Skip to main content

Advertisement

SpringerLink
Log in

Deep eutectic solvents for deacidification of waste biodiesel feedstocks: an experimental study

  • Original Article
  • Published:
Biomass Conversion and Biorefinery Aims and scope Submit manuscript

Abstract

The key challenge in reaching the goals set by Renewable Energy Directive is developing a sustainable, profitable, and environmentally acceptable biodiesel production process. In order to achieve balance between the above criteria, any use of high-quality edible oils as feedstocks needs to be avoided and utilization of waste feedstocks, such as used coffee grounds and waste animal fats, should be encouraged. The main drawback of these waste feedstocks is their high impurity content which usually requires an additional purification step. It is the purpose of this research to investigate the deacidification of three different waste biodiesel feedstocks by means of liquid-liquid extraction with deep eutectic solvents, in order to identify possible connections between solvent properties and extraction efficiency. Eight deep eutectic solvents were chosen to cover a wide range of different properties and the three used feedstocks varied in free fatty acid content. The relationship between solvent properties and extraction efficiency was determined by Spearman’s rank-order correlation. Strong, statistically significant positive correlation was found for solvent pH values, while a strong negative correlation was observed for polarities and molar volumes. The most effective solvent was potassium carbonate/ethylene glycol (1:10, mol.). Depending on the initial total acid number, solvent to feedstock mass ratios 0.1:1 and 0.25:1 were enough to reduce the acidity of waste animal fat below 2 mg of potassium hydroxide/g fat and the solvent was successfully reused up to four times.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12

Similar content being viewed by others

References

  1. Abbott AP, Harris RC, Ryder KS (2007) Application of hole theory to define ionic liquids by their transport properties. J Phys Chem B 111:4910–4913. https://doi.org/10.1021/jp0671998

    Article  Google Scholar 

  2. Alexandri E, Ahmed R, Siddiqui H et al (2017) High resolution NMR spectroscopy as a structural and analytical tool for unsaturated lipids in solution. Molecules:22

  3. Atadashi IM, Aroua MK, Abdul Aziz AR, Sulaiman NMN (2013) The effects of catalysts in biodiesel production: a review. J Ind Eng Chem 19:14–26

    Article  Google Scholar 

  4. Bagh FSSG, Shahbaz K, Mjalli FS et al (2013) Electrical conductivity of ammonium and phosphonium based deep eutectic solvents: measurements and artificial intelligence-based prediction. Fluid Phase Equilib 356:30–37. https://doi.org/10.1016/j.fluid.2013.07.012

    Article  Google Scholar 

  5. Bhosle BM, Subramanian R (2005) New approaches in deacidification of edible oils—a review. J Food Eng 69:481–494. https://doi.org/10.1016/j.jfoodeng.2004.09.003

    Article  Google Scholar 

  6. Brauman JI, Blair LK (1968) Gas-phase acidities of alcohols. Effects of alkyl groups J Am Chem Soc 90:6561–6562. https://doi.org/10.1021/ja01025a083

    Article  Google Scholar 

  7. Brovč EV, Pajk S, Šink R, Mravljak J (2019) Comparison of the NMR and the acid value determination methods for quality control of input polysorbates. Acta Chim Slov 66:934–943. https://doi.org/10.17344/acsi.2019.5144

    Article  Google Scholar 

  8. Castejón D, Mateos-Aparicio I, Molero MD, Cambero MI, Herrera A (2014) Evaluation and optimization of the analysis of fatty acid types in edible oils by 1H-NMR. Food Anal Methods 7:1285–1297. https://doi.org/10.1007/s12161-013-9747-9

    Article  Google Scholar 

  9. Chen Y, Mu T (2019) Application of deep eutectic solvents in biomass pretreatment and conversion. Green Energy Environ 4:95–115. https://doi.org/10.1016/j.gee.2019.01.012

    Article  Google Scholar 

  10. Delaney SP, Nethercott MJ, Mays CJ, Winquist NT, Arthur D, Calahan JL, Sethi M, Pardue DS, Kim J, Amidon G, Munson EJ (2017) Characterization of synthesized and commercial forms of magnesium stearate using differential scanning calorimetry, thermogravimetric analysis, powder X-ray diffraction, and solid-state NMR spectroscopy. J Pharm Sci 106:338–347. https://doi.org/10.1016/j.xphs.2016.10.004

    Article  Google Scholar 

  11. Dimian AC, Kiss AA (2019) Eco-efficient processes for biodiesel production from waste lipids. J Clean Prod 239:118073. https://doi.org/10.1016/j.jclepro.2019.118073

