Computer simulation study of ionic and non ionic hydration

Résumé du document

Liquids, solution, and liquid crystals present local, but not long-range order. On one hand particles in liquids interacts much strongly than the particles in gas, and this prevents the use of the usual statistical mechanics methods to predict their structure and properties since the needed averaging process must be adapted to this situation. On the other hand, they are not as ordered as the atoms in a crystal and this prohibits the quantum mechanical computing of their electronic properties using symmetry considerations. And since several chemical reactions do not occur in gas phase and biological processes are known to take place only in aqueous environments, the understanding of the microscopic structure and behaviour of liquids is essential.In order to apprehend the importance of intermolecular forces the use of computers has become relatively common and much progress has been made in the field. In the present paper we report the results of computer simulations of different systems and the essay is arranged as follows: in Sec. I, we present a brief description of the advantages of the computer simulation and a definition of the radial distribution function (RDF). Section II explains how water behaves in a liquid phase. Section III will focus on the ionic hydration and on the temperature influence; whereas section IV will look into the non ionic hydration of Argon, tetrafluoromethane and nanopore. Finally, in Sec. V, we draw the main conclusion from our work.

Extraits

[...] In fact, with the Ar atom, the water molecules can't stay near the atom because there is a hydrophobic effect. This subject will be briefly described later in the text - Perfluoroalkanes hydration Computer simulations aren't limited anymore to model small systems, interactions with bigger molecules such as perfluoroalkanes can be easily considered. Only 1 tetrafluoromethane (CF4) molecule surrounded by 500 molecules of water was simulated with a Monte Carlo method, thermodynamic variables were set to reproduce liquid water at room temperature. [...]

[...] First of all, we could make a study of the RDFs of Ar, and Cl- with the oxygen of water. The first difference among these RDFs is the charge-bonded solvation shells presented by the ionic species, which are not present for the neutral Ar in gAr-O(r) (Figure 3). This means there is a more definite structure around these species compared to the argon atom, mainly because of the charge, which forces the liquid to a specific distribution. Then, the number of oxygen atoms in the first solvation shell is also significantly different for the ionic species or and for the Ar neutral atom (16). [...]

[...] Such a secondary shell is typically encountered in the liquid structure of polar or nonpolar molecules. This second shell has 20 hydrogen atoms, on average. After this shell it's difficult to identify subsequent solvation shells, as the liquid tends not to possess any recognizable structure. Henceforth, it looks more like a gas, with atoms evenly distributed. The form of the peaks can be explained by considering the detailed nature of the hydrogen bonding interaction. Indeed, this interaction has preferred orientation and distance; molecules that do not meet these stringent geometrical attributes are therefore excluded from the first shell. [...]

[...] Both methods not only allow the isolation and manipulation of individual structural and thermodynamic parameters, but also the visualization of a large set of representations (microscopic level images and animations, graphs and numerical data) simultaneously. Such interrelated visualizations are fundamental to understanding the connections between concepts, variables and phenomena. After the simulation process, thermodynamics and structural properties of the system can be calculated. The radial distribution function (RDF) is the most informative feature of the molecular structure and will use be as the backbone of our work. [...]

[...] MC and MD simulation methods have become a promising tool to study fluids even under extreme conditions and can potentially provide the much needed insight into the physics of fluids under extreme conditions and hence reengineer existing geochemical equations of state or even find new approaches to their formulations Ionic hydration in infinitely dilute solutions Only 1 solute or Cl- ion) surrounded by 200 molecules of water was simulated with a MC method, thermodynamic variables were set to reproduce liquid water at room temperature. The RDFs studied in this part are calculated between the solute and an atom of all 200 solvent water molecules. The radial distribution functions of gNa+-O(r) and gCl--O(r) are shown in Figure 3 and the RDF of gNa+-H(r) and gCl--H(r) in Figure. [...]