Density, Viscosity, and Structure of Equilibrium Solvent Phases in

Article pubs.acs.org/jced

Density, Viscosity, and Structure of Equilibrium Solvent Phases in Butyric Acid Extraction by Phosphonium Ionic Liquid Ján Marták* and Štefan Schlosser Slovak University of Technology, Faculty of Chemical and Food Technology, Institute of Chemical and Environmental Engineering, Radlinského 9, SK-81237 Bratislava, Slovakia S Supporting Information *

ABSTRACT: Density, viscosity, and dynamic light scattering (DLS) measurements have been done for quaternary phases with phosphonium ionic liquid (Cyphos IL-104 + dodecane + BA + water) from the extraction of butyric acid (BA) by a binary solvent (IL + dodecane). Density data show volume contraction in the solvent. Addition of water, BA, or dodecane considerably decreases IL viscosity, while BA decreases viscosity less than water. When increasing BA concentration, viscosity of the solvent phase in equilibrium without dodecane goes through a maximum at BA mass fraction of about 0.2. In solvents with more than 30% in mass fraction of dodecane, almost no influence of BA on viscosity was observed. Viscosity of water saturated solvents with IL concentration below 50% in mass fraction is markedly higher than in solvents containing also BA. This is due to the formation of dodecane-rich micelles in water saturated solvents which increase IL concentration in the bulk of the solvent. No large aggregates with size above 100 nm were observed in quaternary solvent phases from BA extraction in DLS measurements even at high IL concentrations. The size of aggregates in the quaternary solvents decreases with increasing BA concentration.

1. INTRODUCTION Experimental data on the extraction of butyric acid (BA) from aqueous solutions by binary solvents (Cyphos IL-104 + dodecane) and by undiluted IL were presented in a previous paper.1 Commercial phosphonium ionic liquid (IL) trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL-104, Cytec, CA) was used. The new model of BA and water extraction with very good fit to the experimental data has been developed and presented as well. The second part of experimental data on physical properties of the equilibrium solvent phases from BA extraction published by Marták and Schlosser1 are presented in this article. In separation processes with organic solvents as extraction or pertraction through liquid membranes, viscosity of the organic phase is a crucial parameter influencing mass-transfer. It has significant influence on the effectiveness of the separation process. Molecular solvents, for example water or dodecane considerably decrease the viscosity of ILs.2−6 There are no published data on viscosity and other properties of solvents containing hydrophobic ILs loaded with solutes which are mostly quaternary mixtures (IL+diluent+solute+water). Compared to undiluted IL, the decreased viscosity of the membrane containing IL and a water insoluble molecular diluent, e.g., dodecane, may compensate the decreased distribution coefficient so that the transport properties, e.g., mass transfer coefficient of the solute through both membranes can be similar.7 © 2017 American Chemical Society

Composition of the quaternary organic phase (Cyphos IL104 + dodecane + BA + water) from BA extraction depends on BA concentration in the equilibrium aqueous solution. In case of the tested system, increasing BA concentration leads to competitive replacement of water bound to IL with extracted BA and, at the same time, to coextraction of additional amount of water bound to BA.1 All these complicated changes affect the viscosity of the solvent and therefore it is important determine it. For a set of imidazolium ILs it has been found that the interaction energy of an ion pair reflecting the strength of the IL cation−anion interaction is related to the viscosity: ILs with higher interaction energy have higher viscosity and vice versa. The same was found for the energy and number of hydrogen bonds between anions and cations.8 Since the interaction between the solute (BA or water) and IL is based on hydrogen bonds,1 a similar effect can be expected. The presence of reverse micelles was identified in phosphonium3,6 and ammonium9 ILs based on dynamic light scattering (DLS) measurements. Large aggregates in dry phosphonium IL were found in earlier work.6 Aqueous microemulsion particles in imidazolium IL were identified by transition electron microscopy and DLS measurements.10 Received: January 14, 2017 Accepted: September 14, 2017 Published: September 26, 2017 3025

DOI: 10.1021/acs.jced.7b00039 J. Chem. Eng. Data 2017, 62, 3025−3035

Journal of Chemical & Engineering Data

Article

Table 1. Chemical Sample Table chemical name

CAS registry number

Cyphos IL104 Dodecane

465527−59−7

Butyric Acid

107−92−6

Sodium Hydroxide Water

1310−73−2

112−40−3

7732−18−5

initial mass fraction purity

source Cytec (Canada) Fluka (Switzerland) Merck (Germany) Mikrochem (Slovakia) potable water

final mass fraction purity

purification method

analysis method

>0.95

NMR

>0.99

Conditioning: 1. 0.5 kmol.m−3 NaOH 2. deionized water Drying: T = 413 K, P = 0.8 kPa none

-

-

0.991

none

-

>0.98

none

-

Acidobasic titration -

750 μS.m−1

RO plus mixed bed

10 μS.m−1

>0.95

Conductometer

Table 2. Density (ρS), Dynamic Viscosity (ηS), and Hydrodynamic Diameter of Aggregates (d) in the Ternary and Binary Solvent Phase of System (Cyphos IL-104 + Dodecane + Water) at Temperature T and Pressure P = 0.10 MPaa,b,c,d exp. no.

