G. B. Behera

Time-resolved fluorescence studies of aminostyryl pyridinium dyes in organic solvents and surfactant solutions

Absorption, fluorescence emission spectra and fluorescence lifetimes of N-alkyl-4-(p-N,N-dimethyl aminostyryl) pyridinium dyes of varying lengths of alkyl chain have been measured in different organic solvents (protic, dipolar aprotic and aprotic) and in different surfactant solutions. The spectra for the dyes are similar in a single solvent but vary with solvent type. The Stokes shift for the dye does not vary with the solvent parameter as predicted by Lippert–Mataga equation. The fluorescence decay of the dyes shows two lifetimes in organic solvents except in a few solvents where a single or three lifetimes were obtained. The lifetime results are interpreted in terms of emission from cis and quinoid (or intramolecular charge transfer, ICT) forms of the dye in the excited state. The spectra and fluorescence lifetime results for the cis isomer (synthesized as a stable compound) in organic solvents confirmed the coexistence of multiple species. In surfactant solutions, the spectra and fluorescence lifetimes of the dyes depend upon the length of the alkyl chain of the dye and the concentration of the surfactant, namely, premicellar or post micellar. Dye–surfactant complexes were identified in a few cases in SDS and CTAB premicellar solutions. TX-100 surfactant is found to stabilize the cis isomer for all the dyes, whereas both SDS and CTAB surfactants stabilize the trans isomer predominantly.

Micellization of ionic surfactants in tetrahydrofuran-water and acetonitrile-water mixed-solvent systems

Abstract The effect of the dipolar aprotic solvents, tetrahydrofuran (THF) and acetonitrile, on the micellization of ionic surfactants, cetyltrimethylammonium bromide (CTAB) and sodium lauryl sulphate (NaLS) was studied by (i) a spectral method at 25°C and (ii) a conductance method at different temperatures. It was observed that the critical micellar concentration (c.m.c.) is temperature dependent. The c.m.c. is found to increase with increase in mole fraction of acetonitrile and THF in aqueous solution. The increase in c.m.c. is more marked with CTAB than with NaLS in the same solvent and the c.m.c. change is greater in THF than in acetonitrile. The increase in the c.m.c. is discussed on the basis of water structure and hydrophobic interaction. The thermodynamics of micellization are also discussed.

Interaction of N-alkyl styryl pyridinium dyes with TX-100 in aqueous medium: Role of the alkyl chain during solubilisation

Abstract Interaction of a series of N -alkyl styryl pyridinium dyes having varied alkyl chain (C n ) with nonionic surfactant (Triton X-100) solution has been studied by using absorption and emission techniques. The binding constants of the dyes (C 1 –C 18 ) with the surfactant aggregates have been calculated and found to increase with increasing alkyl chain length of the dyes. A decrease in the CMC value of TX-100 in presence of the dyes with increasing chain length has been observed and the phenomenon is attributed to the increasing hydrophobicity of the additive. The relative E T (30) values of the local environment of the dyes in TX-100 micelles with respect to standard solvents have been determined. A solubilisation strategy of the micelle for the dyes has been reported where the cationic head group remains in the oxyethylene cage of the micelle and the hydrophobic tail remains buried in the micellar core due to pulling effect.

Photochemistry in microemulsions: fluorescence quenching of 1- and 2-naphthol by Cu2+

Abstract The fluorescence characteristics of 1- and 2-naphthol in the presence of copper ion have been studied in aqueous medium and sodium lauryl sulphate (NaLS)-isobutanol-hexane-water microemulsion. On excitation 1-naphthol (1-ROH) dissociates completely and 2-naphthol (2-ROH) partially in aqueous medium. The dissociation of 2-ROH is suppressed completely and that of 1-ROH partially in the microemulsion. The extent of dissociation of 1-ROH varies with changing microemulsion formulation. The dissociation behaviour and fluorescence quenching of naphthols by Cu 2+ in the microemulsion suggest a compartmentalization of the fluorophores. Various quenching models have been analysed to explain the quenching efficiency. A generalized model explaining both the dynamic and transient quenching processes has been proposed. The probable number of quenchers present in the quenching sphere of action has been calculated.

Photochemistry in microemulsions: Fluorescence quenching of naphthols and their O-alkyl derivatives by CCl4

Abstract The fluorescence characteristics of 1- and 2-naphthols and their O-alkyl derivatives in presence of carbon tetrachloride have been studied in hexane and in sodium lauryl sulphate (NaLS)-isobutanol-hexane-water microemulsions. The fluorescence quenching of the fluorophore by CCl4 obeys the linear Stern-Volmer equation in most of the cases except in oil-in-water microemulsion. A quenching sphere of action model has been proposed for the deviation and the number of quenchers present in the quenching sphere of action has been calculated. The dielectric constant of the water-in-oil and oil-in-water microemulsion surface has been evaluated to be 27.36 and 34.31, respectively.

Photochemistry in microemulsions: fluorescence quenching of naphthols by some γ-picolinium salts

Abstract The fluorescence quenching behaviour of naphthols by some water-soluble quenchers like (methylene), bis-(γ-picolinium bromide) and 1-hexyl y-picolinium bromide have been studied in aqueous medium, and in both water-rich and oil-rich regions of NaLS (sodium lauryl sulfate)-isobutanol-hexane-water and CTAB (cetyltrimethyl ammonium bromide)-isobutanol-hexane-water microemulsion systems. From the fluorescence quenching behaviour, the localization sites of the fluorescent probes in microemulsions have been identified. The charge on the surfactant head group has been found to have significant effect on the quenching phenomena of the naphthols by the pyridinium salts. Both linearity and deviation from linearity in Stern-Volmer plots have been observed during the quenching process and a sphere of action model has been proposed for positive deviation.

Incorporation of Triphenylmethane Dye Cations into Surfactant Micelles

The incorporation of methyl violet and rosaniline hydrochloride (the triphenylmethane dye cations) has been investigated in sodium lauryl sulphate (NaLS) and TritonX-100 (Txl00) in aqueous solutions at 25°C. Both the anionic and nonionic surfactants incorporate the dye which reaches saturation at their cmc’s. The incorporation constants, Kj, in the anionic and nonionic submicellar aggregates have been calculated for both the dyes at different dye concentrations. The maximum number of dye molecules per aggregate (x) has also been calculated and found to be about 5 and 4.6 for methyl violet in anionic and nonionic surfactants, respectively; whereas for rosaniline hydrochloride these values are 2.7 and 2.0, respectively. The kinetically important species is found to be MD\at low concentrations of the dye.

Evidence for Carbinolamine Intermediate in Schiff Base Hydrolysis

Schiff bases have been synthesized from heterocyclic amines and hydroxy aromatic aldehydes to undertake the hydrolysis in a wide range of pH values in various organized assemblies such as micelles, reversed micelles and microemulsions. The intermediacy of carbinolamine during Schiff base hydrolysis has been investigated by designing monolayer-forming Schiff bases and carrying out: surface studies. Hydrolysis studies in reversed micelles and microemulsions have also supported the formation of carbinolamine intermediate during hydrolysis.