Interfacial systems

Many vastly different interfacial systems with one aqueous phase have been studied by computer simulations with detailed molecular representation of... 

Similar surface terms are commonly used in the description of interfacial systems. They correspond to the idea of a localized interaction with the wall. This has been used in the description of adsorption (see, e.g., [29]), wetting phenomena [30] and interfacial criticality [31],... 

The existing books cover the simple, rather than the advanced, theoretical approaches to interfacial systems. This volume should fill this gap in the literature. It is the purpose of this volume to serve as a comprehensive reference source on theory and simulations of various interfacial systems. Furthermore, it shows the power of statistical thermodynamics that offers a... 

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

Liquid Interfacial Systems Oscillations and Instability, Rudoiph V. Birikh, Vladimir A. Briskman, Manuei G. Veiarde, and Jean-Claude Legros... 

Boundary membranes play a key role in the cells of all contemporary organisms, and simple models of membrane function are therefore of considerable interest. The interface of two immiscible liquids has been widely used for this purpose. For example, the fundamental processes of photosynthesis, biocatalysis, membrane fusion and interactions of cells, ion pumping, and electron transport have all been investigated in such interfacial systems. 

Viscosity, defined as the resistance of a liquid to flow under an applied stress, is not only a property of bulk liquids but of interfacial systems as well. The viscosity of an insoluble monolayer in a fluid-like state may be measured quantitatively by the viscous traction method (Manheimer and Schechter, 1970), wave-damping (Langmuir and Schaefer, 1937), dynamic light scattering (Sauer et al, 1988) or surface canal viscometry (Harkins and Kirkwood, 1938 Washburn and Wakeham, 1938). Of these, the last is the most sensitive and experimentally feasible, and allows for the determination of Newtonian versus non-Newtonian shear flow. 

M.S. Wrighton, M.I.T. Your H202, Br2 generation is a good example of how one can couple one-electron reagents with interfacial systems to do the desired reaction. Most photochemical systems will be one-electron initially, since one photon excites one electron. Also, under what conditions does your system work ... 

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

Molecular predictions of the properties of interfacial systems are now becoming possible as a result of rapid advances in liquid state chemical physics and computer technology. The objectives of this paper are 1) to review the general approaches and models used in Monte Carlo (MC) and molecular dynamics (MD) simulations of interfacial systems, 2) to describe and discuss results from selected simulation studies of interfacial water, and 3) to discuss the major limitations of these techniques and to offer suggestions for overcoming them. 

Monte Carlo Methods. Although several statistical mechanical ensembles may be studied using MC methods (2,12,14), the canonical ensemble has been the most frequently used ensemble for studies of interfacial systems. In the canonical ensemble, the number of molecules (N), cell volume (V) and temperature (T) are fixed. Hence, the canonical ensemble is denoted by the symbols NVT. The choice of ensemble determines which thermodynamic properties can be computed. 

In the NVT ensemble one cannot compute the chemical potential or entropy of the system two properties which are of critical importance for interfacial systems. The choice of an ensemble also determines the sampling algorithm used to generate molecular configurations from random moves of the molecules. 

Equations 3-4 show that the form of the interaction potentials used in simulating interfacial water is critical. Of interest for interfacial systems are both the interaction potential between water molecules and that between the surface and a water molecule. 

The first MC (16) and MD (17) studies were used to simulate the properties of single particle fluids. Although the basic MC (11,12) and MD (12,13) methods have changed little since the earliest simulations, the systems simulated have continually increased in complexity. The ability to simulate complex interfacial systems has resulted partly from improvements in simulation algorithms (15,18) or in the interaction potentials used to model solid surfaces (19). The major reason, however, for this ability has resulted from the increasing sophistication of the interaction potentials used to model liquid-liquid interactions. These advances have involved the use of the following potentials Lennard-Jones 12-6 (20), Rowlinson (21), BNS... 

