Micellar liquid chromatography (MLC) has been used in a variety of applications including separation of mixtures of charged and neutral solutes, direct injection of serum and other physiological fluids, analysis of pharmaceutical compounds, separation of enantiomers, analysis of inorganic organometallics, and a host of others.
For example, the micelles are, by nature, spatially heterogeneous with a hydrocarbon, nearly anhydrous core and a highly solvated, polar head group.
Their surrounding environment (pH, ionic strength, buffer ion, presence of a co-solvent, and temperature) has an influence on their size, shape, critical micelle concentration, aggregation number and other properties.
[1] Fischer and Jandera[7] studied the effect of changing the concentration of methanol on CMC values for three commonly used surfactants.
The two forms will be retained by the stationary phase to different extents, thus allowing the retention to be varied by adjusting the concentration of equilibrant (micelles).
[11] The resulting equation solved for capacity factor in terms of partition coefficients is much the same as that of Armstrong and Nome: Where: Foley used the above equation to determine the solute-micelle association constants and free solute retention factors for a variety of solutes with different surfactants and stationary phases.
A review article by Marina and Garcia with 53 references discusses the usefulness of obtaining solute-micelle association constants.
Foley's model applies to many cases and has been experimentally verified for ionic, neutral, polar and nonpolar solutes; anionic, cationic, and non-ionic surfactants, and C8, C¬18, and cyano stationary phases.
[13] Other models proposed by Arunyanart and Cline-Love and Rodgers and Khaledi describe the effect of pH on the retention of weak acids and bases.
[13] One research group, Rukhadze, et al.[14] derived a first order linear relationship describing the influence of micelle and organic concentration, and pH on the selectivity and resolution of seven barbiturates.
The model was successful in predicting the experimental conditions necessary to achieve a separation for compounds which are traditionally difficult to resolve.
The results showed that the model could be applied to MLC, but better predictive behavior was found with concentrations of surfactant below the CMC, sub-micellar.
[15] A final type of model based on molecular properties of a solute is a branch of quantitative structure-activity relationships (QSAR).
In MLC, the stationary phase become modified by the adsorption of surfactant monomers which are structurally similar to the membranous hydrocarbon chains in the biological model.
To enhance efficiency, the most common approaches have been the addition of small amounts of isopropyl alcohol and increase in temperature.
A review by Berthod[19] studied the combined theories presented above and applied the Knox equation to independently determine the cause of the reduced efficiency.
The Knox equation is commonly used in HPLC to describe the different contributions to overall band broadening of a solute.
Raising the column temperature served to both decrease viscosity of the mobile phase and the amount of adsorbed surfactant.
Again an increase in temperature, now coupled with an addition of alcohol to the mobile phase, drastically decreases the amount of the absorbed surfactant.
Further optimization of efficiency can be gained by reducing the flow rate to one closely matched to that derived from the Knox equation.
Overall, the three proposed theories seemed to have contributing effects of the poor efficiency observed, and can be partially countered by the addition of organic modifiers, particularly alcohol, and increasing the column temperature.
[19] Despite the reduced efficiency verses reversed phase HPLC, hundreds of applications have been reported using MLC.
Micelles have an ability to solubilize proteins which enables MLC to be useful in analyzing untreated biological fluids such as plasma, serum, and urine.
[21] Desferrioxamine (DFO) is a commonly used drug for removal of excess iron in patients with chronic and acute levels.
This study found that direct injection of the serum was possible for MLC, verses an ultrafiltration step necessary in HPLC.
The researcher found that, in this case, reverse phase HPLC, was a better, more sensitive technique despite the time savings in direct injection.
[22] MLC mimics, yet enhances, the selectivity offered by ion-pairing reagents for the separation of active ingredients in pharmaceutical drugs.
Hydrophilic drugs are often unretained using conventional HPLC, are retained by MLC due to solubilization into the micelles.
Commonly found drugs in cold medications such as acetaminophen, L-ascorbic acid, phenylpropanolamine HCL, tipepidine hibenzate, and chlorpheniramine maleate have been successfully separated with good peak shape using MLC.
The use of MLC in the future appears to be extremely advantages in the areas of physiological fluids, pharmaceuticals, and even inorganic ions.