Biren Gohain

Hydrophobicity-induced deprotonation of dye in dye–submicellar surfactant systems

Interaction of aqueous sulfonephthalein dyes, viz., phenol red, cresol red and cresol purple with alkyltrimethylammonium and alkylpyridinium halide surfactants in submicellar concentration ranges of the surfactants has been studied by UV–VIS spectroscopy. The spectral changes of the dyes observed on addition of the surfactants at concentrations far below critical micelle concentration (c.m.c.) indicate the formation of doubly deprotonated dye anions, which otherwise are formed only at much higher pH in the absence of the surfactant. The equilibrium constants for the interactions were determined. The deprotonation has been found to increase considerably with increase in hydrophobicity of the surfactant. It has been suggested that combined electrostatic–hydrophobic interaction increases acidity of the sulfonephthalein dyes in their closely packed dye–surfactant ion-pairs, formed at concentrations of surfactant far below c.m.c., causing deprotonation of the dye.

Premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions: deprotonation of dye in ion pair micelles.

The premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions monitored by surface tension and spectroscopic measurements has been described. The measurements have been made for three anionic sulfonephthalein dyes and cationic surfactants of different chain lengths, head groups, and counterions. The observations have been attributed to the formation of closely packed dye surfactant ion pairs which is similar to nonionic surfactants in very dilute concentrations of the surfactant. These ion pairs dominate in the monolayer at the air-water interface of the aqueous dye surfactant solutions below the CMC of the pure surfactant. It has been shown that the dye in the ion pair deprotonates on micelle formation by the ion pair surfactants at near CMC but submicellar surfactant concentrations. The results of an equilibrium study at varying pH agree with the model of deprotonated 1:1 dye-surfactant ion pair formation in the near CMC submicellar solutions. At concentrations above the CMC of the cationic surfactant the dye is solubilized in normal micelles and the monolayer at the air-water interface consists of the cationic surfactant alone even in the presence of the dyes.

A partition equilibrium study of sulfonephthalein dyes in anionic surfactant systems: determination of CMC in buffered medium

Abstract A spectrophotometric method has been reported which can be used to simultaneously determine partition equilibrium constant of anionic sulfonephthalein (acid–base indicator) dyes between micellar pseudophase and aqueous phase, and critical micelle concentration (CMC) of the surfactants in buffered aqueous anionic surfactant systems. The method is based on two assumptions: (1) only one form of the dye goes into the micellar pseudophase, which holds for similarly charged dye–surfactant systems and (2) the activity terms for the conjugate acid and base forms of the dye can be combined in buffered solution. An absorption band, which decreases with addition of surfactant, is used for the analysis. The method has been found suitable to the systems of sulphonephthalein (anionic) dyes, viz. bromophenol blue (BPB), bromothymol blue (BTB) and thymol blue (TB) with anionic surfactants, viz. sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN). The partition equilibrium constants have been found to be dependent on hydrophobicity of the dye and pH of the solutions. The CMCs have been found to increase with increase in pH for a particular buffer system. The method can conveniently be used for study of interactions in such dye–surfactant systems and to determine CMC of the anionic surfactants in buffered solutions.

Stabilization of diketo tautomer of curcumin by premicellar anionic surfactants: UV-Visible, fluorescence, tensiometric and TD-DFT evidences.

A newly observed UV band of aqueous curcumin, a biologically important molecule, in presence of anionic surfactants, viz., sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), and sodium dodecylsulfonate (SDSN) in buffered aqueous solutions has been studied experimentally and theoretically. The 425 nm absorption band of curcumin disappears and a new UV-band is observed at 355 nm on addition of the surfactants in the submicellar concentration range which is reversed as the surfactant concentration approaches the critical micelle concentration (CMC). The observed spectral absorption, fluorescence intensity and surface tension behavior, under optimal experimental conditions of submicellar concentration ranges of the surfactants in the pH range of 2.00-7.00, indicate that the new band is due to the β-diketo tautomer of curcumin stabilized by interactions between curcumin and the anionic surfactants. The stabilization of the diketo tautomer by submicellar anionic surfactants described here as well as by submicellar cationic surfactant, reported recently, is unique as this is the only such behavior observed in presence of submicellar surfactants of both charge types. The experimental results are in good agreement with the theoretical calculations using ab initio density functional theory combined with time dependent density functional theory (TD-DFT) calculations.

A partition equilibrium study of benzene/water/SDS-butanol oil/water microemulsions

Partition of a sulfonephthalein dye, viz., phenol red, between the pseudophases in oil/water benzene/water/SDS-butanol microemulsions has been studied by UV-VIS spectroscopy and the partition equilibrium constants have been correlated with the microstructure of microemulsions and effect of cosurfactant.

A Visible Spectroscopic Study of Microstructure of O/W Microemulsions of Anionic Surfactants: Effect of Cosurfactant

The microstructure of o/w microemulsions, stabilized by sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) with different cosurfactants, has been studied by partitioning of a dye, phenol red, between the oil‐water interface and bulk water. The cosurfactants used are propan‐1‐ol, propan‐2‐ol, butan‐1‐ol, butan‐2‐ol, pentane‐1‐ol, pentane‐2‐ol, and pentan‐3‐ol. The effects of changing the oil volume fraction and surfactant‐cosurfactant w/w ratio on the oil‐water interface and droplet size have also been discussed. Larger droplet size was predicted for SDS than SDBS. The predicted droplet radius increased with increase in the oil fraction, decrease in the surfactant concentration, increase in the C‐number of the linear cosurfactant, and decrease in branching of the cosurfactant. Surfactant‐cosurfactant ratio and pH did not affect the droplet size significantly. The minimum concentrations of surfactants with which microemulsions were formed were found to be higher for larger oil fraction, smaller C‐number of the alcohol, more branching of the alcohol, and higher pH.