    Article  Google Scholar 

  12. García G, Aparicio S, Ullah R, Atilhan M (2015) Deep eutectic solvents: physicochemical properties and gas separation applications. Energy Fuel 29:2616–2644. https://doi.org/10.1021/ef5028873

    Article  Google Scholar 

  13. Ghaedi H, Ayoub M, Sufian S, Shariff AM, Lal B, Wilfred CD (2018) Density and refractive index measurements of transition-temperature mixture (deep eutectic analogues) based on potassium carbonate with dual hydrogen bond donors for CO2capture. J Chem Thermodyn 118:147–158. https://doi.org/10.1016/j.jct.2017.11.008

    Article  Google Scholar 

  14. Guillén MD, Goicoechea E (2007) Detection of primary and secondary oxidation products by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (NMR) in sunflower oil during storage. J Agric Food Chem 55:10729–10736. https://doi.org/10.1021/jf071712c

    Article  Google Scholar 

  15. Guillén MD, Uriarte PS (2012) Study by 1H NMR spectroscopy of the evolution of extra virgin olive oil composition submitted to frying temperature in an industrial fryer for a prolonged period of time. Food Chem 134:162–172. https://doi.org/10.1016/j.foodchem.2012.02.083

    Article  Google Scholar 

  16. Gurdeniz G, Ozen B, Tokatli F (2010) Comparison of fatty acid profiles and mid-infrared spectral data for classification of olive oils. Eur J Lipid Sci Technol 112:218–226. https://doi.org/10.1002/ejlt.200800229

    Article  Google Scholar 

  17. Harris RC (2009) Physical properties of alcohol based deep eutectic solvents. Thesis Univ Leicester

  18. Hatzakis E, Agiomyrgianaki A, Kostidis S, Dais P (2011) High-resolution NMR spectroscopy: an alternative fast tool for qualitative and quantitative analysis of diacylglycerol (DAG) oil. JAOCS J Am Oil Chem Soc 88:1695–1708. https://doi.org/10.1007/s11746-011-1848-2

    Article  Google Scholar 

  19. Hayyan A, Mjalli FS, Alnashef IM et al (2012) Fruit sugar-based deep eutectic solvents and their physical properties. Thermochim Acta 541:70–75. https://doi.org/10.1016/j.tca.2012.04.030

    Article  Google Scholar 

  20. Ismail AA, van de Voort FR, Emo G, Sedman J (1993) Rapid quantitative determination of free fatty acids in fats and oils by Fourier transform infrared spectroscopy. J Am Oil Chem Soc 70:335–341. https://doi.org/10.1007/BF02552703

    Article  Google Scholar 

  21. Jain S (2019) The production of biodiesel using Karanja (Pongamia pinnata) and Jatropha (Jatropha curcas) oil. Biomass, Biopolymer-Based Materials, and Bioenergy. Elsevier, In, pp 397–408

    Google Scholar 

  22. Jeong KM, Ko J, Zhao J, Jin Y, Yoo DE, Han SY, Lee J (2017) Multi-functioning deep eutectic solvents as extraction and storage media for bioactive natural products that are readily applicable to cosmetic products. J Clean Prod 151:87–95. https://doi.org/10.1016/j.jclepro.2017.03.038

    Article  Google Scholar 

  23. Kato S, Shimizu N, Hanzawa Y et al (2018) Determination of triacylglycerol oxidation mechanisms in canola oil using liquid chromatography–tandem mass spectrometry. npj Sci Food 2:1. https://doi.org/10.1038/s41538-017-0009-x

    Article  Google Scholar 

  24. Kumar R, Bansal V, Tiwari AK, Sharma M, Puri SK, Patel MB, Sarpal AS (2011) Estimation of glycerides and free fatty acid in oils extracted from various seeds from the Indian region by NMR spectroscopy. JAOCS J Am Oil Chem Soc 88:1675–1685. https://doi.org/10.1007/s11746-011-1846-4

    Article  Google Scholar 

  25. Lankhorst PP, Chang AN (2018) The application of NMR in compositional and quantitative analysis of oils and lipids. In: Modern Magnetic Resonance. Springer International Publishing, pp 1743–1764

  26. Ma J, Liu J, Zhang Z, Han B (2012) Mechanisms of ethylene glycol carbonylation with carbon dioxide. Comput Theor Chem 992:103–109. https://doi.org/10.1016/j.comptc.2012.05.010

