Thermodynamic Surfaces

A. W. Neumann and J. K. Spelt, eds.. Applied Surface Thermodynamics. Interfacial Tension and Contact Angles, Marcel Dekker, New York, 1996. 

Although liquid Hg would never be used as a reference (model) surface in surface physics because its liquid state and high vapor pressure do not allow appropriate UHV conditions, this metal turns out to be a reference surface in electrochemistry for precisely the same reasons reproducibility of the surface state, easy cleaning of its surface, and the possibility of measuring the surface tension (surface thermodynamic conditions). In particular, the establishment of a UHV scale for potentials is at present based on data obtained for Hg. 

We have discussed here, very briefly, some recent observations of small particle surfaces and how these relate to geometrical catalytic effects. These demonstrate the general conclusion that high resolution imaging can provide a direct, structural link between bulk LEED analysis and small particle surfaces. Apart from applications to conventional surface science, where the sensitivity of the technique to surface inhomogenieties has already yielded results, there should be many useful applications in catalysis. A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why results comparable to those described cannot be obtained from commercial catalyst systems. 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

A volume-related term (expressed by polarizability) and electrostatics (expressed by partial atomic charge) made minor contributions to intestinal absorption in humans. Lipophilicity, expressed by logP or logD values, shows no correlation with the human absorphon data. Recently, similar results were obtained for 154 passively transported drugs on the basis of surface thermodynamics descriptors [39] ... 

Stampfl C, KreuzerHJ, Payne SH, PfniirH, Scheffler M. 1999. First-principles theory of surface thermodynamics and kinetics. Phys Rev Lett 83 2993. 

Applied Surface Thermodynamics, edited by A. 14/. Neumann and Jan K. Spelt Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal Detergents in the Environment, edited by Milan Johann Schwuger Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda... 

A W Neumann and J K Spelt, Applied surface thermodynamics (New York Marcel Dekker, 1996). 

In the last two sections the formal theory of surface thermodynamics is used to describe material characteristics. The effect of interfaces on some important heterogeneous phase equilibria is summarized in Section 6.2. Here the focus is on the effect of the curvature of the interface. In Section 6.3 adsorption is covered. Physical and chemical adsorption and the effect of interface or surface energies on the segregation of chemical species in the interfacial region are covered. Of special importance again are solid-gas or liquid-gas interfaces and adsorption isotherms, and the thermodynamics of physically adsorbed species is here the main focus. 

On the other hand, there is considerable interest to quantify the similarities between different molecules, in particular, in pharmacology [7], For instance, the search for a new drug may include a comparative analysis of an active molecule with a large molecular library by using combinatorial chemistry. A computational comparison based on the similarity of empirical data (structural parameters, molecular surfaces, thermodynamical data, etc.) is often used as a prescreening. Because the DFT reactivity descriptors measure intrinsic properties of a molecular moiety, they are in fact chemical fingerprints of molecules. These descriptors establish a useful scale of similarity between the members of a large molecular family (see in particular Chapter 15) [18-21],... 

Some of the compounds described in this chapter were studied for specific physical properties. Surface tension measurements with solutions of 9-16 in 0.01 M hydrochloric acid demonstrated that these zwitterionic X5Si-silicates are highly efficient surfactants.21 These compounds contain a polar (zwitterionic) hydrophilic moiety and a long lipophilic z-alkyl group. Increase of the n-alkyl chain length (9-15) was found to result in an increase of surface activity. The equilibrium surface tension vs concentration isotherms for 9 and 16 were analyzed quantitatively and the surface thermodynamics of these surfactants interpreted on the molecular level. Furthermore, preliminary studies demonstrated that aqueous solutions of 9-16 lead to a hydrophobizing of glass surfaces.21... 

Tab. 8.5 Apparent surface thermodynamic properties for goethite and hematite at 298.15K (see Diakonov et al. 1994)...

Felmy, A.R. Rustad, J.R. (1998) Molecular statics calculations of proton binding to goethite surfaces Thermodynamic modeling of the surface charging and protonation of goethite in aqueous solution. Geochim. Cosmochim. Acta 62 25—31... 

Using the same procedure, spherical nanosize particles of hematite were coated with yttrium basic carbonate and showed that various surface thermodynamic properties of these systems were essentially those of yttria (37). 

The Young Equation. The principle of balancing forces used in the derivation of the Laplace equation can also be used to derive another important equation in surface thermodynamics, the Young equation. Consider a liquid droplet in equilibrium... 

There is one exception to the formation of ordered monolayers Fe on W forms three-dimensional crystallites even though surface thermodynamic considerations would predict monolayer formation. 

It is clear that equilibrium measurements of surface thermodynamics cannot predict surface composition under the dynamic conditions of catalytic oxidation. Nevertheless, such measurements will provide a sounder base than bulk thermodynamics for understanding the surface chemistry and permit working backward, from direct measurements of surface chemistry during reaction, to predictions concerning the microenvironment at the surface under reaction conditions. 

Surface Chemkin, which handles surface thermodynamics and chemical kinetics. 

To calculate the surface thermodynamic properties from equations (12.33) to (12.41), we must know how 7 changes with T. Equation (12.42) is a semi-empirical equation that has been used to relate 7 and T. It is given by... 

Since the build-up of multilayers of proteins on a surface is thermodynamically unfavorable (parts of the protein layer exposed towards the ambient solution may act as a kind of a swollen hydrophilic layer, rendering the adsorption of other proteins to the surface thermodynamically unfavorable), competitive adsorption becomes a decisive factor in the cell s response once the protein-coated surface is placed into... 

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