Selectivity In Micellar Electrokinetic Chromatography Research Articles

Glycine-based polymeric surfactants with varied polar head group: II. Chemical selectivity in micellar electrokinetic chromatography using linear solvation energy relationships

A series of four acyl and four alkenoxy glycinates (i.e., mono-, di-, tri-, and tetraderivatives of polysodium N-undecenoyl glycinate (poly-SUGs) as well as polysodium N-undecenoxy carbonyl glycinates (poly-SUCGs)) were compared for simultaneous separation of nonhydrogen bonding (NHB), hydrogen-bond acceptor (HBA), and hydrogen-bond donor (HBD) solutes. An increase in the number of glycine units in the polar head group of polymeric surfactant decreases both the retention and the migration window of all solutes with some changes in separation selectivity. The poly(sodium N-undecenoxy carbonyl-glycinate) (poly-SUCG1) with one glycine unit was the least polar surfactant and has the lowest phase ratio, but this monoglycinate surfactant provided the best simultaneous separation of 10-NHBs and 8-HBAs. On the other hand, 9-HBDs were well separated using any of the six mono-, di-, and triglycinate surfactants compared to the two tetraglycinates. Linear solvation energy relationships (LSERs) and separation of the geometrical isomers studies were also performed to further envisage the selectivity differences. From LSER studies, the phase ratio and hydrogen-bond-donating strength of the poly-SUG series of surfactant were found to increase with an increase in the size of the head group, but no clear trends were observed for poly-SUCG surfactants. The cohesiveness for all poly-SUG and poly-SUCG was positive, but the values were generally lower (with exception of the poly(sodium N-undecenoyl glycyl-glycyl-glycinate)) at a higher number of glycine units. Finally, the poly(sodium N-undecenoyl glycinate) and poly-SUCG1 were found to be the two best polymeric surfactants as they provided relatively higher shape selectivity for separation of two of the three sets of geometrical isomers.

Monomeric and polymeric anionic gemini surfactants and mixed surfactant systems in micellar electrokinetic chromatography. Part II: Characterization of chemical selectivity using two linear solvation energy relationship models

Sodium di(undecenyl) tartarate monomer (SDUT), a vesicle-forming amphiphilic compound possessing two hydrophilic carboxylate headgroups and two hydrophobic undecenyl chains, was prepared and polymerized to form a polymeric vesicle (i.e., poly-SDUT). The anionic surfactants of SDUT and poly-SDUT (carboxylate head group) and sodium dodecyl sulfate, SDS (sulfate head groups) as well as mixed surfactant systems (SDS/SDUT, SDS/poly-SDUT, and SDUT/poly-SDUT) were applied as pseudostationary phases in micellar electrokinetic chromatography (MEKC). Two linear solvation energy relationship (LSER) models, i.e., solvatochromic and solvation parameter models, were successfully applied to investigate the effect of the type and composition of pseudostationary phases on the retention mechanism and selectivity in MEKC. The solvatochromic and solvation parameter models were used to help understand the fundamental nature of the solute-pseudostationary phase interactions and to characterize the properties of the pseudostationary phases (e.g., solute size and hydrogen bond-accepting ability for all pseudostationary phases). The solute types were found to have a significant effect on the LSER system coefficients and on the predicted retention factors. Although both LSER models provide the same information, the solvation parameter model is found to provide much better results both statistically and chemically than the solvatochromic model.

Polymeric sulfated surfactants with varied hydrocarbon tail: II. Chemical selectivity in micellar electrokinetic chromatography using linear solvation energy relationships study

The effect of hydrocarbon chain length on chemical selectivity in micellar electrokinetic chromatography (MEKC) was investigated using polymeric sulfated surfactants: poly-(sodium 7-octenyl sulfate), poly(sodium 8-nonenyl sulfate), poly(sodium 9-decenyl sulfate), and poly(sodium 10-undecenyl sulfate). Linear solvation energy relationships (LSERs) and free energy of transfer studies were conducted to predict the selectivity differences between the four polymeric surfactants. The overall nature of the solute/ polymeric micelle interactions was found to be different despite the fact that all polymeric surfactants have the same head group. The polar character and acidic strength of the polymeric surfactant are found to decrease as the hydrocarbon chain length of the surfactant is increased. On the other hand, the polarizability of the polymeric sulfated surfactants increases (upon interacting with solute lone-pair electrons) with increasing hydrocarbon chain length. The LSER results show that the solute size and hydrogen bond accepting ability play the key roles in MEKC retention.

