr-Al syntan micelles to exist more freely in the hydrophilic group of glycine than that of urea. The special and peculiar properties of urea is invoked. In the presence of urea, the destabilisation or stabilisation of Cr-Al syntan micelles has been observed through disruption or reorganisation of the hydrogen bond, which depends on temperature. It has also been seen that the concentration of Cr-Al syntan above CMC (preferably five to ten times of CMC) is useful for tanning processes whereas below CMC is not practically useful. The main aim for the preparation of Cr-Al syntan was to enhance the fixation of both Cr and Al, high shrinkage temperature and light fastness.