Assessment of Particle Size-Resolved Surface-Charge Density Distributions of Polymer Nanoparticles by Capillary Zone Electrophoresis

Polymeric nanoparticles (PNPs) are widely used in paints, coatings, and as drug carriers. Surface charge density (SCD) is a key performance parameter of PNPs to ensure PNP stability and adhesion to surfaces, but its detailed assessment is challenging as PNP synthesis yields intrinsically heterogeneous results, exhibiting both SCD and particle-size distributions (PSDs). We present a new method for the determination of PNP SCD distributions using capillary zone electrophoresis (CZE) and chemometric deconvolution. For method development, polystyrene NPs of certified size (50-350 nm) were analyzed using a background electrolyte (BGE) of 3.75 mM sodium tetraborate (pH 9.2). Applying Ohshima's model, the electrophoretic mobilities measured for PS NPs were converted into zeta potentials from which, subsequently, the SCDs were derived. Deconvoluting the effect of NP size on electrophoretic mobility, NP SCD distributions were extracted from the obtained CZE peak widths. This method was applied to industrial PNPs made of polyacrylate- and polyurethane-based copolymers comprising acid-functionalized monomers. PSDs of the PNPs were established using hydrodynamic chromatography and combined with CZE results to create informative heat maps displaying SCD distributions as a function of PNP size. The SCDs appeared to be affected by adsorbed ions, which were displaced by adding a neutral surfactant (Brij-35) to the BGE, allowing determination of intrinsic PNP charges. The final CZE results revealed that the SCDs increased with acidic-monomer content, whereas for all tested industrial PNPs, the mean SCD diminished with growing particle size. These findings demonstrate the unique capability of the new method to provide size-resolved SCD distributions of relevant PNPs.