Aggregation of antihistamines in aqueous solution: effect of counterions on self-association of pyridine derivatives.

The effects of electrolytes on the self-association of the antihistaminic drugs, tripelennamine hydrochloride, thenyldiamine hydrochloride, pyrilamine maleate, pheniramine maleate, chlorpheniramine maleate, and brompheniramine maleate, in aqueous solution were examined by light-scattering from tripelennamine bydrochloride and thenyldiamine hydrochloride in 0.154 mole of sodium chloride/kg and 0.150 mole of sodium maleate/kg indicated a micellar pattern of aggregation. Higher aggregation numbers and lower CMC's were determined in the presence of the maleate ion. No significant discontinuity in the concentration dependence of the light scattering of the remaining compounds in either of the two electrolytes was evident, and the aggregation of these compounds was treated using a stepwise association model. Values of the association constants and the limiting number of associating species were, in general, increased by the addition of electrolyte in the order water less than sodium chloride less than sodium maleate. An apparently nonmicellar pattern of aggregation could be induced by chemically changing the counterion from chloride to maleate.