Surface Tension Of Ionic Liquids Research Articles

Further property of ionic liquids: Hildebrand solubility parameter from new molecular thermodynamic model

A molecular thermodynamic-based approach has been previously employed to correlate the surface tension of ionic liquids (ILs). This paper aims to calculate further property of ILs, the solubility parameter of 27 ILs having imidazolium, pyrrolidinium, pyridinium, phosphonium, piperidinium and ammonium cations by the help of that approach along with an ion contribution-based equation of state (EOS). The proposed model calculates the internal pressure of ILs using a statistical mechanical expression and subsequently their solubility parameters through a simple relation. In this respect, contributions to internal pressure from the hard-sphere repulsion, Lennard-Jones dispersion force, and columbic interactions are considered and assumed to be additive in the development of the model. The performance of the proposed model is checked against the literature solubility parameters of ILs over temperature range within 298–358K. The proposed model has a sound basis of statistical-mechanics which incorporates contributions arising from the hard-sphere repulsion, Lennard-Jones dispersion forces. Further, the electrostatic interaction is taken into account using the mean spherical approximation (MSA). The outcomes of our model are also compared with those obtained based on the vaporization enthalpies and molar volumes, for which their values are available in literature. Generally, the new molecular model represents accurately the solubility parameters of studied ILs with uncertainty of the order of ±2.33%. The miscibility of some non-polar and polar hydrocarbons in ILs is also investigated by the use of their solubility parameters and Flory–Huggins interaction parameter.

Effect of water content on physicochemical properties and electrochemical behavior of ionic liquids containing choline chloride, ethylene glycol and hydrated nickel chloride

We investigated the effect of water addition on some physicochemical properties of liquid mixtures containing choline chloride, ethylene glycol, nickel chloride and extra water in the molar ratio of 1:2:1:x (i.e. ethaline+NiCl2+xH2O) where x was equal to 6, 9, 12 or 18. The influence of water upon density, viscosity, conductivity, and surface tension of ionic liquids is established for temperatures between 25 and 80°C. An increase in the water content leads to decreasing density, viscosity and surface tension and to increasing conductivity. All these results are rationalized in terms of hole theory. The conductivity and viscosity of the liquid mixtures are controlled by the ionic mobility and the availability of voids of suitable dimensions. With increasing water content the average hole sizes increases which makes ionic motion considerably easier. There is a strong linear correlation between activation energies for conductivity and viscosity. Cyclic voltammograms recorded on Ni substrate in ionic liquids under consideration show that the electrochemical process of metal deposition is irreversible. An increase in water content in ethaline+NiCl2+xH2O mixtures leads to the deceleration of Ni electrodeposition reaction.

Surface tensions of ionic liquids: Non-regular trend along the number of cyano groups

Ionic liquids (ILs) with cyano-functionalized anions are a set of fluids that are still poorly characterized despite their remarkably low viscosities and potential applications. Aiming at providing a comprehensive study on the influence of the number of –CN groups through the surface tension and surface organization of ILs, the surface tensions of imidazolium-based ILs with cyano-functionalized anions were determined at atmospheric pressure and in the (298.15–343.15) K temperature range. The ILs investigated are based on 1-alkyl-3-methylimidazolium cations (alkyl = ethyl, butyl and hexyl) combined with the [SCN]-, [N(CN)2]−, [C(CN)3]− and [B(CN)4]− anions. Although the well-known trend regarding the surface tension decrease with the increase of the size of the aliphatic moiety at the cation was observed, the order obtained for the anions is more intricate. For a common cation and at a given temperature, the surface tension decreases according to: [N(CN)2]− > [SCN]− > [C(CN)3]− > [B(CN)4]-. Therefore, the surface tension of this homologous series does not decrease with the increase of the number of –CN groups at the anion as has been previously shown by studies performed with a more limited matrix of ILs. A maximum in the surface tension and critical temperature was observed for [N(CN)2]-based ILs. Furthermore, a minimum in the surface entropy, indicative of a highly structured surface, was found for the same class of ILs. All these evidences seem to be a result of stronger hydrogen-bonding interactions occurring in [N(CN)2]-based ILs, when compared with the remaining CN-based counterparts, and as sustained by cation-anion interaction energies derived from the Conductor Like Screening Model for Real Solvents (COSMO-RS).

