The thermodynamic stability of the mixtures of hyperbranched poly(ethyleneimine) and sodium dodecyl sulfate at low surfactant-to-polyelectrolyte ratios

The equilibrium nature of the association between the hyperbranched poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) has been investigated in the presence of excess polyelectrolyte. It was found that the thermodynamic stability of the system considerably depends on the ionization degree of the PEI molecules. In the case of slightly charged PEI molecules, the PEI/SDS mixtures are thermodynamically stable solutions in the pre-precipitation concentration range. In contrast, at low and moderate pH kinetically stable colloidal dispersions of the positively charged PEI/SDS particles can be observed at low surfactant-to-polyelectrolyte ratios. These dispersions are stabilized by the uncompensated charges of the PEI molecules. In addition to the primary PEI/SDS particles, larger aggregates may also appear in the mixtures. The higher the protonation degree of the PEI molecules and the smaller the net charge of the primary PEI/SDS particles, the more likely the aggregate formation becomes.