Interfacial and Solution Behavior of Amphiphilic Drug and Counterion‐Coupled Gemini (COCOGEM) Surfactants

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The critical micelle concentrations (cmc) of mixed systems comprising an amphiphilic drug amitriptyline hydrochloride and counterion‐coupled gemini surfactants 12‐6‐12, 14‐6‐14, or 16‐6‐16 [1,6‐bis(N,N‐alkyldimethylammonium) adipate] were determined using tensiometry. The results were analyzed in the light of different theoretical models from Rosen, Clint, Rubingh, and Motomura. The cmc values decrease with increasing mole fraction of surfactant (α1). The cmcid values (cmc value at ideal mixing conditions) also decrease with α1 but remain above the experimental cmc values. This means that the mixed micelles form as a result of attractive interactions. These interactions are also seen in surface excess concentration (Γmax) and minimum area per molecule (Amin) data: Γmax increases and Amin decreases. Both Rosen's and Rubingh's models indicate synergistic interactions (interaction parameter, βm and βσ, values are negative). The βm values are larger in magnitude than βσ for 14‐6‐14 and 16‐6‐16 systems, whereas the reverse is the case with 12‐6‐12 because the surfactant's short chain makes adjustment in the core difficult for both components.