Surface functionalization dipole moments

A development of novel fabrication approaches is another field of great interest in LED technologies. Assembly and self-assembly methods as bottom up approaches are assumed to be very attractive from the point of view of simplicity and fabrication costs. For these methods the functionality of the species (specific groups, surface charge, dipole moment, etc) to be assembled is of crucial... 

Dubai H-R, Ha T-K, Lewerenz M and Quack M 1989 Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore In CHXg molecules J. Chem. Phys. 91 6698-713... 

Hollenstein H, Marquardt R, Quack M and Suhm M A 1994 Dipole moment function and equilibrium structure of methane In an analytical, anharmonic nine-dimenslonal potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations J. Chem. Phys. 101 3588-602... 

Furthermore, in a series of polyoxyethylene nonylphenol nonionic surfactants, the value of varied linearly with the HLB number of the surfactant. The value of K2 varied linearly with the log of the interfacial tension measured at the surfactant concentration that gives 90% soil removal. Carrying the correlations still further, it was found that from the detergency equation of a single surfactant with three different polar sods, was a function of the sod s dipole moment and a function of the sod s surface tension (81). 

Work function transients of the type shown on Figures 5.10 and 5.11 can be used to estimate initial dipole moments of the spillover dipoles on the catalyst surface.5,36,37... 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

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. 

The surface potential as function of particle area (AV A isotherm) is another indicator of the quality of the monolayer structure. The surface potential at the air-water interface changes as the film-forming molecules reorient themselves during the compression process. For a closely packed monolayer, the surface potential is directly proportional to the surface dipole moment (/r ) by [13] ... 

As it has been already mentioned in numerous cases one should take into account the dipole component in the adsorption — caused change in the work function stemming from availability of dipole moment in adsorption particles. This component alters the value of micropotential of the surface. As it has been already mentioned in section 1.2, the origin of these dipoles can differ ranging from inherent to quantum mechanical one [4]. 

The same effect exists for adsorption on a metal surface from the gas phase. In this case the adsorbate-induced dipole potential changes the work function by an amount A. If nad is the number of adsorbed molecules per unit area, the component mx of the dipole moment of single adsorbed molecule can be inferred from the relation ... 

The difference in the work functions causes the flow of electrons from the metal with the lower work function to that with the higher one, so that a surface dipole moment is created. This effect is similar to the establishment of an outer potential difference at the contact between two different metals (see Chapter 2). An adsorbate layer does not have a work function in the same... 

Table 9.2 Dipole moment, maximum work function change and charge transfer for potassium adsorbed on transition metal surfaces.

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