Article pubs.acs.org/IECR
Dynamic Viscosity of Tetracyanoborate- and TricyanomethanideBased Ionic Liquids by Dynamic Light Scattering Shengshan Bi,†,‡ Thomas M. Koller,†,§ Michael H. Rausch,†,§ Peter Wasserscheid,∥ and Andreas P. Fröba*,†,§ †
Erlangen Graduate School in Advanced Optical Technologies, University of Erlangen-Nuremberg, Paul-Gordan-Straße 6, D-91052 Erlangen, Germany ‡ Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, China § Department of Chemical and Biological Engineering, Institute of Engineering Thermodynamics, University of Erlangen-Nuremberg, Am Weichselgarten 8, D-91058 Erlangen, Germany ∥ Department of Chemical and Biological Engineering, Institute of Chemical Reaction Engineering, University of Erlangen-Nuremberg, Egerlandstraße 3, D-91058 Erlangen, Germany S Supporting Information *
ABSTRACT: Low-viscosity ionic liquids (ILs) based on the anions [B(CN)4]− (tetracyanoborate) and [C(CN)3]− (tricyanomethanide) are currently discussed as alternative working fluids for various applications. The dynamic viscosity of such ILs carrying [1-alkyl-MIM]+ (1-alkyl-3-methylimidazolium) cations was investigated via determination of the translational particle diffusion coefficient by dynamic light scattering (DLS). The long-term stability of suspensions of different particles in the ILs, their particle diameters, and the influence of laser power on the measured particle diffusion coefficient for semitransparent ILs were studied. For temperatures from 283 to 353 K at atmospheric pressure, the dynamic viscosity of the four pure ILs forming stable suspensions was obtained with an uncertainty of less than 5% (k = 2) and agrees with literature. Absorption of CO2 at pressures up to 0.87 MPa induced a distinct decrease in the dynamic viscosity. Differences between the viscosities of the different systems can be explained by varying strength of molecular interactions.
1. INTRODUCTION Besides other transport and thermophysical properties, accurate viscosity data are required for evaluating the role of ionic liquids (ILs) as alternative working fluids in many industrial processes.1,2 For instance, low-viscosity ILs based on the anions [B(CN)4]− (tetracyanoborate) and [C(CN)3]− (tricyanomethanide) carrying [alkyl-MIM]+ (1-alkyl-3-methylimidazolium) cations are characterized by high CO2 absorption capacities. In addition, these systems are currently discussed regarding their potential use as electrolytes in solar cell applications3,4 or in gas separation processes.5,6 In a previous study,7 the viscosity of the transparent [B(CN)4]−-based ILs carrying the cations [EMIM]+ (1-ethyl-3-methylimidazolium) and [HMIM]+ (1-hexyl-3-methylimidazolium) was successfully measured by surface light scattering (SLS) applied in transmission direction, where scattered light is observed in the forward direction near refraction. SLS represents, as the name indicates, the application of the dynamic light scattering (DLS) method to the surface of a liquid or to phase boundaries. In general, the DLS method allows the absolute and contactfree determination of transport and other thermophysical properties in macroscopic thermodynamic equilibrium without the need for imposing any gradients on the investigated sample. Furthermore, it relies on strictly valid working equations without the need for any corrections.8−10 For light-absorbing semitransparent samples, however, the comparatively large incident laser power required for SLS in transmission direction © XXXX American Chemical Society
induces a local change of the sample temperature and, thus, of the measured properties. The application of SLS in reflection direction using small scattering angles could solve this problem because of the smaller laser powers needed. Yet, under these conditions instrumental broadening effects are not negligible and have to be taken into account for data evaluation.11 DLS from suspended particles represents a further possibility to measure the viscosity of semitransparent liquids. It has already been successfully applied for simple fluids12−14 and requires only small incident laser powers because of the comparatively strong particle scattering intensities. Small amounts of monodisperse particles are suspended in the studied liquid, and the translational particle diffusion coefficient is measured by DLS. On the basis of the Stokes−Einstein equation and the calibrated hydrodynamic particle diameter, the dynamic viscosity of the liquid can be determined. In the present study, DLS from suspended particles was tested for measuring the viscosity of the [B(CN)4]− and [C(CN)3]−based ILs carrying the [alkyl-MIM]+ cations [EMIM]+ (ethyl), [BMIM]+ (butyl), [HMIM]+ (hexyl), [OMIM]+ (octyl), and [DMIM]+ (decyl) in the temperature range from 283 to 353 K at atmospheric pressure. For this, the long-term stability of Received: January 7, 2015 Revised: March 6, 2015 Accepted: March 9, 2015
