J. Chem. SOC., Faraday Trans. I , 1982, 78, 137-141 Salt-induced Metachromatic Behaviour of an Azo Dye in Aqueous Solution BY MICHEL DE VYLDER Laboratorium voor Fysische Scheikunde, Rijksuniversiteit Gent, Krijgslaan 27 1, B-9000 Gent, Belgium Received 16th January, 198 1 Addition of substantial amounts of simple electrolytes into basic aqueous methyl orange solutions causes a marked decrease of the intensity of the light absorption of the dye in the visible range and the appearance of a total new absorption band at 358 nm. Addition of a non-electrolyte such as urea shows no such effect. The phenomenon is attributed to interactions between the added ions and the electrons of the chromophoric system. The formation of a new absorption band in the visible range on the short-wavelength side and at the expense of the original absorption band, which can be observed on increasing the concentration of some water-soluble dyes, is known as metachromatic behaviour.Where this phenomenon is not attributable to the presence of aggregated species, interactions between dye-ions and counter-ions are taken into account. Similar spectral changes have been observed on adding foreign ions in large excess into dye solutions of a defined Concentration' but hitherto azo dyes have apparently been devoid of such behaviour. Only the qualitative effect of KC1 and CaC1, on the spectrum of methyl orange (I) has been mentioned, albeit without explanation.2 We report here investigations of the influence of several electrolytes [NaCl, KCl, LiC1, CaCl,, MgCl,, NaNO,, LiNO, and Ca(NO,),] and of a non-electrolyte (urea) on the spectrum of methyl orange in aqueous solution at different temperatures and pH values, we also report our attempts to interpret the results.We observed that in basic solutions the addition of increasing amounts of each electrolyte used causes a proportionally marked decrease in the dye absorbance at 465 nm, while urea does not influence the spectrum at all, even at concentrations as high as 7.5 mol drn-,. However, the concentration required to induce the phenomenon depends on the nature of both the anion and the cation (fig. 1). Upon further salt addition, we noticed the formation of a new absorption band at 358 nm. Fig. 2 shows the typical development of the absorption spectrum as a function ofelectrolyteconcentration. The presence of an isosbestic point proves the simultaneous existence of only two absorbing species in solution.The original spectrum is gradually restored on heating and becomes fully recovered to its state before salt addition at ca. 75 O C . On recooling, the spectrum no longer reverts to its initial state, however (fig- 3)- 137138 METACHROMATIC BEHAVIOUR OF A N AZO DYE 2*5 t 2.5 5 7.5 c/mol d n ~ - ~ FIG. 1.-Decrease in the molar absorptivity ( E ) of the methyl orange anion in basic aqueous solution as a function of the addition of different substances (A = 465 nm, 4.28 x mol dm-, methyl orange, 8 < pH < 10, temp. 20 f 0.5 OC, corresponding salt solutions as reference). 0, MgCl,; 0, CaC1,; + , NaCl; A, Ca(NO,),; 0, KCl; @, NaNO,; x , LiCl; A, LiNO,; a, urea.DISCUSSION Water solubility is conferred to the methyl orange ions by the presence of the lyophilic SO; group and by the hydration of the lyophobic group via hydrogen bonds at the p-azo nitr~gen.~ Interactions resulting in spectral changes are thus only likely after the protecting water shields around the dye ions are broken up. The order of influence of the univalent cations which we found experimentally can be related to the series of their specific powers of breaking the water structure, as determined by Frank and R~binson.~ The greater effect of Mg2+ and Ca2+ follows from their larger size and subsequently greater breaking power. The slighter effectiveness of nitrates with respect to chlorides can be attributed to their having a larger extent of ass~ciation.~ The formation of a new absorption band can be justified in different ways.Thus, shifts in the absorption spectra of various ions have been ascribed to the broadening of the ionic atmosphere resulting from the release of surrounding water molecules.s Such an explanation is not applicable here since thermal agitation should cause at leastM. DE VYLDER 139 500 450 400 350 X/nm FIG. 2.