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
Date Written: March 2002
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
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