Surfactant pseudostationary phases

Regarding other pseudostationary phases for measurement of lipophilicity or lipophilicity-related properties (e.g., intestinal absorption, brain penetration), there are several reports on the use of vesicles such as phospholipid bilayer liposome (56-58), lysophospholipid micelle (59), DTAB/SDS vesicle (60), and double-chain synthetic surfactant vesicle (61), which are described in other chapters. 

Electrokinetic chromatography (EKC) using microemulsion is one of the most powerful tools for the rapid measurement of log P w with high reproducibility. Because it is relatively easy to manipulate the pseudostationary phases of EKC, a lot of phases have been reported for the measurement not only of physicochemical properties but also of the separation selectivity, such as polymer micelles (64) and double-chain surfactant vesicles (56-58,60,61). These phases are also interesting in terms of the correlation to bioactivity. 

S Terabe, H Ozaki, Y Tanaka. New pseudostationary phases for electrokinetic chromatography A high-molecular surfactant and proteins. J Chin Chem Soc 41 251-257, 1994. 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

More recently, Priego-Capote et al. reported on the production of MIP nanoparticles with monoclonal behaviour by miniemulsion polymerisation [63]. In the synthetic method that they employed, they devised to use a polymerisable surfactant that was also able to act as a functional monomer by interacting with the template (Fig. 4). The crosslinker content was optimised at 81% mol/mol (higher or lower contents leading to unstable emulsions). In this way, the authors were able not only to produce rather small particles (80-120 nm in the dry state) but also to locate the imprinted sites on the outer particle surface. The resulting MIP nanobeads were very effective as pseudostationary phases in the analysis of (/ ,S)-propranolol by CEC. 

The micellar pseudostationary phase is produced by adding a surfactant to the buffer at concentrations that exceed its critical micelle concentration... 

Typical polymeric pseudostationary phases include micelle polymers, polymeric surfactants, water-soluble anionic siloxanes and dendrimers [223-231]. Micelle polymers [e.g. poly(sodium 10-undecylenate), poly (sodium 10-undecenylsulfate), poly(sodium undeconylvalinate), etc.] are synthesized from polymerizable surfactant monomers at a concentration above their critical micelle concentration. These polymers have similar structures to micelles without the dynamic nature of the micelle structure. Polymeric surfactants are polymers with surfactant properties [e.g. acrylate copolymers, such as 2-acrylamide-2-methyl-l-propanesulfonic acid and alkyl methacrylamide, alkyl methacrylate or alkyl acrylate, poly (ally lamine)-supported phases, poly(ethyleneimine), etc]. Water-soluble anionic siloxane polymers are copolymers of alkylmethylsiloxane... 

Polymeric micelles form stable pseudostationary phases with a critical micelle concentration of virtually zero (aggregation number of 1), and are tolerant of high organic solvent concentrations in the electrolyte solution. Mass transfer kinetics are slow compared with conventional surfactant micelles, and peak distortion from mass overloading is a problem for some polymer compositions. Preliminary studies indicate that polymeric surfactants are effective pseudostationary phases in micellar electrokinetic chromatography, but only a limited number of practical applications have been demonstrated, and uptake has been slow. 

Minimizing the temperature effects discussed above could be obtained with the use of polymer micelles or polymer surfactants [81-83], whose CMC is zero, and even in nonaqueous solvent, the micelle is stable. Although several polymer surfactants are commercially available, no such surfactant is widely accepted, probably because SDS, CTAB, or CTAC, and bile salts are superior to polymer surfactants as the pseudostationary phase in MEKC. Although microemulsion electrokinetic chromatography (MEEKC) is not discussed in this chapter but covered in Chapter 4 by Altria and colleagues, a similar optimization strategy to that in MEKC applies to MEEKC [84-86]. Since... 

Tickle D.C., Okafo GN., CamiUeri R, Jones R.F.D., Kirby J., Glucopyranoside-based surfactants as pseudostationary phases for chiral separations in capillary electrophoresis. Anal. Chem., 66, 4121-4126 (1994). 

S.H. Edwards and SA. Shamsi, Micellar electrokinetic chromatography of polychlorinated biphenyl congeners using apolymeric surfactant as the pseudostationary phase, J. Chromatogr. A, 903,227-236,... 

Chiral surfactants allow for the separation of enantiomers by MEKC. ° The separation of neutral compounds is dependent on the partitioning coefficient of the analyte into the micelle. The elution order for neutral compounds is often similar to reverse-phase chromatography, again, due to the fact that in the run buffer, the micelle acts like a pseudostationary phase. 

In CZE, only ionic or charged analytes can be separated in principle since its separation mechanism is based on the difference in electrophoretic mobility of the analytes. MEKC has become popular as a powerful technique for the separation not only of neutral analytes but also of charged ones using a conventional CE instrument without any alteration. In MEKC, an ionic surfactant micelle is used as a pseudostationary phase (PS) that corresponds to the stationary phase in conventional chromatography and the surrounding aqueous phase to the mobile phase. The separation principle of MEKC is based on differential partitioning of analytes between the aqueous phase and the micelle phase. 

Anionic alkylglucoside chiral surfactants, such as dodecyl (3-D-glucopyranoside monophosphate and monosulfate, and sodium hexadecyl D-glucopyranoside 6-hydrogen sulfate, were used as chiral pseudostationary phases in MEKC, where several enantiomers (e.g., PTH-dl-AAs and binaphthol) were resolved. 

With respect to chromatographic techniques, cationic surfactant micelles have been used as mobile phase additives in the well-known micellar liquid chromatography (MLC) [4]. They have also been employed as pseudostationary phases in micellar electrokinetic capillary chromatography (MEKC) [5]. 

Among all the variations of capillary electrophoresis (CE), MEKC is the most suited for the separation of PAHs because of its ability to separate uncharged compounds. The separation by MEKC is based on the partition of analytes between aqueous buffer and charged pseudostationary micellar phase [51]. The separation conditions involve the use of a high-pH electrolyte containing relatively high levels of surfactant such as sodium dodecylsulfate (SDS). The attributes of MEKC can be compared with the separating ability and rimtime obtained in HPLC. It is based on differential movement of analytes into micelles while they move within an electric field. There have been a number of separations of PAHs by MEKC [51,52]. 

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