Polyelectrolyte/surfactant interaction: an NMR characterization

Interaction between a cationic copolymer (acrylamide-trimethylaminoethyl acrylate)P(AAm-TMA) and an anionic perfluorinated surfactant (lithium perfluorooctanoate) (LiPFO) was studied using 1H and 19F NMR. To clarify the effect of the charge density, copolymers having a different composition of comonomers have been used. In each copolymer-surfactant system the presence of the polyelectrolyte in solution strongly modifies the 19F chemical shifts. Consequently, a strong polymer-surfactant interaction is evidenced. A complete saturation of the cationic charges of the polymer is clearly detectable, followed by a partial precipitation and subsequent redissolution of the system. The chemical shifts and line widths observed have been interpreted as dependent on two main factors: the aggregation state of the system and the rate of exchange among different sites. 1H spectra, which run at a constant concentration of polymer and different concentrations of surfactant, appear rather broad and poorly defined. The spectral broadening becomes extremely marked at certain value C of the surfactant concentration, where a strong super-Lorentzian line appears, surmounted by weak sharp lines in which the chemical shift is still visible. Increasing the charge density the critical concentration C increases. A further increase in the surfactant concentration causes a sharp precipitation which is accompanied by the disappearance of the super- Lorentzian resonance. This critical concentration value C may be interpreted as being due to the so-called “cac”, i.e., the concentration of critical aggregation. At a higher surfactant concentration, after the resolubilization, a super-Lorentzian 19F line is observed superimposed again by a sharp spectrum. The presence of the super-Lorentzian 19F line can be interpreted as an indicator of the incoming gelification of the mixture. A model of interaction has been proposed on the basis of the results obtained.