Systematic Study on the Viscosity of Ionic Liquids: Measurement and

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A Systematic Study on the Viscosity of Ionic Liquids: Measurement and Prediction Rafael Alcalde, Gregorio Garcia, Mert Atilhan, and Santiago Aparicio Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02713 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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Industrial & Engineering Chemistry Research

A Systematic Study on the Viscosity of Ionic Liquids: Measurement and Prediction Rafael Alcalde,a Gregorio García,a Mert Atilhan,b* and Santiago Aparicioa* a a

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

*

Corresponding authors: [email protected] (M. A.) and [email protected] (S. A.).

ABSTRACT: Dynamic viscosity for twenty-seven ionic liquids involving ions with molecular structures selected to infer the effects of molecular structure on fluids' viscosity is reported in this work as a function of temperature. The viscosity data is analyzed considering these structural features and fitted according to Vogel-Fulcher-Tamman equation. Viscosity data were systematically analyzed to infer anion and cation effects on the property. COSMO-RS method was used to infer relationships between molecular level features and viscosity data for the considered families of ions. A QuantitativeStructure-Property-Relationship model was developed using a Genetic Function approximation from selected molecular descriptors.

KEYWORDS: ionic liquid; viscosity; temperature; Vogel-Fulcher-Tammam; COSMO; QSPR.

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1. INTRODUCTION The interest on ionic liquids, ILs, and related materials such as deep eutectic solvents is widespread both in industry and academia considering their possible applications in many technologies both for advantageous replacing of other materials of for developing new technologies not possible with common fluids.1,2,3,4,5,6 In spite of this interest and the reported promising properties of ILs, which have led to a huge amount of publications,7 applications at industrial level are still scarce. The difficulties for scaling up ILs from laboratory to large scale technologies rise from different sources including large costs of many ILs8 combined with not well characterized environmental and biodegradability problems, 9,10

which have raised some questions about the suitability of ILs.11 Likewise, successful scaling up of

processes involving ILs requires an accurate and reliable knowledge of ILs physicochemical properties for developing suitable process design.12,13 Among the properties required for process design purposes, viscosity is probably one of the most relevant for industrial operations such as mixing or separations and for the design of equipment such as heat exchangers, pipelines, or distillation columns.14,15 The analysis of the available literature have showed relevant problems on the viscosity data for ILs.12,16 First, remarkable differences rising from the poor characterization of the used samples, including uncharacterized effects of impurities and water content.12,17,18,19 Second, the well-known problems on the accuracy of applied measurement methods,20 which are particularly important for ILs because their viscosity is larger than fluids commonly used in the industry.19 Third, the need of analyzing the effects rising from the molecular structures of involved ions in ILs viscosity,21,22 which are required for the development of structure-property relationships18 and for gaining a deeper knowledge of molecular level roots of ILs viscosity.22 This is particularly important considering that roughly 106 possible ILs may be developed,1 which on one side is a technological advantage for ILs in comparison with traditional fluids because it allows to develop task - specific ILs but on the other side hinders to carry out experimental studied for all the possible ILs. Therefore, studies on the viscous behavior of ILs is required in which the effect of considered anions and cations were analyzed systematically for different types of ions. Therefore, an experimental study on the viscosity of twenty-seven commercial ionic liquids as a function of temperature is reported in this work. The selection of ions was carried out to include anions and cations considering different molecular structures, and thus cations belonging to imidazolium, pyridinium, ammonium, sulfonium, pyrrolidinium, piperidinium and phosphonium were considered in combination with tetrafluroborate, hexafluorophosphate, phosphate, sulfate, acetate, nitrate, bis(trifluoromethylsulfonyl)imide, triflate, dicyanamide and chloride anions, Table S1 (Supporting Information). 23-Error! Bookmark not defined. In this way, the main features rising from molecular 23

structures of involved ions and their effect on the ILs viscosity may be systematically analyzed. 2


