Bonded Phases - Application of the Theory to the Adsorption of Aliphatic Alcohols 1
Application of the Theory to the Adsorption of Aliphatic Alcohols
Equation (11) was employed to process the data of Scott and Simpson (29). Using a simple iterative computer program the values of (A) and (B) were calculated that provided the minimum error between the two sides of equation (11) for values that were obtained for the three alcohols. Having determined the values of (A) and (B), the corrected retention volumes were calculated from the retention volume data using the following equation.
The results obtained for the series of aliphatic alcohols are depicted as curves relating the reciprocal of the corrected retention volume to solvent concentration in figure 15 and it is clearly seen that the predicted linearity is precisely realized.
Figure 15. Graph of 1/V' for a Series of Aliphatic Alcohols against their Respective Concentration in the Mobile Phase
Taking the values for the slopes and intercepts from the curve fitting procedure in equations (5) and (6), the effective chromatographic surface area of the reverse phase for all four alcohols was calculated together with the distribution coefficient of each alcohol between water and the reverse phase. The results are given in table 4.
Table 4 Chromatographic Data for Four Aliphatic Alcohols
From table 4 it is seen that the data from all three alcohols provide a mean value for the effective chromatographic surface area of 206.8m2g-1 with a standard deviation of 4.8m2g-1 that is only 2.35% of the mean. The reverse phase employed was ODS3 from Whatman Inc. that was reported by the manufacturers to have been prepared from silica gel having a BET surface area of 350 m2g-1. This indicates that the effective chromatographic surface area of the reverse phase was about 58% of that of the parent silica gel. It should be noted that the fraction of the silanol groups reacted with the silane reagent is normally between 50 and 60%.
The way in which the solvent is deposited on the surface is best shown by the shape of the adsorption isotherm. By rearranging equation 3 an expression for the adsorption isotherm can be obtained
Dividing throughout by (a), the usual form of the expression for the adsorption isotherm is obtained.