T K

wIo

wDo

wWsat,S

ρS

ηS

c Wsat,S

mPa . s

d nm

c Wsat,S

kg . m−3

kmol . m−3

cI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

298 298 298 298 298 298 298 298 298 298 298 310 310 318 318

0.295 0.381 0.381 0.475 0.475 0.584 0.584 0.665 0.674 0.950e 0.950e 0.584 0.950e 0.584 0.950e

0.689 0.599 0.599 0.500 0.500 0.385 0.385 0.300 0.291 0 0 0.385 0 0.385 0

0.0517 0.0606 0.0585 0.0705 0.0729 0.100 0.0954 0.104 0.117 0.156 0.151 0.0950 0.146 0.0896 0.142

802.19 809.24 825.41 845.87 858.87 861.08 909.07 836.40 900.11 828.80 893.78

8.40 10.4 15.8 25.9 35.2 142 15.8 79.7 11.2 55.6

5.70 3.06 0.981 0.247 0.210 0.167 234 -

2.30 2.72 2.63 3.23 3.34 4.71 4.48 4.96 5.59 7.89 7.62 4.41 7.30 4.12 7.04

= zW

7.90 7.25 7.01 6.86 7.11 8.19 7.48 7.41 8.43 8.38 7.86 7.71 7.73 7.23 7.47

a

Initial mass fractions of IL and dodecane (wIo and wDo) in dry solvents (Cyphos IL-104 + dodecane) and experimental (liquid−liquid) equilibrium water mass fraction in the water saturated solvent (wWsat,S). Derived quantities shown are the equilibrium molar concentration of water (cWsat,S) and the loading of ionic liquid with water (zW) in the equilibrium solvent phase. bNew data unpublished in the previous work1 are in bold. c Measurements in italic were evaluated as poor by the DLS software. dStandard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wIo) = ur(wDo) = 0.0025, ur(wWsat,S) = 0.015, ur(ρS) = 0.005, ur(ηS) = 0.05, ur(d) = 0.05, ur(cWsat,S) = 0.015, ur(zW) = 0.035. eUndiluted IL

were carried out by weighting precise amounts of each component to the glass vials. The equilibrium was achieved in a rotational shaking water bath at 298, 310, or 318 K. The frequency of shaking was optimized to ensure the dispersion of phases but at the same time to avoid the formation of an emulsion. In most cases it was about 180 min−1. The time of 10 h was satisfactory to achieve equilibrium. For the analysis of BA in the organic phases, a sample was thoroughly mixed with an aqueous solution of sodium hydroxide containing at least a 1.5-fold molar excess of NaOH for achieving quantitative stripping of BA. The stripped BA was then analyzed by capillary isotachophoresis. During the stripping, the volumes of aqueous and organic phases are changing due to transfer of BA and water from one phase to the other one. The evaluation of experimental data considering these differences is in detail described in SI of the previous work.1 The mass fraction of water in the organic phases was determined by the Karl Fischer titration. Description of the equilibrium experiments and the analytical methods in more detail are presented in the previous work.1 The density of the organic phases at equilibrium temperature was determined using a vibration density meter DMA-5000 (Anton Paar, Austria) with the accuracy of temperature setting of ±0.01K and the precision of measurement of ±0.01 kg.m−3 according to the manufacturer. The calibration of density meter

The aim of this work is to present density, viscosity and dynamic light scattering (DLS) measurements of the equilibrium quaternary organic phases (IL + dodecane + BA + water) from BA extraction by Cyphos IL-104. Data on binary mixtures (IL + BA), (IL + water), and (IL + dodecane)6 are also shown and discussed.

2. MATERIALS AND METHODS Chemicals, their preparation including conditioning and drying of ionic liquid tetradecyl(trihexyl)phosphonium bis(2,4,4trimethylpentyl)phosphinate (Cyphos IL-104) were in detail described in previous work.1 1H, 13C, and 31P NMR spectra (Figures S1 to S5) of conditioned and dried Cyphos IL-104 measured on Varian VNMRS 600 spectrometer (MRL, USA) are shown and discussed in SI. The most important information on the preparation of chemicals for use is collected in Table 1. Water for the preparation of aqueous solutions was treated by passing through a reverse osmosis unit and a mixed-bed ionexchange module (Chezar, Slovakia). The final conductivity of treated water was 10 μS.m−1. Equilibrium solvent phases from liquid−liquid equilibrium experiments with quaternary two-phase system (BA + water + IL + dodecane) described in the previous paper1 were used for the measurements presented in this work. These experiments 3026



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Table 3. Density (ρS), Dynamic Viscosity (ηS), and Hydrodynamic Diameter of Aggregates (d) in the Quaternary or Ternary Solvent Phases from Extraction of BA at Temperature T = 298 K and Pressure P = 0.10 MPaa,b,c,d exp. no.

wIo

wDo

wBA,S

wW,S

kmol . m−3

ρS

ηS

kg . m−3

mPa . s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

0.300 0.300 0.295 0.296 0.295 0.300 0.300 0.285 0.380 0.380 0.475 0.475 0.600 0.600 0.600 0.599 0.600 0.600 0.600 0.599 0.600 0.600 0.600 0.570 0.600 0.674 0.700 0.665 0.950e 0.950e 0.950e 0.950e 0.950e 0.950e 0.950e 0.950e 0.950e 0.950e

0.684 0.684 0.689 0.688 0.689 0.684 0.684 0.700 0.600 0.600 0.500 0.500 0.368 0.368 0.368 0.369 0.368 0.368 0.368 0.369 0.368 0.368 0.368 0.400 0.368 0.291 0.263 0.300 0 0 0 0 0 0 0 0 0 0

0.0137 0.0331 0.0526 0.0771 0.100 0.133 0.159 0.175 0.0775 0.119 0.0928 0.143 0.0344 0.0644 0.109 0.124 0.136 0.156 0.175 0.195 0.206 0.233 0.276 0.292 0.331 0.168 0.180 0.323 0.0502 0.0949 0.148 0.193 0.231 0.263 0.325 0.358 0.397 0.413

0.0331 0.0210 0.0144 0.0084 0.0085 0.0081 0.0086 0.0110 0.0130 0.0100 0.0157 0.0121 0.0708 0.0476 0.0251 0.0152 0.0146 0.0149 0.0200 0.0222 0.0227 0.0259 0.0215 0.0267 0.1088 0.0769 0.0476 0.0306 0.0259 0.0286 0.0349 0.0375 0.0401 0.0434

