Cationic surfactants capillary electrophoresis

The optimum pH for separating cations is pK + 0.30 K. K.-C. Yeung and C. A. Lucy, Isotopic Separation of [14N]- and fI5N] Aniline by Capillary Electrophoresis Using Surfactant-Controlled Reversed Electroosmotic Flow, Anal. Chem 1998, 70. 3286. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

The type of electrophoresis we have been discussing so far is called capillary zone electrophoresis. Separation is based on differences in electrophoretic mobility. If the capillary wall is negative, electroosmotic flow is toward the cathode (Figure 26-20) and the order of elution is cations before neutrals before anions. If the capillary wall charge is reversed by coating it with a cationic surfactant (Figure 26-24) and the instrument polarity is reversed, then the order of elution is anions before neutrals before cations. Neither scheme separates neutral molecules from one another. 

Baryla, N.E. Lucy, C.A. Simultaneous separation of cationic and anionic proteins using zwitterionic surfactants in capillary electrophoresis. Anal. Chem. 2000, 72 (10), 2280-2289. 

Gong, M., Wehmeyer, K.R., Limbach, P.A., and Heimeman, W.R., Unlimited-volume electrokinetic stacking injection in sweeping capillary electrophoresis using a cationic surfactant. Anal. Chem., 78, 6035, 2006. 

Nonaqueous capillary electrophoresis (NACE) involves the separation of analytes in a medium composed of organic solvents. The viscosity and dielectric constants of organic solvents affect both sample ion mobility and the level of EOF. The changes in separation selectivity in nonaqueous conditions contribute to a better separation of some substances that have very small charge-to-mass differences in aqueous phases (linear alkylbenzene sulfonates, or triazines). Further adsorption on the capillary wall and/or ion interactions, which cause solute precipitation (e.g., anionic surfactants with cations), can be avoided using NACE. 

Electrophoretic techniques, mainly capillary zone electrophoresis (CZE) and also capillary iso-tachophoresis (ITP) or micellar electrokinetic chromatography (MEKC) have been used in cosmetic analysis (e.g., determination of cationic surfactants in toiletries, parabens in different cosmetics, fluoride, and polyphosphates in toothpaste, hair dyes, or acid preservatives in cosmetic lotions). However, their use is less extensive than LC, probably because the... 

Phenoxy acid herbicides, sulfonyl ureas, quaternary ammonium derivatives (quats), and aryloxy propanoic acids are the main classes of compounds subjected to capillary zone electrophoresis (CZE). Triazines are also separated using nonaqueous CZE, while low pfCa characterized chlorotriazines require an ion-pair-like solubilization using cationic surfactants (tetradecylammonium bromide, dodecyltrimethyl-ammonium bromide). Chiral selectors are added in CZE for obtaining enantioselectivity. Chiral selectors used for herbicide enantiomeric discrimination are vancomycin, y-cyclodextrin, ethyl carbonate -cyclo-dextrin, cyclohexyl-alkyl-)S-D-maltoside, sulpropyl ether a-cyclodextrin, and hexakis(2,3-di-0-methyl)-a-cyclodextrin. 

CE is a separation technique that may be an alternative to LC to determine alkyl chain distribution of anionic and cationic surfactants. Most of the studies refer to alkylbenzyldimethylammonium with UV absorbance detection (Figure 2). Other uses of CE include the separation of homologs of anionic surfactants (as AS) and cationic (as alkyltrimethylammonium) by isotachophoresis with conductometric detection and homologs of non-UV absorbing surfactants (AS, alkylsulfonate, alkylsarcosinates and dialkyldimethylammonium) by capillary zone electrophoresis using indirect detection. 

C. Wang, C. A. Lucy, Mixed cationic/anionic surfactants for semipermanent wall coatings in capillary electrophoresis. Electrophoresis, 2004, 25, 825-832. 

P. A. Gallagher, N. D. Danielson, Capillary electrophoresis of cationic and anionic surfactants with indirect conductivity detection,/. Chromatogr., 781, 533, 1997. 

Abstract Surfactant mixtures are commonly used in many industrial applications. At the same time, the methods for surfactant analysis are rather laborious and often do not permit the determination of the individual surfactant content in mixed solutions. In the present work capillary zone electrophoresis (CZE) instrumentation was applied for the quantitative analysis of a cationic surfactant (dodecylpyridinium bromide) and a nonionic surfactant (Triton X-100) in aqueous solutions. The linear dependence of the analytical signal (electrophoregram peak area) versus the surfactant concentration was established for both surfactants over a wide concentration range. The analytical signal of an individual surfactant was... 

The synthesis of a new CIL [S-(—)-2-hydroxymethyl-l,l-dimethylpyrrolidinium tetrafluoroborate] derived from L-proline alcohol have been reported by Maier et al. [78]. This CIL was found to be an effective additive to acidic background electrolytes affording the separation of a mixture of five tricyclic antidepressants using capillary zone electrophoresis (CZE). The addition of the CIL to acidic background electrolytes leads to suppression of the magnitude of electroosmotic flow (EOF) and gradually reversed the direction of the EOF. Baseline separation of the five model analytes was achieved. It was observed that the proline-derived CIL offers relatively smaller anodic EOF compared to cationic surfactants that are mostly used for... 

Capillary electrophoresis analyses of cationic and amphoteric surfactants are summarized in Tables 6 and 7. 

TABLE 6 Capillary Electrophoresis Analysis of Cationic Surfactants... 

Surfactants are amphophilic molecules, which consist of a hydrophobic carbohydrate part and a hydrophilic head group. In capillary zone electrophoresis (CZE), different types, i.e., anionic, cationic, but also neutral, tensides are employed. The ability of such molecules to interact with ionic and nonionic species has been used in ion chromatography and, in particular, in SDS-poly-(acrylamide) gel electrophoresis (PAGE) (15). 

The reversal of the direction of the electro-osmotic flow by the adsorption onto the capillary wall of alky-lammonium surfactants and polymeric ion-pair agents incorporated into the electrolyte solution is widely employed in capillary zone electrophoresis (CZE) of organic acids, amino acids, and metal ions. The dependence of the electro-osmotic mobility on the concentration of these additives has been interpreted on the basis of the model proposed by Fuerstenau [6] to explain the adsorption of alkylammonium salts on quartz. According to this model, the adsorption in the Stern layer as individual ions of surfactant molecules in dilute solution results from the electrostatic attraction between the head groups of the surfactant and the ionized silanol groups at the surface of the capillary wall. As the concentration of the surfactant in the solution is increased, the concentration of the adsorbed alkylammonium ions increases too and reaches a critical concentration at which the van der Waals attraction forces between the hydrocarbon chains of adsorbed and free-surfactant molecules in solution cause their association into hemimicelles (i.e., pairs of surfactant molecules with one cationic group directed toward the capillary wall and the other directed out into the solution). 

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