Hydrodynamic Radius of Poly (Ethylene Glycol) 6000 Determined via Characterization of Composition-Dependent Aqueous Two-Phase Equilibrium System

Abstract

This work was carried out to develop a simple method for the determination of hydrodynamic radius of poly (ethylene glycol) 6000 (PEG-6000). The contribution of the activity coefficients of PEG-6000 and a mixture of phosphate and sodium chloride salts to the apparent equilibrium constant of distribution of PEG-6000 in an aqueous two-phase system containing 135 – 270 g dm-3 of PEG-6000 were quantified at temperatures between 15 - 40oC. The volume of the salt-rich phase increases with increasing ionic strength of the solution at constant temperature and fixed PEG-6000 concentration. The contribution of the ionic strength to the activity of the ATPS was determined from the plot of the logarithm of equilibrium distribution of PEG-6000 against the ionic strength according to Specific Ion Theory (SIT) equation. The activity coefficient of PEG-6000 was quantified by expansion of its natural logarithm in terms of PEG-6000 concentration and the two- and three-body interaction coefficients. Under conditions of limiting dilution of ionic strength, at each temperature, the natural logarithm of the activity coefficient of PEG decreases with increasing PEG-6000 concentration to a minimum at critical concentration, ca. 230.23 ± 1.81 g dm-3 and then increases above the critical concentration.  Hydrodynamic radius of PEG-6000 was estimated to be 2.18 ± 0.02 nm. This result is in good agreement with the values of hydrodynamic radius determined using advanced equipment.