Enantioselectivity of structurally modified poly(sodium undecenoyl-L-leucinate) by insertion of Triton X-102 surfactant molecules

The effectiveness of Triton X-102 (TX-102), as a structural modifier of the polymeric surfactant sodium undecanoyl-L-leucinate (L-SUL) was investigated for enhanced enantiomeric recognition of various chiral compounds in micellar electrokinetic chromatography (MEKC). Increasing concentrations of TX-102 were separately added into the micellar solutions of L-SUL and then polymerized to form poly-L-SUL. The resulting polymers were purified by use of 3500 molecular-weight-cutoff (MWCO) dialysis membranes. Fluorescence and pulsed field gradient-nuclear magnetic resonance (PFG-NMR) techniques were used to elucidate the structural effects of TX-102 on poly-L-SUL. Evaluation of data from fluorescence measurements suggested an increase in polarity with increasing concentration of TX-102. However, the polarity decreased at higher concentrations of TX-102. Evaluation of data from PFG-NMR suggested an increase in hydrodynamic radius upon increasing the concentration of TX-102. The racemates of coumarinic and phenythiohydantoin amino acid derivatives, and pindolol were used as test analytes in MEKC. A notable increase in resolution and capacity factors of the test analytes was observed when the modified poly-L-SUL was used in MEKC measurements. Examination of the data obtained from fluorescence, PFG-NMR, and MEKC suggests a strong correlation between the polarity and the hydrodynamic radii of TX-102 modified micelles and the enantiomeric resolution of the test analytes.     

Separation and selectivity in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or Tween 20-modified mixed micelles

The separation and selectivity of eight aromatic compounds ranging from hydrophilic to hydrophobic properties in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or Tween 20-modified mixed micelles were investigated. The effect of different operation conditions such as SDS and Tween 20 modifier surfactant concentration, buffer pH, and applied voltage was studied. The resolution and selectivity of analytes could be markedly affected by changing the SDS micelle concentration or Tween 20 content in the mixed micelles. Applied voltage and pH of running buffers were used mainly to shorten the separation time. Complete separation of eight analytes could be achieved with an appropriate choice of the concentration of SDS micelles or Tween 20-modified mixed micelles. Quicker elution and better precision could be obtained with SDS-Tween 20 mixed micelles than with SDS micelles. The mechanisms that migration order of those analytes was mainly based on their structures and solute-micelle interactions, including hydrophobic, electrostatic, and hydrogen bonding interactions, were discussed.     

On-line sample preconcentration in micellar electrokinetic chromatography by sweeping with anionic-zwitterionic mixed micelles

On-line preconcentration by sweeping in micellar electrokinetic chromatography using mixed micelles of sodium dodecyl sulfate (SDS)-SB-12 is presented. Because of their large micelle radius, they permit increased partitioning of hydrophobic analytes into the core. In addition, they also possess lower negative surface charge relative to pure SDS micelles so anionic analytes can be retained better due to decreased electrostatic repulsion. As the efficiency of sweeping is predicated on the magnitude of retention factors, these advantages translated to better focusing. As much as a 370-fold improvement in detector response, in terms of peak height, was obtained for some neutral steroids, while about a 360-fold improvement was obtained for some phenol derivatives, which were previously not amenable to sweeping by pure SDS micelles.     

Enantioselectivity of alcohol-modified polymeric surfactants in micellar electrokinetic chromatography

A novel method of modifying sodium undecanoyl-L-leucinate (SUL) micelles employed in chiral separation of analytes in micellar electrokinetic chromatography (MEKC) to enhance selectivity toward specific analytes is discussed. The current study aimed at modifying the SUL micelles by introducing different alcohols into the mono-SUL micelles. The micellar solutions were then polymerized in the presence of alcohols followed by postpolymerization extraction of the alcohols to yield alcohol-free polymeric surfactants (poly-L-SUL). The effects of hexanol (C(6)OH) and undecylenyl alcohol (C(11)OH) on micellar properties of this surfactant were investigated by use of surface tensiometry, fluorescence spectroscopy, pulsed field gradient-nuclear magnetic resonance (PFG-NMR), and MEKC. The surface tension and PFG-NMR studies indicated an increase in the critical micelle concentration (cmc) and micellar size upon increasing the alcohol concentration. Fluorescence measurements suggested that alcohols induce closely packed micellar structures. Coumarinic and benzoin derivatives, as well as (+/-)-1, 1'-binaphthyl-2,2'-dihydrogen phosphate (BNP) were used as test analytes for MEKC experiments. Examination of MEKC data showed remarkable resolutions and capacity factors of coumarinic derivatives obtained with modified poly-L-SUL as compared to the unmodified poly-L-SUL. Evaluation of fluorescence, PFG-NMR, and MEKC data suggest a strong correlation between the polarity and hydrodynamic radii of alcohol-modified micelles and the resolution of the test analytes.     

