Dipole-surface charge interaction, induced

In the absence of strong attractive interactions induced by charge transfer between adsorbates and surface atoms, weak attractive interactions can be induced in several ways. When a gas atom or molecule with no permanent dipole moment approaches the surface of a metal in which the conduction electrons constitute a mobile, fluctuating electron gas, the surface charge induces a dipole in the approaching species. This attractive induced dipole-surface-charge interaction is similar to that of a gas molecule with a permanent dipole, and the potential energy of interaction is of the form... 

Image-charge models. These models take into account the discreteness of surface charges, which induces orientation in the adjacent water dipoles [574-577]. Dipoles due to zwit-terionic surface groups, for example, phospholipid headgroups [578], have been also taken into consideration in models of the electrostatic interaction between planar dipole lattices [579-583]. 

For predictions of adsorption of molecules with non-zero dipole moments, equations taking into account long-range interactions, such as molecular dipole-surface charge, dipole-induced dipole, and van der Waals one were deduced. Thus, e.g., the interaction energy of a molecule with a surface charge is [168]... 

In order to explain the interactions between silica surfaces, the polarization model is adapted to poorly-organized surfaces. To account for the disorder induced in water by the rough surfaces of silica, the dipole correlation length Am, which is the main parameter of the polarization model, is allowed to decrease from Am=14.9A obtained for water perfectly organized in ice-like layers in the vicinity of a surface to smaller values. For Am=4A, good agreement with experiment is obtained for reasonable values of the parameters involved (such as surface dipole and charge densities) [7.9],... 

The analysis of the dipole moment curves for the motion of the adsorbate perpendicular to the surface provides additional information about the degree of ionicity of a given surface chemical bond. Moreover, the analysis of the dipole moment curves is also related to the interpretation of variations of the surface work function induced by the presence of the adsorbate. However, the response of the surface to the presence of the adsorbate does not permit to extract directly adsorbate charges from the dipole moment curve. A procedure based in the use of frozen densities has been proposed that permits to avoid the effect of the surface polarization on the dipole moment curve. Unfortunately, this method has not been yet extensively used. To close this short discussion about the different procedures commonly used to interpret the chemical bond between chemical species and the surface of a catalyst in terms of net charges we mention the valence bond reading of Hartree-Fock and Configuration Interaction wave functions. This procedure has been used to interpret the electronic correlation effect on the surface chemical bond. ... 

The matrix p is a combined set of the external electrostatic fields that represent the effects of the QM field on the EFP polarizability tensors and the PCM potential, while w is a combined set of induced dipoles and surface charges. The physical meaning of the supermatrix equation (3) is that the EFP induced dipoles and PCM induced charges are uniquely determined by the external field and potential therefore, the right hand side of Eq. (3) involves only the external field /potential, and the left side involves only the induced EFP dipoles and PCM charges. The interactions among the induced dipoles and charges are implicitly described with the matrix B. The supermatrix Eq. (3) can be solved either with direct inversion or various iterative methods. 

The original formal theory is expressed in terms of quanttun electrodynamics with the continuum mediwn characterized by its spectnun of complex dielectric frequencies. A more recent formulation, derived from this theory, is based on the extension of the reaction field concept to a dipole subject to fluctuations exclusively electric in origin. Another procedme has been formulated starting, as for the repulsion contribution, from the theory of intermolecular forces. Following the scheme commonly exploited to derive the electrostatic contribution to the interaction energy, the molecule B is substituted by a continuum medium, the solvent S, described by a surface charge density as induced by the solute transition densities of M (the equivalent of A) and spreading on the cavity surface. 

The so-called London dispersion or van der Waais interactions are those between molecules that have neither a net charge nor a permanent dipole moment. This interaction is essentially due to the interactions between a transient electrical dipole in one molecule and its induced electrical dipole in the other molecule. TTiis type of electrical interaction plays an important role in biological systems (e.g., in surface tension, stability of biological membranes, condensation properties, adhesion and fusion of biological cell membranes, enzyme-substrate recognition, etc.). 

Table 1. A - Coulomb repulsion between partly charged particles as the origin of electrostatic stabilization. B - Distribution of the surface charge and geometry of the electric field of the neutral metal sphere of radius R, when the adsorbate is a single external charge in the distance L from the centre. The electric potential of this system is equivalent to the superposition of the potential of an external point charge C[ and the induced dipole moment d.C- Attractive dipole-dipole interaction between electrically neutral metal particles.

The quadrupolar splitting, Avq, of headgroup-deuterated PC or PE varies linearly as a function of the total amount of electric surface charge, and changes in opposite direchons are induced by positive and negative surface charges. This indicates that the phosphocholine and the phospho-ethanolamine dipoles are sensitive to electric charges at the membrane surface and function as an electrometer. As an example, the interaction of a-CD2-POPC with an anionic and a cationic amphiphile is shown in Eig. 3. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

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