Micelle dynamic chromatography

In order to separate neutral compounds, Terabe et al. [13] added surfactants to the buffer electrolyte. Above their critical micellar concentration (cmc), these surfactants form micelles in the aqueous solution of the buffer electrolyte. The technique is then called Micellar electrokinetic capillary chromatography, abbreviated as MECC or MEKC. Micelles are dynamic structures consisting of aggregates of surfactant molecules. They are highly hydrophobic in their inner structure and hydrophilic at the outer part. The micelles are usually... 

Vesicles differ considerably from micelles in several ways. Micelles are very dynamic species, so that both monomeric surfactant and solubilized materials are constantly exchanging between the aqueous and micellar pseudophases. However, vesicles are relatively long-lived species, and can be separated from other materials by techniques such as gel-permeation chromatography, and they can be isolated and the structures determined by electron microscopy (159, 160]. In addition the bilayer has an inside and an outside surface, so that the transport of solutes across the bilayer is of considerable interest. 

Tesaf ova et al. [87] used a modified version of SIMUL, which they called SIMULMIC to simulate the separation of neutral analytes in a system with a neutral cyclodextrin and anionic micelles. A number of systems were examined in which various combinations of the inlet and outlet vials and the capillary itself were filled with cyclodextrin. Simulation results were used to examine the micellar/cyclodextrin boundary at various times and concentrations although no simulation results for a chiral separation were reported. To the best of our knowledge, this is the only dynamic simulation of an electrokinetic chromatography (EKC) separation to date. 

The exclusion chromatography of surfactant solutions is a problematic method for analyses, since the micelles are only stable when in equilibrium with monomers of concentration equal to the cmc. The monomer-micelle equilibrium thus can be subjected to chromatographic investigations, under various conditions, e.g. dynamic or equilibrium. In the former case, if one applies a micellar solution with concentration greater than cmc, the surfactant front would move down the column at a speed relative to the size of micelles. However, if the dilution factor in the column is too large, such that only monomers are present, then the front moves at the same speed as the monomer. In these systems the eluant is supposed to be pure solvent. However, if concentration of the surfactant in the eluant is close to the cmc, then the micellar solution moves down the column with little perturbation of the monomer-micelle equilibrium. The surfactant front under these conditions then moves... 

This equilibrium will be perturbated in the chromatography of monomer-micelle systems because of dilution by the eluant (solvent). This process we shall designate as the "dynamic" analysis of micellar solutions by exclusion chromatography. 

If a surfactant solution with concentration >cmc is applied to the chromatography column in an eluant as pure solvent (without any added surfactant), the monomer-micelle equilibrium is driven towards monomer due to dilution during the chromatography process. This system under dynamic conditions has been investigated by various investigators [45,48,52]. 

The dynamic exclusion chromatography of the monomer-micelle equilibria clearly indicates that the dilution factor in the column determines the degree of the shift of this equilibria towards left. 

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