Attractive Forces between Long Saturated Chains at Short Distances

The assumption of locally additive forces is used to describe the London—van der Waals dispersion attraction between long saturated chains. For two linear chains of length L, opposed and parallel, each built out of N identical units of length λ(Nλ=L), the total dispersion energy is given by W=A4λ2D4ρ(3tan−1ρ+ρ1+ρ2), ρ=LD, where D is the mutual distance of the two chains; A is the coefficient of the dispersion interaction between two basic units. At distances D much smaller than the molecular length L, the energy becomes proportional to L (or N) and inversely proportional to the fifth power of the intermolecular distance. The coefficient A is calculated for the interaction between two CH2 units in hydrocarbon chains and the result is used to estimate the sublimation energy at 0°K of paraffin crystals. Sound agreement with experiment is obtained. Finally, the particularities of the attractive forces between long saturated chains are examined in more detail and their great sensitivity to distance is shown by several examples. Various applications to systems of biological interest are also suggested.