Separation selectivity in micellar electrokinetic chromatography using different anionic surfactants as pseudo-stationary phases

Micellar electrokinetic chromatography (MEKC) and ligand exchange-MEKC have been widely used for the separation of achiral and chiral compounds. For the separation of different compounds, one expects to obtain the desired different separation selectivity by choosing different surfactants as pseudo-stationary phases. This paper describes a comparative study on the separation selectivity in MEKC and LE-MEKC using different anionic surfactants such as sodium dodecyl sulfate (SDS), sodium alkyl polyoxyethylene sulfate (AES-Na) and sodium α-sulfonated fatty acid methyl esters (α-SF-Na). Different selectivities were observed with these anionic surfactants. For the separation of aromatic derivatives by MEKC, better selectivity was obtained by using AES-Na than SDS micellar phases. For the simultaneous separation, of positional enantiomers ofo-, m- andp-fluoro-DL-phenylalanine by LE-MEKC, the SDS micellar phase provided the best selectivity among these anionic surfactants. Furthermore, the critical micelle concentrations (CMC) of these surfactants were also estimated by LE-MEKC.

Characterization of chemical selectivity in micellar electrokinetic chromatography: VI. Effects of surfactant counter-ion

Linear solvation energy relationships and free energy of transfer data were used to evaluate the influence of the surfactant counter-ion on selectivity in micellar electrokinetic chromatography. It was determined that selectivity differences are dependent on the valency of the counter-ion but not the type of counter-ion. Monovalent surfactants, sodium dodecyl sulfate (SDS) and lithium dodecyl sulfate, have nearly identical selectivity behavior. The divalent surfactants, magnesium didodecyl sulfate and copper didodecyl sulfate also show very similar behavior. However, when the divalent counter-ion species is compared to SDS under similar conditions, significant differences are observed. Most notably, the utilization of divalent counter-ion species of dodecyl sulfate surfactants causes the micelles to become more hydrophobic and a weaker hydrogen bond donating pseudo-stationary phases. It is believed that the divalent counter-ions reduce the electrostatic repulsion between the surfactant head groups and therefore, increase the chain packing of the monomers in the micelle aggregates. This reduces the degree of hydration of the micellar palisade layer leading to a decreased ability of the micelle to participate in polar/polarizable and hydrogen bonding interactions with solute molecules.

Influence of ester and amide-containing surfactant headgroups on selectivity in micellar electrokinetic chromatography.

The selectivity differences between six anionic surfactants in micellar electrokinetic chromatography (MEKC) are presented and the structural influence of the surfactant head-group is investigated. It was determined that the surfactant structure can have a significant impact on retention and selectivity. Linear solvation energy relationships (LSERs) are used to study the role of solute size, polarity/polarizability, and hydrogen bonding characteristics in determining retention and selectivity. While both the solute size and hydrogen bond accepting ability were found to be the most important factors in solute retention, the hydrogen bonding characteristic of the solutes have the largest influence on selectivity differences between surfactants.

Statistical evaluation of linear solvation energy relationship models used to characterize chemical selectivity in micellar electrokinetic chromatography

Characterization of retention and selectivity differences between surfactants in micellar electrokinetic chromatography (MEKC) using linear solvation energy relationships (LSERs) has been given a significant amount of attention in the last four years. This report evaluates the validity of using the two LSER models that have been used to fit retention in MEKC in the literature. The results and the fit of the revised model and parameters developed by Abraham and coworkers are compared to the original model developed by Kamlet, Taft, and coworkers. LSERs can generally only be used as a comparative tool to describe the selectivity differences between surfactant systems used in MEKC. With this in mind, it was determined that the results of both models essentially provide the same information about these differences. However, the revised model and parameters have been found to yield a statistically better fit of the MEKC retention data as well as providing more chemically sound LSER coefficients.

