Temperature Dependant Mixed Micellization Behavior of a Drug-AOT Mixture in an Aqueous Medium

Abstract

The mixed micellization behavior of an amphiphilic antidepressant drug amitriptyline hydrochloride (AMT) in the presence of the conventional anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was studied at five different temperatures and compositions by the conductometric technique. The critical micelle concentration (cmc) and critical micelle concentration at the ideal state (cmc id ) values show mixed micelle formation between the components (i.e., drug and AOT). The micellar mole fractions of the AOT (X1) values calculated using the Rubingh, Motomura, and Rodenas models show a higher contribution of AOT in the mixed micelles. The interaction parameter (β) is negative at all temperatures and the compositions show attractive interactions between the components. The activity coefficients (f1 and f2) calculated using the different proposed models are always less than unity indicating non-ideality in the systems. The ΔGm Ө values were found to be negative for all the binary mixed systems. However, ΔHm Ө values for the pure drug as well as the drug-AOT mixed systems are negative at lower temperatures (293.15- 303.15 K) and positive at higher temperatures (308.15 K and above). The ΔSm Ө values are positive at all temperatures but their magnitude was higher at T=308.15 K and above. The excess free energy of mixing (ΔGex) determined using the different proposed models also explains the stability of the mixed micelles compared to the pure drug (AMT) and surfactant micelles.