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 cellu lose acetate films from aqueous solution is reported. Sorption isother ms at 75 and 60-degrees-C were linear up to saturation, in agreement w ith previous findings that have led to the conclusion that cellulose a cetate-disperse dye systems are thermodynamically ideal. Thermodynamic analysis of these data also gave results consistent with such previou s findings. The isotherms for 45 and 25-degrees-C, on the other hand, exhibited increasing curvature, in line with similar recent findings f or a variety of hydrophobic polymer-disperse dye systems, and consiste nt with the presence of some strong absorption sites. However, the sit uation appears to be more complex than envisaged by simple dual-mode s orption theory. Early time desorption kinetic data were found to be co nsistent with Fick's law, with no indication of any significant depend ence of the diffusion coefficient D on concentration (in the medium- t o high-concentration range) or on film thickness. On the other hand, D was affected significantly by the history of film formation, the meth od of introducing the dye or heat treatment of the film. The origin of these effects was traced by DSC to definite microstructural differenc es. 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 ques tion were generally more marked at higher temperature or film thicknes s, or when dyeing had been effected from the vapor phase, and were att ributed to slow release of strongly adsorbed dye molecules. (C) 1994 J ohn Wiley & Sons, Inc.