Binding of sodium dodecyl sulfate to some polyethyleneimines and their ethoxylated derivatives at different pH values. Electromotive force and microcalorimetry studies

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 differen t pH's. In all cases the polymers show a remarkable affinity toward SDS. Th e SDS concentration at the onset of binding decreases with decreasing pH. A t low pH's phase separation similar to that observed for strong polyelectro lyte/oppositely charged surfactant systems occurs presumably as a consequen ce of the protonation of the N atoms in the polymer to form a polycation. T he SDS range over which phase separation occurs decreases as the size of th e ethoxylated chains increases, and for the polymer with the longest ethoxy lated chain no precipitation occurs. In many cases where phase separation o ccurs, as more SDS is added the polymer/surfactant complex often resolubili zes and binding proceeds until the polymer becomes saturated with bound SDS . For any given polymer at this limiting stage in the binding, the number o f surfactant molecules bound per mole of polymer is independent of pH. In a ddition both the charged and uncharged N atoms as well as the pendant ethox ylated chains make significant contributions to the binding process.