Critical micelle concentration of surfactants in aqueous buffered and unbuffered systems

Sodium maleopimaric acid as pseudostationary phase for chiral separations of amino acid derivatives by capillary micellar electrokinetic chromatography

A new type of chiral surfactant, sodium maleopimaric acid (SMA), was synthesized, and employed for the enantioselective micellar electrokinetic chromatographic (MEKC) separation of amino acid enantiomers derivatized with naphthalene-2,3-dicarboxaldehyde (NDA-D/L-AAs). The effect of the surfactant concentration, type and concentration of the BGE, and buffer pH on the resolution was studied, and optimized conditions were used to evaluate the ability of this new surfactant to perform chiral separations toward NDA-D/L-AAs by MEKC. Enantiomeric separations of NDA-D/L-AAs were achieved with a running buffer consisting of 100 mM borate (pH 9.5) and 20 mM SMA in a 58.5 cm length x 50 microm id capillary. Under the conditions selected, two pairs of tested amino acid enantiomers including NDA-D/L-trptophan (Trp) and NDA-D/L-kynurenine (Kyn) were resolved.

Correlation between CMC and chromatographic index: simple and effective evaluation of the hydrophobic/hydrophilic balance of bile acids

The discovery that bile acids are involved in the modulation of nuclear steroid receptors has prompted renewed interest in this field of research. Due to the nature of research in this field, a technique that enables simple and effective assessment of the hydrophobic/hydrophilic balance, thus improving and speeding up evaluations of the biological profiles of these compounds, is greatly needed. In this context, both CMC value determination and RP-HPLC mobility evaluation were explored as possible approaches. While the CMC was calculated using the noninvasive conductimetric method, the RP-HPLC mobility was assessed by measuring the retention factor at several mobile phase compositions and extrapolating back to the pure aqueous mobile phase. The correlation of the CMC with the derived chromatographic hydrophobic index phi0 was satisfactory.

MEKC: An update focusing on practical aspects

This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensitivity arising from the use of on-line sample concentration (sweeping, stacking, and combination of both protocols) and derivatization (in-capillary reactions and coupling with flow-injection systems) and improvements in resolution obtained by changing the composition of the BGE (e.g., with organic modifiers, ionic liquids, nonionic and zwitterionic surfactants, mixed micelles, and vesicles) or using coated capillaries are discussed in detail. In addition, MS and LIF spectroscopy are examined in relation to their advantages and restrictions as applied to MEKC analysis. Some thoughts on potential future directions are also expressed.

Comparison of migration models for acidic solutes in micellar electrokinetic chromatography

The validity of two models that explain the migration of ionisable solutes in micellar electrokinetic chromatography (MEKC), mobility model and retention factor model, has been tested. For this purpose, the mobility (mu) and retention factor (k) of a set of 10 phenolic compounds with different hydrophobicity and pKa values have been determined for several sodium dodecyl sulphate (SDS) concentrations and pH values, and fitted to the models. Results show that in general the retention factor model explains better the retention of ionisable solutes, although for hydrophilic compounds at low SDS concentration, mobility model can give better fits. The different drawbacks pointed out by several authors in relation to both models have been checked, and a deep evaluation of each one has been done. As a result we have observed that, while in the retention factor model the variation of k with pH and [SDS] always follows the same trend, the variation of mu with these variables mainly depends on the value of the binding constant of the neutral form of the solutes to the micelles, KHA(m), which plays a critical role in the fit of the mobility model. Also we provide rules and advices to set up the experimental conditions to apply each model to a particular solute.