Complex fluids, molecular simulation

The behavior of simple and molecular ions at the electrolyte/electrode interface is at the core of many electrochemical processes. The complexity of the interactions demands the introduction of simplifying assumptions. In the classical double layer models due to Helmholtz [120], Gouy and Chapman [121,122], and Stern [123], and in most analytic studies, the molecular nature of the solvent has been neglected altogether, or it has been described in a very approximate way, e.g. as a simple dipolar fluid. Computer simulations... 

The radial distribution function plays an important role in the study of liquid systems. In the first place, g(r) is a physical quantity that can be determined experimentally by a number of techniques, for instance X-ray and neutron scattering (for atomic and molecular fluids), light scattering and imaging techniques (in the case of colloidal liquids and other complex fluids). Second, g(r) can also be determined from theoretical approximations and from computer simulations if the pair interparticle potential is known. Third, from the knowledge of g(r) and of the interparticle interactions, the thermodynamic properties of the system can be obtained. These three aspects are discussed in more detail in the following sections. In addition, let us mention that the static structure is also important in determining physical quantities such as the dynamic an other transport properties. Some theoretical approaches for those quantities use as an input precisely this structural information of the system [15-17,30,31]. 

We have seen that the earlier methods of micropore analysis were either essentially empirical or based on questionable assumptions. In contrast, molecular simulation and DFT offer the prospect of a more rigorous treatment since they are based on the fundamental principles of statistical mechanics. However, it must be kept in mind that to solve the statistical mechanical Hamiltonian, it is necessary to know the exact position of the force centres in the solid structure and also the potential functions which govern the solid-fluid and fluid-fluid interactions. In view of the complexity of most porous adsorbents, it is not surprising that so far most attention has been given to the adsorption of small, spherical molecules in pores of uniform geometry -particularly cylindrical or slit-shaped pores (Steele and Bojan, 1997). 

In some cases, one is interested in the structures of complex fluids only at the continuum level, and the detailed molecular structure is not important. For example, long polymer molecules, especially block copolymers, can form phases whose microstructure has length scales ranging from nanometers almost up to microns. Computer simulations of such structures at the level of atoms is not feasible. However, composition field equations can be written that account for the dynamics of some slow variable such as 0 (x), the concentration of one species in a binary polymer blend, or of one block of a diblock copolymer. If an expression for the free energy / of the mixture exists, then a Ginzburg-Landau type of equation can sometimes be written for the time evolution of the variable 0 with or without flow. An example of such an equation is (Ohta et al. 1990 Tanaka 1994 Kodama and Doi 1996)... 

Further, molecular simulation and computational chemistry have evolved, and are evolving, into important tools for developing better characterization techniques where it is not possible to measure all data. Even so, it is precisely the molecular complexity of petroleum fluids that seems to be an inhibiting factor in the use of these methods for developing better characterization methods. However, identification of important functional groups in petroleum fluids applying molecular simulation and/or computational chemistry for use with group contribution methods to predict thermo-physical properties may be an area for further research. 

In Dzwinel and Yuen (2000a,b 2001), it was demonstrated that DPD fits very well for simulating multiresolution structures of complex fluids. Typical examples of complex fluids with large molecular structure include microemulsions, micellar solutions, and colloidal suspensions like blood, ink, milk, fog, paints, and partially crystalline magmatic melts (Larson 1999). 

Confined Complex Fluids. - Because of their important technological relevance, the study of alkanes and polymers under extreme confinement continues to gain popularity in the simulation community. The difficulty in making experimental measurements for nano-confined systems, and the lack of confirmed theoretical models for such systems, makes molecular simulation the ideal tool to explore thermodynamics, structure and transport at such scales. 

Based on the number, diversity and depth of the studies undertaken over the past two years we are confident that the future of molecular simulation is very healthy. In terms of the volume of work undertaken, clearly the growth areas are in complex fluids and confined fluids. The simulation of biomolecular fluids in particular (DNA, proteins in solution, etc) will attract considerable effort in the years to come. New theories continue to be proposed and simulation will play a key part in assessing their validity, particularly when experimental verification is unavailable or even unfeasible at the present time. New results from simulation will also demand a re-working of existing theories, and the validity of new simulation techniques themselves will be judged against available experimental data. There is much to do, and without a doubt much will be reported when the next review of the liquid state will be conducted in two years time. 

In spite of their practical importance, the precise mechanisms by which cryo- and lyoprotectants work are not well understood. Protectant formulations are often conceived through a trial-and-error process. Moreover, the thermophysical property data required to formulate protectant solutions rationally and to design cryopreservation and lyophilization protocols are rarely available. As discussed in this proposal, it is of particular interest to understand how protectants interact with cell membranes. Over the last several years we have conducted a systematic study of the structure, thermod)mamic and transport properties of model cell membranes in liquid and glassy solutions of protectant molecules. Our two-pronged theoretical and experimental approach comprises the development of novel and powerful methods for molecular simulation of complex fluids near the... 

Stevens MJ, Kremer K. Molecular dynamics simulations of charged polymer chains from dilute to semidilute concentrations. In Schmitz KS, ed. Macroion Characterization From Dilute Solutions to Complex Fluids. Washington DC ACS Symposium Series, 1994 57-66. 

The molecular dynamics methods that we have discussed in this chapter, and the examples that have been used to illustrate them, fall into the category of atomistic simulations, in that all of the actual atoms (or at least the non-hydrogen atoms) in the core system are represented explicitly. Atomistic simulations can provide very detailed information about the behaviour of the system, but as we have discussed this typically limits a simulation to the nanosecond timescale. Many processes of interest occur over a longer timescale. In the case of processes which occur on a macroscopic timescale (i.e. of the order of seconds) then rather simple models may often be applicable. Between these two extremes are phenomena that occur on an intermediate scale (of the order of microseconds). This is the realm of the mesoscale Dissipative particle dynamics (DPD) is particularly useful in this region, examples include complex fluids such as surfactants and polymer melts. 

Alda, W., Dzwinel, W., Kitowski, J., Moscinski, J., Pogoda, M., and Yuen, D.A., Complex fluid-dynamical phenomena modeled by large-scale molecular dynamics simulations. Comp. Phys., 12... 

With the ongoing increase of computer performance, molecular modeling and simulation is gaining importance as a tool for predicting the thermodynamic properties for a wide variety of fluids in the chemical industry. One of the major issues of molecular simulation is the development of adequate force fields that are simple enough to be computationally efficient, but complex enough to consider the relevant inter- and intramolecular interactions. There are different approaches to force field development and parameterization. Parameters for molecular force fields can be determined both bottom-up from quantum chemistry and top-down from experimental data. 

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