Jacobo Troncoso

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.

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.

Thermal properties of ionic systems near the liquid-liquid critical point

Isobaric heat capacity per unit volume, C(p), and excess molar enthalpy, h(E), were determined in the vicinity of the critical point for a set of binary systems formed by an ionic liquid and a molecular solvent. Moreover, and, since critical composition had to be accurately determined, liquid-liquid equilibrium curves were also obtained using a calorimetric method. The systems were selected with a view on representing, near room temperature, examples from clearly solvophobic to clearly coulombic behavior, which traditionally was related with the electric permittivity of the solvent. The chosen molecular compounds are: ethanol, 1-butanol, 1-hexanol, 1,3-dichloropropane, and diethylcarbonate, whereas ionic liquids are formed by imidazolium-based cations and tetrafluoroborate or bis-(trifluromethylsulfonyl)amide anions. The results reveal that solvophobic critical behavior-systems with molecular solvents of high dielectric permittivity-is very similar to that found for molecular binary systems. However, coulombic systems-those with low permittivity molecular solvents-show strong deviations from the results usually found for these magnitudes near the liquid-liquid phase transition. They present an extremely small critical anomaly in C(p)-several orders of magnitude lower than those typically obtained for binary mixtures-and extremely low h(E)-for one system even negative, fact not observed, up to date, for any liquid-liquid transition in the nearness of an upper critical solution temperature.

Two ways of looking at Prigogine and Defay's equation

In the search for understanding of several types of abnormal thermodynamic behaviour in the vicinity of critical lines of binary liquid mixtures, we have revisited an apparently forgotten relationship between the pressure dependence of the critical temperature and the second derivatives with respect to the composition of the volumetric and enthalpic properties of the mixture. We refer to an equation originally developed in the fifties by Prigogine and Defay and soon afterwards analysed by others. Under some restrictive assumptions, the T–p slope of the critical locus can simply be inferred from the ratio between vE and hE. The interest and usefulness of this approximate relation is self-evident. Values for any one of the three properties involved,(dT/dp)c, vE or hE, can be assessed based on the availability of the other two. Moreover, the amplitude of the divergence of thermodynamic response functions to criticality are intimately associated with the slope of the critical locus. A link between critical behaviour and solution excess properties is thus established. For instance, double critical points tend to occur if one of the excess properties changes its sign as the temperature or pressure is varied. In this work, we have started a detailed study of the practical limits of validity of the approximate relation. Five binary liquid mixtures were tested, all of them sharing a UCST/LCSP-type of phase transition. Although, from a theoretical perspective, the original second-derivatives approach should perform better, in practice, the direct ratio of the excess properties constitutes a superior strategy for obtaining (dT/dp)c values. The underlying reasons for this are discussed in detail. The T–p critical slope is normally found to play a secondary role in assessing the critical amplitudes of diverging thermodynamic functions.

Thermodynamic consistency near the liquid-liquid critical point.

The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition C(p,x) and the density rho near the liquid-liquid critical point is studied in detail. To this end, C(p,x)(T), rho(T), and the slope of the critical line (dT/dp)(c) for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both C(p,x)(T) and rho(T) data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa(-1) (or 8%) is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for C(p,x)(T) and rho(T) are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of rho accurately: specifically, to get full consistency, higher-order terms in rho(T) are needed; however, the various contributions at play cannot be separated unambiguously. As a consequence, the use of C(p,x)(T) and (dT/dp)(c) for predicting the behavior of rho(T) at near criticality appears to be the best choice at the actual experimental resolution levels. Furthermore, the reasonably good thermodynamic consistency being encountered confirms that previous arguments appealing to the inadequacy of the theoretical expression relating C(p,x) and rho for describing data in the experimentally accessible region must be fairly rejected.

Effect of Molecular Structure on the Thermodynamics of Nitromethane+Butanol Isomers Near the Upper Critical Point

Densities, isentropic compressibilities, and isobaric molar heat capacities were determined over the whole composition range for nitromethane+(2-butanol or isobutanol) at atmospheric pressure and at the temperatures 288.15, 293.15, 298.15, and 308.15 K. These results allowed us to obtain isobaric thermal expansivities, isothermal compressibilities, and isochoric molar heat capacities at the temperature 298.15 K. The excess quantities for the given properties were obtained. In addition, liquid–liquid phase separation temperatures were also determined, locating upper critical solution temperatures near the experimental temperatures. The variation of the properties among isomers is discussed. Also, the effect of the nonrandomness of the mixtures expected near the critical point is discussed.

Note: Evidence against 2D-Ising criticality in aqueous solutions with added salt

Coexistence-curve data in the refractive index-temperature plane for solutions of 3-methyl-pyridine in heavy water with a small amount of added sodium tetraphenylborate have been determined. The analysis of such data indicates that this system belongs to the universality class of the three-dimensional Ising model (3D-Ising). This finding contrasts with previous work by Sadakane et al. [Soft Matter 7, 1334 (2011)] in which 2D-Ising criticality is invoked, but agrees with the recent assessment by Leys et al. [Soft Matter 9, 9326 (2013)].

Heat capacity anomalies along the critical isotherm in fluid-fluid phase transitions

The behavior of the isochoric heat capacity of pure fluids and the isobaric heat capacity at constant composition of binary mixtures along isothermal paths of approach to liquid-gas and liquid-liquid critical points is studied. From the complete scaling formulation of fluid-fluid criticality, explicit expressions for the critical amplitudes of the leading /Y-Y(c)/(-alpha/beta) (where Y can be the density or the mole fraction) contributions are found to reveal previously discovered features of the scaling function, whereas the nature of the most important asymmetry-related terms is characterized. Data for pure toluene and for the binary mixture nitromethane-isobutanol are described within experimental uncertainty using the /Y-Y(c)/(-alpha/beta) singularity plus a linear term. Extensive data for mixtures allow proper visualization of the topological features of the heat capacity-density-temperature surface in the critical region.

Structural and physical properties of a new reversible and continuous thermochromic ionic liquid in a wide temperature interval: [BMIM]4[Ni(NCS)6]

We report spectroscopic, structural, optical and magnetic characterization of tetra(1-butyl-3-methylimidazolium)hexaisothiocyanatonickelate. This paramagnetic ionic liquid exhibits reversible and continuous thermochromism from 298 K up to 400 K and it is solar responsive resisting numerous heating–cooling cycles. Its appearance changes from pale blue to grass green from 298 K to 343 K. Well above these temperatures it becomes brown and gray. Thermochromism is observed both in the solid and liquid phase.