The Mechanism of Chromatographic Retention - Mixed Phases > Page 25

The work of Purnell on mixed stationary phase in GC was confirmed for mixed mobile phase in LC by Katz et al. (7). Reiterating the equation proposed by Purnell, for two solvents (A) and (B) in GC,

V'AB = a(V'A - V'B) + V'B (1)

Now bearing in mind that,

Then KAB = a(KA - KB)) + KB (2)

Katz et al. measured the distribution coefficient of n-pentanol between carbon tetrachloride, toluene, n-heptane, n-chloroheptane and pure water together with mixtures of n-heptane and n-chloroheptane and pure water. These systems were chosen because the solvents were immiscible and virtually mutually insoluble in each other and thus interactions in one phase were not influence significantly by the presence of the other. The results they obtained are shown in Figure 11. The linear relationship between the distribution coefficient and the volume fraction of the respective solvent is clearly established. The distribution coefficient of n-pentanol between water and pure carbon tetrachloride was found to be about 2.2 and similar distribution coefficient for n-pentanol was predicted by calculation to be obtainable from a mixture containing 82%v/v chloroheptane and 18%v/v of n-heptane. The experiment for toluene was repeated using a mixture of 82 %v/v chloroheptane and 18% n-heptane mixture in place of carbon tetrachloride which was, in fact, constituted a ternary mixture comprising of toluene, chloroheptane and n-heptane. The chloroheptane and n-heptane, however, was always in the ratio of 82/18 by volume to simulate the interactive character of carbon tetrachloride. All the points for the n-heptane/n-chloroheptane mixtures fell on the same straight line as that produced using a mixture of carbon tetrachloride and toluene. These experiments can be considered to simulate normal phase chromatography using pure water instead of silica gel, except that the water phase is not modified by the solvents in the way a silica gel surface would be.