Influence of Additives on Capillary Absorption of Aqueous Solutions into Asymmetric Porous Ceramic Substrate

The effect of divalent and trivalent salts (CaCl(2), CaBr(2), MgCl(2), MgBr(2), LaCl(3), CeCl(3), La(NO(3))(3), and Ce(NO(3))(3)) on the micelle formation in C(8)-lecithin solutions was investigated using the techniques of static and dynamic light scattering. The critical micelle concentration (cmc), mean hydrodynamic radius (R(h)), gyration radius (R(g)), and weight-average molecular weight of the micelles were measured as functions of salt identity and concentration, amphiphile concentration, and temperature. It was found that the micelles in solutions of magnesium are less likely to form and less stable; their standard enthalpy is less exothermic as the ionic strength increases. On the contrary, the micelles in solutions of calcium and trivalent salts form easily, and are more stable; their standard enthalpy is also more exothermic as the ionic strength increases. Based on our model of the Gibb's free energy for the salt-added solutions, we obtained the following formula for the effect of salts on cmc: ln(cmc)'=ln(cmc)+k(1) I(1/2)+k(2)I, where (cmc)' and (cmc) are the critical micelle concentrations in salt-added and salt-free solutions, respectively, I is the ionic strength, and k(1) and k(2) are the salt effect parameters. The agreement between the formula and the experimental data for all the systems under study shows that the formula is more satisfactory than those suggested previously by other authors in describing the effect of salts on the cmc in the micellar solutions of not only zwitterionic but also nonionic surfactants. Copyright 2001 Academic Press.     

Computer Simulations of the Formation of Bile Salt Micelles and Bile Salt/DPPC Mixed Micelles in Aqueous Solutions

Brownian dynamics simulations for a coarse-grained model have been performed to study the formation of micelles from bile salts and mixed micelles with dipalmitoyl-phosphatidylcholine (DPPC) in aqueous solutions. The particular association behavior of bile salts as facial surfactants was shown to be caused by their special molecular architecture with a hydrophilic and a hydrophobic side. The experimentally observed smooth transition into the micellar region with increasing concentration is reproduced. Micelle size distributions have been evaluated at different bile salt concentrations. Typical structures of pure bile salt micelles could be identified. The composition and the structure of mixed micelles have been studied in their dependence on the bile salt/lipid concentration ratio in the aqueous solution. We have found that the bile salt fraction in the mixed micelles increases considerably with increasing bile salt/lipid concentration ratio and decreasing micelle size. The structural and thermodynamic features of micelle formation in the aqueous bile salt solutions with DPPC, which we have studied with the coarse-grained model, are in good qualitative agreement with experimental findings.     

Selective transport of amino acids into the gas phase: driving forces for amino acid solubilization in gas-phase reverse micelles

