The role of attractive forces in the structure of simple liquids: a theory for small-angle scattering

Examines the role of attractive forces in determining the static structure factor S(q) for simple liquids. By treating these forces in a random-phase approximation and the repulsive forces in a blip function expansion about a hard-sphere reference system the authors derive explicit results for the coefficients of q 2 , q 3 and q 4 in the expansion of c(q), the Fourier transform of the Ornstein-Zernike direct correlation function of a Lennard-Jones liquid, they show that the attractive forces ensure that alpha , the coefficient of q 2 , is negative for most thermodynamic states. This implies that even near the triple point a simple (argon-like) liquid should exhibit 'residual' Ornstein-Zernike behaviour, i.e. there should be a shallow minimum in S(q) for q approximately=0.25AA -1 . The authors find that the short-range correlation length, R=(- alpha rho ) 1/2 varies weakly with density rho and with temperature. Their calculated value for R at the critical point is in good agreement with results obtained from radiation scattering experiments on Ar and Kr near their critical points. The q 3 term is associated with r -6 asymptotic behaviour of the interatomic potential. The authors analysis suggests that it may be possible to detect such a contribution, if it is present in a real liquid, by X-ray or neutron diffraction.