The cohesive energy of molten salts and its density

Marcus, Yizhak

doi:10.1016/j.jct.2009.07.004

Cited by 29 publications

(7 citation statements)

References 36 publications

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“…We have simulated a primitive model of the NaCl molten salt at 1267 K to allow direct comparison with the ionic liquids. Note that the primary purpose is to illustrate general similarities of ionic systems, and the reader specifically interested in molten salt properties is referred to other work. ,− Analogolous to our simulations of ionic liquids, we investigate the system with and without inclusion of electronic polarization; note that in this analysis polarization is applied only to the “Cl” anion (see Theory and Methods). The scattering and charge-correlation structure factors are shown in Figure a, and charge oscillations q sum an ( r ) and q sum cat ( r ) in the molten salt are shown in Figure b.…”

Section: Resultsmentioning

confidence: 99%

Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

McDaniel

Yethiraj

2019

J. Phys. Chem. B

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The properties of room-temperature ionic liquids (ILs) may be viewed as resulting from a balance of electrostatic interactions that can be tuned at short range but constrained to satisfy universal, asymptotic screening conditions. Short-range interactions and ion packing provide ample opportunity for chemical tunability, while asymptotic sum rules dictate that the long-range structure and charge oscillation be similar to those of molten alkali halide salts. In this work, we study the structure factors and long-range electrostatic interactions in six ILs. The cation in all cases is 1butyl-3-methylimidazolium (BMIM + ), and we study six anions, namely, tetrafluoroborate (BF 4 − ), hexafluorophosphate (PF 6 − ), nitrate (NO 3 − ), triflate (CF 3 SO 3 − ), bisfluorosulfonylimide [(FSO 2 ) 2 N − ], and bistriflimide [(CF 3 SO 2 ) 2 N − ]. To gain insight, we perform similar computer simulations of a primitive molten salt model with and without electronic polarization. We emphasize universal similarities among ionic liquids and molten salts in the long-range ion ordering and the influence of electronic polarization on the screening conditions while also characterizing important differences in the short-range electrostatic interactions. We show that polarization systematically reduces charge oscillations by as much as ∼0.5−1 ion per radial shell, which we argue is general to all room-temperature ILs as well as molten salts. We suggest that a fundamentally important distinction among BMIM-based ionic liquids (with different anions) is the nature of the midrange, ∼1 Å −1 peak in the charge-correlation structure factor; while this correlation is straightforward to analyze in computer simulations, it may often be hidden in X-ray and/or neutron scattering structure factors.

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Section: Resultsmentioning

confidence: 99%

Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

McDaniel

Yethiraj

2019

J. Phys. Chem. B

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“…Furthermore, based upon computationally derived ced C+A values, some ILs have stronger intermolecular interactions than water. However, ILs certainly have significantly weaker intermolecular interactions than classical molten salts; Marcus has estimated ced C+A for molten salts ranging from 6241 for CsI to 64 520 J cm −3 for LiF [ 229 ]. Based upon the values for both ced IP and ced C+A presented here, ILs have very strong intermolecular interactions (many larger than water).…”

Section: Insights From Cohesive Energy Densities Of Ionic Liquidsmentioning

confidence: 99%

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

Lovelock¹

2017

R. Soc. open sci.

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For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, cedIP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, cedC+A, where the ionic vapour constituents are isolated ions. A cedIP dataset is presented for 64 ILs. For the first time an experimental cedC+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. cedC+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between cedIP and the inverse of the molecular volume. A good linear correlation is found between IL cedIP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to cedIP. These findings show that cedIP is very important for understanding IL intermolecular interactions, in spite of cedIP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

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“…Any adequate discussion of IL miscibility must consider the very large cohesive energies of the ILs themselves. Just as alkali-halide salts only dissolve in water because of the large and delicate counterbalance of ionic crystal cohesive energies (e.g., ∼−180 to −185 kcal/mol per ion pair for NaCl) , with the ion solvation free energies in water (e.g., ∼−100 kcal/mol for Na + and ∼−75 kcal/mol for Cl – ), − ILs will only mix with solvents if sufficiently compensating solvation energies counterbalance the large cohesive energy of the IL. Cohesive energies of prototypical ILs have been estimated by both experiment and theory to be ∼−120 ± 10 kcal/mol per ion pair depending on ion type; note that IL vaporization energies are significantly lower than these cohesive energies because of formation of ion pairs in the gas phase. ,, Because the cohesive energies of low-molecular-weight organic solvents are generally on the order of a few to 10 kcal/mol, the mixing energetics of ILs with low dielectric solvents is dominated by the cohesive energy of the pure IL and the ion/solvent interactions (solvation energies) …”

Section: Introductionmentioning

confidence: 99%

On the Miscibility and Immiscibility of Ionic Liquids and Water

McDaniel

Verma

2019

J. Phys. Chem. B

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Although the “like-dissolves-like” rule is often invoked to explain why sodium chloride dissolves in water, hidden behind this explanation is the delicate balance between the very large cohesive energy of the ionic crystal and large solvation energies of the ions. Room-temperature ionic liquids (ILs) are liquid analogues of ionic crystals and, as dictated by a similar energetic balance, may either fully mix with water or be immiscible with water depending on ion type and cation/anion combination. In this work, we study three hydrophobic and three hydrophilic ILs to examine whether a priori prediction of water miscibility is possible based on analysis of bulk properties alone. We find that hydrophilic and hydrophobic ILs exhibit distinct signatures in their (reciprocal space) Coulomb interactions that indicate predisposition to water mixing. Hydrophilic ILs exhibit a prominent peak in their electrostatic interactions at ∼5–8 Å length scale, largely due to repulsion between neighboring anion shells. When mixed with water, this peak is significantly reduced in magnitude, indicating that electrostatic screening by water molecules is an important driving force for mixing. In contrast, hydrophobic ILs show no such peak, indicating no predisposition to mixing. In addition to this analysis, we compute and compare solvation free energies of the six different anions in water, ion-pairing free energies at “infinitely” dilute concentration, and water absorption free energies in the different ILs. Analyzed within the context of empirical data, our calculations suggest that hydrophobicity trends of different ILs are very sensitive to precise water content at dilute conditions. For example, we predict that bis(fluorosulfonyl)imide-based ILs exhibit anomalously large water absorption free energies at zero water content, with increasing hydrophobicity as preferential absorption sites within the IL become saturated.

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The cohesive energy of molten salts and its density

Cited by 29 publications

References 36 publications

Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

On the Miscibility and Immiscibility of Ionic Liquids and Water

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