Modeling of surface tension for ionic liquids using group method of data handling

The group method of data handling (GMDH) was utilized to estimate the surface tension of 59 ionic liquids. In this regard, an extensive experimental data was selected from literature over the range 268.3 to 532.4 K. The GMDH model can predict the surface tension of ionic liquids by a grand polynomial correlation function of molar density, reduced boiling temperature, reduced temperature and pressure, acentric factor, and critical compressibility factor. The values of the GMDH model showed a very good regression with the experimental data. The average absolute relative deviation (AARD%) of the GMDH model for all ionic liquids was 4.59 % which indicated a good precision in comparison with those obtained from the generalized dimensionless equation, Mousazadeh–Faramarzi equation, and Parachor equation with AARDs% of 10.31, 13.02, and 11.89, respectively.

Surface thermodynamic properties of ionic liquids from new molecular thermodynamic model and ion-contribution equation of state

We aim to develop a new molecular thermodynamic model for predicting the surface thermodynamic properties of pure ionic liquids (ILs) based on the statistical mechanical expression developed by Winterfeld et al. (1978. AIChE. J. 24, 1010). In this respect, contributions to surface tension from the hard-sphere repulsion, Lennard–Jones dispersion force, and electrostatic interactions were considered and assumed to be additive in the development of the model. According to Winterfeld et al. approach, the surface tension of ILs is closely related to their pair potential function, pair radial distribution function and liquid densities. The required densities were predicted from our previous ion-contribution equation of state. The reliability of the proposed model was checked by calculating 464 surface tension data points for 13 different ILs over temperature range within 268.6–393K. The overall average absolute deviation of the correlated surface tensions from the experimental ones was found to be 1.55%. This result demonstrates the good performance of the proposed model and the rationality of molecular parameters, i.e., the soft-sphere diameter, σ dispersive energy, ε and inverse shielding length parameter, Γ. The uncertainty of the predicted surface tension was of the order of ±8.14%. Finally, our method has also been employed to estimate the sound velocities in ILs. The uncertainty of the calculated sound velocities was equal to ±9.25%.

Cation Alkyl Side Chain Length and Symmetry Effects on the Surface Tension of Ionic Liquids

Aiming at providing a comprehensive study of the influence of the cation symmetry and alkyl side chain length on the surface tension and surface organization of ionic liquids (ILs), this work addresses the experimental measurements of the surface tension of two extended series of ILs, namely R,R'-dialkylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C(n)C(n)im][NTf2]) and R-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C(n)C(1)im][NTf2]), and their dependence with temperature (from 298 to 343 K). For both series of ILs the surface tension decreases with an increase in the cation side alkyl chain length up to aliphatic chains no longer than hexyl, here labeled as critical alkyl chain length (CACL). For ILs with aliphatic moieties longer than CACL the surface tension displays an almost constant value up to [C12C12im][NTf2] or [C16C1im][NTf2]. These constant values further converge to the surface tension of long chain n-alkanes, indicating that, for sufficiently long alkyl side chains, the surface ordering is strongly dominated by the aliphatic tails present in the IL. The enthalpies and entropies of surface were also derived and the critical temperatures were estimated from the experimental data. The trend of the derived thermodynamic properties highlights the effect of the structural organization of the IL at the surface with visible trend shifts occurring at a well-defined CACL in both symmetric and asymmetric series of ILs. Finally, the structure of a long-alkyl side chain IL at the vacuum-liquid interface was also explored using Molecular Dynamics simulations. In general, it was found that for the symmetric series of ILs, at the outermost polar layers, more cations point one of their aliphatic tails outward and the other inward, relative to the surface, than cations pointing both tails outward. The number of the former, while being the preferred conformation, exceeds the latter by around 75%.

