Spectrophotometric study of the solution interactions between riboflavin, sodium salicylate and caffeine

The self-association of Vitamin B-2 (Riboflavin, RBF) and its hetero-association with Sodium Salicylate (NAS) and Caffeine (CAF) have been studied in aqueous solution by UV-Vis spectrophotometry. Using modified Benesi-Hildebrand approach equilibrium hetero-association constants were obtained for RBF-NAS [K = (25 +/- 4) M-1] and RBF-CAF [K = (69 +/- 4) M-1] systems at T = 298K. Absorption spectra of the vitamin in the mixed solutions with CAF/NAS evidence a bathochromic and hyperchromic shifts which are indicative of the hetero-association process between the dissolved molecules, most likely occurring via vertical stacking of their chromophores. Addition of Urea to the solutions of RBF-NAS and RBF-CAF decreases the stability of the hetero-complexes downto K = (17 +/- 3) M-1 and K = (51 +/- 7) M-1, respectively.