Effect of surfactants on the cloud point of amphiphilic drug solutions

The effect of cationic (alkyltrimethylammonium halide), anionic (sodium alkyl sulfate) and nonionic (Pluronic) surfactants on the cloud point (CP) of amphiphilic drug amitriptyline (AMT) solutions has been investigated in the absence and presence of sodium phosphate (SP). The CP increases on addition of ionic surfactants in the presence of small amounts of SP. Furthermore, the CP increases with the alkyl chain length of the ionic surfactants. Unexpectedly, the rate of increase in CP on addition of anionic surfactants is greater than cationic surfactants in the presence of small amounts of SP because the added anionic surfactants are adsorbed on the AMT micellar surface. The CP becomes less sensitive to addition of ionic surfactants in the presence of large amounts of SP. Addition of Pluronic F-127 (poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–PEO triblock copolymer) to the AMT solution increases the CP, whereas addition of Pluronic 10R-8 (PPO–PEO–PPO triblock copolymer) decreases the CP. These results have been discussed in terms of interactions between AMT micelles and added surfactants.