Free Access
J. Chim. Phys.
Volume 96, Number 8, September 1999
Page(s) 1450 - 1478
DOI: 10.1051/jcp:1999221

J. Chim. Phys. Vol. 96, N°8  p. 1450-1478

Étude densimétrique et calorimétrique des systèmes binaires et ternaires (eau/dioxanne/NaB(Ph)4). Effet "clathrate" et solvatation préférentielle de B(Ph) 4-

J. M'Halla and S. M'Halla

Laboratoire d'Électrochimie des Solutions, Faculté des Sciences, 5000 Monastir, Tunisie

In this work, we experimentally studied the variation of partial molal volumes and relatives partial molal enthalpies of NaB(Ph) 4, water and dioxane with the weight- fraction PD of dioxane (0 = PD = 20%) and with the molality m of the salt (0 = m = 0.4) at 25  $^{\circ}$C and P = 1atm. We also partly achieved the same work for some binary and ternary systems: NaCl/Water/Dioxane, in order th show "clathrate" and preferential solvation effects of B(Ph) 4-. This study shows: a- A molecule of dioxane locks into the hexagonal hydrogen bond network of water so that the volume effects remain weak. Each dioxane molecule is surrounded by 8 water molecules, whereas a water molecule is surrounded by 4H 2O or 3 H 2O and 1 dioxane molecule. b- B(Ph) 4- reinforces the water "clathrate" structure in its vicinity. c- Preferential solvation of B(Ph) 4- by dioxane occurs even at low PD: B(Ph) 4- is solvated respectively by 2, 3, 4 and 5 molecules of dioxane for PD = 5, 10, 15 and 20% and it remains hydrated by 2 water molecules for PD = 20%; while Na+ remains hydrated by 4 water molecules without any dioxane molecules in its first solvation shell. These results agree with those obtained by NMR spectroscopic studies and Ion-solvant mutual transport. d) Partial enthalpy of transfer of NaB(Ph) 4 at infinite dilution is > 0 whereas partial molal free enthalpy of transfer is < 0 for $P_D \leq 20$%. So that the irreversible transfert of B(Ph) 4- from pure water to dioxane-water mixtures ( $P_D \leq 20$%) is due to a positive entropy variation corresponding essentially to the "fusion" of the water "clatharate" structure surrounding B(Ph) 4-.

Dans ce travail, nous avons étudié expérimentalement la variation des volumes molaires partiels et celle des enthalphies molaires partielles de NaB(Ph) 4 de l'eau et du dioxanne en fonction du pourcentage du poids PD du dioxanne ( 0 = PD = 20 %) et de la molalité m du sel (0 = m = 0.4) à 25  $^{\circ}$C et P = 1 atm. Nous avons aussi réalisé en partie le même travail pour des systèmes binaires et ternaires NaCl/Eau/Dioxanne afin de mettre en éficence d'une part la structure "clathrate" de l'eau entourant B(Ph) 4- dans l'eau pure et d'autre part la solvatation préférentielle de cet ion par le dioxanne dans les mélanges riches en eau. Les nombres de solvatation et d'hydratation obtenus d'après cette étude sont compatibles avec ceux déterminés par RMN et par les méthodes de transport mutuel ion-solvant.

Key words: Eau-dioxanne-NaB(Ph) 4- -- densité -- enthalpie de transfert -- clathrate -- solvatation preférentielle

© EDP Sciences 1999

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