Hydrophobic interactions, organic modifiers

Besides the electrostatic potential effect on reactivity, functionalized polyelectrolytes have a variety of interesting features worthy of study. If a polyelectrolyte is covalently modified with highly hydrophobic functional groups, it provides an unusual opportunity to study the chemical reactions of normally otherwise water insoluble functional groups in aqueous solution. Furthermore, a structural organization via hydrophobic interactions may occur in aqueous solution [25 — 31], which is of general scientific importance and is worth studying for its own sake. 

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. 

Solute adsorption can be minimized most effectively by capillary wall coating, thereby decreasing the free energy of hydrophobic or ionic interactions. Coating can be achieved either by covalently bonded organic modifiers, e.g., polyacrylamides, sulfonic acids, polyethylene glycols, maltose, and polyvinyl pyrolidinone, or by dynamic deactivation (i.e., addition of... 

Adsorption. Hydrophobic interactions, which may occur using aqueous mobile phases, usually can be eliminated by the addition of an organic modifier to the aqueous mobile phase (30,33) or by a reduction of ionic strength (3A 25.)- Recently, Haglund and Marsden (36-AO) have undertaken a systematic study on the chromatographic behavior of low molecular weight solutes on Sephadex packings and explained these results in terms of hydrophobic interactions. 

Wei et al. studied the separation of amines (aniline, ephedrine, codeine, cocaine, thebaine) and quaternary ammonium compounds (berberine, jatrorrhi-zine) on a bare silica stationary phase [44], A thorough study of the separation mechanism revealed a complex multifunctional mechanism. Contributions from differential electrophoretic migration were superimposed on hydrophobic, cation-exchange, and normal-phase interactions. Retention was highly dependent on the pH (optimal pH 8.3), ionic strength, and the amount of organic modifier. As the content of acetonitrile exceeded 80%, retention was consistent with a normal-phase mechanism. 

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