Applying parachor method to the prediction of ionic liquids surface tension based on modified group contribution

In this paper parachor method is applied to estimate surface tension of pure imidazolium based ionic liquids at the different temperatures. For this prediction a Modified Group Contribution Method is proposed to estimate the surface tension of ionic liquids covering wide ranges of temperature and chain length based on experimental data collected from literatures. The average relative error obtained from the comparison of experimental and calculated surface tension values for studied ionic liquids (less than 8.5%) shows the model has good accuracy in comparison with other predictive equations.

Estimation of Surface Tension of Ionic Liquid–Cosolvent Binary Mixtures by a Modified Hildebrand–Scott Equation

The correlation model of gas–liquid interfacial tension (surface tension) of nonaqueous mixtures containing ionic liquids (ILs) is still in its infancy. The purpose of this work is to develop a modified Hildebrand–Scott equation based on local composition concept and the group surface area parameters of UNIFAC model as well as to evaluate the capability of this model. The surface tension of 33 IL–cosolvent binary systems were correlated by the modified Hildebrand–Scott equation with two energy parameters and an overall average relative deviation of only 0.92%. The ILs in the study included imidazolium-based, pyridinium-based, and isoquinolinium-based ones, and the cosolvents included methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile, and tetrahydrofuran. Using the energy parameters obtained from the surface tension of a given IL–cosolvent binary system at 298.15 K, the surface tension of the same system at other different temperatures was predicted by the modified Hildebrand–Scott equation, with an overall average relative deviation of 1.06%.

Predicting the surface tensions of ionic liquids by the quantitative structure property relationship method using a topological index

The general topological indexes (TIs) proposed in our previous work were applied to predict the surface tensions of ionic liquids (ILs) by Quantitative Structure Property Relationship (QSPR) method. ILs are a class of molten salts which are composed entirely of cations and anions, therefore the descriptors for ILs are generally calculated from cations and anions separately and the interaction between them is always neglected. In this study, besides the two sets of TIs generated from cation and anion, a third TI was used to depict the interaction of anion and cation. Differing from the previous estimation methods for surface tensions of ILs, the temperature dependence is also correlated by structure parameters. 930 surface tension data points for 115 ILs are applied to develop QSPR model. The correlation coefficient (R2) and the overall average absolute deviation (AAD) are 0.9516 and 3.43%, respectively.

Prediction of surface tension of ionic liquids by molecular approach

Originally, Quantitative Structure Property Relationship (QSPR) models for the surface tension of ionic liquids are developed based on molecular descriptors. A large data set of 930 experimental surface tension data points for 48 ionic liquids is applied to derive the model. Seven descriptors are selected by genetic function approximation to relate the surface tension of ionic liquids to their corresponding anions and cation structures. To capture the nonlinear nature of surface tension, a model based on Least-Squared Supported Vector Machine (LSSVM) is also developed. The derived models are authenticated with several statistical validation techniques.

Volumetric properties of ionic liquids from cubic equation of state: Application to pure and mixture

In this work, we developed a cubic equation of state (CEOS) for modeling the volumetric properties of various ionic liquids (ILs). Two temperature-dependent parameters presented in the CEOS, have been determined from two sets of corresponding states correlations which are based on the critical point data or the surface tension of ILs. The predicted densities of pure ILs were compared with the experimental data over a broad pressure range from 1 to 100MPa. The total average absolute deviations (AADs) of the calculated densities of 948 data points from the experimental data are 1.82% using the critical property and 0.96% using the surface tension and liquid density as scaling parameters. Furthermore, the proposed CEOS was successfully extended to mixtures including IL+IL and IL+solvent systems. 1282 literature data points for mixtures were taken to check the reliability of the mixture version of the proposed CEOS. The AAD of the calculated densities of the mixtures using the surface tension and liquid density as scaling parameter is 0.37%. Furthermore, the excess molar volumes (VE) of the studied binary mixtures have been successfully computed by the use of the proposed CEOS.

