Micellar mobile phase aqueous media

The addition of small percentages of 1-propanol to micellar mobile phases was first recommended by Dorsey et al. [13], to enhance the chromatographic efficiency and decrease the asymmetry of chromatographic peaks. Since then, several organic solvents have been studied as modifiers in MLC. Of these, short and medium chain alcohols (z. e., methanol, ethanol, propanol and butanol) have shown to be the most suitable. Less frequent has been the use of pentanol [14, 15], Only a few rqports have appeared on the MLC behavior of solutes in the presence of other organic solvents commonly employed in conventional RPLC, such as acetonitrile [13, 16, 17], and tetrahydrofiiran [18, 19]. Micellar mobile phases allow the use of organic solvents in aqueous solution at molar concentrations well above their normal solubility limit in water alone. For example, the water solubility of pentanol is ca. 0.30 M, whereas in 0.285 M SDS micellar medimn, it increases to ca. 0.94 M [17]. 

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). 

The effect of the addition of short chain alcohols on the chromatographic selectivity and peak efficiency was extensively exposed in previous chapters. The addition of such alcohols to a micellar solution forms mixed micelles. This is the first step toward the achievement of microemulsions with ionic surfactants. The oil in water microemulsion (LI structure, see Chapter 2) has a continuous aqueous phase containing oil swollen micelles or microdroplets of oil stabilized by an alcohol-surfactant interphase layer. The medium is transparent and stable, however it 1ms a dynamic structure. Then, it is interesting to see if LI microemulsion mobile phases could be useful in... 

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