Hexadecane-Nanoemulsion-Doped Polyacrylamide Hydrogels

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.