EMF measurements on binary mixtures of the cationic surfactants tetradecyltrimethylammonium bromide and tetradecylpyridinium bromide and a range of nonionic polymers have been carried out. The existence of polymer/surfactant complexes was found for those polymers whose microstructures contain side groups with hydrophobic character. Isothermal titration microcalorimetry measurements were also taken to obtain more information about the polymer/cationic surfactant complexes. An examination of the dependence of the monomer surfactant concentration following the binding showed that "free" micelles are formed in solution before the polymers become "saturated" with bound surfactant. In some cases these free micelles occur in solution before the critical micellar concentration of the pure surfactantsan effect which is thought to occur due to an excess counterion concentration in the microenvironment at the vicinity of the complex. Finally a comparison of the present data with the binding behavior of these polymers with sodium dodecyl sulfate (SDS) allows us to identify the roles of electrostatic and hydrophobic interactions that take place to allow the formation of stable polymer/surfactant complexes.
Electromotive force and isothermal titration microcalorimetry measurements have been carried out to study the interactions of sodium dodecyl sulfate (SDS) with polyethyleneimines (PEI's) and some ethoxylated PEI's at different pH's. In all cases the polymers show a remarkable affinity toward SDS. The SDS concentration at the onset of binding decreases with decreasing pH. At low pH's phase separation similar to that observed for strong polyelectrolyte/oppositely charged surfactant systems occurs presumably as a consequence of the protonation of the N atoms in the polymer to form a polycation. The SDS range over which phase separation occurs decreases as the size of the ethoxylated chains increases, and for the polymer with the longest ethoxylated chain no precipitation occurs. In many cases where phase separation occurs, as more SDS is added the polymer/surfactant complex often resolubilizes and binding proceeds until the polymer becomes saturated with bound SDS. For any given polymer at this limiting stage in the binding, the number of surfactant molecules bound per mole of polymer is independent of pH. In addition both the charged and uncharged N atoms as well as the pendant ethoxylated chains make significant contributions to the binding process.
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