Jean-Luc Fortier

Thermodynamic properties of alkali halides. II. Enthalpies of dilution and heat capacities in water at 25°C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25°C with flow microcalorimeters in the concentration range 0.01 to 1m. Apparent molal relative enthalpiesφL, derived from the enthalpies of dilution, can be represented by a parametric equation in molality. CombiningφL with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacitiesφC were fitted with the equationφC=φC°+AC(d0m)1/2+BCm. TheφC° are, in general, additive to better than 1 J-K−1-mole−1 and reflect mostly the structural part of ion-solvent interactions. TakingφC°(H+)=0, conventional ionicφC° are obtained. The parameterBCfor different pairs of ions follows approximately the same trends as the corresponding parameterBVfor apparent molal volumes and seems to reflect structural interactions between the ions.

The apparent molar heat capacities and volumes of aqueous NaCl from 0.01 to 3 mol kg−1 in the temperature range 274.65 to 318.15 K

Abstract The heat capacities per unit volume of aqueous solutions of NaCl were measured with a flow microcalorimeter. The molality and temperature range covered were 0.01 to 3 mol kg −1 and 274.65 to 318.15 K. The derived apparent molar heat capacities C 2 , φ, when extrapolated to infinite dilution, give standard partial molar heat capacities C 2 o which are in excellent agreement with those of Criss and Cobble. The excess apparent molar heat capacities ( C 2 , φ - C 2 o ) can be used to predict the temperature dependence of ( H 2 , φ - H 2 o ), the excess apparent molar enthalpy. The calculated values of ΔH 2 , φ agree within experimental uncertainty with the integral enthalpies of dilution of Ensor and Anderson and of Messikomer and Wood up to 323.15 K. Above this temperature significant differences are observed. The densities of the solutions were also remeasured in the same range of temperature and molality with a flow densimeter, and the derived apparent molar volumes agree with the literature values.

Enthalpies of dilution of electrolyte solutions by flow microcalorimetry

The enthalpies of dilution of NaCl, Me4NBr, andn-Bu4NBr were measured in water at 25°C with a new flow microcalorimeter. The data were analyzed with a polynomial equation, and the derived relative apparent molal enthalpies φL are in good agreement with literature values. Provided care is taken that mixing is complete, flow calorimeters are as reliable and much less time-consuming than cell-type instruments for enthalpies of dilution measurements.

Thermodynamic properties of alkali halides. III. Volumes and heat capacities of transfer from H2O to D2O at 25°C

The volumetric specific heats and densities of alkali fluorides, alkali bromides, and soduum halides were measured in D2O at 25°C in the concentration range 0.05 to 1 aquamolal. The results can be combined with data in H2O to give the corresponding standard and excess transfer functions from H2O to D2O. The volumes and heat capacities of transfer are both negative but, contrary to the hydration functions, show little dependence on ionic size and sign. Also, while heats, entropies, and volumes of transfer are usually small compared with the hydration functions, the heat capacity of transfer is of comparable magnitude. These observations, when interpreted with the Frank and Wen model, suggest that the total number of water molecules in the hydration cosphere is approximately constant for all alkali halides and that heat capacities are more sensitive to structural interactions than volumes and enthalpies. The sign of the excess transfer functions is consistent with the presence of structural ion-ion interactions, but no systematic trend with ionic size can be detected in view of the large experimental uncertainty.

Direct continuous measurements of thermal expansion coefficients of liquids and solids using flow microcalorimetry

A differential heat capacity flow microcalorimeter is used to monitor in a continuous mode the thermal expansion of a sample during a programmed temperature scan. The sample may consist of liquids, suspensions, or bulk solids in a confining liquid and the typical temperature scanning rate is of the order of 1 K/min. The technique has a precision better than 1% and a detection limit of 10(-6) ml s(-1). In contrast to conventional dilatometers, this technique offers variable sensitivity and is not limited by the magnitude of the total volume change during the experiment. Various expansibility data obtained in the temperature range 10-55 degrees C are reported for several systems, namely water, benzene, carbon tetrachloride, and aqueous solutions of sodium chloride. The volume changes for the thermal transition of Teflon and the phase separation of 2-butoxyethanol/water mixtures further illustrate the possibilities of this new technique.