Adsorption of local anesthetics on activated carbon: Freundlich adsorption isotherms

We have shown that adsorbability of local anesthetics onto activated carbon, expressed by the partition coefficients at infinite dilution, correlated well with the pharmacological activity. However, there is no parameter that can singly express the tendency to be adsorbed. Adsorbability is a loosely defined term and its meaning varies with the adsorption model. This study showed that the logarithm of the adsorbed amount of drugs was linearly related to the logarithm of the free drug concentration, in conformity to the Freundlich adsorption isotherms. The slope of the double logarithmic plot is expressed by 1/N in the Freundlich equation and is considered to be inversely related to the drug affinity to the adsorbent. The slope was used to evaluate the tendency to be adsorbed, or "adsorbability" of seven aromatic amine local anesthetics. Phenobarbital was included to compare anionic drugs in contrast to the cationic local anesthetics. The slopes were nearly equal between the cationic and neutral local anesthetics. Apparently, the lower hydrophobicity of the cationic forms is compensated by the electrostatic attraction from the negative charges present on the activated carbon surface. With phenobarbital, the slope value of the anionic form was larger than the neutral form. The lower affinity of the anionic form may be caused by the electrostatic repulsion. The molecular size parameters (i.e., molecular weight, molar refraction, and parachor) showed a linear relationship to the slope values. It may be possible to estimate the affinity-related slope values from these parameters.