0.293 0.292 0.284 0.279 0.272 0.269 0.262 0.244 0.362 0.349 0.453 0.432 0.588 0.584 0.575 0.563 0.565 0.547 0.542 0.526 0.523 0.501 0.474 0.438 0.437 0.610 0.629 0.493 0.938 0.922 0.895 0.865 0.830 0.795 0.721 0.684 0.641 0.619

794.65 794.84 795.44 798.04 801.34 807.16 810.68 811.99 810.37 815.71 825.97 832.59 847.02 846.92 855.32 851.09 857.57 854.61 861.74 859.69 865.67 863.53 869.75 866.24 877.34 863.55 871.58 882.67 907.59 906.33 905.30 907.18 909.85 912.58 917.80 921.14 926.23 926.62

5.82 5.23 5.10 5.00 5.10 5.22 5.18 4.59 7.53 7.60 11.6 11.9 27.3 25.7 24.3 24.1 29.0 29.5 25.7 24.4 14.4 19.0 32.5 20.2 147 156 166 167 158 138 104 83.1 54.4 52.8

cI

d nm

2.67 1.67 1.28 1.02 0.984 0.993 1.07 1.15 0.709 0.647 0.424 0.368 0.199 0.230 0.153 0.175 0.236 0.130 0.165 0.0052 0.0068 0.0069 0.0068 -

0.0480 0.0498 0.0635 0.112 -

46.7 44.6 39.0 48.3 -

a Initial mass fractions of IL and dodecane (wIo and wDo) in dry solvents (Cyphos IL-104 + dodecane), experimental (liquid−liquid) equilibrium mass fraction of butyric acid (wBA,S) and water (wW,S), concentration of IL (cI) in the equilibrium solvent phase. bNew data unpublished in the previous work1 are in bold. cMeasurements in italic were evaluated as poor by the DLS software. dStandard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wIo) = ur(wDo) = 0.0025, ur(wBA,S) = 0.035, ur(wW,S) = 0.015, ur(cI) = 0.078, ur(ρS) = 0.005, ur(ηS) = 0.05, ur(d) = 0.05. eUndiluted IL

was done using liquid density standard ultra pure water (Anton Paar, Austria) in the temperature interval from 288 to 333 K and air was the second calibration fluid. Viscosity of the organic phases was measured by a rheometer Kinexus Pro (Malvern, UK) with a cone and plate system and precise control of the sample temperature with the accuracy of 0.01 K in the range of shear rates from 10 to 200 s−1. The cone and plate were covered with a tempered active hood protecting the sample from contact with wet air during the measurement. Calibration of the instrument was done with standard oil (PRA, UK) with the viscosity of 1.0 Pa.s at the temperature of 298 K in the studied shear rate interval. Mean deviation of the measured viscosities was 0.1%. In the measurements of the solvent samples, eight points were considered at various shear rates for each temperature. For each shear rate, the value of viscosity was taken as the mean of integrated measured values

(5 kHz data streaming) in 3 s after achieving mechanical stability of the instrument. The criterion of stability had to be met for 7 s at the given shear rate. Final viscosity was obtained as the average of the results with the applied motor torque of over 2.5 × 10−6 Nm. Maximum mean deviation of the measured viscosities for one sample at various shear rates was lower than 0.9%. Aggregates in IL solutions in dodecane were analyzed by dynamic laser scattering (DLS) using a Zetasizer Nano (Malvern, UK) at 298 K in a quartz cuvette. Evaluation of the volumetric distribution of the aggregate size by the Zetasizer Software, ver. 7.01 (Malvern, UK) was based on the viscosity of the measured organic phase. Measurements of solvents with IL concentrations higher than ca. 50% in mass fraction were estimated by the DLS software as of poor quality due to the elevated baselines of the correlation function or large 3027



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Table 4. Density (ρS) and Dynamic Viscosity (ηS) of Ternary or Quaternary Solvent Phases from Extraction at Temperature T = 310 K and Pressure P = 0.10 MPaa,b,c exp. no.

wIo

wDo

wBA,S

wW,S

kmol . m−3

ρS

ηS

kg . m−3

mPa . s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

0.100 0.295 0.297 0.300 0.392 0.398 0.399 0.487 0.499 0.499 0.600 0.600 0.600 0.599 0.600 0.600 0.600 0.599 0.600 0.600 0.600 0.600 0.700 0.800 0.799 0.799 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d

0.895 0.689 0.687 0.684 0.587 0.581 0.580 0.487 0.475 0.475 0.368 0.368 0.368 0.369 0.368 0.368 0.368 0.369 0.368 0.368 0.368 0.368 0.263 0.158 0.159 0.159 0 0 0 0 0 0 0 0 0 0 0 0

0.0408 0.0751 0.0846 0.110 0.0910 0.109 0.135 0.109 0.129 0.159 0.0346 0.0647 0.106 0.120 0.140 0.149 0.172 0.183 0.200 0.239 0.280 0.346 0.165 0.147 0.181 0.224 0.0510 0.0975 0.141 0.167 0.189 0.205 0.222 0.253 0.250 0.316 0.359 0.419

0.0654 0.0454 0.0260 0.0184 0.0158 0.0176 0.0198 0.0232 0.0284 0.1127 0.0759 0.0510 0.0354 0.0298 0.0287 0.0329 0.0363 0.0461

0.094 0.265 0.276 0.272 0.351 0.367 0.359 0.435 0.457 0.443 0.585 0.579 0.570 0.554 0.554 0.543 0.537 0.524 0.520 0.492 0.465 0.422 0.635 0.738 0.712 0.678 0.923 0.912 0.891 0.862 0.858 0.833 0.830 0.799 0.790 0.728 0.679 0.604