We report a study on encapsulation of various amino acids into gas-phase sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) reverse micelles, using electrospray ionization guided-ion-beam tandem mass spectrometry. Collision-induced dissociation of mass-selected reverse micellar ions with Xe was performed to probe structures of gas-phase micellar assemblies, identify solute-surfactant interactions, and determine preferential incorporation sites of amino acids. Integration into gas-phase reverse micelles depends upon amino acid hydrophobicity and charge state. For examples, glycine and protonated amino acids (such as protonated tryptophan) are encapsulated within the micellar core via electrostatic interactions; while neutral tryptophan is adsorbed in the surfactant layer. As verified using model polar hydrophobic compounds, the hydrophobic effect and solute-interface hydrogen-bonding do not provide sufficient driving force needed for interfacial solubilization of neutral tryptophan. Neutral tryptophan, with a zwitterionic structure, is intercalated at the micellar interface between surfactant molecules through complementary effects of electrostatic interactions between tryptophan backbone and AOT polar heads, and hydrophobic interactions between tryptophan side chain and AOT alkyl tails. Protonation of tryptophan could significantly improve its incorporation capacity into gas-phase reverse micelles, and displace its incorporation site from the micellar interfacial zone to the core; protonation of glycine, on the other hand, has little effect on its encapsulation capacity. Another interesting observation is that amino acids of different isoelectric points could be selectively encapsulated into, and transported by, reverse micelles from solution to the gas phase, based upon their competition for protonation and subsequent encapsulation within the micellar core.     

Highly efficient separation of isomeric epoxy fatty acids by micellar electrokinetic chromatography

A capillary electrophoresis (CE) method has been developed for simple and direct separation of cis- and trans-12,13-epoxy-9(Z)-octadecenoic acid and 9,10-epoxy-12(Z)-octadecenoic acid isomers. Separation was performed in micellar electrokinetic capillary chromatography (MEKC) using a buffer consisting of 25 mM borate (pH 9.20), 10 mM sodium dodecyl sulfate (SDS) and 10% v/v acetonitrile. The key variables, concentrations of SDS and organic modifier, were optimized by the application of a factorial experimental design. The use of a low micellar concentration, just above critical micelle concentration (CMC), in a background electrolyte containing an organic modifier not only made it possible to dissolve and separate highly hydrophobic fatty acid isomers, but also resulted in improved separation efficiency and selectivity. Separation efficiency up to 4 x 10(5) theoretical plates/m was achieved under an optimized condition. Also investigated were the influence of temperature on separation and the effect of organic modifier concentration on the dynamic exchange of the analytes between micelles and the bulk of the buffer solution. Direct UV was applied for detection of the fatty acids.     

Modification of a micellar system for amino acid separation by MEKC--application for amino acid profiling in formulations for parenteral use

The paper proposes a new method for amino acid determination which can be applied for amino acid profiling in solutions for parenteral nutrition. The MEKC method based on a mixed micellar system was developed for the separation of 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatized amino acids. Background electrolyte was based on tris-borate buffer with high alkaline pH. Sodium dodecyl sulfate micelles were modified using 1,2-hexanediol as a co-surfactant. The effect of the modifier on amino acid migration was studied with respect to hydrophobicity of the analytes. The modifier appeared to be suitable to improve the separation of AQC-tagged amino acids without an adverse effect on buffer ionic strength or EOF velocity. The method was successfully validated and applied for amino acid profiling in medicinal preparations for parenteral nutrition. The results obtained were compared with a reference chromatographic method (amino acid analyser).     

Dynamic light scattering from non-entangled wormlike micellar solutions

The fuzzy cylinder theory, originally proposed for conventional polymer solutions, was applied to wormlike micellar solutions to take into account effects of the intermicellar collision and hydrodynamic interaction on the self-diffusion of wormlike micelles in solution at finite concentrations. Previously reported apparent hydrodynamic radius data obtained by dynamic light scattering for non-entangled wormlike micelles formed in aqueous solution by non-ionic surfactants, polyoxyethylene monoalkyl ethers C(i)E(j), were analyzed by this theory to estimate the persistence length q of the wormlike micelles. The results of q estimated were consistent with those obtained from radius of gyration data obtained by static light scattering.     

Tuning the structure of SDS micelles by substituted anilinium ions

The growth behavior of aggregates formed in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the cationic hydrophobic salts o-toluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) has been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS studies indicate a progressive growth of SDS micelles with addition of less than equimolar concentrations of hydrophobic salts. A prolate ellipsoidal model is used to analyze the DLS data, which is further supported by SANS measurements. We explain the propensity for the strong growth of micelles in the presence of OTHC and MTHC by the high charge neutralization provided by these salts as the aromatic counterions are adsorbed on the surface of the micelles. When the substitution is at the meta position, i.e., for MTHC, micellar growth is favored at lower salt concentrations than for OTHC. The variation in growth behavior is explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. Micellar parameters obtained from SANS data at elevated temperature also support enhanced growth of micelles in the presence of MTHC as compared to OTHC.     