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2. METHODS 2.1. Materials. Samples for the twenty-seven studied ILs were obtained from IOLITEC. The main characteristics of the used samples, together with the impurities are reported in Table S1 (Supporting Information). The water content, which effect on viscosity is very remarkable,16,17,24,25,26,27 was measured before measurements through Karl-Fischer coulometric titration using a Metrohm 831 KF coulometer, for 0.1 g ILs samples, to ±0.3% accuracy in water mass content. Some of the ILs studied in this work are highly hygroscopic, Table S1 (Supporting Information), and they may absorb atmospheric water very quickly. It would be possible to reduce the water content for these ILs through vacuum drying but it was decided to use these samples with high water contents because they would resemble conditions of large scale industrial applications, which do not use to work under dry atmosphere conditions. 2.2. Experimental Measurements. Dynamic viscosity, η, was measured at 1 bar as a function of temperature using an electromagnetic VINCI Tech. EV1000 viscometer. Several studies have showed the main problems for the measurement of ionic liquids viscosity,12,28 but electromagnetic viscometers have been used successfully for different types of ionic liquids.21,29,30,31,32,33 A detailed description of the apparatus may be found in previous works.21 The working principle is based in the relationship between the movement of the electromagnetically driven stainless steel pistons inside a cylindrical chamber filled with the fluid under study and the fluid's viscosity.21 The chemical structures of the ions considered in this study, Table S1 (Supporting Information), led to very different viscosity values, and thus pistons of different sizes were used considering the viscosity values of the studied ILs. The temperature of the measurement chamber was controlled by a external circulating bath (Julabo F32) and measured inside the chamber by a platinum resistance probe to ± 0.01 K. Measurements were carried in the 298 to 348 K temperature range. The calibration of the viscometer was done using certified oils by Koehler Inc., which certified viscosity data for each oil were measured according to ASTM D2162 (± 0.18 % viscosity uncertainty or better, depending on the viscosity range). Therefore, considering all the uncertainty sources, the total calculated uncertainty for the reported dynamic viscosity data was ± 2 %.21 Viscosity measurements for each IL sample in the studied temperature range were developed in a continuous way, i.e. once the sample was placed in the viscometer chamber, and the temperature equilibrated at 298 K, the temperature was increased to 348 K at a 2 K per hour rate and viscosity data were continuously registered. These data for the whole temperature range are reported in Supporting Information (Supporting_Information_I_Experimental_Data), whereas data at 298.15 K are reported in Table S1 (Supporting Information) together with literature data when available for comparison purposes. In this way, a large amount of data is collected for each IL (the whole viscosity-temperature curve in a continuous way instead of measuring viscosity at only some selected 3



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temperatures), which considering that they do not follow and Arrhenius-like behavior were fitted to a Vogel-Fulcher-Tamnam (VFT) model, eq. 1, with the fitting parameters being reported in Table S2 (Supporting Information). ߟ = ‫ ݌ݔ݁ ∙ ܣ‬ቀ

஻

்ି்బ

ቁ

(1)

3. RESULTS 3.1. Experimental Results. The twenty-seven studied ILs allow to infer several relevant structural features in relationship with viscosity. Therefore, in the following sections the main molecular features will be analyzed. 3.1.1. Imidazolium-based ionic liquids. Anion effect. Viscosity for [EMIM]+ containing ILs with seven different anions is reported in Figure 1. Reported data show the large effect of anion type on these ILs, e.g. [EMIM][T

f2N] and [EMIM][HS] led to 29.3 and 1410.8 mPa s at 298.15,

respectively. The seven studied anions follow the viscosity ordering [HS] − > [DEP]− > [Tf] − > [ES] − > [OA] − > [BF4] − > [Tf2N] −, in the 298 to 348 K range. Likewise, the decreasing rate of viscosity with increasing temperature follows different trends for [EMIM]+ based ILs as a function of the anion type. These large differences in ILs viscosity as a function of the anion type show that the strength of anioncation interactions have a pivotal role on ILs viscosity. The anions considered in this section have very different molecular characteristics, which led to the very different behavior of viscosity, these characteristics may be analyzed using COSMO-RS method.34 COSMO-RS calculations were carried out for isolated ions using COSMOtherm X software according to the methodology described in a recent work.35 One of the most relevant properties of COSMO-RS analysis is the so-called σ-profile, which is the histogram function of the charge distribution on the molecular surface. σ-profiles are usually divided in three regions: i) σ < -0.0082 e Å-2, corresponding to the hydrogen bond donor donor (HBD) regions in the molecular surface, ii) -0.0082 < σ < 0.0082 e Å-2, corresponding to the non polar regions (NP), and iii) σ > 0.0082 e Å-2, corresponding to hydrogen bond acceptor (HBA) sites. σprofiles for involved ions in the [EMIM]-based ionic liquids studied in this section are reported in Figure 2. The [EMIM]+ cation shows its main features in the NP region corresponding mainly to the alkylic chains, with relevant features in the HBD region corresponding to the hydrogen atoms in the imidazolium ring. The σ-profiles for the studied anions show their most relevant peaks in the NP and HBA regions, with some minor contributions for [HS]- in the HBD region. σ-profiles have been used in the literature for developing predictive models for physicochemical properties as relevant as density36 or toxicity;37 in these works, molecular descriptors obtained from the area below the σ-profiles, Sσprofiles,

