We studied the effects of a high molecular weight polymer (polyacrylic acid, PAA, $M_w=7.5\mathrm{}\times {10}^5)$ on the coexistence curve of a binary liquid mixture (2,6-lutidine + water, LW). A high precision refractometer was built to measure the temperature dependence of the refractive indexes of the two coexisting phases of phase separated LW samples containing less than 1 mg/cc of PAA. From the refractive indexes we mapped out the coexistence curve and obtained the refractive index difference $▵n$ between the two coexisting phases. At temperature T close to the critical temperature $T_c,$ the critical exponent $\beta$ such that $▵n\propto (T-T_c{)}^{\beta }$ was found to be $0.41\pm 0.02$ which is higher than $(0.31\mathrm{}\pm 0.01)$ that of pure LW.

• Received 21 September 2000

DOI:https://doi.org/10.1103/PhysRevE.63.026108