-Changes in the visible absorption spectrum of the methyl orange anion in basic aqueous solution caused by the addition of CaC1, (4.28 x mol dm-3 methyl orange, pH z 10, temp. 20k0.5 OC, salt solution as reference). Concentration CaCI, (in mol dm-3): (1) 1.2, (2) 1.6, (3) 2.0, (4) 2.4, (5) 2.8. qualitatively similar effects unlike our experimental finding that increasing the temperature causes the spectrum to revert to its shape before the addition of the salt.Ascribing the phenomenon to aggregated species is also unrealistic because of the lack of unequivocal reversibility of the spectrum on varying the temperature (fig. 3). This conviction is further supported by earlier statements revealing that methyl orange fails to show a distinct dimer or polymer band, even at concentrations near saturation. Although similar marked shifts of the absorption band have been observed in other solvents whose dielectric properties are comparable with those prevailing in aqueous electrolyte solutions, and related to the ionizing power of the solvent,8 an analogous interpretation of our results is inadequate in view of the evident differences in the solvation state.Photochromism of methyl orange, resulting in a decrease in the main absorption band, has been demonstrated on irradiating alkaline solutions of the dyeg but, since only a partial conversion could be achieved in this way, a new band due to the cis isomers was not seen. The application by ReeveslO of the theoretical approach elaborated by Fischer displays this eventual absorption band near 360 nm, in140 METACHROMATIC BEHAVIOUR OF AN AZO DYE 1 . 1 500 450 LOO 350 h/nm FIG. 3.-Influence of the temperature on the spectral changes of the methyl orange anion in basic aqueous solution caused by the addition of 2.8 mol dm-3 CaC1, (4.28 x moi dm- dye, pHoz 8.5, CaC1, solution as reference). Curves 1-4 were recorded with increasing temperature (4,20,40 and 55 C) and curves 5 and 6 with decreasing temperature (20 and 4 "C).accordance with the Amax value of the band we report. Nevertheless, the ratio between the absorption intensities of the presumed cis and trans forms calculated at 100% conversion, differs slightly from the ratio we obtain experimentally. The plausibility of trans-cis isomerism of methyl orange in electrolyte solutions should be confirmed by a suitable direct technique. Recent interpretations of laser-excited resonance spectra attribute the bands observed at 1400 and 1300 cm-l to trans- and cis-azo groups, respectively.ll However, we did not observe a similar shift on adding electrolytes but were confronted with a systematic lowering of all but one of the bands, the latter being assigned to the C-N stretching of the aromatic amine.Therefore, a likely explanation should involve strong interactions between the electrons of the chromophoric part of the dye and the added counter-ions. The electron distribution is influenced by the strong repelling effect of the dimethylamino group. Charges induced in this way are of course neutralized by adjacent water dipoles. Added salt ions will take up the positions of the water dipoles after they have broken up the protecting water shields. Related effects caused by alkali metal ions, observed in the absorption band of crown-ether dyes, have been explained in the sameM. DE VYLDER 141 way.I2 The appearance of a new band at 357 nm has only been reported on the addition of Ba2+ salts, however.The involvement of charges is further evidenced on considering the absence of the effect for urea. EXPERIMENTAL The spectrophotometric experiments in the visible range were performed with a Varian Techtron 635 spectrophotometer and carried out on fresh solutions. Identical spectra could usually be obtained within several hours, except in the presence of the highest electrolyte concentrations where turbidity occurred within a few minutes. Resonance Raman spectra were recorded on a Coderg spectrometer. The laser excitation was supplied by a He-Ne laser and the frequency used was 632.8 nm. We thank Mr Tony Haemers (Labo Algemene en Anorganische Scheikunde) for recording the Raman spectra. R. B. McKay and P. J. Hillson, Trans. Faraday Soc., 1965, 61, 1800. V. V. Palchevskii, M. C. Zakharevskii and T. M. Kalvarskaya, Vestn. Leningr. Univ., Ser. Fiz. 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