Monte Carlo simulation associating fluids

If a confined fluid is thermodynamically open to a bulk reservoir, its exposure to a shear strain generally gives rise to an apparent multiplicity of microstates all compatible with a unique macrostate of the fluid. To illustrate the associated problem, consider the normal stress which can be computed for various substrate separations in grand canonical ensemble Monte Carlo simulations. A typical curve, plotted in Fig. 16, shows the oscillatory decay discussed in Sec. IV A 2. Suppose that instead... 

Finally, in Sec. IV, two examples of the application of the Monte Carlo simulation to investigate the structure and thermodynamic properties of adlayers of an associating fluid are given. The results of simulations are compared with those from theoretical approaches. In conclusion, we discuss some methodological perspectives in the discussed area of research. 

FIG. 1 Total local density p(z) for bulk density p = 0.821 and e /k T = 4.25. The solid line is for PYl theory, the dashed line is for HNCl approximation and the points denote the Monte Carlo simulation results. (Reprinted from S. Sokolowski, D. Henderson, A. Trokhymchuk, O. Pizio. Density profiles of associating fluid near a hard wall PY/EMSA and HNC/EMSA singlet theory, Physica A, 220, 22-32. (1995), with permission from Elsevier Science.)... 

Associating Fluids from Osmotic Monte Carlo Simulation... 

To test the results of the chemical potential evaluation, the grand canonical ensemble Monte Carlo simulation of the bulk associating fluid has also been performed. The algorithm of this simulation was identical to that described in Ref. 172. All the calculations have been performed for states far from the liquid-gas coexistence curve [173]. 

To conclude, the introduction of species-selective membranes into the simulation box results in the osmotic equilibrium between a part of the system containing the products of association and a part in which only a one-component Lennard-Jones fluid is present. The density of the fluid in the nonreactive part of the system is lower than in the reactive part, at osmotic equilibrium. This makes the calculations of the chemical potential efficient. The quahty of the results is similar to those from the grand canonical Monte Carlo simulation. The method is neither restricted to dimerization nor to spherically symmetric associative interactions. Even in the presence of higher-order complexes in large amounts, the proposed approach remains successful. 

P. Bryk, A. Patrykiejew, O. Pizio, S. Sokolowski. The chemical potential of Lennard-Jones associating fluids from osmotic Monte Carlo simulations. Mol Phys 92 949, 1997 A method for the determination of chemical potential for associating liquids. Mol Phys 90 665, 1997. 

Designing an effective recipe to conduct a Monte Carlo simulation of a simple fluid is, in most situations, fairly straightforward (see Monte Carlo Simulations for Complex Fluids and Monte Carlo Simulations for Liquids). Unfortunately, as the number of internal degrees of freedom of a molecule increases, so does the complexity of the Monte Carlo recipe. This article is intended to provide an overview of several algorithms that have been devised to surmount some of the difficulties associated with the simulation of polymeric fluids. 

Various equations of state have been developed to treat association ia supercritical fluids. Two of the most often used are the statistical association fluid theory (SAET) (60,61) and the lattice fluid hydrogen bonding model (LEHB) (62). These models iaclude parameters that describe the enthalpy and entropy of association. The most detailed description of association ia supercritical water has been obtained usiag molecular dynamics and Monte Carlo computer simulations (63), but this requires much larger amounts of computer time (64—66). 

Johnson, J.K., Panagiotopoulos, A.Z., and Gubbins, K.E. (1994). Reactive canonical Monte Carlo a new simulation technique for reacting and associating fluids. Mol. Phys., 81, 717-33. 

Figure 2. Comparison of theory and simulation for an associating fluid with one bonding site. Details of the theory and interaction potential are given in Ref. 2. The circles are simulation results from the RCMC method and the triangles are from conventional Monte Carlo...

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