Analysis of Experimental Heats of Dilution of Aqueous Solutions of Nabph by Use of the Mean Spherical Approximation and Molecular Dynamics Simulations

Science Direct Working Paper No S1574-0331(04)70230-1

31 Pages Posted: 23 May 2017 Last revised: 9 Dec 2017

See all articles by Jalel M'halla

Jalel M'halla

Laboratoire d’Electrochimie des Solutions - Faculté des Sciences

S. M'halla

Laboratoire d’Electrochimie des Solutions - Faculté des Sciences

G. Wipff

Université Louis Pasteur - Laboratoire MSM

Date Written: March 2002

Abstract

Calorimetric measurements of heats of dilution: Q = - n ϕ, of aqueous solutions of NaBPh are determined at 25C in the concentration range: 0 < C < 1M. Results show a large positive deviation between experimental ϕ and ϕ calculated from exact MSA theory. This deviation can be interpreted as an indirect solute-solute attraction due to the specific hydration of the BPh ion in presence of Na. In order to present a deeper physicochemical analysis of the dilution process of NaBPh, we used Molecular Dynamics Simulations of different mixtures of NaBPh (0 ≤ ≤ 64) and TIP3P water molecules (2473 ≤ ≤ 4024). ϕ are calculated directly in terms of molar energies or indirectly in terms of molar energies. MD results show that the sign of the slope: dϕ / dC is very sensitive to the charge distribution on the BPh ion. On the other hand, according to our analysis this slope depend on a specific contribution Φ defined by: m dΦ / dm ≈ - [e - (e) ], m is the molecularity and is the hydration number. The condition dΦ/dm >> 0, implies that the energy of hydration molecules: e is more negative than the molecular energy (e) of water molecules in the bulk solution. This condition is satisfied in the case of strong ion-solvent interaction or in the case of hydrophobic hydration. MD results concerning the separation of e in terms of e and e energies, show that the H bonds of water molecules around BPh ions are partially broken: contrarily to the Na+ ion, the BPh ion cannot orient completely the water dipoles in the direction of its local field. However, it seems that the TIP3P model cannot explain completely the enhancement of the H bonds of water molecules in the second shell around the BPh ion, by comparison to the H bonds in bulk solution. This analysis is important from the fact that, the hydrophobic interaction is governed by the potential of between water molecules via the influence of the ion-water interactions. Calculations show that this last interactions are to the charge distributions of both the polyatomic aqua-ions and the hydration molecules. Consequently, the “extrathermodynamic” hypothesis concerning polyatomic ions require more precise justifications than those usually given by the literature.

Keywords: Physical Chemistry > Computational Chemistry, physchem/0203016

Suggested Citation

M'halla, Jalel and M'halla, S. and Wipff, G., Analysis of Experimental Heats of Dilution of Aqueous Solutions of Nabph by Use of the Mean Spherical Approximation and Molecular Dynamics Simulations (March 2002). Chemistry Preprint Archive Vol. 2002, Issue 3, pp 240-270. Available at SSRN: https://ssrn.com/abstract=2969233

Jalel M'halla (Contact Author)

Laboratoire d’Electrochimie des Solutions - Faculté des Sciences ( email )

5000 Monastir
Tunisia

S. M'halla

Laboratoire d’Electrochimie des Solutions - Faculté des Sciences

5000 Monastir
Tunisia

G. Wipff

Université Louis Pasteur - Laboratoire MSM

UMR 7751
4 rue Blaise Pascal
670000 Strasbourg
France

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