Solution Non‐Randomness and the W‐Shape Concentration Dependence of C^E_p for Hexafluorobenzene and Chloronaphthalene in Hydrocarbons

Abstract

H m t Cupacitja Liquids / ThermodynamicsExcess heat capacities, C,' have been obtained through the concentration range at 25-C for five sets of systems of type A, + 9 : hexafluorobenzene + 2.2,4-trimethylpentane and 2,2,4,4,6,8,8-heptamethylnonane; hexafluorobenzene + cyclohexane (10' , W) and dicyclohexyl (W); I-chloronaphthalene + 2,2-dimethylbutane, 2,2,4-trimethylpentane (W) and 2,2,4,4,6,8,8-heptamethylnonane (W). I -chloronaphthalene + n-hexadecane: I-chloronaphthalene + cyclohexane and dicyclohexyl (IS?, W). For the systems indicated, C,' has a W-shape, i.e., two minima separated by a maximum or two changes of sign of the curvature. The W-shape does not occur for hexafluorobenzene + cyclohexane at 25 but appears at lo?. Temperature and molecular size act on C," through the degree of solution non-randomness as indicated by G ' and the concentration-concentration correlation function, S,,, which is a good predictor for the W-shape and of the concentration at which the maximum appears. The W-shape corresponds to an S,, value greater than 0.7 and G E > 800 J mol-I, the temperature being within 100 K of the UCST.New absolute measurements of the thermal conductivity of aqueous NaCI, KCl and CaCI2 solutions are presented. The concentrations studied were 10% and 20% by weight of NaCl and KCl, and 10, 20 and 30% by weight of CaCI2. The measurements were performed in a transient hot-wire apparatus employing two thin tantalum wires coated with an anodic tantalum pentoxide insulation layer. The temperature range examined was 290 to 345 K and the pressure was atmospheric. The uncertainty of the measurements is estimated to be +0.5%, an estimate confirmed by the measurement of the thermal conductivity of water. -Existing correlations for the thermal conductivity of aqueous inorganic salt solutions are also examined. The measurements have been used to develop a simple consistent correlative and predictive scheme that allows the correlation of the thermal conductivity of these solutions with an uncertainty of f0.7% and the prediction of the thermal conductivity to high pressures with an uncertainty of +2.5%.

W-shape and solution non-randomness: hydrocarbons + tetraalkoxysilanes

“…The instrument measures the difference ACp/V in volumetric heat capacity between liquid A in the working cell and liquid B in the reference cell. However, as in previous work ( 5 ) with systems with large values of H~ a "boundary effect" leads to a different AC,/Vvalue. If liquids A and B are interchanged then the "correct" value is the average of the two.…”

“…[4] and [5] is therefore go, which must be fitted to experiment. One possibility is to fit to GE for c-C6 + Si(OMe), or to the corresponding value of Scc, i.e.…”

“…The second contribution is due to has been found to exist between Scc and the magnitude of the W-shape, a value of 0.7 in Scc being suggested as the threshold for the occurrence of the W-shape, rather than the normal parabolic, CE. Scc is also obtainable from the concentration dependence of GE for the system and a simple Flory-Huggins expression (4,5) for G and G~ relates Scc to molecular size of the components and their interaction. It predicts that Scc and hence the W-shaped CF should be enhanced by reduction of T and increase of molecular size of the components.…”

Concentration Fluctuations and Surface Adsorption in Hydrogen-Bonded Mixtures

“…The connection between bulk and surface segregation is clearly pointed out by considering that concentration fluctuations and nonrandomness in binary mixtures can be described in terms of the concentration-concentration correlation function S cc (i.e., the structure factor at zero wavevector), 5,6 which is directly related to the osmotic compressibility: 7 S cc is also closely related to the so-called Kirkwood-Buff integrals that fully describe the equilibrium properties of fluid mixtures. [8][9][10] Strey et al 11 have shown that assuming ideal behavior to calculate the osmotic compressibility may be inadequate when calculating surface adsorption, even for dilute water-alcohol mixtures, thus an estimation of µ 2 is necessary. In refs 2, 3, and 11, the activity of the solute a 2 [µ 2 (x 2 , T) ) µ 2 0 (T) + RT ln a 2 (x 2 ,T), with µ 2 0 being the chemical potential at the reference state] has been estimated from the vapor pressure data.…”

“…Therefore, Γ 21 can be calculated if the composition's dependence on γ and of μ 2 is known. The connection between bulk and surface segregation is clearly pointed out by considering that concentration fluctuations and nonrandomness in binary mixtures can be described in terms of the concentration−concentration correlation function S cc (i.e., the structure factor at zero wavevector), , which is directly related to the osmotic compressibility: S cc is also closely related to the so-called Kirkwood−Buff integrals that fully describe the equilibrium properties of fluid mixtures. − …”