Asian Journal of Physical and Chemical Sciences

The hydrodynamic radius of poly(ethylene glycol) 8000 (PEG8K) was estimated using an aqueous two-phase equilibrium approach that requires no advanced analytical instrumentation. PEG8K solutions were prepared in phosphate buffer at pH 7.4 over an ionic-strength range of 1.2–2.0 mol dm^-3 and examined at temperatures from 15 to 40°C. The apparent equilibrium distribution constant (K_D) was determined from the phase-volume distribution at fixed PEG8K concentration and temperature. The contribution of electrolyte concentration to solution non-ideality was described using the Setschenov relationship, and extrapolation of log K_D to zero ionic strength provided the equilibrium-distribution term for the electrolyte-free system at each PEG8K concentration. The concentration dependence of this term was then analysed using a McMillan–Mayer virial expansion to quantify PEG8K self-interaction. The fitted dependence passed through a maximum, defining a critical PEG8K concentration at which the attractive and repulsive interaction terms were taken to balance. The critical concentration reported in Table 1 was 182.3 ± 2.7 g dm^-3. Treating PEG8K molecules as approximately hard spherical particles at this concentration yielded a hydrodynamic radius of 2.59 ± 0.04 nm. This value falls within the range discussed in the manuscript for PEG8K values obtained using established methods. The approach therefore provides a simple volumetric basis for estimating the hydrodynamic radius of PEG8K under the investigated high-ionic-strength conditions.