A DETAILED STUDY OF SORPTION EQUILIBRIA AND KINETICS OF A MODEL DISPERSE DYE IN CELLULOSE-ACETATE

A detailed investigation of the sorption of 4-aminoazobenzene by cellulose acetate films from aqueous solution is reported. Sorption isotherms at 75 and 60-degrees-C were linear up to saturation, in agreement with previous findings that have led to the conclusion that cellulose acetate-disperse dye systems are thermodynamically ideal. Thermodynamic analysis of these data also gave results consistent with such previous findings. The isotherms for 45 and 25-degrees-C, on the other hand, exhibited increasing curvature, in line with similar recent findings for a variety of hydrophobic polymer-disperse dye systems, and consistent with the presence of some strong absorption sites. However, the situation appears to be more complex than envisaged by simple dual-mode sorption theory. Early time desorption kinetic data were found to be consistent with Fick's law, with no indication of any significant dependence of the diffusion coefficient D on concentration (in the medium- to high-concentration range) or on film thickness. On the other hand, D was affected significantly by the history of film formation, the method of introducing the dye or heat treatment of the film. The origin of these effects was traced by DSC to definite microstructural differences. Late time kinetic data deviated significantly from the theoretical predictions based on the corresponding early time data, indicating a progressive slowdown of the desorption process. The deviations in question were generally more marked at higher temperature or film thickness, or when dyeing had been effected from the vapor phase, and were attributed to slow release of strongly adsorbed dye molecules. (C) 1994 John Wiley & Sons, Inc.