Deuterium Isotope Effects in Binary Critical Mixtures Study of the Systems Methanol/Isooctane and Acetonitril/Water

The shift of the critical temperature of binary mixtures with an upper critical point caused by a replacement of hydrogen atoms by deuterium in different parts of the molecules is studied. The systems are methanol (methanol, methanol‐d1, methanol‐d4)/isooctane and acetonitrile (acetonitrile, acetonitrile‐d3)/water (H2O, D2O). It is found that replacement of hydrogen atoms involved in hydrogen bonding (OH‐group of methanol, OH‐group of water) by deuterium leads to an increase of the miscibility gap.