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DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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coulometer) with an estimated expanded uncertainty (k = 2) of less than 20%. For the DLS experiments at atmospheric pressure, glass pipettes were used for transferring about 3 mL of the studied suspension into sealable square cuvettes made of quartz glass with a cross section of 1 cm2 and a volume of 3.5 mL. The cuvettes were inserted into the thermostated cuvette holder with four optical accesses shown in Figure 1a. Its main body
different particles suspended in the ILs as well as their particle diameters had to be analyzed in advance. For some semitransparent [C(CN)3]−-based ILs, the influence of the applied laser power on the measured particle diffusion coefficient was studied to exclude laser heating effects. Finally, the influence of CO2 absorbed in [BMIM][B(CN)4] on the dynamic viscosity was investigated at (293 ± 0.4) K and CO2 pressures up to 0.87 MPa.
2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Because of their excellent chemical stability and high monodispersity, commercial native silica, melamine resin, and polystyrene particles were tested regarding their ability of forming stable suspensions with the studied ILs. The employed charge of native silica particles (Monosphere 250, provided by Merck KGaA, Germany) with a nominal diameter of 250 nm and a density of 2.65 g·cm−3 at 293.15 K had already contributed successfully to the determination of the dynamic viscosity of acetone, ethanol, and 1-undecanol in 1995.13 To check the effect of particle size, also smaller native silica particles (Monosphere 100, provided by Merck KGaA) with a nominal diameter of 100 nm were used. The melamine resin particles (MF-F-S1898, nominal diameter 460 nm, standard deviation 30 nm, density 1.51 g· cm−3 at 293.15 K) and the polystyrene particles (PS-F-0.25, nominal diameter 248 nm, standard deviation 8 nm, density 1.05 g·cm−3 at 293.15 K) were purchased from microParticles GmbH, Germany. For the calibration of the particle diameter with liquids of well-known viscosities, the stability of suspensions of the above particles in high purity water deionized by an ion exchange water system (SG 2800 SK, SG Wasseraufbereitung und Regenerierstation, Germany), toluene (Merck KGaA, nominal purity of ≥99.9%), and ndodecane (Merck Schuchardt, nominal purity of ≥99%) was studied. All [B(CN)4]−-based ILs were synthesized from their corresponding chloride salts [alkyl-MIM]Cl according to the procedure described by Koller et al.15 For these ILs, purities of more than 99% on a molar basis were verified by 1H NMR analysis (JEOL, ECX-400 spectrometer) with the solvent dimethylsulfoxide-d6. The [C(CN)3]−-based ILs were provided by IoLiTec Ionic Liquid Technologies, Germany. Their purities of more than 98 mol % stated by the supplier are based on ion chromatography measurements. All ILs were filtered with PTFE syringe filters with a pore size of 0.20 μm and degassed for at least 3 h at about 323.15 K on a vacuum line (60 Pa) with an oil-sealed vacuum pump and a liquid nitrogen trap. For the preparation of suspensions, the liquids and the particles were weighed (Sartorius, BP110S) with a precision of 0.1 mg and an estimated expanded uncertainty (k = 2) of 1 mg, mixed in flasks, and exposed to ultrasonic treatment for at least 2 h to break up agglomerates and improve dispersion. With density data for the ILs7,15,16 and the particle material, volume fractions ϕ of the particles in the suspensions were calculated. Suspensions with maximum particle volume fractions of 0.01% were prepared to minimize influences of multiple scattering or particle interactions.12 On the basis of the sample preparation procedure, the uncertainty of the reported particle volume fractions is estimated to be smaller than 10%. The water content on a mass basis in the stable suspensions was determined directly before and after the viscosity measurements by Karl Fischer coulometric titration (Metrohm, 756 KF
Figure 1. (a) Cuvette holder for measurements at atmospheric pressure. (b) Sample cell for measurements with CO2 absorbed in [BMIM][B(CN)4].
