Excess volumes for binary mixtures of fused nitrates

Abstract

A direct method has been used to measure the excess volumes of 18 approximately equimolar binary mixtures of the fused nitrates of Li, Na, K, Rb, Cs, Ag and Tl. The method is more accurate than the density method for a given uncertainty in the temperature measurement. The excess volume is positive for all mixtures studied except AgNO₃+ TlNO₃. For twelve of the mixtures V^E and H^E have opposite signs, showing that first-order conformal solution theory is not obeyed. In accordance with the second-order theory of Reiss, Katz and Kleppa, V^E/X₁X₂ is roughly proportional to δ²₁₂=[(d₁–d₂)/d₁d₂]², where d₁ and d₂ are the anion-cation distances in the pure components. Deviations from this relationship are attributed to van der Waals or polarization interactions between nearest neighbour anions and cations. For mixtures of AgNO₃ or TlNO₃ with alkali nitrates, V_E is large even when δ²₁₂∼0. Van der Waals and polarization interactions do not account for the results for these mixtures, and it is suggested that the excess volume arises from a change in the covalent character of the Ag—O or Tl—O bond when AgNO₃ or TlNO₃ are mixed with an alkali nitrate.

The excess volumes have been used to calculate the excess internal energies at constant total volume from the excess heats at atmospheric pressure. For alkali nitrate mixtures, U^E_v=–400 δ²₁₂kJ/mol; for alkali nitrate + silver nitrate mixtures, U^E_V=–0.1 + 36 δ₁₂–400 δ²₁₂ kJ/mol; and for alkali nitrate + thallous nitrate mixtures, U^E_V= 22 δ₁₂– 400 δ²₁₂ kJ/mol (δ₁₂ in 10¹⁰ m^–1).