Two Assumptions in the Theory of Attractive Forces between Long Saturated Chains

Abstract

A recent calculation by Salem of the attractive forces between two long parallel hydrocarbon chains was based on two assumptions: (1) pairwise additivity of dispersion forces between individual groups on each chain, and (2) isotropy of the polarizability of a group on the chain. The validity of these assumptions is examined here by means of a Drude model calculation of the interaction between two parallel linear lattices of dispersion oscillators. We describe an exact calculation on this model, and also a calculation based on applying Salem's assumptions to the same model. We conclude that the assumptions are not valid for the Drude model, because of strong induced‐dipole interactions between neighbors along the lattice. Salem's procedure leads to essentially correct numerical results, however, due to use of an experimentally determined bond polarizability at a crucial stage of the calculation. This allows for the effects of interactions between neighbors. Our results are applicable to real hydrocarbon chains only to the extent that the Drude model is valid.