228 JSDC VOLUME 115 JULY/AUGUST 1999 The influence of metal ions on the aggregation and hydrophobicity of dyes in solutions K W Yeung and S M Shang Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong The effect of the addition of metal ions to dye solutions in promoting dye aggregation and in changing dye association in solution has been studied. Experimental findings indicate that the promoting effects of calcium and magnesium ions are greater than that of sodium ions. It is expected that any change in dye hydrophobicity would have a direct influence on the extent of dye aggregation. Under acid conditions, a higher concentration of metal ions would result in an increase in dye aggregation with a reduction in dye hydrophobicity. On the other hand, while the dye aggregation is increased under alkaline conditions under the influence of metal ions, hydrophobicity was shown to increase. These changes in dye hydrophobicity could be used to explain the dyeing behaviour of reactive dyes in the dyeing of silk. INTRODUCTION In the study of dye adsorption properties, the possibility of aggregation of dye molecules in solution must be taken into account [1]. Numerous studies have been conducted on the dye aggregation of anionic, cationic and disperse dyes [2–4]. Although highly sophisticated analytical methods are available, such as small-angle X-ray scattering [5], spectro- photometric techniques have been extensively employed in the study of dye aggregation phenomena [6,7]. However, such techniques suffer from the shortcoming that the broad adsorption curves obtained with monomers, dimers and large aggregates cannot be sufficiently resolved [8]. It is found that, for a given pH, any change of dye hydrophobicity in solution also changes the dye association. The hydrophobicity of a compound is defined as its relative tendency to be readily soluble in most nonpolar solvents, but only sparingly soluble in water [9], and plays an important role in physico-chemical, biological and environmental phenomena. The hydrophobic effect causes the separation of a nonpolar solute from aqueous solution, and is the driving force for liquid–liquid distribution processes, micelle formation, passive membrane transport, soil sorption and bioconcentration of environmental pollutants [10]. Dyeing carried out in the aqueous phase can be considered as a transfer of the dye from the water to the passive membrane of a fibre. It has been found that changes of the hydrophobic tendency of a fibre also influence dye adsorption [11–13]. The change in the hydrophobicity of monoazo disperse dyes caused by different alkyl chain lengths can affect the equilibrium of dyeing [14]. It has also been demonstrated that dye-resist effects are highly dependent on the hydrophilic/ hydrophobic characteristics of both the dye and the substrate [15,16]. The present paper investigates the effects of metal ions on the aggregation and the hydrophobic behaviour of a typical reactive dye. The effect of such behaviour on the exhaustion and fixation properties on silk is also studied. EXPERIMENTAL Dyeing Procion Red MX-5B (BASF, CI Reactive Red 2) was purified by dimethyl formamide (DMF) dissolution and acetone precipitation [17]. In all experiments, 1 g of scoured silk fabric was placed in 40 ml of 0.4 g/l dye solutions with the known amounts of calcium, magnesium or sodium ions. Dyeings were conducted in acidic conditions (adjusted with formic acid to pH 2.5), elevating the temperature from 40 to 85 ° C over 45 min and maintaining at 85 ° C for 45 min, and also in alkaline conditions (adjusted with sodium carbonate to pH 10), at 50 ° C for 90 min. Measurement of dye exhaustion and fixation The percentage exhaustion ( E) and the percentage fixation (F) of dye on fibre were obtained by measuring the absorbance of the dye solutions according to Eqns 1 and 2: E M M M = - × o f o 100 (1) F M M M M = - - × o f w o 100 (2) where M o and M f are the mass of dye present in the initial dyebath and in the final dyebath respectively, and M w is the mass of the unfixed dye removed from the fibre by stripping at the boil for 15 min in 1:1 DMF/water solution [18]. The