Intrinsic surface energy

Note that in core-level photoelectron spectroscopy, it is often found that the surface atoms have a different binding energy than the bulk atoms. These are called surface core-level shifts (SCLS), and should not be confiised with intrinsic surface states. Au SCLS is observed because the atom is in a chemically different enviromuent than the bulk atoms, but the core-level state that is being monitored is one that is present in all of the atoms in the material. A surface state, on the other hand, exists only at the particular surface. 

The van der Waals and other non-covalent interactions are universally present in any adhesive bond, and the contribution of these forces is quantified in terms of two material properties, namely, the surface and interfacial energies. The surface and interfacial energies are macroscopic intrinsic material properties. The surface energy of a material, y, is the energy required to create a unit area of the surface of a material in a thermodynamically reversible manner. As per the definition of Dupre [14], the surface and interfacial properties determine the intrinsic or thermodynamic work of adhesion, W, of an interface. For two identical surfaces in contact ... 

The starting points for the continuity and energy equations are again 21.5-1 and 21.5-6 (adiabatic operation), respectively, but the rate quantity7 (—rA) must be properly interpreted. In 21.5-1 and 21.5-6, the implication is that the rate is the intrinsic surface reaction rate, ( rA)int. For a heterogeneous model, we interpret it as an overall observed rate, (—rA)obs, incorporating the transport effects responsible for the gradients in concentration and temperature. As developed in Section 8.5, these effects are lumped into a particle effectiveness factor, 77, or an overall effectiveness factor, r]0. Thus, equations 21.5-1 and 21.5-6 are rewritten as... 

Thus it would seem that the actual intrinsic surface tension of biological liquids, i.e. the one that plays a role in the interactions between cells among one another, and between cells and biopolymers, must be close to that of serum or plasma ultrafiltrates, i.e. y 70 dyn/cm or the intrinsic interfacial free energy of the interstitial mammalian liquid, serum or plasma, or AF —140 ergs/cm2. 

From a comparison of the optical absorption and excitation data for the oxides (Table V), it is clear that the energy decreases with increasing cation size along the series Mg to Ba. The bulk exciton transitions of these oxides also decrease in a similar manner (Table VI). It is possible to make a semi-quantitative calculation of the intrinsic surface energy states using the approach of Levine and Mark (151) where the ions in an ideal surface are considered to be equivalent to bulk ions except for their reduced Madelung... 

Monoenergetic photons excite a core-hole. The modulation of the adsorption cross-section with energy 100-500 eV above the excitation threshold yields information on the radial distances to neighboring atoms. The cross-section can be monitored by fluorescence as core-holes decay or by the attenuation of the transmitted photon beam. EXAFS is one of many "fine-structure" techniques. This is not intrinsicly surface sensitive (see SEXAFS). 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

The best strategy to be followed in order to get accurate sets of A values has not been defined, so at present more or less complex statistical elaborations of some data are used. Among the numerical data that have been used we mention solvation and solvent transfer energies, intrinsic solute properties (electron isodensity surfaces, isopotential electronic surfaces, multipole expansions of local charge distribution), isoenergy surfaces for the interaction with selected probes (water, helium atoms), Monte Carlo simulations with solutes of various nature. All these sets of data deserve comments, that are here severely limited not to unduly extend this Section. 

Adsorption is defined as the concentration of gas molecules near the surface of a solid material. The adsorbed gas is called adsorbate, and the solid where adsorption takes place is known as the adsorbent. Adsorption is a physical phenomenon (usually called physisorption) that occurs at any environmental condition (pressure and temperature), but it becomes measurable only at very low temperatures. Thus physisorption experiments are performed at very low temperatures, usually at the boiling temperature of liquid nitrogen at atmospheric pressure. Adsorption takes place because of the presence of an intrinsic surface energy. When a material is exposed to a gas, an attractive force acts between the exposed surface of the solid and the gas molecules. The result of these forces is characterized as physical (or van der Waals) adsorption, in contrast to the stronger chemical attractions associated with chemisorption. The surface area of a solid includes both the external surface and the internal surface of the pores. 

The solute and solvent molecules present in any solution have different intensities of attractive force fields, and also have different molecular volumes and shapes. A concentration difference between the surface region and the bulk solution occurs because the molecules that have the greater fields of force tend to pass into the interior, and those with the smaller force fields remain at the surface. The Gibbs surface layer of a solution is more concentrated in the constituents that have smaller attractive force fields, and thus whose intrinsic surface free energy is smaller than the interior. As we stated in Section 3.3, this concentration difference of one constituent of a solution at the surface is termed adsorption. In qualitative terms, if the solution has a smaller surface tension than its pure solvent, the solute is concentrated in the surface layer indicating a positive adsorption according to... 

Abstract. An isolated individual molecule clearly has only one ionization energy. For ordered molecular assemblies, however, multiple values have been found depending on the orientation of the molecules relative to a supporting substrate. This intriguing observation is rationalized here for the prototypical molecule pentacene, in terms of intrinsic surface dipoles built into the molecules, which collectively give rise to the orientation dependence of the molecular ionization energy. 

The work function (<[>) is defined as the energy difference between the Fermi level (Ef) and the vacuum level above a sample (Evac).It is known that < > of metals depends on the crystal face exposed to vacuum [1-3], e.g., ( > spans a range of 0.5 eV for copper (100), (110), and (111) surfaces [1,2], As EF is constant, this observation has been explained by crystal face dependent intrinsic surface dipoles . Differences in the geometric and, consequently, electronic structure cause a different amount of the electronic cloud to spill out of the bulk into the vacuum [3,4], The resulting dipoles change Evao and thus impact <]>. 

Fig. 12. Photoelectron energy distribution for Si lll 2 X 1. The filled surface state curve represents the difference between clean and oxidized surface curves and depicts the optical density of intrinsic surface states (after Eastman and Grobman [154]).

Finally, we introduce the coneept of intrinsic surface stress [52Her, 1876] which is equal to the work required to deform a surface. It is direetly related to the surfaee energy and its derivative with respect to strain. For crystalhne surfaces, strain is a tensor, and thus the surface stress, Xy, is also a tensor of second... 

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