Counterion Effect of Cationic Surfactants Upon the Interaction with Poly(methacrylic acid)

This work investigates decylammonium salts, with inorganic (chloride and thiocyanate) and organic (acetate and butyrate) counterions, in interaction with poly(methacrylic acid) (PMA). The study is conducted by means of surface tension, relative viscosity and pH measurements. The role of the methyl group in PMA appears by comparison with analogous mixtures of poly(acrylic acid) (PAA). In aqueous solution, a non-cooperative interaction between the surfactant and the hydrophobic microdomains (HMD) of PMA starts first. With surfactant addition, micellar aggregates bound onto the polymer chain appear at a concentration T ₁, which is higher than for the analogous PAA systems, but lower than the respective CMC. T ₁ is lower for chloride than for thiocyanate and lower for acetate than for butyrate. Depending on the counterion, the HMD conceal or expose carboxyl groups, with influence upon the interaction strength and the bulk and surface behavior. Stronger interactions and maxima of surface tension and relative viscosity appear for the systems with organic counterions. An estimation of the interaction strength shows weaker surfactant interactions with PMA than with PAA, and that acetate induces the strongest interaction while thiocyanate the weakest.