Enantiomeric separation by MEKC using dodecyl thioglycoside surfactants: Importance of an equatorially oriented hydroxy group at C‐2 position in separation of dansylated amino acids

To investigate the influence of stereogenic centers of sugar‐based surfactants for enantiomeric separation, four n‐dodecyl thioglycoside sulfates (CMC 1.5–3.6 mM) were chosen as micelle‐forming surfactants and five dansylated hydrophobic amino acids were used as test analytes. The analytes were mutually separated by these micelles exhibiting almost similar migration times independent of the used surfactant. Baseline separations of all enantiomers were achieved using both β‐D ‐glucose and β‐D ‐galactose derivates that have an equatorially oriented hydroxy group at C‐2 position. In contrast, the ability of enantioseparation was markedly decreased in the case of β‐D ‐mannose and 2‐deoxy‐β‐D ‐glucose derivatives. These results suggested that the structure of C‐2 position of the sugar unit, namely presence of an equatorially oriented hydroxy group, is highly important for the enantiomeric separation of the chosen hydrophobic dansylated amino acids.     

Effects of hydrophobic chain length on the characteristics of the micelles of octaoxyethylene tetradecyl C(14)E(8), hexadecyl C(16)E(8), and octadecyl C(18)E(8) ethers

Size, shape, and flexibility of micelles of octaoxyethylene tetradecyl C(14)E(8), hexadecyl C(16)E(8), and octadecyl C(18)E(8) ethers in dilute aqueous solutions were determined at finite surfactant concentrations c by static light scattering (SLS) and dynamic light scattering experiments at several temperatures T below the critical points. The SLS results were successfully analyzed with the aid of the thermodynamic theory formulated with wormlike spherocylinder model for SLS of micelle solutions. The analysis yielded the molar mass M(w) of the micelles as a function of c and the cross-sectional diameter d. The hydrodynamic radius R(H) and the radius of gyration S(2)(1/2) of the micelles as functions of M(w) were found to be also well-described by the corresponding theories for the wormlike spherocylinder or wormlike chain models. The results of the stiffness parameter lambda(-1) have revealed that the micelles are far from rigid rods but rather stiff compared with typical flexible polymers and they grow in size with increasing T to greater length for longer hydrophobic chains, i.e., alkyl groups of the surfactants. As the alkyl group becomes longer, the d value increased, while the spacings s between adjacent hexaoxyethylene chains on the micellar surface were found to remain substantially constant.     

Online transient micellar phase concentration of anions using CTAB in CE

A transient micellar phase extractor using CTAB was described for the online sample concentration of various anionic analytes (drugs and herbicides) in CE. Stacking and separation was performed at neutral pH in coelectroosmotic flow in a hexadimethrine bromide coated fused‐silica capillary. A micellar plug (e.g. 10 mM CTAB) was injected prior to hydrodynamic injection of the analytes prepared in aqueous organic solvent (e.g. with 30% ACN). In the presence of an electric field, the micelles interacted with the anions inside the capillary. This was followed by selective analyte focusing via the mechanism of micelle to solvent stacking. The micelles acted as transient extractor because the stacking ends when the injected micelles completely migrated through the boundary between the sample and micellar plug. Fundamental studies were performed (effect of surfactant concentration, etc.) and the technique yielded 13‐ to 30‐fold improvements in peak height. A stacking CE method in conjunction with liquid–liquid extraction was also tested for the detection of the herbicides fenoprop and mecoprop in fortified drinking water at analyte concentration levels relevant to Australian Drinking Water Guidelines.     

Micellization and intermicellar interactions in aqueous sodium dodecyl benzene sulfonate solutions

Quasielastic light scattering was used to investigate the size and shape of sodium dodecyl benzene sulfonate (SDBS) micelles in aqueous solutions. Measurements were made as a function of temperature and NaCl and SDBS concentrations. Light scattering data indicate that the micelles are spherocylinders with a semiminor axis of 22 Å. The length of the micelles increases strongly with salt concentration and decreases with temperature. The minimum micelle has an aggregation number of n⁰ = 28 and a hydrodynamic radius of R = 22 Å. Diffusion coefficient and intensity data were analyzed using a thermodynamic theory of micellar aggregation and a model based on DLVO theory of interactions.     