A new fragment contribution‐corresponding states method for physicochemical properties prediction of ionic liquids

A new fragment contribution‐corresponding states (FC—CS) method based on the group contribution method and the corresponding states principle is developed to predict critical properties of ionic liquids (ILs). There are 46 fragments specially classified for ILs considering the ionic features of ILs, and the corresponding fragment increments are determined using the experimental density data. The accuracy of the developed method is verified indirectly via predicting density and surface tension of ILs. The results show that the FC—CS method is reasonable with an average absolute relative deviation less than 4%. With the calculated critical properties, corresponding states heat capacity (CSHC) and corresponding states thermal conductivity (CSTC) correlations are proposed to predict heat capacity and thermal conductivity of ILs, respectively. The predicted results agreed well with the experimental data. The proposed FC—CS method and the two corresponding states correlations are important for design, simulation, and analysis of new ionic liquid processes. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1348–1359, 2013

A simple corresponding-states group-contribution method for estimating surface tension of ionic liquids

A corresponding-states group-contribution method is proposed to estimate the surface tension of ionic liquids covering wide ranges of temperature (268.29–533.20K) and surface tension (15.50–76.20mN/m) based on experimental data collected from literatures. A good agreement between the predicted and experimental data from literature was obtained for total 1224 data points of 105 ionic liquids with an average absolute deviation (AAD) of 4.95%. The corresponding-states group-contribution method developed in this paper offers a simple but reliable approach for estimating surface tension of new ionic liquids in wide ranges of temperature at atmospheric pressure.

Group contribution model for estimation of surface tension of ionic liquids

In this communication, a reliable group contribution method is developed for estimation of surface tension of ionic liquids. A data set including 920 experimental data for 51 ionic liquids collected from various references is used to develop and assess the predictive capability of the model. The data set covers a temperature range of 268.3 through 743.7K and a surface tension range of 0.02499 through 0.0647Nm−1. It employs a total of 19 sub-structures plus temperature to predict the surface tension. To better distinct the effects of anion and cation on the surface tension of ionic liquids, 12 sub-structures related to chemical structure of anion, and 7 substructures related to the chemical structure of cation were implemented. The results of this method show an average absolute relative deviation (AARD) of 3.6% from experimental data.

Surface tension of ionic liquids and ionic liquid solutions

Some of the most active scientific research fronts of the past decade are centered on ionic liquids. These fluids present characteristic surface behavior and distinctive trends of their surface tension versus temperature. One way to explore and understand their unique nature is to study their surface properties. This critical review analyses most of the surface tension data reported between 2001 and 2010 (187 references).

Corresponding states theory for the prediction of surface tension of ionic liquids

The surface tension of ionic liquids is studied according to phenomenological scaling and the law of corresponding states. The reduced coordinates γ*–T*, where γ* represents the reduced surface tension and T* is the reduced temperature, are introduced for the prediction of the surface tension from the melting point up to boiling point. It has been shown that the correlation can be expressed as a unique straight-line plot with a linear correlation coefficient of 0.984 that requires only the melting and boiling point parameters and can predict the surface tension accurately.

Evaluation of Cation−Anion Interaction Strength in Ionic Liquids

Electrospray ionization mass spectrometry with variable collision induced dissociation of the isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) ions of imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ionic liquids (ILs) combined with a large set of anions, such as chloride, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, and bis[(trifluoromethyl)sulfonyl]imide, was used to carry out a systematic and comprehensive study on the ionic liquids relative interaction energies. The results are interpreted in terms of main influences derived from the structural characteristics of both anion and cation. On the basis of quantum chemical calculations, the effect of the anion upon the dissociation energies of the ionic liquid pair, and isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) aggregates, were estimated and are in good agreement with the experimental data. Both experimental and computational results indicate an energetic differentiation between the cation and the anion to the ionic pair. Moreover, it was found that the quantum chemical calculations can describe the trend obtained for the electrostatic cation-anion attraction potential. The impact of the cation-anion interaction strengths in the surface tension of ionic liquids is further discussed. The surface tensions dependence on the cation alkyl chain length, and on the anion nature, follows an analogous pattern to that of the relative cation-anion interaction energies determined by mass spectrometry.