Characterization of chemical selectivity in micellar electrokinetic chromatography: V. The effect of the surfactant hydrophobic chain

The influence of the surfactant hydrophobic chain on selectivity in micellar electrokinetic chromatography was investigated using two sets of surfactants. A series of three sarcosinate surfactants (sodium N-lauroyl sarcosinate, sodium n−myristoyl sarcosinate, and sodium N-parmitoyl sarcosinate) with C11, C13, and C15 hydrocarbon tails were compared. In addition, sodium dodecyl sulfate and sodium tetradecyl sulfate were investigated to determine the chain length effect for sodium sulfate surfactants. Linear solvation energy relationships as well as free energy of transfer studies were used to predict the selectivity differences. The results suggest that although the surfactant chain length can have some effect, the magnitude of this influence seems to be dependent on the head group. © 2000 John Wiley & Sons, Inc. J Micro Sep 12: 433–441, 2000

Characterization of chemical selectivity in micellar electrokinetic chromatography. 4. Effect of surfactant headgroup.

The influence of surfactant headgroups on migration behavior in micellar electrokinetic chromatography is examined. Using linear solvation energy relationships (LSER) and functional group selectivity studies, the effect of six anionic headgroups on chemical selectivity is characterized. The sodium dodecyl surfactants of the sulfate [SO4-], sulfonate [SO3-], carboxylate [CO2-], carbonyl valine [OC(O)NHCH(CH(CH3)2)CO2-], and sulfoacetate [OC(O)CH2SO3-] anions are investigated. Solute size and the hydrogen-bond-donating ability of the micellar phase play the most significant roles in solute retention in all of the surfactants studied. While solute-micelle hydrogen bonding plays a dominant role in the observed selectivity, the dipolarity and polarizability of the micellar phase also have a small influence. The results also suggest that the hydrogen-bond-accepting ability for surfactants is inversely proportional to the proton acidity (pKa) of its headgroup. The observed hydrogen-bond-donating ability and dipolarity of surfactant systems are believed to be a result of the water that resides near the micelle surface.

Characterization of mixed micellar pseudostationary phases in electrokinetic chromatography using linear solvation energy relationships

The influence of mixed micellar systems on retention and selectivity in micellar electrokinetic chromatography is examined using linear solvation energy relationships (LSER). Systems that were investigated include mixed bile salts [sodium deoxycholate (SDC) and sodium cholate (SC)] and mixed sodium dodecyl sulfate (SDS)–bile salt systems (e.g., SDS–SC and SDS–SDC). The retention behavior in individual and mixed micellar systems is primarily determined by size and hydrogen bond acceptor strengths of solutes. Through a comparative study of the LSER coefficients in the individual and mixed micellar systems, it was concluded that hydrogen bonding interactions have a significant effect on selectivity of these pseudostationary phases in electrokinetic chromatography. The interactive properties of the mixed micelles are different from the constituent individual micelles, however, the overall characteristics are closer to one of the bile salt micelles in the mixture even at the equimolar compositions.

Recommendations for the determination of selectivity in micellar electrokinetic chromatography

A general approach for characterizing selectivity in micellar electrokinetic chromatography based on the solvation parameter model is recommended. Individual surfactants are characterized by their cohesion and capacity for polar interactions indicated as lone pair–lone pair electron attraction, dipole-type interactions, and hydrogen bond acidity and basicity. The statistical and chemical validity of the solvation parameter model requires that retention data are determined for a collection of 20–40 varied solutes with a wide range of retention factors and that clustering of values and cross-correlation among the solute descriptors are absent. Since micelles are interfacial solvents with properties that can vary with changes in their external environment (buffer composition, concentration, pH, temperature, etc.) a generic set of experimental conditions are recommended for the measurement of anionic surfactant selectivity under standard conditions. The system constants used to characterize surfactant selectivity are calculated for 12 common surfactants used in micellar electrokinetic chromatography. Of these surfactants sodium dodecyl sulfate, sodium cholate, lithium perfluorooctanesulfonate, sodium N-dodecanoyl-N-methyltaurine and tetradecyltrimethylammonium bromide are identified as providing a useful range of selectivity differences for methods development in micellar electrokinetic chromatography.

