No abstract

As a first step toward understanding the electroacoustic response of polyelectrolyte hydrogel nanocomposites, we systematically vary the linear charge density of acrylic acid-co-acrylamide polymer networks (at a fixed total polymer concentration) to ascertain how the charge and mobility affect the electrokinetic sonic amplitude (ESA) spectra. A weakly charged polymer registers a negative ESA (real part), which we attribute to dynamics of the elastic network and its charge, whereas a highly charged polymer registers a positive ESA, attributed to sodium counterions. A mechanistic theoretical model is proposed that attributes the transition to the subtle manner in which the hydrodynamic friction of the network strands scale with their linear charge density. Further insights are gained from the electrical conductivity and dynamic shear moduli (in the linear viscoelastic regime). The electrical conductivity is dominated by the mobility of the sodium counterions and is diminished by network hindrance and counterion condensation, and the storage modulus decreases systematically with the linear charge density, suggesting a dominant role of interchain electrostatics, and possibly electro-steric influence on the polymerization and cross-linking.

Dynamic mobility spectra of sodium dodecyl sulphate (SDS)-stabilized hexadecane nanodrops in aqueous NaCl electrolytes are measured using the electrokineticsonic amplitude, and interpreted using a recently proposed theory for highly charged drops with thin double layers. This novel interpretation shows that emulsion drops exhibit fluidlike dynamics at megahertz frequencies, whereas such drops have conventionally been assumed to behave as rigid spheres because of the interfacial "freezing" effects arising from interfacial Maxwell and Marangoni stresses. Our results lend support to a view of SDSdecorated emulsion dropsin the ∼100−1000 nm rangeas being very highly charged, in a colloidal regime for which the standard electrokinetic model predicts two ζ-potentials for a single steady electrophoretic mobility. Although electrophoretic mobility measurements carried out with light-scattering electrophoresis are conventionally converted to a ζpotential using the Smoluchowski formula, the present experimental and theoretical interpretations suggest that the Smoluchowski ζ-potential obtained with the Guoy− Chapman model may erroneously predict the surface charge density, perhaps explaining whydespite decades of researchit has been so challenging to reach a consensus on how to resolve electrokinetic and thermodynamic studies of SDS-decorated oil−water interfaces. The present work identifies new challenges in interpreting the electrokinetic dynamics at surfactant concentrations above the critical micelle concentration, a regime in which electrostatic screening is not well understood.Article pubs.acs.org/JPCC

Hydrogels are promising supports for nanoemulsion drops since the skeleton provides a physical barrier to coalescence and possibly a thermodynamic barrier to Ostwald ripening. How these factors play a role in the encapsulation of oil drops in polymer networks for drug-delivery applications is largely unknown, owing to the challenge of measuring in situ drop size and understanding the drop–hydrogel interface. In this study, the electrokinetic sonic amplitude of sodium dodecyl sulfate (SDS)-stabilized hexadecane-in-water nanoemulsion drops in polyacrylamide hydrogels is harnessed to ascertain dynamic mobility spectra, which we interpret using a recently proposed model for highly charged nanodrops. The nanodrop radius in hydrogels was found to be close to their nanoemulsion counterparts (∼100–600 nm), with a frequency-independent shear viscosity (∼0.5 mPas at megahertz frequencies) and a frequency-independent shear modulus (∼1 kPa). From a macrorheological perspective, the low-frequency plateau modulus was concluded to increase systematically with the hexadecane volume fraction according to an Einstein intrinsic shear modulus having a quadratic correction for elastic interactions at finite volume fractions. The electroacoustic and micro- and macro-rheological analyses are complemented with swelling, confocal imaging, and electrical conductivity characterization. Together, these point to an “ideal behaving” microstructure that may be tailored for advanced drug-delivery systems. The electroacoustic testing platform developed in this work provides a beneficial tool for noninvasively characterizing the microstructure of nanoemulsion-doped hydrogels.