    Article  Google Scholar 

  27. Mamtani K, Shahbaz K, Farid MM (2021) Glycerolysis of free fatty acids: a review. Renew Sust Energ Rev 137:110501. https://doi.org/10.1016/j.rser.2020.110501

    Article  Google Scholar 

  28. Martínez-Yusta A, Guillén MD (2016) Monitoring compositional changes in sunflower oil-derived deep-frying media by 1H nuclear magnetic resonance. Eur J Lipid Sci Technol 118:984–996. https://doi.org/10.1002/ejlt.201500270

    Article  Google Scholar 

  29. Martínez-Yusta A, Goicoechea E, Guillén MD (2014) A review of thermo-oxidative degradation of food lipids studied by 1H NMR spectroscopy: influence of degradative conditions and food lipid nature. Compr Rev Food Sci Food Saf 13:838–859. https://doi.org/10.1111/1541-4337.12090

    Article  Google Scholar 

  30. Mjalli FS, Naser J, Jibril B, al-Hatmi SS, Gano ZS (2014) Ionic liquids analogues based on potassium carbonate. Thermochim Acta 575:135–143. https://doi.org/10.1016/j.tca.2013.10.028

    Article  Google Scholar 

  31. Naser J, Mjalli F, Jibril B, al-Hatmi S, Gano Z (2013) Potassium carbonate as a salt for deep eutectic solvents. Int J Chem Eng Appl 4:114–118. https://doi.org/10.7763/IJCEA.2013.V4.275

    Article  Google Scholar 

  32. Nieva-Echevarría B, Goicoechea E, Manzanos MJ, Guillén MD (2014) A method based on 1H NMR spectral data useful to evaluate the hydrolysis level in complex lipid mixtures. Food Res Int 66:379–387. https://doi.org/10.1016/j.foodres.2014.09.031

    Article  Google Scholar 

  33. Ogihara W, Aoyama T, Ohno H (2004) Polarity measurement for ionic liquids containing dissociable protons. Chem Lett 33:1414–1415. https://doi.org/10.1246/cl.2004.1414

    Article  Google Scholar 

  34. Park KS, Kim YJ (2019) Choe EK (2019) Composition characterization of fatty acid zinc salts by chromatographic and NMR spectroscopic analyses on their fatty acid methyl esters. J Anal Methods Chem 2019:1–11. https://doi.org/10.1155/2019/7594767

    Article  Google Scholar 

  35. Petračić A, Sander A, Cvetnić M (2019) A novel approach for the removal of trace elements from waste fats and oils. Sep Sci Technol 55:3487–3501. https://doi.org/10.1080/01496395.2019.1706575

    Article  Google Scholar 

  36. Popescu AM, Constantin V (2014) Synthesis, characterization and thermophysical properties of three neoteric solvents-ionic liquids based on choline chloride. Chem Res Chin Univ 30:119–124. https://doi.org/10.1007/s40242-014-3346-1

    Article  Google Scholar 

  37. Raba DN, Poiana MA, Borozan AB, Stef M, Radu F, Popa MV (2015) Investigation on crude and high-temperature heated coffee oil by ATR-FTIR spectroscopy along with antioxidant and antimicrobial properties. PLoS One 10:e0138080. https://doi.org/10.1371/journal.pone.0138080

    Article  Google Scholar 

  38. Reichardt C (1994) Solvatochromic dyes as solvent polarity indicators. Chem Rev 94:2319–2358. https://doi.org/10.1021/cr00032a005

    Article  Google Scholar 

  39. Sander A, Antonije Košćak M, Kosir D, Milosavljević N, Parlov Vuković J, Magić L (2018) The influence of animal fat type and purification conditions on biodiesel quality. Renew Energy 118:752–760. https://doi.org/10.1016/j.renene.2017.11.068

    Article  Google Scholar 

  40. Sander A, Petračić A, Vuković JP, Husinec L (2020) From coffee to biodiesel—deep eutectic solvents for feedstock and biodiesel purification. Separations 7. https://doi.org/10.3390/separations7020022

  41. Santacesaria E, Tesser R, Di Serio M et al (2007) Kinetics and mass transfer of free fatty acids esterification with methanol in a tubular packed bed reactor: a key pretreatment in biodiesel production. Ind Eng Chem Res 46:5113–5121. https://doi.org/10.1021/ie061642j

    Article  Google Scholar 

  42. Satyarthi JK, Srinivas D, Ratnasamy P (2009) Estimation of free fatty acid content in oils, fats, and biodiesel by 1H NMR spectroscopy. Energy Fuel 23:2273–2277. https://doi.org/10.1021/ef801011v

