Association of hydrophobically modified positively charged N-isopropylacrylamide copolymers with the nonionic surfactant Triton X-100

The association in dilute aqueous solution of the nonionic surfactant t-octylphenoxy polyoxyethanol (Triton X-100) with hydrophobically modified copolymers based on an N-isopropylacrylamide (NIPAM) backbone is presented. The NIPAM-based copolymers contain also N,N-[(dimethylamino)propyl] methacrylamide (MADAP), the content x of MADAP varying from 0 up to 25 mol%, completely alkylated with dodecylbromide or octadecylbromide. The highly hydrophobic copolymers are characterised by a compact conformation in aqueous solution, due to the formation of intrachain hydrophobic aggregates. Association with Triton X-100 leads to the destruction of these intrachain aggregates and to their replacement by mixed alkyl/surfactant ones, revealed macroscopically by a gradual viscosity increase upon addition of surfactant. Fluorescence probing studies and dialysis equilibrium experiments confirmed that polymer/surfactant association starts in the vicinity of the critical micelle concentration (CMC) of Triton X-100.