made of copper and insulated by a PTFE shell contains flow channels for water circulated by a lab thermostat. A calibrated Pt 100 Ω resistance probe with an absolute uncertainty (k = 2) of 0.02 K is used for measuring the sample temperature. It is directly inserted into the sample through a bore in the lid of the cuvette. In this study, temperatures in the range from about 283−353 K were adjusted in steps of 10 K, where the temperature stability was better than ±0.02 K during each measurement. After completion of the measurements for a given temperature, the studied suspension was redispersed by immersing the cuvette into an ultrasonic bath for about 5 min. For the viscosity measurement of [BMIM][B(CN)4] containing dissolved CO2 at (293 ± 0.4) K, approximately 6 mL of a stable suspension with 0.0025 vol % of silica particles was filled into the block-shaped and gold-coated measurement cell with four optical accesses shown in Figure 1b. The upper inlet of the cell was connected to a gas buffer containing CO2 with a purity of 99.999 vol % provided by Linde, Germany. The tubing system between the gas buffer and the cell was flushed with CO2 several times. Then the valve between the gas buffer and the cell was opened to allow for equilibration of the ILCO2 system at approximately constant pressure. A pressure transducer with an absolute uncertainty (k = 2) of 1500 Pa was used for recording the applied CO2 pressures ranging from 0.435 to 0.869 MPa. The temperature on the outside of the cell was measured with the same resistance probe used for measuring the sample temperature in the cuvettes. Because of the lack of solubility data for CO2 in [BMIM][B(CN)4] in the literature, a procedure based on solubility data for CO2 in [EMIM][B(CN)4] and [HMIM][B(CN)4] detailed by Rausch B
DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research et al.17 was applied to estimate the mole fraction of CO2 in [BMIM][B(CN)4] with an expanded uncertainty (k = 2) of 0.04. 2.2. Dynamic Viscosity by Light Scattering from Suspended Particles. DLS analyzes the temporal behavior of the scattered light intensity reflecting the dynamics of microscopic fluctuations in pressure, temperature, and concentration in fluids in macroscopic thermodynamic equilibrium.8−10 It has been successfully used for the contact-free measurement of various transport and thermophysical properties of numerous fluid types including ILs.7,16−21 Its application for the determination of the dynamic viscosity via measurement of the translational particle diffusion coefficient of seed particles suspended in fluids is described in detail elsewhere12−14 and is only briefly summarized in the following. The decay of microscopic fluctuations in the particle concentration of a macroscopically equilibrated suspension is governed by the particle diffusion coefficient DP and reflected in the time-dependent intensity correlation function of light scattered by the particles. In the ideal case of monodisperse and freely diffusing particles, the normalized homodyne intensity correlation function takes the form g(2)(τ ) = A + B exp(−2τ /τC)
Figure 2. Optical and electronic setup. Abbreviations in the figure are explained in the text.
