ELSEVIER ANALYTICA CHIMICA ACTA Analytica Chimica Acta 306 (1995) 81-89 Comparison between solvatochromic and chromatographic studies of anthraquinones in binary aqueous mixtures M.R. Reta, J.D. Anunziata, R.I. Cattana, J.J. Silber * Depar~amen~o de Quimica y Fisica. Universidad National de Rio Cuarto, Estafeta N 9, 5800, Rio Cuarto, Argentina Received 31 May 1994; revised 8 November 1994; accepted 11 November 1994 Abstract The solvatochromic studies on 9,10-anthraquinone (AQ), and its symmetric dihydroxy derivatives namely l,Sdihydroxy- anthraquinone (1,5-DHAQ) and 2,6_dihydroxyanthraquinone (2,6-DHAQ) in aqueous solvent mixtures of methanol, acetonitrile, tetrahydrofuran and n-propanol are reported. Preferential solvation is detected in every case. However, a remarkable variation in the magnitude of the preferential solvation constant KAB is observed when the composition of the mixture is changed. This effect is attributed to the solvent-solvent interaction between components. Moreover, and although stronger dipolar interactions are to be expected between the solute and water, only preferential solvation by the organic solvent is detected in every case. These effects may be explained in terms of self association of water through hydrogen bonding and the “microheterogeneity” of the binary mixtures. Moreover, differences are observed in the solvatochromic behavior of the solutes. In the case of 2,6-DHAQ the so-called synergistic effect is observed and explained as a function of the strong hydrogen bond solute-solvents interactions. Reversed-phase liquid chromatographic (RPLC) studies of AQ and 2,6-DHAQ in aqueous mixtures of methanol, acetonitrile and tetrahydrofuran were also performed. The results were interpreted by the Kamlet-Taft solvatochromic comparison method showing that the solvatochromic results can be correlated with certain success with RPLC data. 1. Introduction The problem of solute-solvent interactions is par- ticularly interesting since most of the important chemical processes take place in solution. Central to the study of this problem is the question of how the free energy of a solute changes due to the presence of surrounding interacting solvent molecules. Ap- proaches to the problem at the molecular level are difficult and in most cases the problem is beyond the reach of theoretical tools [l]. Also, intermolecular * Corresponding author. interactions can be studied experimentally by a vari- ety of methods, mostly spectroscopic. The interac- tions reveal themselves in the spectra of molecules in different ways such as shifts in absorption and emis- sion bands, changes in intensity and shape of bands [2]. These solvent effects on the electronic spectrum of a molecule are referred as “solvatochromism.” The solvatochromic shifts associated with electronic transitions in solution, result from different solvation energies of the initial and final electronic states. The study of solvatochromic shifts gives relevant infor- mation concerning the kind of solute-medium inter- actions [3] and can be quite revealing for studies in solvent mixtures. 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI OOO3-2670(94)00595-8