Chain Length Dependence of the Thermodynamic Properties of Linear and Cyclic Alkanes

摘要

The specific heat capacity and the heats of the solid-liquid and the solid-solid transitionsrnwere measured with step-scan DSC for linear alkanes from pentane (C_5H_(12)) to nonadecanern(C_(19)H_(40)) and for several cyclic alkanes. For the linear alkanes, the specific heat capacity in thernequilibrium liquid state decreases as chain length increases; above a carbon number N of 10rn(decane) the specific heat asymptotes to a constant value. In addition, only the solid-liquidrntransition is observed in the even numbered alkanes, while additional solid-solid transitions arernobserved before the solid-liquid transition in the odd-numbered alkanes from nonane (C_9H_(20)) tornnonadecane (C_(19)H_(40)). It is found that the sum of the entropy of the solid-solid and solid-liquidrntransitions in each odd-numbered alkane is the same as the entropy of the solid-liquid transitionrnin even-numbered alkanes and is independent of the chain length. For the cyclic alkanes wernstudied, the heat capacity in the equilibrium liquid state is lower than that of the correspondingrnlinear chains, and increases with increasing chain length. The thermal expansion coefficient ofrnthe linear and cyclic alkanes were calculated from literature data and compared with the trends inrnthe specific heats. For the linear alkanes the thermal expansion coefficient decreases withrnincreasing carbon number in a similar manner as the specific heat. For the cyclic alkanes thernthermal expansion coefficient also decreases with increasing carbon number; this is trend isrnopposite that observed for the specific heat.