Bonded Phases - Application of the Theory to the Adsorption of Aliphatic Acids

Consider a solvent mixture of methanol /ethanol /water in contact with a reverse phase. If both alcohols are present at low concentrations, then the surface will be predominantly covered with the more strongly adsorbed ethanol. If the concentration of methanol is now increased, there will be strong interactions between the ethanol and methanol in the mobile phase tending to remove some of the ethanol on the surface into the mobile phase.

Figure 19. The Adsorption Isotherms of Acetic acid and Propionic Acid over the Concentration Range of 0 to 0.4

In addition, the higher concentration of methanol will render the probability of the methanol striking the reverse phase surface and adhering much greater. Consequently, as the methanol concentration increases, the reverse phase surface becomes predominantly covered with methanol and the ethanol concentration on the surface decreases. This means that the distribution coefficient of the ethanol with respect to the stationary phase is reduced and, as a result, if chromatographed as a solute, its retention volume would also be reduced.

If the ethanol concentration is increased and that of the methanol decreased, the converse will occur. At higher concentrations, the ethanol will completely cover the surface although at lower concentrations some of the surface would be covered with methanol. This replacement will result, in part, from the increased interactions between the alcohols in the mobile phase increasing the probability of the ethanol striking the reverse phase surface and adhering, consequently, leaving no available space for the methanol. In this case, the methanol is behaving as a solute and would be eluted progressively more rapidly as the concentration of ethanol was increased.

In the practice of chromatography it is important to understand that there is no basic difference in the way solutes and solvents interact with the surface. In general the solutes are components in the chromatographic system that interact more strongly with the stationary phase while the solvents, being at a higher concentrations interact more frequently with the surface. In the successful elution of the sample, the solvent concentration must be chosen such that the greater frequency but weaker interactions of the solvent molecules with the surface compensates for the less frequent but stronger interactions of the solute molecules with the surface. Under such conditions, the components will be eluted discretely and in a reasonable time.