Isotope Effects on Molar Volume and Surface Tension: Simple Theoretical Model and Experimental Data for Hydrocarbons
Experimental measurements of molar volumes and surface tensions of benzene, toluene, cyclohexane, and methylcyclohexane and of their completely deuterated analogs are presented. The lighter species have a greater volume by about 0.3%. Surface tensions of the lighter species are greater, also, and increase from 0.2% to 1% as the number of hydrogens per carbon increases. Results are interpreted on the basis of the bond-stretching anharmonicity which makes C-H bonds longer and more polarizable than C—D bonds. The ``steric'' and polarizability effects are comparable in magnitude but opposite in sign in their influence on cohesive properties. It is shown by a simple theoretical model that the ``pure-mass effect'' on molar volume arising from zero-point translational motions of the molecules as a whole is negligible in comparison with the effects stemming from zero-point motions of the C-H and C—D bonds.