    Article  Google Scholar 

  43. Shahbaz K, Baroutian S, Mjalli FS, Hashim MA, AlNashef IM (2012) Densities of ammonium and phosphonium based deep eutectic solvents: prediction using artificial intelligence and group contribution techniques. Thermochim Acta 527:59–66. https://doi.org/10.1016/j.tca.2011.10.010

    Article  Google Scholar 

  44. Skulcova A, Russ A, Jablonsky M, Sima J (2018) The pH behavior of seventeen deep eutectic solvents. BioResources 13:5042–5051. https://doi.org/10.15376/biores.13.3.5042-5051

    Article  Google Scholar 

  45. Soderholm CG, Younker RS, Albrecht JJ, Wells GE (2015) Partial neutralization of free fatty acid mixtures with potassium, livestock feed compositions including them, and methods of making same. 1–14

  46. Suppes GJ, Bockwinkel K, Lucas S, Botts JB, Mason MH, Heppert JA (2001) Calcium carbonate catalyzed alcoholysis of fats and oils. JAOCS J Am Oil Chem Soc 78:139–145. https://doi.org/10.1007/s11746-001-0234-y

    Article  Google Scholar 

  47. Tang W, Row KH (2020) Design and evaluation of polarity controlled and recyclable deep eutectic solvent based biphasic system for the polarity driven extraction and separation of compounds. J Clean Prod 268:122306. https://doi.org/10.1016/j.jclepro.2020.122306

    Article  Google Scholar 

  48. Troter DZ, Todorović ZB, Dokić-Stojanović DR et al (2016) Application of ionic liquids and deep eutectic solvents in biodiesel production: a review. Renew Sust Energ Rev 61:473–500. https://doi.org/10.1016/j.rser.2016.04.011

    Article  Google Scholar 

  49. Vaisali C, Charanyaa S, Belur PD, Regupathi I (2015) Refining of edible oils: a critical appraisal of current and potential technologies. Int J Food Sci Technol 50:13–23. https://doi.org/10.1111/ijfs.12657

    Article  Google Scholar 

  50. Verevkin SP, Sazonova AY, Frolkova AK, Zaitsau DH, Prikhodko IV, Held C (2015) Separation performance of BioRenewable deep eutectic solvents. Ind Eng Chem Res 54:3498–3504. https://doi.org/10.1021/acs.iecr.5b00357

    Article  Google Scholar 

  51. Vuksanović JM, Todorović NM, Kijevčanin ML et al (2017) Experimental investigation and modeling of thermophysical and extraction properties of choline chloride + DL-malic acid based deep eutectic solvent. J Serbian Chem Soc 82:1287–1302. https://doi.org/10.2298/JSC170316054V

    Article  Google Scholar 

  52. Yadav A, Pandey S (2014) Densities and viscosities of (choline chloride + urea) deep eutectic solvent and its aqueous mixtures in the temperature range 293.15 K to 363.15 K. J Chem Eng Data 59:2221–2229. https://doi.org/10.1021/je5001796

    Article  Google Scholar 

  53. Yadav A, Trivedi S, Rai R, Pandey S (2014) Densities and dynamic viscosities of (choline chloride+glycerol) deep eutectic solvent and its aqueous mixtures in the temperature range (283.15-363.15)K. Fluid Phase Equilib 367:135–142. https://doi.org/10.1016/j.fluid.2014.01.028

    Article  Google Scholar 

  54. Yang CM, Grey AA, Archer MC, Bruce WR (1998) Rapid quantitation of thermal oxidation products in fats and oils by 1H-NMR spectroscopy. Nutr Cancer 30:64–68. https://doi.org/10.1080/01635589809514642

    Article  Google Scholar 

  55. Zahrina I, Nasikin M, Krisanti E, Mulia K (2018) Deacidification of palm oil using betaine monohydrate-based natural deep eutectic solvents. Food Chem 240:490–495. https://doi.org/10.1016/j.foodchem.2017.07.132

    Article  Google Scholar 

  56. Zahrina I, Mulia K, Yanuar A, Nasikin M (2018) Molecular interactions in the betaine monohydrate-polyol deep eutectic solvents: experimental and computational studies. J Mol Struct 1158:133–138. https://doi.org/10.1016/j.molstruc.2017.11.064

    Article  Google Scholar 

  57. Zahrina I, Nasikin M, Mulia K, Prajanto M, Yanuar A (2018) Molecular interactions between betaine monohydrate-glycerol deep eutectic solvents and palmitic acid: computational and experimental studies. J Mol Liq 251:28–34. https://doi.org/10.1016/j.molliq.2017.12.016