762.07 789.74 792.22 796.43 806.19 810.67 814.61 822.09 827.97 831.99 837.60 838.08 846.27 841.71 848.24 845.03 852.17 849.29 856.29 854.72 861.04 868.81 861.13 875.49 877.90 881.87 898.44 897.68 898.06 899.55 899.84 901.94 902.26 904.78 905.30 910.06 913.25 918.69

17.8 18.7 18.2 18.7 18.9 15.5 15.2 13.9 11.8 81.7 83.5 90.4 91.2 87.1 77.2 57.5 46.9 30.5

cI

a Initial mass fractions of IL and dodecane in dry (Cyphos IL-104 + dodecane) solvents (wIo and wDo), experimental (liquid−liquid) equilibrium mass fraction of butyric acid (wBA,S), water (wW,S), and concentration of IL (cI) in the equilibrium solvent phase. bNew data unpublished in the previous work1 are in bold. cStandard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wIo) = ur(wDo) = 0.0025, ur(wBA,S) = 0.035, ur(wW,S) = 0.015, ur(cI) = 0.078, ur(ρS) = 0.005, ur(ηS) = 0.05. dUndiluted IL

volume contraction was observed since the measured densities are higher than the calculated ones based on ideal volume mixing of undiluted substances, represented by small open symbols in Figure 1a and Figure S6a and S6c in SI. Similar volume contraction also follows from the density measurements of binary mixtures (IL + water) and (IL + BA) as shown in Figure 2a and Figure S7a and S7c in SI. The volume contraction was quantitatively expressed as a difference between volume of mixture and sum of volumes of individual components per unit mass of mixture defined by relation

polydispersity index leading to measurements uncertainty. Some of them are shown for illustration.

3. RESULTS AND DISCUSSION In this article are presented physical properties of the organic phases from equilibrium extraction of BA and water from aqueous binary solutions by binary solvent (IL + dodecane) or undiluted Cyphos IL-104 (previous work1). Results for the ternary organic phases without BA are shown in Table 2, those for the quaternary organic phases with BA are listed in Tables 3, 4, and 5 and those for single phase binary mixtures are in Tables 6 and 7. Additional data are presented in Supporting Information (SI). 3.1. Density. Densities of equilibrium solvent phases at 298 K are shown in Figure 1a and for 310 and 318 K in Figure S6a and S6c in SI. For undiluted IL with wIo = 0.95 (purity of IL),

ΔVSm =

1 − ρS

∑ i

wi ρi

(1)

where ρS is density of the mixture and wi and ρi are mass fraction and density of the individual components. ΔVmS is a 3028



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Table 5. Density (ρS) and Dynamic Viscosity (ηS) of Quaternary or Ternary Solvent Phases from BA Extraction at Temperature T = 318 K and Pressure P = 0.10 MPaa,b,c exp. no.

wIo

wDo

wBA,S

wW,S

kmol . m−3

ρS

ηS

kg . m−3

mPa . s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.600 0.600 0.600 0.600 0.600 0.600 0.600 0.600 0.600 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d 0.950d

0.368 0.368 0.368 0.368 0.368 0.368 0.368 0.368 0.368 0 0 0 0 0 0 0 0 0

0.0354 0.0665 0.106 0.140 0.169 0.185 0.238 0.280 0.341 0.0509 0.0997 0.143 0.184 0.215 0.245 0.308 0.354 0.412

0.0711 0.0482 0.0247 0.0209 0.0193 0.0181 0.0218 0.0254 0.0288 0.106 0.0748 0.0499 0.0342 0.0311 0.0290 0.0361 0.0377 0.0464

0.581 0.575 0.566 0.549 0.533 0.523 0.490 0.462 0.423 0.925 0.906 0.885 0.859 0.830 0.802 0.728 0.678 0.607

838.19 837.69 839.81 842.71 845.80 845.22 852.23 857.72 864.54 893.78 893.03 892.46 894.44 896.90 899.42 903.68 906.77 911.83

14.0 13.6 16.7 16.4 13.6 11.9 12.0 11.0 9.21 57.5 59.8 69.5 73.3 68.8 61.7 44.5 35.4 24.4

cI

a

Initial mass fractions of IL and dodecane in dry (IL + dodecane) solvents (wIo and wDo), experimental (liquid−liquid) equilibrium mass fraction of butyric acid (wBA,S) and water (wW,S), and concentration of IL (cI) in the equilibrium solvent phase. bNew data unpublished in the previous work1 are in bold. cStandard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wIo) = ur(wDo) = 0.0025, ur(wBA,S) = 0.035, ur(wW,S) = 0.015, ur(cI) = 0.078, ur(ρS) = 0.005, ur(ηS) = 0.05. dUndiluted IL.

Table 6. Density (ρI,BA) and Viscosity (ηI,BA) of Binary Mixtures (IL + Butyric Acid) at Temperature T and Pressure P = 0.10 MPaa,b T/K wI

wBA

wW.102c

0.950 0.938 0.917 0.917 0.884 0.816 0.718 0.662 0.590 0.529 0.480 0.264 0

0 0.0122 0.0346 0.0349 0.0684 0.140 0.242 0.301 0.375 0.440 0.491 0.715 0.991

0.0470 0.0489 0.0523 0.0524 0.0576 0.0686 0.0844 0.0934 0.105 0.115 0.123 0.157 0.200

298

310

318

298

kg . m−3

884.09 886.44 892.23 889.81 893.84 899.77 909.13 914.05 919.80 924.13 927.26 939.25 953.50

876.93 879.26 884.71 882.50 886.39 892.33 901.49 906.22 911.62 915.64 918.52 929.27 941.63