at several σ values or ranges are used. In this work, Sσ-profiles are calculated for each ion in the 4


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HBD, NP and HBA ranges and reported in Table 1. Likewise, σ-profiles for anion-cation pairs are reasonably defined as the sum of anion and cation contributions,37 and thus, a correlative equation was developed for viscosity using Sσ-profiles for HBD, NP and HBA using the sum of ion parameters reported in Table 1 for the corresponding ion pairs in ILs containing [EMIM]+, Figure 3a, leading to R2 = 0.95. The equation reported in Figure 3a shows that the main contribution to viscosity from this COSMO-RS analysis raised from the SHBD term with minor contributions from NP and HBA terms. Therefore, the presence of HBD groups in [EMIM]+ cation, and in minor contribution (but also relevant) in [HS]-, [DEP]- and [Tf]- anions, tend to increase viscosity. NP contribution also led to larger viscosities although the contribution of NP terms in Sσ-profiles is three-orders of magnitude lower than HBD one. Likewise, HBA term tends to decrease viscosity. Nevertheless, the accuracy of the viscosity predictions by the considered model is not too high for low viscous ionic liquids, Figure 3b. 3.1.2. Imidazolium-based ionic liquids. Effect of alkyl chain lengths in cation. As it may be expected increasing alkyl chain length in imidazolium cation leads to a remarkable increase of viscosity, Figure 4, which may be justified from the COSMO-RS approach by the increasing contribution of NP terms to σ-profiles.36 Nevertheless, this increase on the viscosity also depends on the anion with which imidazolium cations is paired. The calculated rates of change for viscosity at 298.15 K as a function of 1-alkyl-3-methylimidazolium alkyl chain length, divided by the viscosity value for the corresponding 1-butyl-3-methylimnidazolium IL, are 0.50, 0.47, 0.18 and 0.23 per number of carbon atoms in the alkyl chain for 1-alkyl-3-methylimidazolium - based ILs with [BF4]-, [PF6]-, [Tf2N]and [Tf]- anions respectively. Therefore, ILs with [BF4]- and [PF6]- anions increase viscosity with increasing alkyl chain length in the cation at more than the double rate of ILs with [Tf2N]- and [Tf]anions. 3.1.3. Cation effect. [Tf2N]-- based ionic liquids. The effect of cation type on ILs viscosity for a common anion is reported in Figure 5 for ILs containing [Tf2N]- anion. Cations of imidazolium, pyridinium, pyrrolidinium, piperidinium, sulfonium and ammonium types are considered, which allow to infer the effect of molecular characteristics of the cation on ILs viscosity. The viscosity of [O3MN][Tf2N] is remarkably larger than for the other considered ILs, as it may be expected from the large alkyl chains. Piperazinium-based ILs are also remarkably more viscous than the other types of cations, whereas the remaining cations leads to ILS with viscosities in the same range. The σ-profiles for the considered cations are reported in Figure 6a, showing their most remarkable featured in the NP region with all of them extending their features toward the HBD region. The σ-profile for [O3MN]+ in the NP region is larger and wider than for any of the other cations, which would justify the very large viscosity of [O3MN]+ containing ILs. The Sσ-profile, calculated as the area under the profiles in the whole range (-0.025 < σ < 0.025 e Å-2), is reported in Figure 6b showing a good linear correlation with 5



