Molecular-scale events

Defect formation and dynamics in the crystal and at the melt-crystal interface are molecular-scale events that are only adequately simulated by lattice-scale models. A discussion of lattice-scale equilibrium and calculations of molecular dynamics is beyond the scope of this chapter. 

In these balance equations all terms should be described at the same level of accuracy. It certainly does not pay to have the finest description of one term in the balance equations if the others can only be very crudely described. Current demands for increased selectivity and volumetric productivity require more precise reactor models, and also force reactor operation to churn turbulent flow which to a great extent is uncharted territory. An improvement in accuracy and a more detailed description of the molecular scale events describing the rate of generation terms in the heat- and mass balance equations has... 

Even at the microscale level, molecular-scale events are described by a continuous process. 

Figure 1. If there is a connection between the molecular, macromolecular and macroscopic scale changes in polymeric materials, then macromolecular and macroscopic scale changes can be predicted from the molecular scale events.

Thus far we have explored the field of classical thermodynamics. As mentioned previously, this field describes large systems consisting of billions of molecules. The understanding that we gain from thermodynamics allows us to predict whether or not a reaction will occur, the amount of heat that will be generated, the equilibrium position of the reaction, and ways to drive a reaction to produce higher yields. This otherwise powerful tool does not allow us to accurately describe events at a molecular scale. It is at the molecular scale that we can explore mechanisms and reaction rates. Events at the molecular scale are defined by what occurs at the atomic and subatomic scale. What we need is a way to connect these different scales into a cohesive picture so that we can describe everything about a system. The field that connects the atomic and molecular descriptions of matter with thermodynamics is known as statistical thermodynamics. 

While D issuing from these experiments is not strictly the diffusion coefficient of water per se, but rather that of H throughout the ensemble of environments in the hydration microstructure, these authors rationalized that it could in fact be identified with D at both high and low water contents. It should be appreciated that self-diffusion coefficients measured in this way reflect fundamental hopping events on a molecular scale. 

Spectroscopic techniques, such as ultra-violet (9), Infrared (25), Nuclear Magnetic Resonance (24), and Fluorescence spectroscopies (5-8), constitute direct probes of specific events occurring at the molecular scale. When a quantitative interpretation is possible, spectroscopy provides very detailed microscopic information. Unfortunately however, the interpretation of spectra in terms of molecular events is often complex. Yet another approach that probes events at the molecular scale involves the use of tracers, such as chromophores (1-225). Again, the complexity of the tracer imposes limitations on the extent to which the data can be interpreted quantitatively. 

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

The range of scales important in environmental toxicology varies from the few angstroms of molecular interactions to the hundreds of thousands of square kilometers affected by large-scale events. Figure 2.4 presents some of... 

The main objective of this work is to discuss recent developments in molecular simulation, multiscale simulation and multiscale systems engineering, and how these developments enable the targeted design of processes and products at the molecular scale. The control of events at the molecular scale is critical to product quality in many new applications in medicine, computers and manufacturing. 

Most processes in which control of events at the molecular scale is important have only one or two manipulated variables available during processing. For example, the applied potential is the only manipulated variable during the electrodeposition of copper to form an interconnect. A challenge for constructing predictive models for such systems is how to excite the dynamics during model identification experiments when so few manipulated variables are available. 

The course of reactions is determined by events at the molecular scale, whether or not the reactive molecules are in the liquid, solid, or gas phase when they enter the reaction zone. As in the case of homogeneous reactions, the course of a complex reaction will be determined by local molar ratios and chemical kinetics. The degree of deviation from expected kinetic behavior is determined by the reaction rate relative to the rates of mass transfer and mixing. Possible mixing... 

The bedrocks of the theoretical and computational methods that allow study of relationships between molecular and mesoscopic scale events and system properties are quantum and statistical mechanics. Thus, this volume comprises chapters that describe the development and application of quantum and statistical mechanical methods to various problems of technological relevance. The application areas include catalysis and reaction engineering, processing of materials for microelectronic applications, polymer science and engineering, fluid phase equilibrium, and combinatorial methods for materials discovery. The theoretical methods that are discussed in the various... 

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