310

318

ηI,BA

ρI,BA

mPa . s

872.20 874.50 879.80 877.64 881.50 887.48 896.40 901.00 906.17 909.99 912.71 922.63 933.77

1023 952 785 787 562 348 216 159 92.7 56.9 38.8 7.55 1.69

521 465 389 388 278 181 118 87.9 53.5 34.4 24.6 5.51 1.39

345 304 257 255 185 123 82.2 62.1 38.9 25.5 18.8 4.56 1.23

a Mass fractions of cyphos IL-104 (wI), butyric acid (wBA), and water (wW) in the solvent phase. bStandard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wI) = ur(wBA) = 0.0025, ur(wW) = 0.015, ur(ρI,BA) = 0.005, ur(ηI,BA) = 0.05. cResidual water in BA and dried IL preparation.

measurements. This tendency is in an agreement with the behavior of the binary mixtures (IL + dodecane) where, as it can be seen in Figure 2b and Figures S7b and S7d in SI, at lower IL concentrations, wI < 0.7, the absolute value of ΔVmS are below 4 cm3·kg−1 However, at higher IL concentration, these values are rather higher with maximum of about 16 cm3·kg−1 at wI about 0.85. This indicates that unbranched dodecane chains can interlock with the vacant space of hydrophobic domains which cannot be occupied by alkyl chains of the IL cation and anion, probably due to a steric barrier since the alkyls in IL are not free but bound to the central phosphorus atoms. Moreover, the alkyls of the phosphinate anion are branched. 3.2. Viscosity. Initially, it was assumed that the influence of water, BA and dodecane on the viscosity of the solvent phase

volume change due to mixing related to unit mass of mixture. It is shown in Figures 1b 2b and for 310 and 318 K in Figures S6b, S6d, S7b and S7d in SI. The volume contraction indicates that the small molecules of water and BA and large IL ions are arranged in more packed arrangement than in individual liquids and that water and BA are able to occupy a part of the space between the polar heads of the IL cation and anion which cannot be occupied in undiluted dried IL. For the equilibrium solvent phases, with the decreasing IL concentration the volume change, ΔVmS (eq 1), decreases (Figure 1b). Four distant points at wIo = 0.6 resulted from an unsatisfactory accuracy of density measurement connected with the improper calibration of density meter after moving it to another place, which was identified and corrected in other 3029



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Table 7. Density (ρI,W) and Viscosity (ηI,W) of Binary Mixtures (IL + water) at Temperature T and Pressure P = 0.10 MPa for Selected Mass Fractions of IL (wI) and Water (wW)a T/K wI

wW.102

0.950 0.950 0.949 0.948 0.945 0.936 0.920 0.873 0.802c 0.812c 0.815c 0

0 0.047 0.122 0.289 0.582 1.47 3.17 8.10 15.6 14.6 14.2 100

298

310

318

298

kg . m−3

883.68b 884.09 884.98 886.11 886.78 887.75 890.65 898.06 909.07 997.04

310

318

ηI,W

ρI,W

mPa . s

876.52b 876.93 877.82 878.88 879.48 880.36 883.04 890.20 900.11 993.35

871.79b 872.20 873.07 874.07 874.64 875.46 878.08 884.95 893.78 990.21

1023 996 924 796 646 431 261 142 0.890

521 512 476 401 321 211 125 79.7 0.686

345 337 313 263 210 137 81.6 55.6 0.581

a

Standard uncertainties u are u(T) = 0.1 K, ur(P) = 0.06, ur(wI) = 0.0025, ur (wW) = 0.015, ur(ρI,W) = 0.005, ur(ηI,W) = 0.05. bTheoretical density of dry IL calculated by subtraction of residual water from conditioned and dried IL considering the volume additivity rule according to the formula

ρE =

ρI,cond.dried (1 − w W,cond.dried) 1 − w W,cond.driedρI,cond.dried / ρ W

. cIonic liquid saturated with water

Figure 2. Density (a) and volume change due to mixing related to unit mass of mixture (eq 1) (b) of binary mixtures of (IL + water) (dark blue circle), (IL + BA) (red square), and (IL + dodecane) (green triangle)6 at 298 K. Lines in Figure 2a represent the values of densities based on ideal mixing of individual components.

Figure 1. Density (a) and volume change due to mixing related to unit mass of mixture (eq 1) (b) of ternary and quaternary organic phases from equilibrium extraction of BA by undiluted IL and binary solvents (IL + dodecane): wIo = 0.3 (dark blue circle), 0.4 (light blue square), 0.5 (orange pentagon), 0.6 (green diamond), 0.7 (pink star), and 0.95 (undiluted IL, red triangle) at 298 K. Small open symbols represent the values of densities considering the volume additivity rule when mixing individual components.

with IL can be evaluated if the solutions with the same volume fractions of molecular solutes are compared. However, since the density measurements of the mixtures show volume contraction, the exact determination of volume fractions in them is difficult. Therefore, it was decided to use mass fractions instead 3030



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of volume fractions because the densities of IL, water, and BA are rather similar and thus the errors originating from their differences are expected as less significant. Viscosities of the equilibrated solvent phases from BA extraction at 298 K are shown in Figure 3 and measurements

Figure 4. Viscosity of ternary organic phases from equilibrium extraction of BA by undiluted IL (red triangle) and mass fraction of water in the solvent phases (dark blue circle), vs aqueous BA concentration. Viscosity of binary mixture (IL + BA) for BA mass fraction equal to the sum of water and BA mass fractions in the ternary organic phases from extraction (green triangle). Temperature was 298 K.

