Poly((N,N,N-trimethylammonio)ethyl acrylate chloride salt) (PCMA)−SDS Complex Formation in Dilute Aqueous Solution. Light Scattering and Time-Resolved Fluorescence Quenching Measurements

Abstract

This paper describes changes in the cationic polyelectrolyte PCMA coil conformation when the anionic surfactant SDS is added in the presence of a simple salt. Four different SDS/PCMA mass concentration ratios (0.074, 0.149, 0.223, and 0.298, corresponding to 0.05, 0.10, 0.15, and 0.20 molar concentration ratios, respectively) have been examined at different NaCl concentrations. Higher ratios than the above give precipitation. When the polyelectrolyte−surfactant complex is formed, the hydrodynamic radius, Rh, is smaller than that for the pure polymer as a result of a combination of electrostatic and hydrophobic interactions. The decrease in Rh on addition of SDS to the PCMA/NaCl solutions is strong even at these low ratios. Inverse Laplace transformation (ILT) of the autocorrelation functions showed that at low salt concentrations there are two modes corresponding to the presence of aggregates (clusters) and the single-coil polymer−surfactant complex. The slow component is distinguished from the analogous component in the earlier studied NaPSS−CTAB system for which the relative intensity of the cluster mode increases strongly on addition of a simple salt. Static light scattering results in 100 mM NaCl show a decrease in the radius of gyration, Rg, as the SDS concentration is increased, in agreement with the observed changes in Rh. The concomitant decrease in the second virial coefficient, A2, when the surfactant/polyelectrolyte ratio is increased, reflects the reduction in the interaction potential when the complex approaches precipitation. Aggregation numbers, Ns, for the bound SDS micelles were measured at different SDS/PCMA ratios in 50 and 100 mM NaCl using time-resolved fluorescence quenching. All measurements resulted in higher Ns values than those for SDS in the absence of PCMA at the same concentration of NaCl.