Figure 6.17 Freundlich adsorption isotherms of local anaesthetics on activated carbon at pH 7.0 and 25°C.

Reproduced from I. Abe, H. Kamaya and I. Ueda, J. Pharm. Sci., 79, 354 (1990) with permission.

occurs. This empirical rule is termed Lundelius's rule. There are numerous examples of the applicability of this rule; for example, in Lundelius's original work it was noted that the adsorption of iodine onto carbon from CCl4, CHCl3 and CS2 was 1:2: 4.5, respectively. These ratios are close to the inverse ratios for the solubilities of iodine in the respective solvents. The effect of solubility on adsorption might be expected since, in order for adsorption to occur, solute-solvent bonds must first be broken. The greater the solubility, the stronger are these bonds and hence the smaller the extent of adsorption.

For homologous series, adsorption from solution increases as the series is ascended and the molecules become more hydrophobic. There is, for example, a good correlation between the Freundlich adsorption constant, 1/n (related to the extent of adsorption) and the molecular weight of the local anaesthetics discussed above (Fig. 6.18). The data for phenobarbital deviated from this linear relationship possibly as a result of the difficulty of adhesion

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