Characterization of Surfactant Selectivity in Micellar Electrokinetic Chromatography

The solvation parameter model is used to delineate the contribution of individual intermolecular interactions to the retention properties of seven common surfactants used in micellar electrokinetic chromatography. Buffer composition and concentration, pH, temperature, and voltage had only a small influence on selectivity for sodium cholate micelles in the normal experimental range for separations. Surfactant concentration was found to affect retention by changing the phase ratio without significantly changing selectivity while the addition of organic solvent in low concentration (up to 10% v/v) has an influence on the solvophobic properties of the buffer (lowers cohesion and reduces its capacity as a hydrogen-bond acid) in a manner which seems to depend very little on solvent identity. The bile salt surfactants, sodium cholate, sodium deoxycholate, sodium taurocholate, and sodium deoxytaurocholate, are quite similar as a group, exhibiting modest differences in their cohesion and hydrogen-bond acidity. Sodium dodecyl sulfate has complementary properties to the bile salt surfactants; it is less cohesive and a weaker hydrogen-bond base and a stronger hydrogen-bond acid. The sodium salt of N-dodeconyl-N-methyltaurine is a stronger hydrogen-bond base than the support buffer and weak hydrogen-bond acid; again properties that set it apart from the bile salts and sodium dodecyl sulfate for the separation of hydrogen-bonding solutes. Hexamethyltrimethylammonium bromide is the least cohesive and the strongest hydrogen-bond base of the surfactants studied but its relatively small migration window may make it less useful in practice for complex separations. Lithium perfluorooctanesulfonate has rather unique selectivity (high cohesion, significant capacity for dipole-type interactions and strong hydrogen-bond acidity) that would make it a good choice with the other surfactants for general selectivity optimization in methods development.

Influence of Composition on the Selectivity of a Mixed-micellar Buffer in Micellar Electrokinetic Chromatography

S. K. Poole and C. F. Poole, Anal. Commun., 1997, 34, 57 DOI: 10.1039/A607645I

Chemical selectivity in micellar electrokinetic chromatography II. Rationalization of elution patterns in different surfactant systems

Retention behavior in micellar electrokinetic chromatography (MEKC) is investigated using linear solvation energy relationships (LSERs) for two pseudo-stationary phases, one consisting of cationic micelles of tetradecyltrimethylammonium bromide (C 14TAB) and the other of an anionic triblock copolymer, poly(methyl methacrylate-ethyl acrylate-methacrylic acid) (Elvacite 2669). It was found that solutes' migration behaviors in these two MEKC systems are mainly influenced by their si/e ( V/100) and hydrogen bonding acceptor (HBA) strength (β). However, solutes' hydrogen bonding donor (HBD) strength (α) has minor effects on their migration in MEKC. The characteristics of these two systems were compared to three other previously reported anionic micellar systems of sodium dodecyl sulfate (SDS) (anionic hydrocarbon), sodium cholate (SC) (anionic bile salt) and lithium perfluorooctane sulfonate (LiPFOS) (anionic fluorocarbon). It was concluded that hydrogen bonding interactions play a major role in providing different chemical selectivity among these five MEKC systems. Both C 14TAB micelles and the ionic polymer of Elvacite 2669 provide hydrogen bonding acceptor (HBA) sites for solutes, which is similar to SC micelles. In fact, C 14TAB is the strongest HBA, while Elvacite 2669 has HBA strength similar to that of SC micelles. On the other hand, the fluorocarbon micelles of LiPFOS are the strongest hydrogen bond donor (HBD) micelles, followed by the weak HBD SDS micelles. In general, cavity formation has little or no effect on chemical selectivity among hydrocarbon surfactant MEKC systems (i.e., SDS, SC and C 14TAB). Information obtained from the LSER analysis is used to rationalize the elution patterns in MEKC with different types of pseudo-stationary phases.

Control of selectivity in micellar electrokinetic chromatography by modification of sodium dodecyl sulfate micelles with organic hydroxy compounds

Modification of micellar phases with solubilized solutes was examined to control the separation in micellar electrokinetic chromatography (MEKC). The addition of organic hydroxy compounds (1-hexanol, cyclohexanol and phenol) as modifiers to a micellar solution of sodium dodecyl sulfate specifically decreased the capacity factor of some aromatic analytes possessing hydrophilic functional groups. The effect of phenol was different in selectivity to that of 1-hexanol and cyclohexanol. The effects of these modifiers were mainly explained in terms of the saturation of the micellar palisade layer with the modifiers and the hydrogen-bond interaction between the modifier and analyte molecules in the micellar phase. Such micellar modification were applied to improve the MEKC separation.

