Osmotic
coefficient measurements have been reported for binary
aqueous solutions of urea (0.11400 to 1.03192 mol·kg–1) and ternary aqueous solutions containing a fixed concentration
(0.10065 mol·kg–1) of polyethylene glycol 4000
(PEG-4000) and various concentrations of urea (0.03882 to 0.70509
mol·kg–1) at 298.15 K using vapor pressure
osmometry. For the binary (urea + water) system, using experimental
osmotic coefficient data, the activity and activity coefficient of
urea and water are calculated, and further activity data are processed
to obtain the Gibbs free energy change of mixing and excess Gibbs
free energy change of mixing. The water activities for the studied
ternary system (water + 0.10065 mol·kg–1 PEG-4000
+ urea) were calculated and used to obtain the activity coefficient
of PEG-4000 and urea. The transfer free energy changes of PEG-4000
from water to aqueous urea solutions (ΔG
tr
1) and that of urea from water to aqueous PEG-4000
solutions (ΔG
tr
2) have
been calculated using the activity data. The pair and triplet interaction
parameters are estimated by applying McMillan–Mayer theory
of virial coefficients to the data of transfer Gibbs free energy.
The salting constant (k
s) value and equilibrium
constant (log K) value have been estimated for PEG-4000–urea
complexation at 298.15 K. All these results are discussed in light
of binding equilibria and water structure alteration concepts. It
has been shown that the hydrophilic interaction of urea with the backbone
of a polymer is also contributing to solution properties.