Effect of polymer chain in coexisting liquid phases by refractive index measurements.

The behavior of polyethylene oxide (PEO, molecular weight, M(w) = 9 x 10(5), as an impurity) was studied in the critical binary mixture of nitroethane (NE) + 3-methylpentane (MP) by refractive index measurements. The measurements were performed at three different PEO concentrations (C = 0.373, 0.759, and 1.509 mg/cc) in the near critical composition of NEMP. We observed that the coexisting phase region shifts down with increasing PEO concentration and the critical temperature (T(c)) decreases linearly with C. At temperatures T close enough to T(c), the critical exponent beta [defined by the relation (n1-n2) proportional (T(c)-T)beta, with n1 and n2 being the refractive indices of the coexisting phases] was found to decrease from 0.456 to 0.372 when the PEO concentration changes from 0.373 to 1.509 mg/cc. These values are higher than that of 0.345+/-0.015 of pure NEMP, which is compatible with the three-dimensional Ising value beta = 0.325. It appears that the shape of the PEO in NEMP coexistence curves is similar from that of pure NE + MP.