Apparent surface dipole moment

Surface Potential Studies. Surface potential measurements were made on all compounds at the same time surface pressure was measured. Since it is preferable to have a parameter which reflects only changes in the fatty acid and surface water dipole moments and not changes resulting from merely increasing the number of dipoles per unit area, surface potential values, AV, were converted to an apparent surface dipole moment, (i), where... 

Fio. 13. Apparent quantum efficiency of the photochemical dccomiiOHition of films of stearanilide on 5 Af HjS04. The points show the experimental results the full line is calculated from the surface dipole moment, mb, of the molecules in the film. As the film is compressed, the orientation of the anilide chromophore group changes. This causes both an increase in and an alteration in mo (Uideal and Mitchell, 42). 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

Over the last years, the basic concepts embedded within the SCRF formalism have undergone some significant improvements, and there are several commonly used variants on this idea. To exemplify the different methods and how their results differ, one recent work from this group [52] considered the sensitivity of results to the particular variant chosen. Due to its dependence upon only the dipole moment of the solute, the older approach is referred to herein as the dipole variant. The dipole method is also crude in the sense that the solute is placed in a spherical cavity within the solute medium, not a very realistic shape in most cases. The polarizable continuum method (PCM) [53,54,55] embeds the solute in a cavity that more accurately mimics the shape of the molecule, created by a series of overlapping spheres. The reaction field is represented by an apparent surface charge approach. The standard PCM approach utilizes an integral equation formulation (IEF) [56,57], A variant of this method is the conductor-polarized continuum model (CPCM) [58] wherein the apparent charges distributed on the cavity surface are such that the total electrostatic potential cancels on the surface. The self-consistent isodensity PCM procedure [59] determines the cavity self-consistently from an isodensity surface. The UAHF (United Atom model for Hartree-Fock/6-31 G ) definition [60] was used for the construction of the solute cavity. 

SchrOder (1979) was able to improve this approach by using a refined expression of the Gibbs free energy of adhesion as a function of the various intermolecular attraction energies between the liquid and the solid. In this way, by using up to 10 different immersion liquids with known parameters, he has calculated the apparent dipole moment and polarizability characterizing the immersion behaviour of various pigment surfaces (e.g. rutile, iron oxide and several phtalocyanines). 

At small coverages, the change A in the work function is proportional to the surface concentration. At higher coverages, the variation of with Ns becomes less rapid in some cases it may even pass through a maximum before reaching the value for a saturated monolayer. This apparent decrease of the dipole moment is caused by the Coulomb interaction between the adsorbed atoms, which makes large dipole moments unfavorable.62... 

When very accurate dipole moments are deduced, it is proper to query the significance of a breakdown of the Bom-Oppenheimer approximation. This approximation justifies the assignment of molecular property tensors, such as dipole moments and polarizabilities, to specific directions in a molecule-fixed frame and supports the use of a property function or surface representing the variation of the property with nuclear position. The dipole moment of HD (5.85 X 1(T4 D)26 arises solely from the breakdown of the approximation and may have the sense H D-.27-29 In HC1 and DC1, there is an isotope effect on the dipole moment that has been attributed to a violation of the Born-Oppen-heimer approximation30 there is an apparent difference of 0.0010 0.0002 D between the dipole functions of HC1 and DC1, with HC1 having the bigger moment. This result is in accord with a recent theoretical analysis by Bunker.31... 

Equation (9.1) shows that P is the dipole moment per unit volume, a vector quantity that has its direction parallel to Ei for an isotropie medium. Imagine a small cylindrical volume of length d/ parallel to the polarisation and of cross-sectional area dA. Let the apparent surface charges at the two ends of the cylinder be dq. It then follows that P = dq dl/dv, where do is the volume of the small cylinder. However, do = dl dA, so that P = dq/dA = a, where a is the apparent surface charge per unit area normal to the polarisation. (Note a stands for conductivity in section 9.3.)... 

The most direct method of finding the coefficient en would be to fill the space between the metallic coaxial cylinders with a liquid crystal having pear-shaped molecules, with the surfaces of the cylinders having been pretreated for homeotropic orientation and to measure the potential difference between the cylinders. In fact, because of the difference in radii of the cylinders, the nematic liquid crystal structure proves to be splay deformed, and if the molecules have even a small longitudinal dipole moment the plates of the coaxial capacitor would be charged. However, despite its apparent simplicity, this experiment is, in fact, complicated because of the screening of the potential caused by the flexoelectric effect by firee charges from the liquid crystal and the atmosphere. 

---