Mechanism of Electrokinetic Separations of Hydrophobic Compounds with Sodium Dodecyl Sulfate in Acetonitrile−Water Mixtures

Abstract

The effect of the addition of 0−70% (v/v) acetonitrile (AcCN) as organic modifier, into a buffer solution containing 0−60 mmol L-1 sodium dodecyl sulfate (SDS), on the electrokinetic behavior of neutral hydrophobic compounds has been studied experimentally. The effective mobilities of the hydrophobic compounds sharply decreased above a certain AcCN concentration, depending to some extent on the SDS concentration. In solutions with up to 40% AcCN, with SDS concentrations below the literature values of the critical micelle concentration, neutral hydrophobic analytes still showed a considerable electrophoretic mobility. The observed mobilities were too high to be attributed to solvophobic interaction of the analytes with single SDS ions. From conductivity measurements, a strong indication was found that increasing the fraction of AcCN in the SDS solution does not lead to a complete disintegration of the micelles. Even with high AcCN concentrations, the mechanism of the separation was found to be a distribution between the aqueous phase and micelle-like SDS aggregates. The influence of the addition of AcCN on the mobilities could be explained as the combined effect of a decrease of the micellar volume fraction, a change of the micellar properties, and a decrease of the distribution constants of the analytes.

Separation of atropisomeric polychlorinated biphenyls by cyclodextrin modified micellar electrokinetic chromatography

“…The has similar velocity ep within the range 0᎐20% acetonitrile, then decreases sharply at 30% acetonitrile. This phenomenon agrees w x with the result given by Seifar et al 28 . They investigated the separation mechanism of hydrophobic compounds by MEKC in acetonitrile ᎐water mixture.…”

Investigation of solvent effects in capillary electrophoresis for the separation of biological porphyrin methyl esters

“…According to literature data, SDS micelles (with SDS concentration up to 60 mM) are believed to be completely disintegrated in 30% ACN, and only individual SDS ions are present in the CE running buffer solution [15][16][17]. Interestingly, the sharp break in migration time when the ACN content was increased from 20% ( Fig.…”

“…However, the question as to whether or not the SDS micelles "completely" break up into individual SDS surfactant monomers is debatable. Note that a number of research groups [17][18][19] have pointed out that SDS micelles and/or SDS micelle-like aggregates could exist at very high ACN concentrations. For example, Kok et al [17] have stated that in mixed hydroorganic solvents with up to 40% ACN, the MEKC separation of a number of hydrophobic antioxidants and alkyl aryl phenones was mainly due to their differential interaction with SDS micelles and/or SDS micelle-like aggregates (rather than free SDS monomers), even with ACN content as high as 40%.…”

“…2D), was relatively strong (i.e., probably in the form of SDS micelles and/or SDS micellelike aggregates) [17,18]. In other words, when the concentration of ACN was lower than 40%, the interaction that might occur between various porphyrin methyl esters and individual SDS surfactant ions (i.e., the so-called solvophobic interaction of which the strength of interaction is relatively weak in nature [17,18]), may only play a minor role, if any, in the observed separation performances, especially for data shown in Figs. 2B-D.…”

Sensitive capillary electrophoresis microchip determination of trinitroaromatic explosives in nonaqueous electrolyte following solid phase extraction

“…At these concentrations, especially in nonaqueous media, the formation of micelles is unlikely, and the separations may be attributed to solvophobic interactions between the analytes and individual surfactant ions [26][27][28].…”