Figure 3. Viscosities of ternary and quaternary organic phases from equilibrium extraction of BA by undiluted IL and binary solvents (IL + dodecane) at 298 K: for higher IL concentrations, wIo = 0.6 (green diamond), 0.7 (pink star), and 0.95 (undiluted IL, red triangle) (a) and for lower IL concentrations, wIo = 0.3 (dark blue circle), 0.4 (light blue square), 0.5 (orange pentagon) (b).

for 310 and 318 K are shown in Figure S8 in SI. On the first sight it can be said that the viscosity markedly decreases as the concentration of IL decreases and temperature increases. In BA extraction by undiluted IL with wBA, S increasing from (0 to 0.2), viscosity increases to the maximum at about wBA, S = 0.2 as shown in Figure 3a and Figure S8 in SI. This maximum coincides with the minimum in dependence of water concentration in the solvent, Figure 4 and Figure S9 in SI, which is due to the replacement of water bound at active Hbonding sites on an IL anion by BA1 (Figure 5). At this minimum the sum of BA and water loadings of IL drops from about 8 to 4 as shown in Figure S10 in SI. In the water saturated solvent, about eight molecules of water are bound to one IL ion pair (Figure 5a). The minimums in water content and (BA + water) loading of IL connected with the viscosity maximum is reached because even four water molecules are released when the first molecule of BA is extracted (Figure 5b) and the rest of water is released when the second one is

Figure 5. Schematic illustration of hypothetical structures: Cyphos IL104 saturated with water (a), loaded with one BA in the complex which substituted 4H2O (b), and loaded with two BA with two molecules of hydration water shared by two complexes which are bound to BA but not to IL (c). 3031



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extracted. However, little amount of water remains bound to BA and not to IL (Figure 5c). This water is shared by two complexes as it was shown in the first part1 and as it is shown in Figure 5b and c. The complexes schematically illustrated in Figure 5 are hypothetical structures proposed on the basis of the model presented in the previous work.1 Viscosity of binary mixture (IL + BA) with wBA equal to the sum of (wBA, S + wW, S) in ternary organic phase from equilibrium is given in Figure 4 and S9 (green triangles) in SI; for low wBA, S the viscosity of binary mixture is higher than that of the ternary phase. This indicates that BA reduces the viscosity of the solvent less than water, which can be supported also by the viscosity measurements of binary mixtures (IL + water) and (IL + BA) shown in Figure 6 and Figures S11 in SI.

be found also for other phosphonium ionic liquids, e. g. IL with tetradecyltrihexylphosphonium cation and chloride, dicyanamide, bis-triflamide anions as well as for ammonium ILs, e.g., tetraalkylammonium chloride and bis(2,4,4-trimethylpentyl)phosphinate.9 Dry tetradecyltrihexylphosphonium decanoate is paste-like at room temperature but it is liquid with viscosity of 96.1 mPa·s when saturated with water at 298 K. Dodecane considerably decreases the viscosity of dried IL as it can be seen in Figures 6 and Figures S11 and S12 in SI. The same can be said also for water saturated IL as it is illustrated in Figure 5. In the earlier work6 it was presented that the addition of 30% in mass fraction of dodecane (wIo = 0.7) reduces the viscosity of dried IL 26.3 times while it is only three times in water saturated ILs (Figure 7). In the extraction by a binary

Figure 6. Viscosity of binary mixtures of (IL + water) (dark blue circle), (IL + BA) (red square), and (IL + dodecane) (green triangle)6 at 298 K. Line represents empirical correlation according to eq 2.

Figure 7. Viscosity of water saturated ternary mixtures (IL + dodecane + water); temperatures: 298 K (dark blue circle), 310 K (green triangle), and 318 K (red square). Shaded area designates the interval ́ of wIo where three phases are formed according to RodriguezEscontrela et al.11 (solid borders) and Marták12 (dashed borders) both at 298 K.

The same data are shown also in Figure S12; the difference is the use of logarithmic scale for the vertical axes. For the same mass fraction of the molecular solute, viscosity of the binary mixture containing water is considerably lower compared to that of the mixture with BA. This difference is rather high and cannot be attributed to the differences in the BA and water density values which are, at 298 K, 953.5 and 997.04 kg·m−3, respectively. Higher viscosity of (IL + BA) mixture compared to (IL + water) mixture can be explained by the fact that the hydrogen bond between BA and IL is stronger than that between water and IL as it has been shown in the previous work.1 Moreover, in addition to polar interactions, also hydrophobic van der Waals interactions are supposed between C3 alkyl group of BA and hydrophobic parts of IL anion and cation. To obtain binary data in Figure 4 and Figure S9 in SI, experimental data of dynamic viscosities (in mPa·s) for binary mixture (IL + BA) were correlated using the empirical equation:

log ηI,BA = AwBA + B

solvent with IL concentration of 60% in mass fraction, the viscosity maximum in the dependence on BA concentration is much less pronounced (Figure 3a and Figure S8 in SI) than in case of undiluted IL. At IL concentrations below 50% in mass fraction, no maximum solvent viscosity was observed (Figure 3b) but viscosity of the water saturated solvent without BA was considerably higher. This is probably connected with the limited miscibility of IL with dodecane at water saturated conditions as it is elucidated in the following section. 3.3. DLS Measurements and Phase Diagram. Results of DLS measurements are shown in Figures 8−12. In water saturated undiluted IL aggregates with average hydrodynamic diameter d = 234 nm have been indicated (Figures 8 and 9). However, the measurement response refers to rather high polydispersity suggesting the presence of diverse aggregates of IL with d in the interval at least ±150 nm with respect to the average value. These aggregates can be effectively split by addition of dodecane to the water saturated IL, as it follows from Figure 8. Compared to undiluted IL, for the water saturated binary solvent with IL concentration wIo = 0.67 the mean hydrodynamic diameter decreased from 234 nm to almost zero. However, in water saturated binary solvents at the IL concentrations wIo < 0.6, nanoaggregates are formed (Figure 10) and their size with the decreasing IL concentration increases (Figures 8 and 10) despite the water concentration in

