Stoichiometry of poly(ethylene glycol) in water and benzene by excess volume

In this work, we studied the excess volume of solutions of low molecular weight poly(ethylene glycol)s (200, 400, and 600) in water and in benzene. We noted that there is a higher contraction of the volume for water solutions in all compositions. Solutions of these polymers in benzene also undergo a volume contraction. However, the magnitude of this effect is lower for benzene solution than for water. Moreover, the stoichiometry of the system at the composition corresponding to the maximum volume contraction is different in each case: one water molecule per two oxyethyelene (EO) units, and one benzene molecule per four EO units. The results are compared with the mixing enthalpy measured by Lakanpal et al.(29) As reported, they observed that while the system PEG/water is exothermic in all compositions, the system PEG/benzene is endothermic, which shows that although in both solvents there is volume contraction, the driving forces for these dissolutions are different. (C) 1997 John Wiley & Sons, Inc.