Micellar selectivity triangle for classification of chemical selectivity in electrokinetic chromatography

A novel micellar selectivity triangle (MST) is developed and used to characterize and classify the chemical selectivities of pseudo-phases in electrokinetic chromatography (EKC). The MST scheme is, in concept, similar to the widely known solvent selectivity triangle (SST) originally developed by Snyder. However, the MST is based on linear solvation energy relationships. Thus it incorporates the solvation characteristics of both the pseudo-phase and the bulk solvent; while the SST is basically for classification of pure solvents. The similarities and differences of these pseudo-phases are determined by the relative scales of hydrogen bond donating ability (X_b), hydrogen bond accepting ability (X_a) and dipolarity (X_s). The MST scheme is used for characterization and classification of a wide range of pseudo-phases such as micelles, polymers, vesicles, liposomes, as well as mixed systems such as mixed micelles, mixed polymer–surfactants, organically modified pseudo-phases, etc. Over seventy pseudo-phases were examined and four clusters of pseudo-phases with different selectivity patterns are recognized that include pseudo-phases with strong hydrogen bond acidities (e.g. fluorinated micelles or micelles modified with fluorinated alcohols), strong hydrogen bond acceptor pseudo-phases (such as bile salts, liposomes, microemulsions, as well as biphasic octanol–water system), strong dipolar phase of a class of polymeric pseudo-phase, and pseudo-phases with intermediate hydrogen bonding and dipolarity [like sodium dodecyl sulfate (SDS) and its analogs as well as organically modified SDS]. The MST scheme is also useful in identifying pseudo-phases that closely resemble the selectivities of octanol–water for determination of octanol–water partition coefficients by EKC.     

Utilization of solid-phase microextraction–high-performance liquid chromatography in the determination of aromatic analyte partitioning to imidazolium-based ionic liquid micelles

A solid-phase microextraction (SPME)–high-performance liquid chromatography (HPLC) approach is used, for the first time, to study the partitioning behavior of eight aromatic analytes to three imidazolium-based ionic liquid micelles, namely, 1-hexadecyl-3-methylimidazolium bromide (HDMIm-Br), 1-hexadecyl-3-butylimidazolium bromide (HDBIm-Br), and 1,3-didodecylimidazolium bromide (DDDDIm-Br). The model used to calculate the partition coefficients is improved by determining the accurate critical micelle concentration (CMC) value of the studied IL-micelles, which considers the nature and amount of organic modifier used in the experiments. Proper CMC values in the model improve the quality of the results and decrease the differences between theoretical and experimental intercepts. Surface tensiometry has been utilized to determine the CMC values for the micelles at different acetonitrile contents (1% and 1.5%, v/v). The calculated partition coefficient values for polycyclic aromatic hydrocarbons (PAHs) oscillate between 631 and 5980, whereas aromatic analytes with a lower number of fused rings in their structures suffer non-partitioning to any of the IL-micelles. The obtained partition coefficients to IL-micelles were highest with the DDDDIm-Br IL and were always higher than those obtained with the traditional surfactant cetyltrimethyl ammonium bromide (CTAB).     

Enhanced Stability of Electrochemical Detection with Surfactant Containing Mobile Phases in Liquid Chromatography and Flow Injection Analysis

Abstract

The use of surfactant containing mobile phases to prevent or reduce the effects of adsorptive fouling of glassy carbon electrodes is reported. Both cationie and antonic surfactants are studied at concentrations above and below the critical micelle concentration. For the oxidative reactions studied here, anionic surfactants have little effect on the fouling problem, likely because of electrostatic attraction of the generated cattonic intermediate to surfactant adsorbed on the electrode surface. Cationic surfactants, however, have the desired effect. Two cationic surfactants, cetyltrimethylammonium chloride and n-decylamine were studied with solutes p-nitrophenol, phenylenediamine and chlorpromazine. With these surfactants present in the mobile phase there was generally no loss of electrochemical response after up to 55 sequential injections. Adsorption of the electroactive specie prior to the electron-transfer process is shown to be a significant cause of poor chromatographic efficiency for some solutes.     

孔雀绿与CTAB胶束的相互作用

以CTAB胶束模拟生命体系,用UV-Vis和荧光光谱等技术研究了孔雀绿与CTAB胶束的相互作用.结果表明,孔雀绿自发地定位于CTAB胶束栅栏层,使得胶束聚集数增加,胶束的微环境极性I1/I3下降.孔雀绿与CTAB胶束之间的结合常数K和孔雀绿在胶束相与水连续相之间的分配系数KD均随孔雀绿浓度增加而降低,表明孔雀绿与CTAB胶束之间的相互作用随孔雀绿浓度增加而降低.     

