Effect of Polymer Architecture on the Phase Behavior and Structure of Polyelectrolyte/Microemulsion Complexes (PEMECs)

We studied polyelectrolyte/microemulsion complexes (PEMECs) formed by the biopolymer sodium hyaluronate (NaHA) and cationic oil-in-water (O/W) microemulsion droplets. Around equimolar charge conditions, a two-phase region with a liquid–liquid phase separation (coacervate formation) is observed, while mixed complexes are formed at polyelectrolyte excess. The largest complexes are found close to the phase boundary. The detailed structure of these complexes was determined by a combination of static and dynamic light scattering (SLS and DLS), small-angle neutron scattering (SANS), and cryo-transmission electron cryomicroscopy (cryo-TEM). Interestingly, these complexes formed with NaHA are much more elongated compared to previously studied complexes formed with sodium polyacrylate (NaPA). Similar observations were made for another polysaccharide, sodium carboxymethyl cellulose (NaCMC). Apparently, the persistence length of the complexes is largely proportional to the persistence length of the polyelectrolyte, which was determined by analyzing the SANS data with a specifically developed model. Accordingly, the size of the complexes formed with stiffer biopolyelectrolytes becomes much larger, increasing with the molecular weight (M w) of the polyelectrolyte. In summary, we conclude that the size and structure of the PEMECs can be controlled by the type of polyelectrolyte, mixing ratio, size of the microemulsion droplets, and M w of the polyelectrolyte. This means that the polyelectrolyte addition can be used as a key structuring element for the size and shape of such complexes.