Evidence of Overcharging in the Complexation between Oppositely Charged Polymers and Surfactants

Details Evidence of Overcharging in the Complexation between Oppositely Charged Polymers and Surfactants Evidence of Overcharging in the Complexation between Oppositely Charged Polymers and Surfactants

2005-07-07

arxiv.org/abs/cond-mat/0507176v2

THE JOURNAL OF CHEMICAL PHYSICS 123, 164703 2005

Physics

Condensed Matter

Soft Condensed Matter

20 pages, 7 Figures, 6 Tables

Scientific paper

10.1063/1.2031167

We report on the complexation between charged-neutral block copolymers and oppositely charged surfactants studied by small-angle neutron scattering. Two block copolymers/surfactant systems are investigated, poly(acrylic acid)-b-poly(acrylamide) with dodecyltrimethylammonium bromide and poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide) with sodium dodecyl sulfate. The two systems are similar in terms of structure and molecular weight but have different electrostatic charges. The neutron scattering data have been interpreted in terms of a model that assumes the formation of mixed polymer-surfactant aggregates, also called colloidal complexes. These complexes exhibit a core-shell microstructure, where the core is a dense coacervate microphase of micelles surrounded by neutral blocks. Here, we are taking advantage of the fact that the complexation results in finite-size aggregates to shed some light on the complexation mechanisms. In order to analyze quantitatively the neutron data, we develop two different approaches to derive the number of surfactant micelles per polymer in the mixed aggregates and the distributions of aggregation numbers. With these results, we show that the formation of the colloidal complex is in agreement with the overcharging predictions. In both systems, the amount of polyelectrolytes needed to build the core-shell colloids always exceeds the number that would be necessary to compensate the charge of the micelles. For the two polymer-surfactant systems investigated, the overcharging ratios are 0.66 and 0.38.

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