UV–VIS investigation of methyl red in presence of sodium dodecyl sulfate/methanol/ethanol/water system

Abstract

This paper reports room temperature UV–VIS study of interaction of methyl red (MR) dye with sodium dodecyl sulfate (SDS)/methanol/ethanol/water systems in different ratios of the solvent components (R = 0, 10, 20, 30, and 40); where the concentration of water is represented by a R parameter. The concentration of SDS was varied from 1.4 × 10-4 to 2.8 × 10-3molL-1 for the SDS/methanol/water system and 2.2 × 10-4 to 4.4 × 10-3molL-1 for SDS/ethanol/water system. In all experiments, MR concentration was fixed to 7.4 × 10-6molL-1. The MR spectrum was very similar at lower R values. The solution appeared cloudy at R = 30. For R = 40, MR absorption maximum (λmax) strongly red-shifted from 412 nm to 493–495 nm on increasing the concentration of SDS in the methanol system but no such shift was observed in ethanol. The transparency of the solution again vanished at R = 50 and the experiment was not carried above. These observations suggested that MR exists in hydrazone and azo form in SDS/methanol and SDS/ethanol system; respectively. On increasing concentration of SDS, the MR absorbance increased in the methanol system (hyperchromic shift) and decreased in ethanol system (hypochromic shift). The distribution constant (K) and binding constant (K’) were evaluated through the NLREG procedure and the modified Benesi-Hildebrand equation was also introduced to calculate the binding constant (Kb) and Gibbs free energy of binding (ΔG0). With increasing R, the ΔG0 value became more negative in methanol system; whereas it became less negative in the ethanol system. The negative value of ΔG0 indicated the spontaneity of SDS binding with MR.