Two-step surfactant binding of solvated and cross-linked poly(N-isopropylacrylamide-co- (2-acrylamido-2-methyl propane sulfonic acid))

Abstract

 A solvated and cross-linked copolymer of N-isopropylacrylamide (IPAAm) and 2-(acrylamido)-2-methyl propane sulfonic acid (AMPS) was synthesized and its interaction with cationic surfactant lauryl-pyridinium chloride (C₁₂PyCl) was investigated. The solvated copolymer exhibited a lower critical solution temperature (LCST) in water, which was extensively shifted to a higher temperature due to the increase of hydrophilicity introduced by AMPS. In C₁₂PyCl solution, LCST of the copolymer was dramatically decreased due to the binding of C₁₂PyCl to AMPS unit, forming a stoichiometric complex. However, in the concentrated C₁₂PyCl solution, its LCST increased due to the non-stoichiometric complex formation. This phenomenon was further examined in the cross-linked copolymer, analyzed by binding isotherms. Two-step binding of surfactant was demonstrated followed by gel shrinking and re-swelling. This binding mechanism was further discussed regarding the effect of charge density and the hydro-phobicity of the main-chain backbone in terms of electrostatic and hydrophobic interactions.