Prediction of Critical Micelle Concentration Using a Quantitative Structure-Property Relationship Approach

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations.

The photodecomposition of the rhodamine 6G (R6G) dye dissolved in n -alcohols and aqueous sodium dodecylsulphate (SDS) micellar solution was investigated under continuous UV-visible illumination. The results indicate that in alcohol solution the photodecomposition efficiency increases with the dye concentration. It also depends on the nature of the alcohol the lowest efficiency being observed in ethyleneglycol. The mechanism involves the electron transfer reaction of the triplet excited state with a ground state dye molecule (T + S 0 → R + + R − . In aqueous SDS micellar solution, the R6G cation is embedded in the hydrophobic interior of the micelle close to the negatively charged sulphate end groups. We show that a rather high concentration (3 × 10 −3 M) of R6G can be dissolved in 10 −1 M SDS aqueous solution, protecting R6G from the electron transfer reaction and secondary decomposition reactions.

Photophysical and photochemical properties of rhodamine 6G in alcoholic and aqueous sodium dodecylsulphate micellar solutions

The hydrophobic–hydrophilic segment geometries of 98 anionic surfactants were fully optimized and calculated by ab initio RHF/6-31G(d), quantum chemical data such as the charge density, the energy of molecular orbital and the dipole moment were obtained. The anionic surfactants employed including sodium alkyl sulfates, sodium alkyl sulfonates, sodium alkyl benzenesulfonates, and potassium alkyl carboxylates with a wide variety of hydrophobic structures. Based on one constitutional descriptor and two quantum chemical descriptors, a significant quantitative structure–property relationship (QSPR) model for the critical micelle concentration of anionic surfactants was obtained by using the multiple linear regression technique. The good correlation coefficient of R2 (0.980) and cross-validation correlation coefficient Rcv2 (0.978) indicate the excellent capability and stability of the regression equation developed. It was found out that the total atom number (NT) in the surfactant hydrophobic–hydrophilic segment plays a major role in the model, the dipole moment (μ) of the surfactant segment and the max net atomic charge on C atom (QC-max) in the surfactant segment are also important. Nine anionic surfactants were employed to test the model developed, the calculated critical micelle concentration was consistent with the experimental one.

Estimation of critical micelle concentration of anionic surfactants with QSPR approach

Surfactant assemblies such as micelles, reverse micelles and solloids or hemimicelles, are fluctuatingand disordered assemblages of surfactants. These assemblies are formed when the concentration ofthe surfactants, in polar or nonpolar solvents exceeds a threshold value. Because of their vastapplications in both industrial and house-hold affairs, there has been considerable research on theformation and properties of surfactant assemblies in solution and at solid–liquid interfaces.Despite many studies of these organized assemblies using X-ray, NMR, light scattering, conductivityand calorimetry, information on their nanostructures, particularly at solid–liquid interfacesis still very limited. In this article, we review the application of fluorescence techniques to investigateaggregate characteristics such as size, micropolarity, nanoviscosity and aggregation number.

Fluorescence Probing of the Surfactant Assemblies in Solutions and at Solid–Liquid Interfaces

The cationic xanthene dyes thiopyronine and selenopyronine are solubilized in the Stern regions of anionic micelles. Upon photoexcitation, the excited triplet states of the dyes are observed to decay via unimolecular processes ( k , z 2 X lo3 s-l) for the case of one dye per micelle and via bimolecular processes (k2 5 X lo6 S I ) for the case of more than one dye per micelle. If the number of dye triplet states that undergo slow decay is measured, the number of micelles can be obtained and thus the micellar aggregation numbers can be evaluated. The aggregation numbers of several surfactant alkanesulfonates, 2,5-dialkylbenzenesulfonates with a total of 14 carbon atoms in the alkyl chains, and other detergents, which are determined in this manner, are in agreement with corresponding values determined by using other photophysical methods and with literature values. The aggregation numbers of the dialkylbenzenesulfonate surfactants increase from 38 to 56 with increasing effective chain length. The fluorescence lifetime and fluorescence depolarization of thiopyronine solubilized in the dialkylbenzenesulfonate micelles are consistent with a low local polarity and high local viscosity for the interior of these micelles, compared to micelles of aliphatic detergents. Investigations using sodium 1 l-(3-hexyl1 -indolyl)undecyl sulfate (6-In-1 1) as a fluorescence probe suggest that the polarity of the interior of the dialkylbenzenesulfonate micelles is similar to that of liquid hydrocarbons. These results suggest a compact structure for the dialkylbenzenesulfonate micelles with low water penetration into the micelles.

W. Ortman

Papers

Photochemical primary process of xanthene dyes. 7. Xanthene dyes as probes for the characterization of anionic micelles