Interaction of partially ionized poly(acrylic acid) with a surfactant counterion at constant pH values

Binding of a cationic surfactant ion, dodecylpyridinium ion, to poly(acrylic acids) of low charge densities was examined by potentiometry using surfactant-selective electrodes in the solutions, where the pH was kept constant by employing a pH buffering system. The binding of the surfactant counterions was thus able to be studied at a constant pH during the binding process. The binding took place in two steps, the first cooperative binding step and the second gradual binding step. The critical association concentration decreased as the pH increased, indicating the predominant role of the electric interaction in the binding. The binding isotherms obtained at different but constant pH values were analyzed by the matrix method, taking into account the nearest-neighbor interactions among three different kinds of sites on the polymer: ionized, protonated, and surfactant-bound. The theoretical analysis could describe only the first step but could not explain the second step. A relatively large cooperativity parameter, u, was found for the first step and it can be between 3 × 10³ and 1 × 10⁴. When the ionic strength was decreased tenfold, the cooperativity of the binding decreased (u∼1 × 10³). The binding constants of the isolated site were 5.5–6.0 × 10⁴ kg mol⁻¹ and slightly increased to 6.5 × 10⁴ kg mol⁻¹ as the ionic strength decreased. The deviation of the second step from the theoretical analysis was supposed to arise from a change of proton dissociation constant in the nonpolar space formed by the bound surfactants. Received: 29 November 2000/Accepted: 24 January 2001