Isotopic binary liquid mixtures are universally characterized by an upper critical solution temperature as a consequence of zero-point motion in conjunction with the anharmonicity of the interatomic potential. This quantum effect is predicted, and demonstrated by small-angle neutron-scattering measurements, to be manifest at ambient temperatures for mixtures of deuterated and protonated polymers. Prior evidence of such phase behavior has been restricted to liquid mixtures of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.

• Received 8 May 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.1429