Chemical Selectivity in Micellar Electrokinetic Chromatography: Characterization of Solute-Micelle Interactions for Classification of Surfactants

The influence of surfactant type on migration behavior and chemical selectivity in micellar electrokinetic chromatography (MEKC) is investigated through linear solvation energy relationships (LSER) and functional group selectivities. In LSER modeling, solutes' capacity factors are correlated with their structural descriptors such as size, dipolarity, and hydrogen-bonding abilities. Using the LSER methodology, useful information about the nature of solute interactions with different types of surfactant aggregates can be obtained since capacity factor in MEKC is directly related to solute distribution between the bulk aqueous solvent and micelles. High correlations were observed for different LSER models of migration behavior in MEKC for a group of 60 uncharged aromatic compounds of non-hydrogen bonding (NHB), hydrogen-bonding acceptor (HBA) bases, and hydrogen-bonding donor (HBD) acids. In two anionic, hydrocarbon micellar systems of sodium dodecyl sulfate (SDS) and sodium cholate (SC), retention is primarily influenced by the size of molecules and their hydrogen bond accepting basicity. Their dipolarity/polarizability and hydrogen bond donating acidity play minor roles. Capacity factors of solutes in SDS and SC systems increase with their size and decrease for stronger hydrogen bond acceptor bases. These results are similar to those observed for other systems where hydrophobic interactions play a major role, e.g., solute distribution in the 1-octanol-water solvent system or retention in reversed phase LC. In MEKC with an anionic fluorocarbon surfactant, lithium perfluorooctanesulfonate (LiPFOS), however, size and solute HBD acidity are the two predominant factors. The LSER results indicate that compounds find the SDS micellar environments slightly less cohesive (i.e., more apolar) than the SC micelles, while the LiPFOS micelles are the most cohesive among the three surfactant aggregates and 1-octanol provides the least cohesive environment. The fluorocarbon micelles of LiPFOS, on the other hand, are the strongest hydrogen bond donor acids, followed by SDS, SC, and 1-octanol, respectively. The SC micelles have the most hydrogen bond acceptor basic characteristics, followed by 1-octanol, SDS, and LiPFOS micelles. It can be concluded that selectivity differences between these surfactant types in MEKC is primarily due to hydrogen-bonding interactions rather than the dipolar interactions. Comparing the perfluorinated and the hydrocarbon surfactants, even solute size can play a role in selective migration patterns. In addition, information from polar and hydrophobic group selectivities confirm the LSER conclusions about the underlying interactions that control migration behavior and chemical selectivity in MEKC.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of the dodecyl sulfate counterion on selectivity and resolution in micellar electrokinetic chromatography: II. Organic counterions

Various tetraalkylammonium ions, such as tetramethyl-, tetrapropyl-, tetrapentyl-, and cetyltrimethylammonium, can be added to sodium dodecyl sulfate (SDS) micellar solutions to achieve unique selectivities in micellar electrokinetic chromatography (MEKC). The cetyltrimethylammonium/SDS buffer system showed the greatest increase in methylene selectivity, while both tetrapropylammonium/SDS and cetyltrimethylammonium/SDS buffer systems had the greatest selectivities for the solutes studied. In addition, the buffer systems that used symmetrical tetraalkylammonium ions showed reductions in retention specific for the carbonyl functional group. The tetramethylammonium/SDS buffer system had the best resolution due to both improved selectivity and an elution range that was 2.5 times that of a comparable SDS buffer system.

Application of electrokinetic chromatography to pharmaceutical analysis

Electrokinetic chromatography (EKC) is one branch of capillary electrophoresis that permits the separation of electrically neutral solutes by the electrophoretic technique. The separation principle of EKC is based on that of chromatography, and various modes of EKC have been developed along with the partition mechanism. Micellar electrokinetic chromatography (MEKC), in which an ionic surfactant solution is employed instead of a buffer solution in capillary zone electrophoresis (CZE) at a higher concentration than the critical micelle concentration (CMC), has become the most popular technique among various EKC modes. Besides the separation of the electrically neutral or non-ionic solutes, MEKC is also effective for the separation of ionic solutes as well as CZE, hence it suits the analysis of pharmaceuticals, including cationic, anionic and neutral. Separation selectivity in MEKC can be manipulated easily through the modification of the buffer as well as changing the surfactants. Direct enantiomeric separation is successful by using chiral surfactants or chiral additives. Determination of drugs in plasma is also successfully achieved by a direct sample injection method, similar to micellar HPLC. In this paper, we summarize the principle separation characteristics of MEKC and some applications to pharmaceutical analysis, including direct enantiomeric separation and direct assay of drugs in plasma.