Thermodynamic investigations on monomer and micellar interactions of promethazine/ondansetron with CTAB in aqueous solutions

This study investigated the volumetric and acoustic behavior of two antiemetic drugs, promethazine (PRH) and ondansetron hydrochloride (OSHD), in aqueous cetyltrimethylammonium bromide (CTAB) solutions. Apparent and partial molar volumes and compressibilities were calculated from experimental density and speed of sound data at 303.15 K and 0.101 MPa. Additionally, acoustic properties (isentropic compressibility, specific acoustic impedance, and relative association) and optical properties (refractive indices) were computed for these systems in both pre-micellar and post-micellar regions. Our findings indicate that the cationic surfactant CTAB significantly influences the drugs' hydration behavior via drug-surfactant interactions. Standard transfer volumes and compressibility results suggest that hydrophobic-hydrophobic interactions primarily dominate over ion-ion/ion-hydrophilic interactions between the drug and surfactant. The observation of the smallest partial molar volume and compressibility values, alongside large negative standard transfer properties for both drugs in the CMC region (0.0010 mol·kg⁻¹ CTAB), is indicative of binding interactions and the inclusion of PRH/OSHD drugs within the surfactant micelle. Furthermore, PRH demonstrates monomer-monomer, monomer-micellar, and mixed micellar interactions, whereas OSHD exhibits only monomer-monomer and monomer-micellar interactions with CTAB. Overall, the partial molar and standard transfer property data reveal that OSHD interacts more strongly with CTAB than PRH in aqueous solutions.