Hydrophobic Complexation of Poly(vinyl caprolactam) with Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Bromide in Solution

Abstract

The interaction of poly(vinyl caprolactam) (PVCAP) with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solutions has been studied systematically by measuring the phase separation temperature, hydrodynamic radius, pyrene solubility, and surfactant binding isotherms. Both surfactants were observed to elevate the solution cloud point of PVCAP and cause the polymer to undergo a coil to globule transition. This transition occurs at a concentration about 1/10 of the critical micelle concentration (cmc) for SDS in the absence of polymer but at the cmc with DTAB. The results indicate that PVCAP interacts with SDS monomers but only with micelles in the case of DTAB. The phase behavior of the PVCAP/SDS/DTAB ternary system shows that the binding of SDS to PVCAP is reversible on changing the concentration of the free surfactant monomer in solution. Potentiometric titration of PVCAP and measurements of pyrene solubility in its mixtures with surfactants suggest that complexation of PVCAP and SDS is due to a combination of ion−dipole and hydrophobic effects. PVCAP and DTAB micelles interact through hydrophobic inclusion of polymer segments into the DTAB micelles. Pyrene is not solubilized by PVCAP in solution alone. Addition of SDS to PVCAP solutions induces marked pyrene solubilization well below the cmc, characterized by a region indicating saturation adsorption of the pyrene to the PVCAP/SDS complex. Above the SDS cmc, solubilization of pyrene increases linearly with SDS concentration, corresponding to inclusion of the pyrene into the SDS micelles. In contrast to SDS, the addition of DTAB to a solution of PVCAP shows no pyrene solubilization until the cmc is reached.