Milling-Induced Chitosan–HPMC Microstructural Organisation Determines the Stability Window of Concentrated Azelaic Acid Nanosuspensions

Objectives: The physical stability of concentrated azelaic acid nanosuspensions cannot be predicted solely from interfacial stabilisation but reflects the interplay between particle size-dependent thermodynamic driving forces, polymer-mediated rheological structuring and the resulting bulk microstructural organisation. This study employed an A-optimal design of experiments (18 runs) to investigate formulation–process relationships in concentrated azelaic acid nanosuspensions (10–20% w/w) stabilised with a dual HPMC–chitosan system. Methods: Particle size, ζ-potential, rheological behaviour, solid-state properties and in vitro permeation across Strat-M® membranes were evaluated, and formulation–process relationships were analysed using multivariate modelling and logistic regression. Results: Particle size (342–1118 nm; R2 = 0.97) was primarily governed by the applied milling regime. Logistic regression demonstrated a size-dependent probability of crystal growth, with the estimated particle-size transition point (predicted probability = 0.5) shifting from approximately 424 nm after preparation to 594 nm following accelerated storage at 40 °C. CHI concentration controlled ζ-potential (+18.5 to +47.9 mV), although ζ-potential alone did not adequately explain the observed storage stability. Where measurable, zero-shear viscosity (68–45,462 mPa·s) reflected substantial differences in low-shear rheological structuring, while formulations containing higher HPMC concentrations generally exhibited improved stability, consistent with polymer-mediated kinetic constraints on crystal growth. In vitro permeation studies using Strat-M® membranes demonstrated permeation behaviour consistent with structured diffusion-controlled systems, exhibiting lower flux but more uniform permeation profiles than the reference formulation under the applied experimental conditions. Conclusions: Integration of multivariate modelling with rheological, solid-state and permeation characterisation provided an integrated understanding of the formulation–process relationships governing the short-term physical stability and comparative in vitro transport behaviour of concentrated dermal AZA nanosuspensions.