The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Page 40

It is seen that the distribution coefficient of all three solutes is predominantly controlled by the amount of unassociated methanol in the aqueous solvent mixture. In addition, the distribution coefficient increases linearly with the concentration of unassociated methanol for all three solutes over the entire concentration range.

Solvent/Solute Interactions with Adsorbent Surfaces

Most LC stationary phases are basically adsorbents mostly made from either silica gel or cross linked polystyrene beads. Appropriate interacting moieties can be chemically attached to the surface of silica producing the well known bonded phases. In a similar manner the polystyrene surface can be chemically modified to produced unique interacting moieties.

Silica gel, per se, is not frequently used as a stationary phase in LC, the bonded phases being a more popular choice for most analysts. This is because the efficient use of silica gel demands a fairly wide range of solvents to provide adequate interactive versatility which may include n-alkanes, chlorinated hydrocarbons, ethers, esters and aliphatic alcohols. The bonded phases utilize aqueous solvents (i.e., methanol/water or acetonitrile/water mixtures) as the mobile phase which are simpler systems using less expensive solvents. Nevertheless, silica gel has certain areas of application for which it is particularly useful. As silica gel is a polar stationary phase, it is useful for the separation of polarizable substances such as the aromatic hydrocarbons and mixtures of solutes of weak polarity such as phenols, esters and aliphatic ethers.

In all chromatography systems both the solvent and the solutes interact with the stationary phase. Thus, when the silica surface is in contact with a solvent, the surface is covered with a layer of the solvent molecules. If the mobile phase consists of a mixture of solvents the surface is partly covered by one solvent and partly with the other (11). Thus, any solute interacting with the stationary phase may well be presented with two (or more), quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent.