The present work reports a photophysical study of norfloxacin (NOF), an efficient chemotherapeuticantibacterial drug, in well-characterized biomimetic micellar aggregates of anionic surfactants. Themajor focus of the present investigation lies in deciphering the effect of the surfactant chain length on thestructural dynamics of the drug within a micelle-encapsulated state through steady-state and time-resolvedfluorescence spectroscopic techniques. The anionic surfactants employed for the purpose are decyl sodiumsulfate (S₁₀S), dodecyl sodium sulfate (S₁₂S) and tetradecyl sodium sulfate (S₁₄S). Our experimental resultsevince that increasing hydrophobic surfactant tail length exerts a profound influence on promoting a specificprototropic form of NOF within the micellar aggregates; an apparently enigmatic observation is thatincreasing hydrophobicity of the micellar microheterogeneous system indeed favors the cationic species ofthe drug. This is interpreted on the basis of electrostatic stabilization between the drug (cationic form) andanionic surface charge of the as-employed micellar assemblies, while the micellar hydration model fails torationalize the experimental findings. Particular emphasis is also given on delineating the probable locationand the modulated rotational-relaxation dynamics of the drug molecule within the micellar aggregates.

The interaction thermodynamics and dynamics of norfloxacin within anionic micelles is critically dependent on the tail length of the surfactant such that the micellar hydration model is found to be inadequate to explain the photophysics of the drug whereas electrostatic interaction appears to play the governing role.