Hydrodynamic radius of poly (ethylene glycol) 70000 determined under strongly non-ideal solution conditions

Hydrodynamic radius of highly concentrated poly (ethylene glycol) 70000 (PEG70K) molecules in phosphate buffer pH 7.4 (total ionic strength between 0.2 – 1.4 mole dm ^-3 ) was determined using turbidity measurement at 600 nm. Analyses of the distribution equilibrium data of the experiment was carried out by eliminating the effect of high ionic strength on the polymer solution. The maximum of the dependence of natural logarithm of activity coefficient contribution of PEG70K to the phase separation gives the critical concentration (the PEG70K concentration at which the activity coefficient is maximum). The critical concentration was determined by equating the derivative of the quadratic equation of the best-fit curve through the experimental data with respect to concentration to zero. Assumption was made that PEG70K is composed of closely packed, spherical, impenetrable molecules whose energy of attraction is equal to that of repulsion at critical concentration. The hydrodynamic radius value of (9.11±0.32) nm here reported is found to be in good agreement with theoretical estimated value of 8.78 nm for unfolded protein of equivalent molecular weight. This value also lies between 6.59 nm for PEG35000 and 11.95 for PEG100000 experimentally determined from viscosity measurement.