We consider the asymptotic decay of structural correlations in pure fluids, fluid mixtures, and fluids subject to various types of inhomogeneity. For short ranged potentials, both the form and the amplitude of the longest range decay are determined by leading order poles in the complex Fourier transform of the bulk structure factor. Generically, for such potentials, asymptotic decay falls into two classes: (i) controlled by a single simple pole on the imaginary axis (monotonic exponential decay) and (ii) controlled by a conjugate pair of simple poles (exponentially damped oscillatory decay). General expressions are given for the decay length, the amplitude, and [in class (ii)] the wavelength and phase involved. In the case of fluid mixtures, we find that there is only one decay length and (if applicable) one oscillatory wavelength required to specify the asymptotic decay of all the component density profiles and all the partial radial distribution functions gij(r). Moreover, simple amplitude relations link the amplitudes associated with the decay of correlation of individual components. We give explicit results for the case of binary systems, expanding on and partially correcting recent work by Martynov. In addition, numerical results for g(r) for the pure fluid square-well model and for gij(r) for binary hard sphere mixtures are presented in order to illustrate the fact that the asymptotic forms remain remarkably accurate at intermediate range. This is seen to arise because the higher order poles are typically well-separated from the low order ones. We also discuss why the asymptotics of solvation forces for confined fluids and of density profiles of inhomogeneous fluids (embracing wetting phenomena) fall within the same theoretical framework. Finally, we comment on possible modifications to the theory arising from the presence of power-law attractive potentials (dispersion forces).

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The decay of structural correlations in the classical one-component plasma is analyzed by calculating the poles of the Fourier transform of the total ͑pairwise͒ correlation function h(r) for two integral equation theories, the soft mean spherical approximation and the hypernetted chain ͑HNC͒. We show that for all except the largest values of the plasma coupling constant ⌫, the leading-order pole contribution provides an accurate description of h(r) at intermediate range, as well as the ultimate asymptotic decay. The crossover from monotonic decay at weak coupling to exponentially damped oscillatory decay at strong coupling is shown to arise from the same mechanism as that which occurs for charge correlations in binary ionic fluids. We calculate the values of ⌫ at which the crossover occurs in the two theories. The role of higher-order poles and ͑within the HNC͒ other singularities in determining the intermediate range behavior of h(r) for strong coupling is discussed. We investigate the properties of the solutions of the integral equations in the strong coupling, ⌫ →ϱ, asymptotic high-density limit ͑AHDL͒. Padé approximants are employed in order to test the validity of the scaling laws proposed for the potential energy, direct correlation function, and for the poles and their contributions to h(r) in the AHDL. Our numerical results provide strong support for the validity of the theoretical predictions concerning the AHDL.

Monte Carlo and molecular dynamics computer simulations have been used to study the structure and dynamics of the interlayer aqueous solution in a colloidal sodium laponite clay at 277 K. The system studied has a clay-clay spacing of 34.06 Å, and contains 1200 interlayer water molecules and 24 sodium counterions. The density profiles for interlayer species show two distinct layers of surface water as one moves away from the clay particles. The innermost of these layers is strongly oriented to form hydrogen bonds to the surface oxygen atoms. Radially averaged pair distributions have been calculated as a function of distance from the clay surfaces, and show that throughout our system the water structure is significantly perturbed from the bulk. In particular, we observe an increase in the second nearest-neighbor oxygen-oxygen distance, similar to that reported for low-density water at 268 K ͓A. K. Soper and M. A. Ricci, Phys. Rev. Lett. 84, 2881 ͑2000͔͒. The majority of the sodium counterions are fully hydrated by six water molecules. These hydrated ions have a strong tendency to remain close to the solid surfaces, as so-called ''outer-sphere'' complexes. However, we also observe cations further from the clay sheets, in the diffuse layer. Diffusion of water and cations in the plane of the clay sheets is comparable to that in the bulk, but is significantly reduced normal to the clay sheets.