laser power. A scattering angle of Θ = 90° was adjusted by back-reflection from the walls of the used cuvettes or the glass windows of the pressure cell. A single-mode fiber (Thorlabs, Tp00865331) connected to the cuvette holder or installed directly in front of the respective window of the pressure cell was used for guiding the scattered light to two photomultiplier tubes (PMTs). Their signals were amplified, discriminated, and fed to a digital single-tau correlator featuring 2048 equally spaced channels. After a constant sample temperature was reached in the cuvette holder or equilibration could be assumed in the pressurized cell, at least six single consecutive experimental runs were performed. For each run, typical measurement times between 10 and 30 min were chosen to achieve a low statistical uncertainty of the recorded correlation function. The decay time τC was obtained from least-squares fitting of eq 1 to the correlator data. The expanded uncertainty (k = 2) of the dynamic viscosity or of the hydrodynamic particle diameter determined from eq 4 was calculated for each experimental run according to the law of propagation of uncertainties. For this, the expanded uncertainty of τC is represented by the double standard deviation of the data fit. The uncertainties in the calibrated hydrodynamic diameter d of the suspended particles and the dynamic viscosity η of the reference fluids used for the calibration of d are discussed in section 3.2. Furthermore, expanded uncertainties of 0.5° and 0.0005 for Θ and n were considered. The measurement procedure and the results for the refractive index of the suspensions studied by DLS are summarized as Supporting Information. In addition to the uncertainty propagation calculations, the double standard deviations of the consecutively measured dynamic viscosities or hydrodynamic diameters were calculated to describe the stability of the measurements. For stable suspensions, the data points in the plots symbolize the average of at least six individual measurement results for the corresponding thermodynamic state. The error bars shown in the figures represent either the mean calculated expanded uncertainty or the double standard deviation of all individual measurements contributing to the respective data point, where always the larger value is used.
(1)
with the experimental constants A and B. In a homodyne detection scheme, in principle only light scattered from the particles is analyzed. The decay time τC is equivalent to the mean decay behavior of particle fluctuations observed at a defined wave vector. It is related to the translational particle diffusion coefficient according to DP =
1 q2τC
(2)
In eq 2, the modulus of the scattering vector q=
4πn Θ sin λ0 2
(3)
which is identical to the wave vector of the fluctuations observed, is given in terms of the refractive index of the fluid n, the laser wavelength in vacuo λ0, and the scattering angle Θ between the directions of the incident laser beam and the analyzed scattered light. According to the Stokes−Einstein equation DP =
kT 3πηd
(4)
DP is related to the dynamic viscosity of the fluid η via Boltzmann’s constant k, the temperature T, and the hydrodynamic diameter of the suspended particles d. The last has to be calibrated by measuring DP for suspensions of the particles of interest in a reference liquid with known viscosity.12 The experimental and electronic setup is schematically illustrated in Figure 2. A He−Ne laser (Spectra Physics 05LHP991, λ0 = 632.8 nm, maximum output power of 30 mW) and a diode-pumped solid-state (DPSS) laser (Cobolt Samba, λ0 = 532.1 nm, maximum output power 1000 mW) were used as coherent light sources. The laser beam was guided to the sample cell by the mirrors M1−M3 and focused into the sample liquid by a lens (f = 500 mm). Two polarization filters were used for controlling the power of the incident light entering the sample. A power meter (Newport 842-PE, uncertainty 0.5%) was used for measuring the corresponding C
DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. Correlation functions for suspensions with 0.01 vol % of silica particles in (a) [BMIM][C(CN)3] and (b) [HMIM][B(CN)4] at T ≈ 343 K.