Direct preparation of core cross-linked micelles in a nonselective solvent

This is the first light scattering study demonstrating that the size of micelles, the aggregation number, and the mobility of the core blocks of the micelles could be controlled by the length of the cross-linker in the micellar cores. The core cross-linked micelles were prepared using a poly[(4-pyridinemethoxy-methyl)styrene]-block-polystyrene (PPySt-b-PSt) diblock copolymer and perfluoroalkyl dicarboxylic acid. The PPySt-b-PSt copolymer formed the micelles in THF, a nonselective solvent, in the presence of the perfluoroalkyl dicarboxylic acid. The light scattering studies demonstrated that the micellar size and aggregation number were dependent on the chain length of the perfluoroalkyl dicarboxylic acid. Perfluoroazelaic acid produced micelles with a larger hydrodynamic radius and higher aggregation number than tetrafluorosuccinic acid. The micellization proceeded through the formation of the pyridinium carboxylate and the cross-linkage between the PPySt blocks via the dicarboxylic acid. The core cross-linked micelles were thermally stable and maintained its structure with changes in the temperature. A ¹H NMR analysis revealed that the micelles prepared by perfluoroazelaic acid had more mobility of the core blocks than those by tetrafluorosuccinic acid.     

Micellization of polystyrene-graft-poly(ethylene oxide) and its mixtures with polystyrene homopolymer in ethanol

Micellization behaviors of polystyrene-graft-poly(ethylene oxide) (PS-graft-PEO) and its mixtures with PS homopolymer in ethanol were investigated by means of nuclear magnetic resonance (¹H NMR) spectroscopy, transmission electron microscopy (TEM), and viscometry. It was revealed that PS-graft-PEO could self-assemble to form polymeric micelles with a core-shell structure in the shape of spherical. Micelle formation of PS-graft-PEO in ethanol was strongly dependent on the concentration, temperature, and the PS chain contents in PS-graft-PEO. The introduction of PS homopolymer not only decreases the critical micelle concentration, but also changes the morphology of the micelles.     

Attractive interactions in critical scattering from non-ionic micelles

We have studied the temperature-dependent critical scattering of both light and neutrons from aqueous solutions ofn-octyl pentaoxyethylene glycol monoether (C₈E₅). We show that the assumption of a short-ranged temperature-dependent attractive pair potential between approximately spherical micelles of constant size permits a quantitative analysis of the neutron scattering data. The analysis, which is undertaken using current liquid-state theory and is in analytic form, contains only one free parameter, the depth of the attractive potential. We find that a potential with a range of only a fraction of a nm is sufficient to generate spatial correlations over tens of nm as the attractive potential deepens on approaching the critical temperature. The analysis also provides a semi-quantitative understanding of the light scattering data as a function of concentration and temperature, and leads to a qualitative prediction of the form of the phase diagram. Numerical values obtained are consistent with the hypothesis that the primary effect of raising the temperature is to lower the degree of structure of water near the micelle surface, allowing increased van der Waals attraction due to closer contact.     

The aggregation ofn-dodecanephosphonic acid in water

Several techniques were employed to study the aggregation ofn-Dodecanephosphonic Acid (DPA) in water. At 22°C, the solubillity of DPA increases, probably due to the formation of small premicellar aggregates. The CMC is (5.4±2.4) ×10^–4 mol·dm^–3 and the solubility reaches the CMC value at 26°C. At 30°C and at a concentration of about 9×10^–3 mol·dm^–3, a lamellar mesophase appears. Both micelles and liquid crystal lamellae are almost uncharged. Their polar heads have strong hydrogen bonds between them. The ionization of DPA molecules in micelles and mesophase structures is strongly reduced in comparison with monomerically dissolved molecules.     

Triton X-100在乙二醇中的胶体性质

表面活性剂在非水溶剂中聚集态的研究所见的报导与水体系相比要少得多．然而近年来对非水体系的研究日益增多．这是因为表面活性剂在非水溶剂中的应用日趋增加，但与水体系相反，对非水体系的性质了解甚少．对表面活性剂非水体系的研究还有助于对其聚集体的共溶剂效应有更深的认识．TritonX－100（以下简写为TX－100）是典型的非离子表面活性剂，在工业上有着广泛的应用．乙二醇在多醇中与水的化学结构最相近问，然而它却是水结构的破坏剂，加入少量的乙二醇会使水的冰点急剧下降．乙二醇又是蛋白质的变性溶剂．表面活性剂在乙二醇中的胶…