Anisotropic orientation in molecular monolayers by infrared spectroscopy

A set of best values for the temperatures and enthalpies of fusion and transition for the n -paraffins is presented. From an analysis of these data a general qualitative theory of the phase behavior of the n -paraffins is developed. Four distinct crystal structures—hexagonal, triclinic, monoclinic, and orthorhombic—describe the solid phases of all n -paraffins with more than nine carbon atoms in the chain. The latter two structures become equivalent at longer chain lengths. Odd-even differences are resolved in terms of reasonable differences in end group packing, and the smooth increase in melting and transition temperature with increasing chain length is attributed to a decrease in the ratio of end groups to chain groups. Double transitions are predicted for several pure n -paraffins above n -C 34 H 70 . Impurity effects are isolated from the pure n -paraffin properties and discussed. The equation, T M (°K) = 414.3 ( n −1.5)/( n + 5.0) is presented as a correct description of the melting temperatures ( T M ) of all n -paraffins above n -C44H 90 . Sufficient data to permit an accurate extrapolation of the enthalpies and entropies of fusion to the infinite-chain limit are not available.