Unusual conductivity changes for sodium dodecyl sulfate solutions in the presence of polyethyleneimine and polyvinylamine

The interaction of two polyamines, hyperbranched polyethyleneimine (PEI) and linear poly(vinylamine) (PVAm), with the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution was studied at high pH by conductometry and potentiometry. At high pH, these polymers are protonated to only a very small degree. As very weak polyelectrolytes, they should behave in a manner similar to that for nonionic polymers. Upon titration of the PEI and PVAm aqueous solutions with SDS, the conductivity increases over the whole range of SDS concentrations. The character of the conductivity changes, however, is different from that of typical nonionic polymers, such as poly(ethylene oxide) or paly(vinylpyrrolidone). Unlike these nonionic polymer-surfactant systems, the absolute values of the specific conductivity of aqueous SDS solutions in the presence of PET and PVAm are higher than that for pure SDS over the whole range of SDS concentrations. Potentiometric measurements showed that binding of SDS to PEI and PVAm is accompanied by the consumption of protons and, as a consequence, by an increase in the pH of the solutions. It has been shown that the contribution of OH- ions to the increase in the conductivity of PEI-SDS and PVAm-SDS systems should be taken into account. Some, other possible mechanisms of the unusual behavior of the conductivity versus [SDS] plots in the presence of PEI and PVAm have been also proposed.