Luis Romaní

A detailed thermodynamic analysis of [C4mim][BF4]+ water as a case study to model ionic liquid aqueous solutions

Since determining experimentally a wide variety of thermophysical properties—even for a very small portion of the already known room temperature ionic liquids (and their mixtures and solutions)—is an impossible goal, it is imperative that reliable predictive methods be developed. In turn, these methods might offer us clues to understanding the underlying ion–ion and ion–molecule interactions. 1-Butyl-3-methylimidazolium tetrafluoroborate, one of the most thoroughly investigated ionic liquids, together with water, the greenest of the solvents, have been chosen in this work in order to use their mixtures as a case study to model other, greener, ionic liquid aqueous solutions. We focus our attention both on very simple methodologies that permit one to calculate accurately the mixtures molar volumes and heat capacities as well as more sophisticated theories to predict excess properties, pressure and isotope effects in the phase diagrams, and anomalies in some response functions to criticality, with a minimum of information. In regard to experimental work, we have determined: (a) densities as a function of temperature (278.15 < T/K < 333.15), pressure (1 < p/bar < 600), and composition (0 < xIL < 1), thus also excess molar volumes; (b) heat capacities and excess molar enthalpies as a function of temperature (278.15 < T/K < 333.15) and composition (0 < xIL < 1); and (c) liquid–liquid phase diagrams and their pressure (1 < p/bar < 700) and isotopic (H2O/D2O) dependences. The evolution of some of the aforementioned properties in their approach to the critical region has deserved particular attention.

Isobaric thermal expansivity and thermophysical characterization of liquids and liquid mixturesElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b104891k/

A methodology for the thermophysical characterization of liquids and liquid mixtures based on measurements of density, sound speed and isobaric heat capacity per unit volume at atmospheric pressure as a function of temperature is proposed. Density and sound speed data are used to determine the isentropic compressibility from the Laplace equation. The precision in density measurements allows one to obtain the isobaric thermal expansivity at different temperatures using an incremental procedure with quite acceptable accuracy and precision. The isothermal compressibility and isochoric molar heat capacity are both obtained from the previous properties, using well-known thermodynamic relations. The accuracy of the proposed methodology was checked by determining the above-mentioned properties for liquid n-hexane, n-heptane, n-octane, n-dodecane, n-hexadecane, cyclohexane, and toluene over the temperature range (288.15–333.15) K and comparing the results with selected reported data. The average absolute deviations from the latter showed data obtained with the proposed methodology to be reasonably accurate. The excess quantities for nine binary mixtures of the cyclohexane + n-dodecane system were also determined with a view to assessing precision, which was found to be quite good as regards the dependence on both composition and temperature.

Highly precise determination of the heat capacity of liquids by DSC: calibration and measurement

Abstract The heat capacities of various liquids were determined with a high precision using differential scanning calorimetry. The calibration and measurement procedures, as implemented on a Setaram Micro DSC II calorimeter based on Calvet’s differential detection of the heat flow rate in the measuring and reference vessels are analysed and discussed. An experimental procedure for determining heat capacities based on the scanning method is proposed. Calibration is done with liquids of known heat capacity. The proposed method was checked against various standard liquids. The results of ensuing scanning method are compared with those obtained by using the isothermal step method.

Effect of temperature on W-shaped excess molar heat capacities and volumetric properties : Oxaalkane-nonane systems

Excess molar volumesVmf of the mixtures diglyme (2,5,8-trioxanonane: TON), triglyme (2.5,8,11-tetraoxadodecane: TODD), or tetraglyme (2,5,8,11,14-pen-taoxapentadecane: POPD; E181)+n-nonane have been obtained from density measurements at 278.15, 288.15,298.15, and 308. 15 K. In addition, a micro DSC II differential scanning calorimeter was used to obtain excess molar heat capacitiesCpE at constant pressure for the same mixtures except for TON+n-nonane and at the same temperatures except for 278.15 K. These results allowed us to calculate the following mixing quantities in the complete range of concentration: α(∂Vmυ/∂T)p, and (∂Hυ/∂p)nT at 298.15K. The excess molar volumes are positive with large maximum values located in the central concentration range with the exception of POPD +n-nonane at 278.15 K, which has a central miscibility gap. For these mixtures,CpF has a W-shaped concentration dependence: two minima separated by a maximum.

