Study of interaction of poly(ethylene imine) with sodium dodecyl sulfate in aqueous solution by light scattering, conductometry, NMR, and microcalorimetry

Light scattering studies show that in aqueous solution poly(ethylene imine) (PEI) exists largely in the form of individual macromolecules plus a small fraction of aggregates. The aggregates make a large contribution to the sc attering signal but only a very small contribution to the solution viscosit y. Addition of sodium dodecyl sulfate (SDS) to the solution has a number of interesting consequences. Microcalorimetry experiments show that well belo w the critical micelle concentration of SDS, individual SDS molecules add t o the PEI through an exothermic process. At higher SDS concentrations, ther e is a noncooperative adsorption, which is endothermic in nature, of SDS mi celles onto the polymer chains. The surfactant-polymer complex likely conta ins several polymer molecules. These solutions are characterized by a highe r specific conductivity than can be explained by the sum of the conductivit ies of all. the individual ions in solution, even if the Na+ and DS- ions w ere free in solution and not bound to the polymer. Pulsed-gradient NMR meas urements were carried out to examine the Na+ and DS- ion mobility in the so lutions. These measurements showed that surfactant binding to the polymer r eleased sodium ions from the SDS micelles. The increase in pH showed that t his binding also releases a small amount of OH- into the solution. These tw o effects by themselves are not large enough to account for the measured co nductivity of the solutions. We speculate that there is high ionic mobility inside the polymer-surfactant complex that adds to the overall conductivit y of the solution.