Laser Light Scattering Study on Aggregation of Cellulose Diacetate in Acetone

We have investigated the properties of cellulose diacetate in solution by using laser light scattering. The cellulose diacetate molecules can form micelles and micellar clusters in acetone besides the individual chains. As the concentration increases, the average hydrodynamic radius （Rh） linearly increases, whereas the ratio of gyration radius to hydrodynamic radins 〈Rg〉/〈Rh〉 linearly decreases. It indicates that the micelles associate and form micellar clusters due to the intermolecular interactions.     

动态光散射法研究季铵盐Gemini表面活性剂的胶团化行为

用动态光散射技术在10～70℃温度范围内,通过测定胶团的平均流体力学半径随温度、盐浓度和联接基团长度的变化情况,研究联接基团为聚亚甲基链的阳离子季铵盐Gemini表面活性剂胶团在无机盐介质中的长大规律.实验结果表明,增加盐量、降低温度和减小联接基团的长度均使平均流体力学半径变大,Gemini表面活性剂胶团长大时是由球状转变为棒状.从实验测定的平流体力学半径求算了支配球-棒转变的平衡常数及热力学函数值,并用NNLS(non-negatively constrainedleast square)算法对胶团的粒径分布情况进行了分析.此外,对具有短联接基团的Gemini表面活性剂胶团长大所具有的独特性质进行了讨论.     

Determination of amino acids by micellar EKC: recent advances in method development and novel applications to different matrices

The extensive use of CE for the analysis of amino acids has been well documented in a series of research articles and reviews. Aim of this report is to address the attention of the reader on the recent advances of micellar electrokinetic chromatography for the separation and determination of these analytes. Enhancements in selectivity of this technique through the use of pseudostationary phases containing mixed micelles, polymers, and chiral selectors are presented. Selected applications concerning separation and quantitation of even minute amounts of amino acids in: (i) biological fluids; (ii) microdialysates; (iii) plant cells; (iv) food stuff; and (v) pharmaceutical formulations have also been covered. Advantages of MEKC over other techniques for the amino acid analysis have been underlined.     

Morphological characterization of self-assembled peptide nucleic acid amphiphiles

Peptide nucleic acid amphiphiles (PNAA) are a promising set of materials for sequence-specific separation of nucleic acids from complex mixtures. To implement PNAA in micellar separations, the morphology and size of PNAA micelles in the presence and absence of a sodium dodecyl sulfate (SDS) cosurfactant have been studied by small-angle X-ray scattering and dynamic light scattering. We find that a 6-mer PNAA with a 12-carbon n-alkane tail forms ellipsoidal micelles (a = 5.15 nm; b = 3.20 nm) above its critical micelle concentration (CMC) of 110.9 microM. On addition of a stoichiometric amount of complementary DNA, PNAA hybridizes to DNA, suppressing the formation of PNAA micelles. At a ratio of 19:1 SDS/PNAA (total concentration = 20 mM), spherical micelles are formed with outer radius Rs = 2.67 nm, slightly larger than spherical micelles of pure SDS. Capillary electrophoresis studies show that PNAA/DNA duplexes do not comicellize with SDS micelles. No such effects are observed using noncomplementary DNA. The shape and size of the PNAA micelles is also verified by dynamic light scattering (DLS) studies. These results provide an interesting case study with competing electrostatic, hydrophobic, and hydrogen-bonding interactions in micellar systems and make possible the use of PNAA in micellar separations of DNA oligomers.     

Specific alkali cation effects in the transition from micelles to vesicles through salt addition

The transition of ionic micelles to vesicles with added salts is explored in this paper. The catanionic surfactant solution was comprised of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with an excess of SDS. The micellar size increased with concentration for all salts. No anion specificity was found, probably because of the excess of SDS. However, when the cation of the added salt was varied, large differences were observed in the hydrodynamic radii of the aggregates. A classification of the cations according to their ability to increase the measured hydrodynamic radii follows a Hofmeister series. The change in aggregate size can be explained by modified counterion binding and dehydration of the surfactant headgroups.