The Effect of the Molecular Size and Shape on the Volume Behavior of Binary Liquid Mixtures. Branched and Cyclic Alkanes in Heptane at 298.15 K

Excess molar volumes V (E) for 40 mixtures of heptane with a liquid alkane and apparent molar volumes in heptane for eight solid alkanes have been obtained at 298.15 K. They include five linear, 30 branched-chain, and 13 cyclic alkanes. Almost all systems exhibit negative V (E) values. For mixtures with open chain alkanes, V (E) increases from C5 to C7 and then decreases. A similar trend is shown by mixtures with cycloalkanes. V (E) values are compared with known H (E) data for mixtures with heptane and tetrachloromethane. Signs and trends of V (E) and H (E) are correlated with the free volume and interactional terms of the Flory theory. The partial molar volumes at infinite dilution in heptane, VA degrees, have also been obtained and discussed together with literature data on other hydrocarbons and polar compounds. The calculated contributions to VA degrees by CH3, CH2, CH and C groups are compared with previously determined contributions of polar groups. The lower contributions of the latter groups are explained with the volume contraction caused by dipole-induced dipole interaction. The volume effects associated with branching and cyclization have been evaluated and compared with the corresponding effects on solvation enthalpy. The branching effect, in the order of magnitude of few cm(3)center dot mol(-1), and the larger negative values of cyclization volumes, down to -24 cm(3)center dot mol(-1), are discussed in terms of packing and solute-solvent interactions, in analogy to polar organic solutes either in heptane and tetrachloromethane. A negative cyclization effect is also exhibited by the solvation enthalpies.