Volumetric, acoustic, electrochemical and spectroscopic investigation of norfloxacin–ionic surfactant interactions

Rising levels of bacterial resistance to antibiotics is a persistent and dynamic concern in the field of medicine and demand design and discovery of new and more effective antibiotics. One of the major hurdles in designing new antibiotics which should be capable of passively diffusing across biological membranes is the scarcity of knowledge regarding their biophysical interaction with these membranes. Surfactant systems have been frequently reported in the past decade as potential pseudo-membrane models to understand biophysical interactions of drugs with biological membranes at the molecular level. The present work is aimed at understanding the interaction of norfloxacin (NRF), a fluoroquinolone drug, with dodecyltrimethylammonium bromide (DTAB, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant) at molecular level. Volumetric and acoustic studies of DTAB and SDS in buffered solutions of NRF at physiological pH (7.4) and a temperature range of 298.15 to 313.15 K were performed to calculate apparent molar volume (ɸV), isentropic compressibility (Ks), apparent molar isentropic compressibility (ɸK), partial molar volume (ɸVo), partial molar expansibility (ɸEo), specific acoustic impedance (Z), relative association (RA), intermolecular free length (Lf) and sound velocity number (U). The variation in these quantities as a function of surfactant concentration is interpreted in terms of NRF-ionic surfactant interaction using a cosphere overlap model. Cyclic voltammetry and UV–visible spectroscopy were used to calculate the partition coefficient (Kc) and binding constant (Kb) of NRF with ionic micelles. Free energies calculated from corresponding Kc and Kb values revealed the spontaneity of partitioning and binding of NRF with ionic micelles.