Isotope Effects on Molar Volume and Surface Tension: Simple Theoretical Model and Experimental Data for Hydrocarbons

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Abstract
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.