SOLVATED INTERFACES

The majority of the population changes take place within the first 10 picoseconds. The active surface function groups are the singly coordinated sites. Triply coordinated surface sites have nearly constant populations throughout the simulation. This illustrates how the gas-phase calculations can be misleading. Because the gas phase proton affinities 

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Reviakine, I., Johannsmann, D., Richter, R.P., 2011. Hearing what you cannot see and visuahzing what you hear interpreting quartz crystal microbalance data from solvated interfaces. Anal. Chem. 83, 8838—8848. 

Figure 3.8 The adsorption energy of sulfate anion over Pt(lll) at (a) vacuum and (b) partially solvated interfaces. The diamond ( ) with solid line represents results from the linear free energy model (Model 2a. 1), the square with dotted line represents ( ) the linear free energy model with dipole correction (Model 2a.2), and the triangle (a) with dashed line represents the electric field model (Model 2a.3).

A similar expression can be written for an oxidation reaction. The free energy of the adsorbed species may be calculated at a vacuum, micro-solvated, or fully solvated interface. Corrections to the energy difference between the two adsorbed species may be made using the dipole-field correction (Model 2a.2), and applied electric field (Model 2a.3), or the double-reference method (Model 2b. 1). 

Zimdars D, Dadap J I, Eisenthal K B and Heinz T F 1999 Femtosecond dynamics of solvation at the air/water interface Chem. Phys. Lett. 301 112-20... 

HyperChem uses th e ril 31 water m odel for solvation. You can place th e solute in a box of T1P3P water m oleeules an d impose periodic boun dary eon dition s. You may then turn off the boundary conditions for specific geometry optimi/.aiion or molecular dynamics calculations. However, th is produces undesirable edge effects at the solvent-vacuum interface. 

M. Iwamatsu. A molecular theory of solvation force oscillations in nonpolar Uquids. J Colloid Interface Sci 204 374-388, 1998. 

From the experimental results and theoretical approaches we learn that even the simplest interface investigated in electrochemistry is still a very complicated system. To describe the structure of this interface we have to tackle several difficulties. It is a many-component system. Between the components there are different kinds of interactions. Some of them have a long range while others are short ranged but very strong. In addition, if the solution side can be treated by using classical statistical mechanics the description of the metal side requires the use of quantum methods. The main feature of the experimental quantities, e.g., differential capacitance, is their nonlinear dependence on the polarization of the electrode. There are such sophisticated phenomena as ionic solvation and electrostriction invoked in the attempts of interpretation of this nonlinear behavior [2]. 

Certainly these approaches represent a progress in our understanding of the interfacial properties. All the phenomena taken into account, e.g., the coupling with the metal side, the degree of solvation of ions, etc., play a role in the interfacial structure. However, it appears that the theoretical predictions are very sensitive to the details of the interaction potentials between the various species present at the interface and also to the approximations used in the statistical treatment of the model. In what follows we focus on a small number of basic phenomena which, probably, determine the interfacial properties, and we try to use very transparent approximations to estimate the role of these phenomena. 

Fig. 20.11 Two types of arrangement of ions at a metal/solution interface, (a) Arrangement O solvated ions in the O.H.P. and surface of electrode covered with water dipoles, (b) Arrangement I desolvated ions in the I.H.P. (after Bockris and Reddy )...

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

It is thus clear from the previous discussion that the absolute electrode potential is not a property of the electrode material (as it does not depend on electrode material) but is a property of the solid electrolyte and of the gas composition. To the extent that equilibrium is established at the metal-solid electrolyte interface the Fermi levels in the two materials are equal (Fig. 7.10) and thus eU 2 (abs) also expresses the energy of transfering an electron from the Fermi level of the YSZ solid electrolyte, in equilibrium with po2=l atm, to a point outside the electrolyte surface. It thus also expresses the energy of solvation of an electron from vacuum to the Fermi level of the solid electrolyte. 

Quaternary Ammonium or Phosphonium Salts. In the above-mentioned case of NaCN, the uncatalyzed reaction does not take place because the CN ions cannot cross the interface between the two phases, except in very low concentration. The reason is that the Na ions are solvated by the water, and this solvation energy would not be present in the organic phase. The CN ions... 

There are other forces that come into play when the thickness of the liquid fihn is in the nanometer range and the size of the molecule is no longer negligible. Short-range oscillatory forces arise then because the liquid molecules feel the presence of the walls of the substrate and are forced to form a layered structure near the interface. These forces are also called structural or solvation forces [6]. 

Differential scanning calorimetry measurements have shown a marked cooling/heat-ing cycle hysteresis and that water entrapped in AOT-reversed micelles is only partially freezable. Moreover, the freezable fraction displays strong supercooling behavior as an effect of the very small size of the aqueous micellar core. The nonfreezable water fraction has been recognized as the water located at the water/surfactant interface engaged in solvation of the surfactant head groups [97,98]. 

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