Liquid-phase dynamics transition

As a consequence of this, from the fluid dynamic point of view, the use of solvent mixtures of EtOH and acetone caimot be used to alter the mixing behavior of the injected jet as it is case for example for using mixtures of DMSO and AC as published by De Marco et al. [18,38]. A manipulation of the final properties of SAS processed particles by either precipitating from a liquid phase, a transition phase or a single phase for the system EtOH/AC/C02 does not seem to be achievable by varying the solvent composition with respect to the jet behavior. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

Phase Dynamics utilizes a unique, patented microwave concept to diagnose and measure molecular transformation process parameters with high sensitivity and accuracy (Phase Dynamics 1992). While originally developed for fluid measurements, the instrumentation is adaptable to most pumpable process lines and to some batch applications. The technique has been utilized for compositional analyses of true solutions as well as complex solid-liquid systems such as colloids and emulsions. Monitoring of molecular transitions which occur in cooking processes, hydrogenation, gelatinization and hydrolysis can also be monitored. 

It should be mentioned that DSC and NMR do not measure the same parameters, and in this way, these techniques are complementary. DSC is a dynamic method, which gives information about the transitions between different phases of lipids, whereas NMR allows quantitation of liquid and solid phases at equilibrium. Indeed, NMR and DSC methods give different values for the solid fat index (SFI) (Walker and Bosin, 1971 Norris and Taylor, 1977) NMR values are much lower than those given by DSC below 20°C. For example, for milk fat at 5°C, DSC and NMR indicate 78.1% and 43.7% solid fat, respectively. The observed difference can be explained by the presence of an amorphous phase which, due to its melting enthalpy, is seen as a solid by the DSC method. Using time-domain NMR, Le Botlan et al. (1999) showed that in milk fat samples, an intermediate component exists between the solid and liquid phases, constituting about 6% of an aged milk fat. 

The dynamics of a chemical process can change considerably in going from the gas phase to the liquid phase. One fundamental reason for such differences is that liquids are able to solvate chemical species. For example, solvation might stabilize the transition state in a chemical reaction to a greater extent than it stabilizes the reactants, thereby accelerating the reaction rate. Of course, solvation itself is a dynamic process, which has important implications for chemical processes in solution. If the lifetime of a transition state is shorter than the inherent dynamic time scale of the solvent, for instance, solvation will not be able to stabilize the transition state to the fullest possible extent. The above example illustrates the importance of gaining a molecular-level understanding of the dynamics of solvents. 

First of all, liquid-phase studies generally do not obtain data which allows static and dynamic solvent effects to be separated [96,97], Static solvent effects produce changes in activation barriers. Dynamic solvent effects induce barrier recrossing and can lead to modification of rate constants without changing the barrier height. Dynamic solvent effects are temperature and viscosity dependent. In some cases they can cause a breakdown in transition state theory [96]. 

Our overall conclusion, therefore, is that for mesoporous glasses adsorption, hysteresis is a dynamic phenomenon that is not simply related to a capillary vapor-liquid phase transition. Slow dynamics for long times makes the states accessible in experiments in the hysteresis loop appear equilibrated and quite reproducible. Mean field theory and Monte Carlo simulations in the grand ensemble provide a physically realistic description of these phenomena. 

Water is well known for its unusual properties, which are the so-called "anomalies" of the pure liquid, as well as for its special behavior as solvent, such as the hydrophobic hydration effects. During the past few years, a wealth of new insights into the origin of these features has been obtained by various experimental approaches and from computer simulation studies. In this review, we discuss points of special interest in the current water research. These points comprise the unusual properties of supercooled water, including the occurrence of liquid-liquid phase transitions, the related structural changes, and the onset of the unusual temperature dependence of the dynamics of the water molecules. The problem of the hydrogen-bond network in the pure liquid, in aqueous mixtures and in solutions, can be approached by percolation theory. The properties of ionic and hydrophobic solvation are discussed in detail. 

The multiple time step propagation scheme is expected to be useful whenever a mixed quantum-classical molecular simulation is performed where only a few degrees of freedom are necessarily described within quantum mechanics and the force calculations in the classical subsystem is the time-limiting step. These conditions hold, for example, in molecular dynamics simulations of electron-and/or proton-transfer processes in the complex photosynthetic centre or in liquid phase. Furthermore, since the RPS is time-reversible, it is possible to calculate quantum reaction rates by propagating mixed quantum-classical trajectories located on the transition state back and forward in time. This opens a wide range of applications. 

The principle of Le Chatelier-Braun states that any reaction or phase transition, molecular transformation or chemical reaction that is accompanied by a volume decrease of the medium will be favored by HP, while reactions that involve an increase in volume will be inhibited. Qn the other hand, the State Transition Theory points out that the rate constant of a reaction in a liquid phase is proportional to the quasi-equilibrium constant for the formation of active reactants (Mozhaev et al., 1994 Bordarias, 1995 Lopez-Malo et al., 2000). To fully imderstand the dynamic behavior of biomolecules, the study of the combined effect of temperature and pressure is necessary. The Le Chatelier-Braim Principle states that changes in pressure and temperature cause volume and energy changes dependent on the magnitude of pressure and temperature levels and on the physicochemical properties of the system such as compressibility. "If y is a quantity characteristic of equilibrium or rate process, then the influence of temperature (7 and pressure (P) can be written as ... 

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