Towards an understanding of the heat capacity of liquids. A simple two-state model for molecular association

A model for the temperature dependence of the isobaric heat capacity of associated pure liquids C(p,m)(o)(T) is proposed. Taking the ideal gas as a reference state, the residual heat capacity is divided into nonspecific C(p) (res,ns) and associational C(p) (res,ass) contributions. Statistical mechanics is used to obtain C(p)(res,ass) by means of a two-state model. All the experimentally observed C(p,m)(o)(T) types of curves in the literature are qualitatively described from the combination of the ideal gas heat capacity C(p)(id)(T) and C(p)(res,ass)(T). The existence of C(p,m)(o)(T) curves with a maximum is predicted and experimentally observed, for the first time, through the measurement of C(p,m)(o)(T) for highly sterically hindered alcohols. A detailed quantitative analysis of C(p,m)(o)(T) for several series of substances (n-alkanes, linear and branched alcohols, and thiols) is made. All the basic features of C(p,m)(o)(T) at atmospheric and high pressures are successfully described, the model parameters being physically meaningful. In particular, the molecular association energies and the C(p)(res,ns) values from the proposed model are found to be in agreement with those obtained through quantum mechanical ab initio calculations and the Flory model, respectively. It is concluded that C(p,m)(o)(T) is governed by the association energy between molecules, their self-association capability and molecular size.

Excess volumes and excess heat capacities of nitromethane+(1-propanol or 2-propanol)

Abstract Densities and heat capacities of binary mixtures containing nitromethane+(1-propanol or 2-propanol) were determined at the temperatures (288.15, 293.15, 298.15, and 308.15) K and atmospheric pressure, over the whole composition range. Excess molar volumes and excess molar isobaric heat capacities were calculated from the results thus obtained. The effect of specific interactions on the excess properties, and the dependence on the position of the OH group in the alkanol, are analysed.

On the isobaric thermal expansivity of liquids

The temperature and pressure dependence of isobaric thermal expansivity, α(p), in liquids is discussed in this paper. Reported literature data allow general trends in this property that are consistent with experimental evidence to be established. Thus, a negative pressure dependence is to be expected except around the critical point. On the other hand, α(p) exhibits broad regions of negative and positive temperature dependence in the (T, p) plane depending on the nature of the particular liquid. These trends are rationalized here in terms of various molecular-based equations of state. The analysis of the Lennard-Jones, hard sphere square well and restricted primitive model equations allows understanding the differences in the α(p) behavior between liquids of diverse chemical nature (polar, nonpolar, and ionic): broader regions of negative temperature and positive pressure dependencies are obtained for liquids characterized by larger ranges of the interparticle potential. Also, using the statistical associating fluid theory (SAFT) allowed the behavior of more complex systems (basically, those potentially involving chain and association effects) to be described. The effect of chain length is rather simple: increasing it is apparently equivalent to raise the interaction range. By contrast, association presents a quite complex effect on α(p), which comes from a balance between the dispersive and associative parts of the interaction potential. Thus, if SAFT parameters are adjusted to obtain low association ability, α(p) is affected by each mechanism at clearly separate regions, one at low temperature, due to association, and the other to dispersive forces, which has its origin in fluctuations related with vapor-liquid transition.

Second-order excess derivatives for the 1,3-dichloropropane + n-dodecane system

An experimental study of the behaviour of the second-order excess derivatives against composition and temperature for the 1,3-dichloropropane + n-dodecane system is presented. To this end, densities and speeds of sound were obtained in the interval 278.15-328.15 K with a step of 1 K. In addition, isobaric molar heat capacities were obtained in the temperature range 283.15-323.15 K. Measurements were performed over the whole composition range and at atmospheric pressure. Isentropic compressibilities were determined using the Laplace equation, and isobaric thermal expansivities were obtained in the interval 283.15-323.15 K from density data. Isothermal compressibilities and isochoric molar heat capacities within 283.15-323.15 K were derived from the above-mentioned properties via appropriate thermodynamic relations. The excess quantities for these properties were calculated using the ideality criterion first proposed by Benson and Kiyohara. Destruction of the 1,3-dichloropropane dipolar order during mixing appears to be the main effect to which the excess properties are sensitive.

Temperature dependence of the volumetric properties of binary mixtures containing oxaalkane + n-heptane

Excess molar volumes of mixtures of n-heptane + 2,5-dioxahexane and n-heptane + 2,5,8-trioxanonane were determined from density measurentents at 5, 15, 25 and 35°C. These results allowed the following mixing quantities to be reported in all range of concentrations: α, (δvE/δT)P and (δhE/δP)T, at 25°C. The obtained values were then compared with the calculated values by using the Flory theory and the Nitta-Chao theory of liquid mixtures. The results are discussed in terms of order or disorder creation.

Excess quantities of dialkyl carbonate + cyclohexane mixtures at a variable temperature

The density at 288.15, 293.15, 298.15 and 308.15 K, sound speed at 298.15 K, and isobaric molar heat capacity at 288.15, 298.15 and 308.15 K, of binary mixtures of cyclohexane with dimethyl carbonate or diethyl carbonate at atmospheric pressure were measured throughout the composition range. The data were used to calculate the excess quantities for the following properties: molar volumes, isentropic and isothermal compressibilities, isobaric thermal expansivity, and isobaric and isochoric molar heat capacities. As a rule, these excess quantities were substantially greater for the mixtures containing dimethyl carbonate than for those of diethyl carbonate. The excess isobaric molar heat capacity of both mixtures was found to exhibit a W-shaped variation with composition. Unlike its variation with the excess volumes, changes in this quantity as a function of temperature are non-linear.