Hydrodynamic approximation of particle anisometry in solid lipid nanoparticles: Influence of lipid and emulsifier

Solid lipid nanoparticles (SLNPs) may be used as carriers for lipophilic drugs but also offer potential for the formulation of hydrophilic biomacromolecules via surface adsorption. Regarding the latter, specific surface area and particle anisometry are key parameters, yet quantitative methods to assess them for non-spherical particles remain limited. In this study, a hydrodynamic approach based on Perrin's theory for oblate ellipsoids was applied to approximate particle anisometry from dynamic light scattering data. Using this approach, the influence of matrix lipid type, particle size, emulsifier type, and emulsifier concentration on the shape of SLNPs was systematically investigated. Along the triglyceride series trilaurin to tristearin, particle anisometry increased with chain length, consistent with the formation of thinner, more platelet-like particles. Smaller SLNPs exhibited lower anisometry than larger particles. Interfacial effects were pronounced: post-modification with different emulsifiers resulted in marked differences in hydrodynamically approximated anisometry, with sucrose monolaurate-treated systems showing the highest and poloxamer 407-treated systems the lowest apparent anisometry. Increasing the concentration of poloxamer 188 reduced anisometry, whereas increasing the concentration of sucrose monolaurate increased it. Electron microscopic imaging qualitatively confirmed the calculated trends in particle shape. Overall, the presented method provides a practical screening tool for comparing relative changes in SLNP anisometry. It also enables the assessment of formulation-dependent differences in particle shape characteristics, which may be highly relevant for optimizing the adsorption of hydrophilic macromolecules to nanoparticle surfaces.