| Abstract
| - A poly(ethylene oxide) derivative of polyethyleneimine behaves like a strong polyelectrolyte at pH = 2.5and a neutral polymer at pH 10. Both the charged and uncharged versions of this polymer bind stronglyto the surfactant sodium dodecyl sulfate (SDS) with no phase separation taking place. Binding isothermswere measured using a dodecyl sulfate electrode, and these data were complemented with isothermaltitration calorimetry (ITC) measurements. Small-angle neutron scattering measurements were also carriedout at some specific concentrations in the binding region at pHs 2.5, 5.5, and 10. With the exception ofone measurement, bound micelles were detected and their aggregation numbers could be evaluated. Forthe SDS/polymer system at pH 10, the polymer/surfactant complex contains 6−8 bound SDS micelles perpolymer molecule at the binding limit. In a solution of 6.5 mM SDS/0.5% w/v polymer at pH 10, boundSDS exists in a nonaggregated form. A detailed examination of the ITC data for the SDS/0.5% w/v polymersystem shows at pH 10 that this spot solution occurs in a narrow SDS concentration range immediatelyfollowing the onset of binding and proceeding until the formation of proper bound micellar aggregates wasdetectable, whose presence and growth were characterized in the ITC experiments by a steplike decreasein the enthalpy per injection as a function of increasing SDS concentration. These data suggest that theabsence of proper micellar aggregates is an inherent consequence of the binding mechanism in the earlystages of SDS binding to ethoxylated polyethyleneimines.
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