Binding of Sodium Dodecyl Sulfate to Some Polyethyleneimines and Their Ethoxylated Derivatives at Different pH Values. Electromotive Force and Microcalorimetry Studies

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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.