Thermodynamics of simple models of associating fluids: primitive models of ammonia, methanol, ethanol and water

Exploiting limited valence patchy particles to understand autocatalytic kinetics

Autocatalysis, i.e., the speeding up of a reaction through the very same molecule which is produced, is common in chemistry, biophysics, and material science. Rate-equation-based approaches are often used to model the time dependence of products, but the key physical mechanisms behind the reaction cannot be properly recognized. Here, we develop a patchy particle model inspired by a bicomponent reactive mixture and endowed with adjustable autocatalytic ability. Such a coarse-grained model captures all general features of an autocatalytic aggregation process that takes place under controlled and realistic conditions, including crowded environments. Simulation reveals that a full understanding of the kinetics involves an unexpected effect that eludes the chemistry of the reaction, and which is crucially related to the presence of an activation barrier. The resulting analytical description can be exported to real systems, as confirmed by experimental data on epoxy-amine polymerizations, solving a long-standing issue in their mechanistic description.

Coarse-Grained Molecular Models of Water: A Review

Coarse-grained (CG) models have proven to be very effective tools in the study of phenomena or systems that involve large time- and length-scales. By decreasing the degrees of freedom in the system and using softer interactions than seen in atomistic models, larger timesteps can be used and much longer simulation times can be studied. CG simulations are widely used to study systems of biological importance that are beyond the reach of atomistic simulation, necessitating a computationally efficient and accurate CG model for water. In this review, we discuss the methods used for developing CG water models and the relative advantages and disadvantages of the resulting models. In general, CG water models differ with regards to how many waters each CG group or bead represents, whether analytical or tabular potentials have been used to describe the interactions, and how the model incorporates electrostatic interactions. Finally, how the models are parameterized depends on their application, so, while some are fitted to experimental properties such as surface tension and density, others are fitted to radial distribution functions extracted from atomistic simulations.

Water and aqueous solutions: simple non-speculative model approach

Different ways of molecular modeling of water are analyzed and their similarities and differences identified. An up-to-date summary of achievements of a general approach to common rigid site-site interaction models of molecular fluids applied to water and aqueous solutions is then presented and discussed. The method is based on considering only a short-range part of a total realistic potential (such as SPC/E or TIPxP) which determines the structure of water (and fluids in general). A simplification of the interactions at short intermolecular separations leads then to simple models, called primitive models. Quite accurate results in an analytic form for the thermodynamic properties of the models are obtained using the thermodynamic perturbation theory. It is shown that the properly constructed primitive models reproduce, qualitatively, anomalies of pure water and basic characteristics of hydrophobic hydration. The concept of an extended excluded volume, based on pseudo-hard bodies, is introduced and exemplified by considering the partial molar volume of apolar solutes. Potential future development towards a theory of water based on the primitive models as a reference with the long-range contributions added as a perturbation is discussed.

Development of the trappe force field for ammonia

The transferable potentials for phase equilibria (TraPPE) force field is extended through the development of a non-polarizable five-site ammonia model. In this model, the electrostatic interactions are represented by three positive partial charges placed at the hydrogen position and a compensating partial charge placed on an M site that is located on the C3 molecular axis and displaced from the nitrogen atom toward the hydrogen atoms. The repulsive and dispersive interactions are represented by placing a single Lennard–Jones site at the position of the nitrogen atom. Starting from the five-site model by Impey and Klein (Chem. Phys. Lett. 1984, 104, 579), this work optimizes the Lennard–Jones parameters and the magnitude of the partial charges for three values of the M site displacement. This parameterization is done by fitting to the vapor–liquid coexistence curve of neat ammonia. The accuracy of the three resulting models (differing in the displacement of the M site) is assessed through computation of the binary vapor–liquid equilibria with methane, the structure and the dielectric constant of liquid ammonia. The five-site model with an intermediate displacement of 0.08 Å for the M site yields a much better value for the dielectric constant, whereas differences in the other properties are quite small.

Toward a statistical mechanical theory for water: Analytical theory for a short-ranged reference system

Starting from a realistic Hamiltonian and making use of recent findings that the properties of associating fluids are determined primarily by short-ranged interactions, this methodology has been implemented using statistical mechanical approaches and thermodynamic perturbation theory for the TIP4P model of water. We focus on the short-range reference system for which an analytic expression for the Helmholtz free energy is derived. It is found that the model (reference system) exhibits, in addition to a faithful representation of the structure of water, the same features that are characteristic for real water, namely, (i) the temperature of the density maximum and its pressure dependence, including the inflection point at high pressures and (ii) the temperature minima of the constant pressure heat capacity and the coefficient of isothermal compressibility.