3. RESULTS AND DISCUSSION 3.1. Suspension Stability. Stable suspensions are essential for performing reliable viscosity measurements by DLS. Particle agglomeration, particle swelling due to penetrating liquid, or other interactions between the particles and the surrounding liquid would result in changing particles size distributions and additional terms in the correlation function. Thus, not only visual observations but in particular the recorded correlation functions have to be taken into account for evaluating the stability of suspensions. For instance, changing τC values with increasing time after sample preparation as well as systematic deviations of the correlation function from a single exponential according to eq 1 indicate such effects. In a first step, the stability of suspensions of the selected particles with volume fractions ranging from 0.0025% to 0.01% in the ILs was tested at (293.15 ± 0.02) K. No stable suspensions could be found using the polystyrene particles. Here, flocculation of the particles was observed. The silica particles formed stable suspensions with [BMIM][B(CN)4], [HMIM][B(CN)4], and [BMIM][C(CN)3]. For the same particle volume concentrations, stronger light scattering signals were found for the suspensions with the larger particle diameter. Thus, only the silica particles with a nominal diameter of 250 nm were considered for further experiments. Melamine particles could only be successfully suspended in [EMIM][C(CN)3]. In most of the other combinations of ILs with silica or melamine resin particles, sedimentation indicated agglomeration of the particles. Smaller agglomerates that did not sediment could be identified by analyzing recorded correlation functions. Hereby, formation of agglomerates at temperatures higher than 333 K was also identified for silica particles suspended in [HMIM][B(CN)4]. In Figure 3, exemplary correlation functions obtained for suspensions of [BMIM][C(CN)3] (a) and [HMIM][B(CN)4] (b) with 0.01 vol % of silica particles at about 343 K and the corresponding residual plots are compared. In addition, the corresponding decay times τC and their expanded uncertainties (k = 2) are given. For [BMIM][C(CN)3], the correlation function can be
represented well by a single exponential function, which is confirmed by the statistical behavior in the residual plot. In contrast, the residual plot for [HMIM][B(CN)4] indicates remarkable contributions from further terms in the long-time domain which are probably related to the onset of particle agglomeration at the probed temperature. This assumption is supported by the τC values determined from six consecutive measurements performed with the He− Ne laser and measurement periods of 10 min. Figure 4 shows
Figure 4. Temporal behavior of τC for consecutive measurements using suspensions with 0.01 vol % silica particles in [BMIM][C(CN)3] and [HMIM][B(CN)4] at T ≈ 343 K. The duration of each individual measurement was 10 min. The given temperatures represent average values over the complete measurement duration.
the good accordance between all single measurements for [BMIM][C(CN)3]. For [HMIM][B(CN)4], the decay time distinctly increases during the experiment, which could be explained by the formation of agglomerates. Because of these observations, the probed temperature range was limited to the range from 283 to 333 K for [HMIM][B(CN)4], whereas temperatures up to about 353 K could be studied for [EMIM][C(CN) 3 ], [BMIM][C(CN) 3 ], and [BMIM] [B(CN)4]. D
DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research 3.2. Particle Size Calibration. The hydrodynamic diameter d of the suspended particles is required for the determination of the dynamic viscosity of the ILs from the DLS experiments, cf. Stokes−Einstein eq 4. The diameter can be calibrated by applying the DLS technique to stable suspensions of the studied particles in liquids with well-known viscosities and having similar properties as the investigated ILs. In the present study, the stability of silica and melamine resin particles suspended with concentrations of up to 0.01 vol % in deionized water, toluene, and n-dodecane was tested. Only water-based suspensions of both particle materials proved to be stable for several days and could be used for the calibration. In toluene, sedimentation clearly indicated the instability of suspensions of both particle materials. The same statement holds for melamine resin particles in n-dodecane. Silica particles seemed to be stable in n-dodecane, but no light scattering signals could be observed. This finding may be related to the similar refractive indices of the silica particles and n-dodecane. For evaluation of the calibration measurements, reference viscosity data for pure water with an expanded uncertainty (k = 2) of 1% were used.22 Furthermore, the required refractive index data for the water-based suspensions were measured within the present study and are summarized in the Supporting Information. It should be mentioned here that the refractive indices measured for the water suspensions and pure water agree within combined uncertainties. This suggests the use of refractive index data for pure water for future studies with low particle concentrations. For the calibration, silica particle concentrations of 0.0025, 0.005, and 0.01 vol % in water were studied in the temperature range from about 283 to 333 K using the He−Ne laser. For all concentrations and temperatures, the recorded correlation functions could be fitted well with the theoretical model according to eq 1 and did not show any dependence on the time elapsed after sample preparation. Figure 5 depicts that the
