In concentrated protein solutions, short-range attractions (SRAs) contribute to clustering, high viscosity, and liquid−liquid phase separation (LLPS) as a function of temperature and salinity, particularly when the long-range repulsions (LRR) are low near the isoelectric point. Herein, we study how SRA and solution morphology vary with the approach to LLPS from increased SRA for two monoclonal antibodies (mAbs) as salt concentration is reduced near the pI. In addition, we examine a set of mAbs at low ionic strength and at a pH far from the pI, where LRR are strong. The SRA are then compared with LRR across multiple pH and buffer types to understand the effect of formulation on viscosity. These properties are quantified using small-angle X-ray scattering (SAXS) interpreted via coarse-grained (CG) molecular dynamics (MD) simulations and compared low-concentration static and dynamic light scattering. Experimental structure factors are fit with a library of MD simulations for a CG 12-bead mAb model to determine the SRA strength (K) and cluster size distributions. Proximity to LLPS and clustering characteristics in mAb solutions are impacted by both net charge, which are modified by pH, and the strength of anisotropic electrostatic SRA (charge−charge, charge−dipole, hydrogen bonding, etc.), which are screened and weakened by added salts. The trends in LLPS are consistent with the reduced diffusion interaction parameter kD/B22ex for dilute solutions. For systems with stronger LRR, the buffer moiety and pH affect the solution viscosity as electrostatic repulsions are weakened from increasing the buffer screening strength or by increasing the pH toward the pI.