(2)

where A = −2.83, −2.60, and −2.46 and B = 2.99, 2.68, and 2.49 are empirical parameters for temperatures 298, 310, or 318 K, respectively. In the equilibrated organic phases, increasing wBA, S above 0.2 causes the viscosity decrease due to the increasing content of both BA and water bound to BA1 and not to IL (see Figure 5c). Viscosity of dried IL-104 at 298 K is 1023 mPa·s (Figure 6 and Tables 6 and 7) and it decreases to 142 mPa·s (Table 2), a 7.2-fold decrease, for water saturated IL. Similar behavior can 3032



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Figure 10. Distribution of aggregate sizes in water saturated ternary mixture (IL + dodecane + water): wIo = 0.3 (dark blue circle), 0.4 (light blue square), and 0.5 (orange pentagon) at 298 K. Figure 8. Mean hydrodynamic diameter of aggregates in water saturated ternary mixture (IL + dodecane + water) at 298 K. DLS measurements were evaluated by the DLS software as good (full symbols) or poor due to high polydispersity (open symbols). Shaded area designates the interval of wIo where three phases are formed ́ according to Rodriguez-Escontrela et al.11 (solid borders) and 12 Marták (dashed borders).

the second dodecane rich organic phase forms in the interval of IL concentrations from about 38 to 10 % in mass fraction at 298 K as shown in the phase diagram in Figure S13 in SI. Aggregates with the increasing size are formed at IL concentrations from the three phase region, Figure 8 and Figures S13 and S14 in SI. These aggregates are probably dodecane-rich reverse micelles, formation of which in fact extracts dodecane from the bulk of the IL-rich phase resulting in the increase of its viscosity, as shown in Figure 3b. With the increasing concentration of BA in the solvent (wBA, S) the size of aggregates (Figures 9a and 11) and viscosity (Figure 3b)

Figure 11. Distribution of aggregate sizes in quaternary organic phases from equilibrium extraction of BA by binary solvent (IL + dodecane) with wIo = 0.30. wBA, S = 0 (red circle), 0.033 (green circle), and 0.10 (dark blue circle). Temperature was 298 K.

decreases. DLS measurements in solvents with higher IL concentrations are doubtful, as discussed in Section 2, and their results are shown in Figures 8 and 9 only to provide approximate information. With the increasing temperature, the interval of IL concentrations at which a three phase system is formed decreases, Figure S14 in SI. Two sets of data were compared in this figure, data from ref 11 and data from Figure S13.12 In the equilibrium solvent phases from BA extraction, the formation of a second organic phase even at IL concentrations below 38% in mass fraction (298 K) was not observed and their viscosity was lower than for the corresponding water saturated ternary mixture (IL+dodecane+water) as follows from Figure 3b. From DLS measurements of the quaternary organic phases from equilibrium extraction of BA by binary solvent (IL + dodecane) follows that no large aggregates with the hydrodynamic diameter above 100 nm (Figures 9, 11, and 12), even

Figure 9. Mean hydrodynamic diameter of aggregates in quaternary or ternary organic phases from equilibrium extraction of BA by the binary solvents (IL + dodecane) with lower IL concentration, wIo = 0.3 (dark blue circle), 0.4 (light blue square), 0.5 (orange pentagon, ⬠) (a), and by the solvents with higher IL concentration, wIo = 0.6 (◊), 0.7 (☆), and 0.95 (undiluted IL, Δ) (b). Temperature 298 K. DLS measurements were evaluated by the DLS software as good (full symbols) or poor due to high polydispersity (open symbols).

the solvent decreases (Table 2). This can result from the demixing of the water saturated solvent (IL+dodecane) when 3033



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massive release of water replaced by the extracted BA and the fact that BA reduces the solvent viscosity less than water. For the BA concentration above 20% in mass fraction, in the solvent without a diluent the viscosity decreases due to the coextraction of BA with water. Viscosity of the solvent phases loaded with BA and containing a dodecane diluent is much lower than that of undiluted ILs and is nearly independent of the IL loading with BA, which is favorable from the technological point of view. Viscosity of the dry and water saturated solvents with 70% in mass fraction of IL is more than 26.3 and three times lower than that of the dry and water saturated undiluted IL, respectively. Viscosity of water saturated solvent with IL concentration below 50% in mass fraction (no BA) is markedly higher compared to that of solvents equilibrated with BA aqueous solutions. This is possibly due to the formation of dodecanerich micelles. At IL concentrations below 38% in mass fraction, the percolation limit is achieved and the micelles aggregate to form a separate dodecane-rich phase. No large aggregates with hydrodynamic diameter above 100 nm were observed in DLS measurements of quaternary solvent phases from BA extraction by binary solvent (IL + dodecane) even at high IL concentrations. Hydrodynamic diameter of the aggregates (reverse micelles) in the quaternary solvents from BA extraction decreases with the increasing BA concentration.

Figure 12. Distribution of aggregate sizes in quaternary organic phases from equilibrium extraction of BA by binary solvent (IL + dodecane) with wIo = 0.6, zBA = 2.3, wBA, S = 0.14 (green diamond); wIo = 0.7, zBA = 2.7, wBA, S = 0.17 (pink star); and wIo = 0.95, zBA = 2.3, wBA, S = 0.19 (red triangle). Temperature was 298 K.