Surface Potentials. Consider the form of the surface-water Interaction potential for an interfacial system with a hydrophobic surface. The oxygen atom of any water molecule is acted upon by an explicitly uncharged surface directly below it via the Lennard-Jones potential (U j) ... 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

Previous work by us has led to the synthesis of silicon, germanium and tin polyethers utilizing Interfacial systems (for instance 5-9). The tin-cotton products should possess an analogous structure from previously reported similarities in reactivity of soluble cellulosic hydroxyls with organic acid chlorides... 

A related, relatively unexplored topic is the importance of many-body forces in the simulations of interfacial systems. The development of water-polarizable models has reached some level of maturity, but one needs to explore how these models must be modified to take into account the interactions with the metal surface atoms and the polarizable nature of the metal itself... 

The basic common denominator for all these applications is qualitatively well understood surfactants and their aggregates permit mixing, or at least close interaction, between phases or substances that are per se immiscible with each other -mostly oil and water. This is how grease is washed off from our hands when we use soap, the removal being mediated by micelles. In turn, micelles and vesicles permit the formation of an extraordinarily efficient interfacial system. Figure 9.3 gives a dramatic demonstration of this, showing that the total surface of a concentrated soap solution in your sink may well correspond to the surface of a stadium ... 

For interfacial systems, potential functions should ideally be transferrable from the gas-phase to the condensed phase. Aqueous-mineral interfaces are not in the gas phase (although they may be close, see (7)), but both the water molecules and the atoms/ions in the substrate are in contact with an environment that is very different from their bulk environment. The easiest different environment to test, especially when comparing with electronic structure calculations, is a vacuum, so there is likely to be a great deal of information available on either the surface of the solid or the gas-phase polynuclear ion or the gas-phase aquo complex (i.e., Fe(H20)63+, C03(H20)62-). The gas-phase transfer-ability requirements on potential functions are challenging, but it is difficult to imagine constructing effective potential functions for such systems without using gas-phase systems in the construction process. This means that any water molecules used on these complexes must also transfer from the gas phase to the condensed phase. A fundamental aspect of this transferability is polarization. 

Thus, the tables of standard electrode potentials predict those processes that tend to occur spontaneously if any pair of listed interfacial systems are built into an electrochemical cell that with the lower (algebraically, i.e., more negative) standard potential will spontaneously undergo deelectronation (oxidation), while that with the higher potential (i.e., more positive) will spontaneously undergo electronation (reduction). 

Equation D3.5.13 illustrated that the free energy of an interfacial system can be expressed in terms of the interfacial tension and chemical potential of the overall system. A simple differentiation or alternatively the reutilization of the definition of the interfacial tension used in Equation D3.5.7 at constant pressure and temperature yields ... 

An understanding of equilibrium phenomena in naturally occurring aqueous systems must, in the final analysis, involve understanding the interaction between solutes and water, both in bulk and in interfacial systems. To achieve this goal, it is reasonable to attempt to describe the structure of water, and when and if this can be achieved, to proceed to the problems of water structure in aqueous solutions and solvent-solute interactions for both electrolytes and nonelectrolytes. This paper is particularly concerned with two aspects of these problems—current views of the structure of water and solute-solvent interactions (primarily ion hydration). It is not possible here to give an exhaustive account of all the current structural models of water instead, we shall describe only those which may concern the nature of some reported thermal anomalies in the properties of water and aqueous solutions. Hence, the discussion begins with a brief presentation of these anomalies, followed by a review of current water structure models, and a discussion of some properties of aqueous electrolyte solutions. Finally, solute-solvent interactions in such solutions are discussed in terms of our present understanding of the structural properties of water. 

The interfacial synthesis of bisphenol-A carbonate oligomers is conducted by passing phosgene into an agitated, two-phase mixture of a water-insoluble organic solvent and an alkaline solution of bisphenol-A. Reactions 1-4 shown on p. 260 illustrate some of the reactions believed to occur in the interfacial system. 

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