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viscosity (R2 = 0.97) and confirming how Sσ-profile capture the most relevant molecular features in relationship with viscosity although the subtle differences in viscosity for those ILs with viscosities in the same range (imidazolium, pyridinium and pyrrolidinium) requires a deeper analysis not possible with the minimalistic approach used considering only isolated ions. 3.1.4. Anion effect for pyridinium and pyrrolidinium - based ionic liquids. The viscosity of ILs may be tuned by the selection of suitable anions for a fixed family of cations, Figures 7a and 8. In the case of pyrrolidinium - based ILs, changing from [Tf2N]- anion to [DCA]- decreases viscosity in a 50 %. This behavior is in direct relationship with the behavior of σ-profiles reported in Figure 7b, σ-profile for [DCA]show peaks both in the NP and HBA regions, but their intensities are remarkably lower than those for [Tf2N]-, and thus, [DCA]- - containing ILs are low viscous fluids for different types of cations, Figure 9. In the case of pyridinium - based ILs, the effect of the type of anion is larger than for imidazolium - based ILs as a comparison of results in Figures 1 and 8 allow to infer for [BF4]- and [Tf2N]- anions. Pyridinium-based ILs are more viscous than imidazolium - based ones, Figure 5, and thus the effect of anion is larger, showing the synergistic effect of cation and anions on ILs viscosity. 3.1.5. Phosphonium - based ionic liquids. The viscosity for [H3O3PHO][Cl] IL was measured in this work, Figure 10a. This is extremely viscous IL (2138 mPa s at 298.15), which would hinder many technological applications,

although its viscosity decreases to reasonable values at higher

temperatures (109 mPa s at 348.15 K). The large viscosity of [H3O3PHO]+ rises from the large alkyl chains leading to a very intense and wide peak in the NP region of σ-profiles, Figure 10b. 3.2. QSPR Modelling. The analysis of viscosity data reported in the previous sections showed relationships between the molecular properties of involved ions and viscosity. Therefore, a Quantitative-Structure-Property-Relationship (QSPR) predictive model was developed in this work for the viscosity of the studied ILs. Zhao et al.38 reported a QSPR study on 89 different ionic liquids using experimental data from many different literature sources. In this work, only those experimental data reported here are included for model development. The molecular structures of the corresponding anion-cation ionic pairs were optimized at B3LYP-D2/6-311+G(d,p) theoretical level using gaussian 09.39 The development of QSPR model was done starting from 251 molecular descriptors, which were calculated for every anion-cation optimized ionic pair using the QSAR module of Materials Studio software,40 Table S3 (Supporting Information). The selection of significant descriptors was carried out using the Genetic Function approximation, and thus, the selected descriptors were used for the development of a predictive model for the experimental viscosity data obtained in this work at 298.15 K, Table S1 (Supporting Information), using the genetic algorithm approach in Materials Studio leading to a 6-parameters model. This number of parameters is reasonable considering the number of studied 6


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ILs and avoids data overfitting. Statistical parameters of the QSPR model are reported in Table 2. The developed QSPR model leads to excellent correlative ability, Figure 11, R2 = 0.99, but also to high predictive ability, cross validated R2 = 0.99.

CONCLUSIONS Experimental viscosity for twenty-seven ionic liquids comprising different types of anions and cations, selected to infer molecular level factors in relationship with viscosity, was measured as a function of temperature. The results were systematically analyzed considering anion and cation structural effects and in the framework of COSMO-RS approach allowing to infer the most relevant molecular levels features for viscosity determination purposes. Likewise, a QSPR model considering six molecular parameters was developed, leading to suitable correlative and predictive ability.

ACKNOWLEDGEMENTS This work was made possible by NPRP grant # 6-330-2-140 from the Qatar National Research Fund (a member of Qatar Foundation), Ministerio de Economía y Competitividad (Spain, project CTQ201340476-R) and Junta de Castilla y León (Spain, project BU324U14). Gregorio García acknowledges the funding by Junta de Castilla y León, cofunded by European Social Fund, for a postdoctoral contract. The statements made herein are solely the responsibility of the authors.

ASSOCIATED CONTENT Supporting Information Supporting_Information_I (experimental viscosity data as a function of temperature); comparison of viscosity data obtained in this work with literature data (Table S1); fitting parameters of VFT model (Table S2); molecular descriptor used for QSPR model (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1. Calculated values of Sσ-profile in the reported ranges (σ values in e/Å2), obtained from the σ-profiles calculated for independent anions and [EMIM]+ cation Sσ-profile

ion [EMIM] [HS]

-

[DEP] [Tf]

-

-

[ES]

-

[OA]

-

[BF4]

-

[Tf2N]

-

+

H-bond donor

non polar

H-bond acceptor

σ

Systematic Study on the Viscosity of Ionic Liquids: Measurement and

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