UV-Vis spectroscopic and chemometric study on the aggregation of ionic dyes in water

The monomer-dimer equilibrium in several ionic dyes (Methylene Blue, Acridi ne Orange, Nile Blue A, Neutral Red, Rhodamine 6G and Safranine O) has been investigated by means of UV-Vis spectroscopy. The data have been processed by a recently developed method for quantitative analysis of undefined mixt ures, based on simultaneous resolution of the overlapping bands in the whol e set of absorption spectra. In the cases of Acridine Orange a second chemo metric approach has been used as a reference. It is based on a decompositio n of the recorded spectra into a product of target and projection matrices using non iterative partial least squares (NIPALS). The matrices are then r otated to give the correct concentrations, spectral profiles of the compone nts and the equilibrium constant. The dimeric constants determined by the t wo methods were in excellent agreement, evidencing the accuracy of the anal ysis. From the calculated dimeric constant and monomer and dimer spectra, t he structures of the dimeric forms of the studied dyes are estimated. (C) 1 999 Elsevier Science B.V. All rights reserved.