Electrostatic control of soft interfaces: charge stabilization in colloids, emulsions and polymer blends
Professor Johannes Zwanikken, University of Massachusetts Lowell
Electrolytes play a vital role in numerous biological processes, and are key to the stability of many systems in Soft Condensed Matter, such as colloids, emulsions, and solutions of charged macromolecules. It is well known that ions 'screen' the interactions between charged solutes, typically over a few nanometers in aqueous solutions. This screening length can, however, extend over enormous distances in apolar media, exceeding tens of micrometers, and support the stability of additive-free water-in-oil emulsions. I will discuss several mechanisms that electrify an intrinsically neutral interface, based on ion partitioning and ion-ion correlations near the interface.
Recently, we used Car-Parrinello Molecular Dynamics and liquid state theory to interrogate electrolyte distributions in narrow confinement, elucidating the structure at the ionic length scale. We find clear signatures in the pair correlation functions and scattering functions of a dense liquid-like structure and preferential adsorption to the interface, even in aqueous solutions of monovalent ions, for concentrations above 100 mM. These results explain the versatile aspects of charge stabilization from a molecular perspective.
We apply the insights to interrogate the phase behavior of polyelectrolyte networks and charge-neutral diblock-copolymers, and identify charge as a powerful parameter to tune the phase diagram in multiple directions, and even enable phases that are inaccessible to neutral polymers of a similar topology [2,3].
External electric fields offer another independent tuning parameter, as will be demonstrated for hydrogels under oscillatory electric fields. Understanding the molecular origin of charge stabilization may provide enhanced control and tunability in the development of soft ionic materials, and enable additional directions for their design.