at high IL concentrations (Figures 9b and 12) have been found. Such aggregates were observed especially in the dry binary solvents (IL+dodecane).6 This is reflected also in 1 order of magnitude lower viscosity of quaternary solvent phases from BA extraction compared to dry binary solvents. 3.4. Environmental and Application Considerations. All greater changes in industrial and energy resources consumption require the development of new separation processes and reactors. Transformation of raw material platform to biomass will require development of new processes to transform biomass to fuels, energy and chemicals in economically competitive way. The development of new separation and hybrid production-separation processes is required.13,14 Separation of relatively diluted solutions will be needed where application of new solvents based on ionic liquids may play significant role. It has been observed that the toxicity of organic substances to living systems increases with increasing number of carbon atoms in their alkyl chains.15 However, this observation is valid for alkyls with limited length, at most eight, possibly nine carbons. Substances with C10 and longer alkyls can be substantially less toxic than shorter ones. For example, decanol is much less toxic than octanol and similar effect has been found also for structural analogues of ammonium ILs with C8 and C10 alkyls in their cations.15 Since Cyphos IL-104 contains C14 alkyl and its solubility in water is very low (9.1 g.m−3)3 rather good environmental compatibility of this IL can be supposed. ILs have almost zero vapor pressure so that volatile solutes, e.g., BA, can be effectively recovered from them by short-path distillation. Compared to classical stripping with alkali solution, main advantage of this process is that the product is in the form of free acid and not its salt can be produced, and the consumption of chemicals is reduced.16

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00039. NMR spectra of conditioned and dried Cyphos IL-104; additional figures depicting data on quaternary, ternary, and binary solvent phases from BA extraction in equilibrium and binary mixtures at 310 and 318 K; phase diagram of ternary mixture (IL-104 + dodecane + water) at 298 and 308 K not published in the main article (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ján Marták: 0000-0003-0190-8099 Funding

Supports of the Slovak grant agencies under projects VEGA 1/ 0757/14 and APVV-15-0494 are acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Tibor Liptaj from the Central Laboratory for measuring and interpretation of NMR spectra. Authors are grateful to Cytec (CA) for kindly providing a sample of Cyphos IL-104.

4. CONCLUSIONS The measured densities of quaternary equilibrated solvent phases (IL + dodecane + BA + water) show the volume contraction. Viscosity of the equilibrated solvent phase in BA extraction markedly decreases with the increasing concentration of the dodecane diluent and with the increasing temperature. In the extraction by pure IL, viscosity of the equilibrium solvent phase increases to the maximum at the IL loading with BA of about 3 with the increasing BA concentration in the solvent from 0 to 20% in mass fraction. This is a consequence of the

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REFERENCES

(1) Marták, J.; Schlosser, Š. New Mechanism and Model of Butyric Acid Extraction by Phosphonium Ionic Liquid. J. Chem. Eng. Data 2016, 61, 2979−2996.

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(2) Seddon, K. R.; Stark, A.; Torres, M. J. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl. Chem. 2000, 72, 2275−2287. (3) Marták, J.; Schlosser, Š. Extraction of lactic acid by phosphonium ionic liquids. Sep. Purif. Technol. 2007, 57, 483−494. (4) Yoo, B.; Afzal, W.; Prausnitz, J. M. Effect of Water on the Densities and Viscosities of Some Ionic Liquids Containing a Phosphonium Cation. Z. Phys. Chem. 2013, 227, 157−165. (5) Awwad, N.; Liu, X. Y.; Afzal, W.; Prausnitz, J. M. Viscosities and Densities of Phosphonium-Based Ionic Liquids Mixed with Dodecane. Z. Phys. Chem. 2014, 228, 839−850. (6) Blahušiak, M.; Schlosser, Š. Physical properties of phosphonium ionic liquid and its mixtures with dodecane and water. J. Chem. Thermodyn. 2014, 72, 54−64. (7) Marták, J.; Schlosser, Š.; Vlčková, S. Pertraction of lactic acid through supported liquid membranes containing phosphonium ionic liquid. J. Membr. Sci. 2008, 318, 298−310. (8) Zhang, X. C.; Huo, F.; Liu, X. M.; Dong, K.; He, H. Y.; Yao, X. Q.; Zhang, S. J. Influence of Microstructure and Interaction on Viscosity of Ionic Liquids. Ind. Eng. Chem. Res. 2015, 54, 3505−3514. (9) Blahušiak, M.; Schlosser, Š.; Marták, J. Extraction of butyric acid with a solvent containing ammonium ionic liquid. Sep. Purif. Technol. 2013, 119, 102−111. (10) Kashin, A. S.; Galkin, K. I.; Khokhlova, E. A.; Ananikov, V. P. Direct Observation of Self-Organized Water-Containing Structures in the Liquid Phase and Their Influence on 5-(Hydroxymethyl)furfural Formation in Ionic Liquids. Angew. Chem., Int. Ed. 2016, 55, 2161− 2166. (11) Rodríguez-Escontrela, I.; Rodríguez-Palmeiro, I.; Rodríguez, O.; Arce, A.; Soto, A. Phase behavior of the surfactant ionic liquid trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate with water and dodecane. Colloids Surf., A 2015, 480, 50−59. (12) Marták, J. New Types of Solvents and their Application in Twoand Three-Phase Contactors. Ph.D. Thesis, Slovak University of Technology, November 2007. (13) Huang, H.-J.; Ramaswamy, S.; Tschirner, U. W.; Ramarao, B. V. A review of separation technologies in current and future biorefineries. Sep. Purif. Technol. 2008, 62, 1−21. (14) Schlosser, Š.; Blahušiak, M. Biorefinery for production of chemicals, energy and fuels. Elektroenergetika 2011, 4, 8−16. (15) Piotrowska, A.; Syguda, A.; Wyrwas, B.; Chrzanowski, L.; Heipieper, H. J. Toxicity evaluation of selected ammonium-based ionic liquid forms with MCPP and dicamba moieties on Pseudomonas putida. Chemosphere 2017, 167, 114−119. (16) Blahušiak, M.; Schlosser, Š.; Cvengroš, J. Simulation of a new regeneration process of solvents with ionic liquid by short-path distillation. Sep. Purif. Technol. 2012, 97, 186−194.

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