Surface tension of 1-alkyl-3-methylimidazolium based ionic liquids with trifluoromethanesulfonate and tetrafluoroborate anion

The amount of available accurate experimental data on the surface tension of ionic liquids is still limited; in many cases the data are rare or even absent. In the present study, air–liquid interfacial tension data were determined experimentally for five 1-Cn-3-methylimidazolium based ionic liquids (n=2, 4, and 6), three with trifluoromethanesulfonate and two with tetrafluoroborate anion, at atmospheric pressure in the temperature range from 268 to 356K. The resultant surface tension data are average values of the measurements repeated many times at each set point temperature. The accuracy of the results, was confirmed by employing the Wilhelmy plate and the du Noüy ring methods in parallel, using the Krüss K100MK2 tensiometer. For the Wilhelmy plate data the combined standard uncertainty is estimated to be about 0.05mNm−1. The data obtained by du Noüy method show about up to seven times greater scatter than those obtained by the Wilhelmy plate method. To the 50 up to now published surface tension values for the five studied ionic liquids the present study adds further 175 data points. In contrast to that of n-alkanes, the surface tension of 1-alkyl-3-methylimidazolium based ionic liquids decreases and their surface entropy increases with the cation alkyl chain length.

Correlation between ultrasonic velocity, surface tension, density and viscosity of ionic liquids

Ultrasonic velocity, density, viscosity and surface tension of ionic liquids 1-butyl-3-methyl imadazolium hexafluorophosphate [BMIM][PF 6], 1-butyl-3-methyl imadazolium octyl sulfate [BMIM][OcSO 4] and 1-ethyl-3-methyl imadazolium methane sulfonate [EMIM][MeSO 3] have been measured from 303 K to 333 K. Using these experimental data, inter relationships among ultrasonic velocity, density, viscosity and surface tension have been examined for these ionic liquids. It has been found that (a) ultrasonic velocity ( u), surface tension ( σ) and density ( ρ) can be correlated linearly using equation, log u = ψ log σ/ ρ + ξ (with typical values of ψ = 1.61 and ξ = 10.459) (b) surface tension is linearly correlated with viscosity by the relation ln σ = ln a + bη −0.125, where ln a and b are constants. These correlations among physicochemical properties of the ionic liquids can be used to evaluate the properties (viz. ultrasonic velocity) not available in literature.

Density and Surface Tension of Ionic Liquids

We measured the density and surface tension of 9 bis[(trifluoromethyl)sulfonyl]imide ([Tf(2)N](-))-based and 12 1-methyl-3-octylimidazolium ([C(8)C(1)Im](+))-based ionic liquids (ILs) with the vibrating tube and the pendant drop method, respectively. This comprehensive set of ILs was chosen to probe the influence of the cations and anions on density and surface tension. When the alkyl chain length in the [C(n)C(1)Im][Tf(2)N] series (n = 1, 2, 4, 6, 8, 10, 12) is increased, a decrease in density is observed. The surface tension initially also decreases but reaches a plateau for alkyl chain lengths greater than n = 8. Functionalizing the alkyl chains with ethylene glycol groups results in a higher density as well as a higher surface tension. For the dependence of density and surface tension on the chemical nature of the anion, relations are only found for subgroups of the studied ILs. Density and surface tension values are discussed with respect to intermolecular interactions and surface composition as determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The absence of nonvolatile surface-active contaminants was proven by ARXPS.