Abstract
We describe a new methodology to prepare loaded polyelectrolyte/surfactant films at the air/water interface by exploiting Marangoni spreading resulting from the dynamic dissociation of hydrophobic neutral aggregates dispensed from an aqueous dispersion. The system studied is mixtures of poly(sodium styrene sulfonate) with dodecyl trimethylammonium bromide. Our approach results in the interfacial confinement of more than one third of the macromolecules in the system even though they are not even surface-active without the surfactant. The interfacial stoichiometry of the films was resolved during measurements of surface pressure isotherms in situ for the first time using a new implementation of neutron reflectometry. The interfacial coverage is determined by the minimum surface area reached when the films are compressed beyond a single complete surface layer. The films exhibit linear ripples on a length scale of hundreds of micrometers during the squeezing out of material, after which they behave as perfectly insoluble membranes with consistent stoichiometric charge binding. We discuss our findings in terms of scope for the preparation of loaded membranes for encapsulation applications and in deposition-based technologies.
Highlights
Charged polyelectrolyte/surfactant (P/S) mixtures have generated interest for more than one hundred years, primarily because of the huge commercial value of the formulations we use in our everyday lives.[1,2,3]. Their properties at the air/water interface have been the focus of several substantial bodies of work over the last decades, the results of which were typically set in the context of P/S co-adsorption or complex adsorption at equilibrium.[4,5,6]
It was inferred that the aggregates closer to charge neutrality, more abundant in the two phase region yet not detected intact at the interface, must dissociate and spread material upon contact with the surface layer
As the equilibrium state in the two phase region is complete precipitation,[7,8,9] with coagulation of the aggregates taking up to several weeks or even longer,[10,14] their fast, spontaneous dissociation at the air/water interface highlights the important role of surface forces in the dynamic spreading process
Summary
Charged polyelectrolyte/surfactant (P/S) mixtures have generated interest for more than one hundred years, primarily because of the huge commercial value of the formulations we use in our everyday lives.[1,2,3] Their properties at the air/water interface have been the focus of several substantial bodies of work over the last decades, the results of which were typically set in the context of P/S co-adsorption or complex adsorption at equilibrium.[4,5,6] The equilibrium bulk phase behavior of these systems has been thoroughly investigated as well, and can comprise a two phase region where complexes with low charge lack colloidal stability and aggregate, precipitate and either sediment or cream.[7,8,9]The key roles that P/S aggregates can play in determining the properties of the air/water interface have been recently highlighted.
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