Richard L. Reeves

The nature of species giving spectral changes in an azo dye on interaction with cationic surfactants below the critical micelle concentration

Abstract 4-Phenylazo-l-naphthol-2,4′-disulfonate gives a shift in the hydrazone-azo tautomeric equilibrium on interaction with dodecyl (DTAB) and hexadecyltrimethylammonium bromide (HTAB) that is related to a transfer from an aqueous to a nonaqueous environment. The shift in the equilibrium gives a spectral change and occurs at surfactant concentrations below the surfactant CMCs. Spectral, solubility, and stopped-flow kinetic studies were used to show that the tautomeric shift does not result from formation of normal micelles or the sparingly soluble salts, but from the formation of aggregates in which surfactant is in small excess over dye. These aggregates are formed rapidly by a kinetically controlled process and can yield the insoluble salts on equilibration for periods of days. The spectral shift does not result from the initial dye-surfactant interaction but from subsequent growth to an aggregate where the microenvironment of the dye is largely nonaqueous. The results are interpreted in terms of a spectrum of mixed micelles beginning with surfactant-poor micelles which form at surfactant concentrations far below the CMC for homomicelles and proceeding to normal surfactant-rich micelles near the CMC.

Surface tensions of aqueous solutions of some azo dye sulfonates and analogs

Abstract Surface tension measurements have been made on aqueous solutions of a series of highly purified azo dye sulfonates of systematically varied structure to determine the effect of structure on the hydrophobicity of the nonionic moieties. Variations included the effect of the size of terminal alkyl groups and the effect of substitution of heteroatoms with hydrogen or isoelectronic methine groups. Measurements of C6–C8 n-alkanesulfonates were included to provide a scale of surface tension lowering for the dyes in terms of equivalent alkane chain length. All-aromatic dye structures containing no pendent alkyl groups are weakly surface active at concentrations up to saturation with Δγ values equivalent to those of a C6 alkanesulfonate. Substitution of the nitrogen atoms of the azo group with carbon atoms increases the surface tension lowering by an amount equivalent to the addition of 2–3 methylenes to an alkane chain. The most pronounced increase in surface activity came from the addition of small alkyl groups. The effect of dye aggregation on the results is discussed. There appears to be little correlation between hydrophobicity and the tendency of the dyes to form small aggregates.

Mixed Micelles of Methyl Orange Dye and Cationic Surfactants

Interaction of the anionic dye with cationic surfactants gives a sharp new absorption band at surfactant concentrations below the cmc for surfactant homomicelles. The band is shown to result from a dye-dye stacking interaction rather than from a change in dye geometry. Variation of the surfactant: dye (S:D) ratio gives distinct absorption bands characteristic of three disperse states: (1) a band similar to that of free dye at S:D ratios near the equivalence point that is due to a microcrystalline suspension of the insoluble salt; (2) the dye aggregate band at larger S:D ratios characteristic of mixed micelles having a significant population of dye molecules occurring side-by-side with their molecular axes nearly parallel; and (3) a band similar to that of dye dissolved in an organic medium at surfactant concentrations near the cmc. The intermediate mixed micelles are unstable and slowly reogranize to the insoluble salt. The dye is weakly surface active. Dye-CTAB mixtures are more surface active than CTAB alone at surfactant concentration

The effect of cation size on ion pairing of methyl orange dye with tetraalkylammonium and benzyltrialkylammonium ions in aqueous solution

near the cmc. The results are discussed in terms of possible structures of the mixed micelles.

A critical spectrophotometric analysis of the dimerization of some ionic azo dyes in aqueous solution

Formation constants have been measured by a solvent distribution method for the ion pairing of an arene sulfonate, methyl orange dye, with two series of quaternary ammonium ions: R4N+(R=Et,n-Pr,n-Bu, andn-Pent) and C6H5CH2R3N+ (R=Me, Et,n-Pr,n-Bu,n-Pent, andn-Hex). Ion pairing increases dramatically as the length of the R group increases beyond butyl. Using a hard-sphere model for contact ion pairs, it is estimated that coulombic attraction contributes about −kT to the binding free energy and decreases slightly with increasing size of R4N+. Other factors related to solvation effects, of which cosphere overlap predominates, contribute from −2kT to −7kT of binding energy. Plots of logK for association as a function of cation size show an inflection with decreasing slope between R=propyl and R=butyl. Possible causes for the inflection are considered.