Micellar Liquid Chromatography

The combination of hydrophobic and hydrophilic properties confers some special characteristics on micellar systems in aqueous solution. This has made these systems applicable in different areas of analytical chemistry, highlighting the increasing interest in their use in separation methods. The ability of micel-lar systems to solubilize hydrophobic compounds in aqueous solutions, to improve different analytical methodologies, or to develop new analytical methods due to the possibility of increasing sensitivity or selectivity should be emphasized.

In 1980, Armstrong and Henry showed the possibility of employing solutions of surfactants at a concentration above their c.m.c. as mobile phases for high performance liquid chromatography (HPLC), giving rise to micellar liquid chromatography (MLC). This technique, in which nonpolar chemically bonded stationary phases are generally used, constitutes an interesting alternative to aqueous-organic mobile phases in HPLC since micellar mobile phases are low cost and have low toxicity as compared with aqueous-organic mobile phases.

The great variety of interactions that are possible in MLC separations, for example electrostatic, hydro-phobic and esteric, and the modification of stationary phase by adsorption of monomeric surfactants, make these systems more complicated than conventional reversed-phase HPLC (RP-HPLC) with aqueous-organic mobile phases. Micelles are not static species, but they exist above the c.m.c. in equilibrium with surfactant monomers. In a chromatographic column, surfactant monomers can be adsorbed on the surface of the stationary phase. For most surfactants and stationary phases, the amount of surfactant adsorbed remains constant after equilibrium between mobile and stationary phase is reached. The adsorption of a surfactant on a silica-bonded stationary phase, such as C1, C8 and C18, can occur in two ways:

1. Hydrophobic adsorption. The alkyl hydrophobic chain of the surfactant is adsorbed and the ionic head is in contact with the polar solution, conferring on the stationary phase some ion-exchange properties when charged solutes are separated.

2. Silanophilic adsorption. The ionic head of the surfactant is adsorbed by the stationary phase, thus acquiring a more hydrophobic character.

Figure 1 shows the different equilibria existing in MLC. First, a solute can be partitioned between the aqueous mobile phase and the micellar mobile pseudophase, this equilibrium being controlled by a distribution coefficient PMW. Second, this solute can be in equilibrium between the stationary phase and the micellar pseudophase, which is characterized by a distribution coefficient PSM, and finally, a third

Figure 1 Distribution equilibria of a solute in micellar liquid chromatography. (Reproduced with permission from Marina ML and Garcia MA (1997) Journal of Chromatography A 780: 103-116, copyright Elsevier Science Publishers B.V.)

equilibrium can be established for the solute distribution between stationary and aqueous mobile phases (psw).

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