Bonded Phases - Application of the Theory to the Adsorption of Aliphatic Alcohols 2
Employing the data given in table (4) together with equation (6) the adsorption isotherm equations can be calculated for all four alcohols.
Methanol ..................... (13)
Ethanol .................. (14)
Propanol .................. (15)
Butanol ................. (16)
Equations (13), (14), (15) and (16) were employed to calculate the adsorption isotherms for all four alcohols that are shown in figure 16.
The effect of the dispersive hydrocarbon chain of an alcohol on the magnitude of its adsorption on a reverse phase is clearly indicated by the relative shape of the curves. The most strongly adsorbed alcohol, butanol, has only a four carbon atoms in its chain and yet the surface is completely covered when the solvent concentration is only about 2%w/v. This implies that any component of a solvent mixture with a hydrocarbon chain length of four or more, will be quickly adsorbed and will extensively modify the reverse phase surface.
Figure 16. The Adsorption Isotherms of a Homologous Series of Aliphatic Alcohols over the Concentration Range of 0 to 0.1 g.ml-1
Consequently, the magnitude of any other solute retention will also be significantly changed. It will be discussed later how this property can be exploited to adsorb ion exchange materials on the surface of a reverse phase as a simple alternative to the use of conventional ion exchange media. The curves shown in figure 16 only cover a range of 0 to 0.1 g.ml-1. In order to illustrate the shapes of the adsorption isotherms for the higher alcohols in proportion to those of the lower alcohols with greater clarity, the same curves are shown in figure 17 for an alcohol concentration range of 0-100% (which is approximately 0-0.8g/ml). The weak character of the methanol adsorption compared with the other alcohols is clearly shown and it is clear that the reverse phase surface is being continually modified over one third of the methanol concentration range. This is one reason why methanol is one of the more popular solvents for use in reverse phase chromatography. When using methanol/water mixtures as the mobile phase the reverse phase surface can be modified in a controlled manner, over the range of 0 to about 40% methanol. However, between methanol concentrations of 40% and 100% the interactive nature of the reverse phase surface remains sensibly constant and consequently only the interactions in the mobile phase are progressively modified.
It should be noted that the mass adsorbed is expressed as mg.cm-2
Figure 17. The Adsorption Isotherms of a Homologous Series of Aliphatic Alcohols
Acetonitrile and tetrahydrofuran are similar in character but their adsorption isotherms are closer in magnitude to those of ethanol than of methanol.