Solubilization of salicylamide and acetaminophen by antihistamines in aqueous solution

The effect of self-association of the antihistaminic drugs pheniramine, chlorpheniramine, and brompheniramine as their maleate salts on the solubilization of salicylamide and acetaminophen in aqueous solution has been investigated. The total solubility of salicylamide increased nonlinearly at lower antihistamine concentrations (less than 0.4 M), but reached limiting linearity (slope = 0.34 mol/mol of antihistamine) at higher concentrations (up to 0.8 M). Salicylamide solubility increases are approximately 10-fold at high concentrations (0.6-0.8 M) of antihistamine, while acetaminophen solubility increases are about fivefold at similar antihistamine concentrations. The solubilization data were analyzed with a stepwise self-association model. Based on a dimer model, the experimental and theoretical log excess salicylamide solubility profiles were in good agreement (r2 = 0.982) except at the lowest chlorpheniramine maleate concentrations. Such deviation at the lowest concentrations increased when trimer and 11-mer models were utilized. To account for this deviation, a monomer-dimer model based on interaction with both the chlorpheniramine maleate monomer and dimer was proposed. This model was in excellent agreement (r2 = 0.996) with the solubility data.