Bonded stationary phases hydroxyl group

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

Nevertheless, silica gel is the material of choice for the production of the vast majority of LC stationary phases. Due to the reactive character of the hydroxyl groups on the surface of silica gel, various organic groups can be bonded to the surface using standard silicon chemistry. Consequently, the silica gel surface can be modified to encompass the complete range of interactive properties necessary for LC ranging from the highly polar to almost completely dispersive. 

A number of ketones, pharmaceutical compounds, alcohols and hydroxy acids have also been resolved on this phase [724,765-767]. A chiral polysiloxane phase with tartramide substituents has been used for the separation of enantiomers capable of hydrogen bonding interactions with the stationary phase, such as enantiomers containing carboxylic, hydroxyl and amine functional groups [768]. 

By covalently bonding organic functions via the hydroxyl groups at the surface of silica or alumina, the chromatographic behavior of the solid stationary phase can be drastically modified and the problems mentioned above seem to be less important with such bonded phases. 

Common chiral stationary phases for gas chromatography have cyclodextrins bonded to a conventional polysiloxane stationary phase.7-8 Cyclodextrins are naturally occurring cyclic sugars. P-Cyclodextrin has a 0.78-nm-diameter opening into a chiral, hydrophobic cavity. The hydroxyls are capped with alkyl groups to decrease the polarity of the faces.9... 

Organic solvents also induce changes in the properties of surfactant-coated stationary phases, such as polarity, surface area, or pore volume. Several studies have demonstrated that /i-alcohols interpenetrate the Cig-bonded alkyl chains to form a single monolayer, structurally similar to an opened micelle (i.e., the hydroxyl group orientated toward the aqueous phase). The competition between organic solvent and surfactant molecules for the active sites on the column explains the reduction of adsorbed surfactant at increasing concentration of organic solvent in the mobile phase. For ionic surfactants, this... 

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