values illustrated in Figure 5 was calculated to be 5.47 nm and included as dashed lines. This value is larger than both the average of all expanded uncertainties determined by propagation calculations and the average of the double standard deviations of the individual measurements contributing to each data point. Accounting for possible disturbing effects due to multiple scattering or particle interactions for the larger melamine resin particles, eight low particle concentrations ranging from 0.000 0625 to 0.001 25 vol % were chosen for first calibration experiments at 293.15 K using the He−Ne laser. The results shown in Figure 6a indicate a concentration dependence of the determined particle diameter for particle concentrations ≥0.0005 vol %. Therefore, concentrations of 0.000 25 and 0.000 375 vol % were chosen for calibration in the temperature range from about 293 to 353 K. Figure 6b shows the very good agreement of the results for the individual measurement conditions. An average diameter of 493.50 nm with an expanded uncertainty (k = 2) of 14.41 nm could be determined. Here, the stated uncertainty represents the mean value of the results from the uncertainty propagation calculations performed for the individual measurements, yielding a larger value than the other two procedures already described for the silica particle size calibration. 3.3. Investigation of Light Absorption Effects. In semitransparent samples such as the [C(CN)3]−-based IL samples studied here, absorption of the incident laser light can cause a local change of the sample temperature and, thus, of the measured viscosity. Furthermore, convection resulting in additional terms in the correlation function hinders the reliable analysis of the particle diffusion coefficient. Such effects were studied by varying the power P of the incident laser beam entering the sample from 1.5 to 10.8 mW at a constant sample temperature. In Figure 7, the decay times obtained for [BMIM][B(CN)4] and [BMIM][C(CN)3] with 0.01 vol % of silica particles at T ≈ 293.2 K and normalized with respect to the decay times found for P = 1.5 mW are shown. Furthermore, photographs of a He−Ne laser beam with a power of about 10 mW guided through the samples from the right to the left-hand side are presented. In the transparent [BMIM][B(CN)4] sample, the laser beam is visible due to particle scattering. Its constant diameter indicates the absence of absorption effects, which is further confirmed by the independence of the measured decay time from the applied laser power. On the contrary, the laser beam widens in [BMIM][C(CN)3], which can be explained by local heating and a thermal lens effect caused by the induced refractive index gradients. Distinctly decreasing decay times measured for increasing laser powers larger than 4 mW point out that the dynamic viscosity decreases in the light absorption area because of local heating. Consequently, measurements for semitransparent ILs were restricted to a laser power of 1.5 mW. Because such low laser powers imply weak DLS signals, satisfactory signal statistics had to be ensured by longer measurement durations of up to 30 min. To estimate remaining light absorption effects, the local temperature change in the irradiated fluid volume was estimated. For a laser power of 1.5 mW, the local superheating was approximated to be smaller than 0.15 K. Taking into account the temperature-dependent viscosity data obtained in the present study, this local temperature increase affects the measured dynamic viscosity by less than 1%. 3.4. Dynamic Viscosity of Pure ILs. In consequence of the results from the preliminary investigations, the measure-
Figure 5. Silica particle diameter d as a function of temperature T measured for different particle volume concentrations.
d values determined for different particle volume fractions and temperatures agree well considering the error bars. Here, the latter represent the average of the expanded uncertainties (k = 2) calculated according to the law of propagation of uncertainties for the at least six individual measurements contributing to each data point. An average diameter of 220.75 nm could be determined taking into account all particle volume concentrations and temperatures. For estimating the uncertainty of the calibrated silica particle diameter on a confidence level of more than 95%, the double standard deviation of the d E
DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Melamine resin particle diameter d (a) as a function of the particle volume concentration ϕ at T = 293.15 K and (b) as a function of temperature T for two selected volume concentrations.
