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 VE and HE 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, VE/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, VE 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, UEv=–400 δ212kJ/mol; for alkali nitrate + silver nitrate mixtures, UEV=–0.1 + 36 δ12–400 δ212 kJ/mol; and for alkali nitrate + thallous nitrate mixtures, UEV= 22 δ12– 400 δ212 kJ/mol (δ12 in 1010 m–1).