Interface Effect on Dipole-Dipole Interaction
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INTERFACE EFFECT ON DIPOLE-DIPOLE INTERACTION
MICHAEL URBAKH AND JOSEPH KLAFTER School of Chemistry, Tel-Aviv University, Tel Aviv 69978, Israel
ABSTRACT The nature of the pair interaction between dipoles embedded in a liquid in the vicinity of a non-metallic interface is investigated in the continuum limit. It is shown that the dipole-dipole interaction can be significantly modified in the presence of a boundary in liquids characterized by a nonlocal dielectric function or in liquids whose dielectric properties change due to geometrical restrictions. Different limits are studied and relationships to experimental observables are discussed.
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INTRODUCTION
Recent experimental and theoretical studies [1-5] have demonstrated that spatial restrictions can strongly affect the dynamic and thermodynamic properties of embedded liquids and molecules. The effects observed and predicted include the influence of interfaces on the pair-interaction energy between charged or uncharged particles [6-7], translational and rotational diffusion of probe molecules [4,8] as well as chemical reactions and energy transfer properties [1,8]. The influence of boundaries on molecular properties in their vicinity is of course related to the nature of the intermolecular forces and to how they are modified near an interface. These modifications can directly influence processes such as adsorption and electron, or energy, transfer at interfaces. In this paper we investigate changes in the interaction between point dipoles embedded in a liquid near a nonmetallic interface when compared to the bulk liquid. We follow Ref.[9] where we represented the liquid in the continuum approximation, in terms of a nonlocal dielectric function c(k,w), and calculated the change in the dielectric friction due to the presence of a boundary. In analogy to this previous work the effect of the interface is introduced through the concept of additional boundary conditions [10]. We analyze the dependence of the dipole-dipole interaction on the distance between the dipoles and between the dipoles and the boundary as well as on the dielectric parameters that characterize the interface region. The approach used here is suitable for both static and dynamic limits. Although the continuum framework does not explicitly include molecular level details, it enables to derive closed expressions that relate microscopic quantities to measurable observables such as dielectric functions [11-13]. Continuum approximations and their modifications have been shown to be powerful in unraveling leading physical processes in complex systems. The nonlocal nature of the liquid defines a length scale, A, which is a measure of spatial correlations in the liquid. This parameter can be estimated on the base of diffraction experiments [14] and on molecular dynamic simulations of liquids [15,16]. For instance, in aqueous solutions the correlation length, A, is of the order of the extension of local hydrogen-bonded clusters. The introduction of this new length scale is of particular importance in th
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