Analysis of the Visual Spectrum of Methyl Orange in Solvents and in Hydrophobic Binding Sites

Abstract

The absorption curves of methyl orange (MO) and analogous p-aminophenylazobenzenes in organic and aqueous organic solvents are shown to consist of two severely overlapping bands. The curves have been resolved into two skewed component bands using a regression method. The blue shift of the absorption maximum obtained when organic solvents are added to aqueous solutions of MO, or when MO is bound to bovine serum albumin or a surfactant micelle, is the result of a change in relative intensities of the component bands. The low-frequency component is assigned to a n, -. nlx transition of a solvate in which there is specific hydrogen-bonding interaction between solvent and the azo nitrogens, and the high-frequency component to a n1 + nl* transition of a solvate in which the interaction is absent. The low-frequency component is favored by aqueous solvent compositions and by low temperatures. The free energies of interconversion of the species in various hydrogen-bonding solvents are correlated by the solvent surface tension but not by the dielectric constant. The results show that the shift in absorption maximum accompanying binding to a protein or micelle should be interpreted as a shift in an equilibrium rather than as a shift in transition energy.Les courbes d'absorption du methylorange (MO) et des p-aminophCnylazobenzenes analogues, en solvants organiques et organiques aqueux, comprennent deux bandes fortement enchevktrees. La resolution de ces courbes en deux bandes gauches elementaires a ete faite par la methode de regression. Le d6placement vers le bleu du nlaxilnuin d'absorption, obtenu par addition de solvants organiques aux solutions aqueuses de MO ou lorsque le MO est lie a l'albumine du serum bovin ou a une micelle surfactive, est le resultat du changement des intensites relatives des bandes composantes. La bande a basse frequence est attribue a la transition n, + nl*, d'un produit de solvatation dans lequel il y a interaction specifique par liaison hydrogene entre le solvant et les azotes de l'azo; la bande a haute frequence correspond a la transition nl + n," du produit de solvatation sans interaction. La bande a basse frequence est favorisee par la composition en eau des solvants et aux basses temperatures. Les energies libres de l'interconversion des diverses especes dans differents solvants a liaison hydrogene, ont ete reliies a la tension superficielle du solvant et non a la constante dielectrique. Les rtsultats montrent que le deplacement du maximum d'absorption consecutif a une liaison a une protkine ou ~nicelle devraient &tre interpretes pluttit comme un deplacement d'equilibre qu'un diplacement de 1'Cnergie de transition.[Traduit par le journal]Can. J. Chem.. 51. 628 (1973) Methyl orange (MO, 1) has been used as a probe for studying the interaction of organic anions with proteins (1-9), synthetic polymers (10, 1 I), polyelectrolytes (12), and surfactants (12-14) in aqueous solution. The interaction is usually accompanied by spectral changes that are not associated with the acid-base indicator p...

Rupture of Pluronic Micelles by Di-Methylated β-Cyclodextrin Is Not Due to Polypseudorotaxane Formation

“…MO is a dye with a strong absorbance in the visible region of the spectrum, sensitive to the nature of the microenvironment in which it is present, 29 such as micelles and cyclodextrins. 11,29 The absorbance spectra of MO in water, DIMEB, and F127 ( Figure 1A, (1À3)) reveals a strong solvatochromism. In each case, the spectral contour of the main band can be fitted to a sum of two Gaussians (Supporting Information Figure SI 2).…”

Intramolecular Electron Density Redistribution upon Hydrogen Bond Formation in the Anion Methyl Orange at the Water/1,2-Dichloroethane Interface Probed by Phase Interference Second Harmonic Generation

“…The equilibrium constant K hyd a (MO/H 2 O)/a (MO) between the hydrated and the nonhydrated forms has been evaluated in the past to be about 100 for 4,4'-diaminoazobenzene, another member of this family of dyes, and closer to unity for MO. [19,20] From the UV/Vis spectra in bulk water for MO concentrations ranging from 5 mm to 10 mm, we determined a value of 1.2 AE 0.1 for the equilibrium constant with a fit to a double Gaussian function. The extinction coefficient of the hydrogen bonded MO/H 2 O form at 467 nm was measured to be 29 700 m À1 cm À1 .…”

“…[15] This red shift has already been observed for Methyl Orange in water ± organic solvent mixtures and has been discussed in terms of a long range ordering of the aqueous medium with the introduction of a small amount of the organic solvent. [20] This can be related to the modifications of the solvent ± solvent interactions on the aqueous side of the interface as discussed in molecular dynamics calculations at the water/DCE interface in terms of the strengthening of the hydrogen bonds between water molecules at the interface. [22] Hence, the change in the solvent ± solvent interaction is reflected in the solute ± solvent interaction at the interface provided a sensitive probe is used.…”

Investigating the Halochromic Properties of Azo Dyes in an Aqueous Environment by Using a Combined Experimental and Theoretical Approach

“…Rinuy et al showed that the nitrogen atoms of the azo group can be hydrogen bonded to a water molecule, which has also been examined previously. [49,50] An equilibrium between the hydrogenbonded and non-hydrogen bonded molecules lies on the origin of the two bands in the UV/Vis spectrum of methyl orange. In addition, the hydrogen-bonding interactions also result in a shift to higher absorption wavelengths.…”