Figure 7. Visual observations and influence of applied laser power P on the decay time τC for [BMIM][B(CN)4] and [BMIM][C(CN)3] with 0.01 vol % silica particles at T ≈ 293.2 K.
Figure 8. Deviation of experimental dynamic viscosities η for [BMIM][B(CN)4] from the correlation according to eq 5 as a function of temperature T.
ment of the dynamic viscosity was performed for stable suspensions of [BMIM][B(CN)4], [HMIM][B(CN)4], and [BMIM][C(CN)3] with silica particles as well as with melamine resin particles suspended in [EMIM][C(CN)3]. For most of these systems, experimental variations were performed to increase the validity of the results. In the following, the experimental results obtained for each IL are summarized and compared with available literature data in deviation plots. In the latter, the experimental or literature data η are related to the dynamic viscosity data calculated from the final correlation, ηcalc. Dashed lines indicate the estimated expanded (k = 2) uncertainties of the dynamic viscosity provided by the correlation. The viscosity data considered for the correlation and the obtained correlation parameters as well as a comparison of the viscosity results for the different pure ILs will be given at the end of this section. The influence of the laser wavelength was tested by using both the red He−Ne laser (λ0 = 632.8 nm) and the green DPSS laser (λ0 = 532.1 nm) at a constant laser power of 10.8 mW for the transparent [BMIM][B(CN)4] with 0.01 vol % of silica particles. Water contents of 0.12 and 0.21 wt % were measured before and after the DLS experiments. Figure 8 illustrates that for temperatures up to 343 K, the two data sets agree within their uncertainties. In consequence, the experimental data obtained with the different lasers as well as the calculated uncertainties and standard deviations were averaged and used for data correlation. At a temperature of 351.99 K, only the
He−Ne laser was used. To our knowledge, no viscosity data for [BMIM][B(CN)4] are available in the literature so far. For [HMIM][B(CN)4] with an initial water content of 0.12 wt %, silica particle volume fractions of 0.0025%, 0.005%, and 0.01% were studied using the He−Ne laser at P = 10 mW. The water contents in the corresponding samples after the measurements were 0.26, 0.31, and 0.38 wt %, respectively. Figure 9 shows that the viscosity results agree well within their uncertainties and indicate the absence of any particle concentration effects. Nevertheless, the smallest uncertainties and standard deviations were found for the highest particle volume fraction because of the highest scattered light intensities. Therefore, only the results for 0.01 vol % of silica particles were considered for data correlation. Three experimental data sets reported in the literature are included in Figure 9 for comparison. Koller et al.7 measured the viscosity of a [HMIM][B(CN)4] sample provided by Merck KGaA (nominal purity of >99%) by using SLS. For the measurements performed at temperatures ranging from 283.15 to 333.15 K, an expanded uncertainty (k = 2) of less than 3% was specified, and the water content increased from 0.039 to 0.046 wt %. Most of the SLS results are within the uncertainty of our correlation. For 303.15 and 313.15 K, their values are somewhat larger but still within combined uncertainties. The data published by Mota-Martinez et al.23 and Neves et al.24 are about 5−6% larger than those calculated from our correlation. Both studies were F
DOI: 10.1021/acs.iecr.5b00086 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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experiments. All literature data were obtained with Anton Paar SVM 3000 Stabinger viscometers and very low stated uncertainties. Although they show similar trends regarding the temperature dependence, the viscosity data from literature clearly differ outside their combined uncertainties. The data reported by Labropoulos et al.,27 which were measured for a sample provided by IoLiTec with a Cl content below 1% and which have a stated uncertainty of 0.005 mPa·s, agree very well with our results. This is also valid for the data given by Carvalho et al.28 who studied a sample from Merck KGaA (purity of >99 wt %, water content of 0.0016 wt %) and stated an uncertainty of 0.35%. The viscosities measured by Neves et al.24 for a sample provided by Merck KGaA (purity of >98 wt %, water content of 99% and water content of 98 wt % and water content of 98 wt %, water content of