    Article  Google Scholar 

  58. Zhang Q, De Oliveira VK, Royer S, Jérôme F (2012) Deep eutectic solvents: syntheses, properties and applications. Chem Soc Rev 41:7108–7146. https://doi.org/10.1039/c2cs35178a

    Article  Google Scholar 

  59. Zullaikah S, Wibowo ND, Wahyudi IMGED, Rachimoellah M (2019) Deacidification of crude rice bran oil (Crbo) to remove ffa and preserve γ-oryzanol using deep eutectic solvents (des). Materials Science Forum. Trans Tech Publications Ltd, In, pp 109–114

    Google Scholar 

Download references

Availability of data and material

Not applicable

Code availability

Not applicable

Funding

This research was funded under the auspices of the European Regional Development Fund, Operational Programme Competitiveness and Cohesion 2014-2020, [project number KK.01.1.1.04.0070].

Author information

Authors and Affiliations

  1. Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000, Zagreb, Croatia

    Ana Petračić & Aleksandra Sander

  2. INA – Industrija nafte d.d., Lovinčićeva bb, 10000, Zagreb, Croatia

    Jelena Parlov Vuković

Authors
  1. Ana Petračić
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Aleksandra Sander
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Jelena Parlov Vuković
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Aleksandra Sander.

Ethics declarations

Ethics approval and consent to participate

Not applicable

Consent for publication

Not applicable

Conflict of interest

The authors declare that they have no conflict of interest.

Additional information

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Appendix

Appendix

Table 5 Viscosity, conductivity, pH, and refractive index model parameters
Full size table

1.1 Viscosity of DESs

The influence of temperature on the DESs’ viscosity is presented in Fig. 13a; (Standard uncertainties u are: for low viscosity DESs, 6.4·10−6 Pas ≤ u(η) ≤ 3.3·10−5 Pas; for high viscosity DESs, 0.1 Pas ≤ u(η) ≤ 1.5 Pas). At room temperature, viscosity of DESs is considerably higher than viscosity of water and organic solvents most frequently used in separation processes. Viscosity is significantly reduced at higher temperatures. This is very important for possible commercial application of this type of solvents. As mentioned previously, low viscosity is one of the most important requirements that a solvent has to fulfil. Lower viscosity means better hydrodynamic conditions for dispersion and lower heat and mass transfer resistances. On the other hand, the most important advantage of liquid–liquid extraction lies in its mild operating conditions (room temperature and atmospheric pressure). However, DESs exhibit other favorable properties, like non-volatility and solvation of different types of compounds, so conducting experiments at moderately increased temperature can be acceptable. Moreover, when feedstock is in solid state, like waste animal fats, the process must be conducted at an elevated temperature.

Fig. 13
figure 13

The influence of temperature on properties of selected DESs. a Dynamic viscosity. b Conductivity. c pH. d Refractive index

Full size image

In general, based on the experimentally obtained viscosities, selected DESs can be divided in two groups: low viscosity DESs (DES 4, DES 5, DES 6, and DES 8) and high viscosity DESs (DES 1, DES 2, DES 3, and DES 7) [12]. Potassium carbonate-based DESs are more viscous than choline chloride-based DESs, when the same HBD is used. Even though molar ratios are not the same, the obtained results can be compared, since DES viscosity is reduced by the increase of glycerol amount [31]. Similar values for viscosity of DES7 were measured by Mjalli et al. [30] and for DES 6 by Popescu and Constantin [36]. When DESs with the same salt and different HBD are compared, like DES 4 and DES 5, or DES 7 and DES 8, it can be concluded that DESs with less viscous HBD have lower viscosity (viscosity of ethylene glycol is significantly lower than viscosity of glycerol). Sugar-based DES has the highest viscosity, and it is followed by acid-based DESs. Therefore, the type of salt and HBD highly influence the viscosity of DES. From the viscosity point of view, it may be expected that DESs with ethylene glycol (DES 5 and DES 8) will have the lowest resistances to momentum, heat, and mass transfer. Low viscosity also means that these solvents can be easily dispersed so they can provide large specific surface.

The viscosity of the selected DESs was fitted with an Arrhenius type equation:

$$ \eta ={\eta}_0\cdot \exp \left({E}_{\eta }/ RT\right) $$
(5)

where η is the dynamic viscosity in Pas, η0 is the pre-exponential constant in Pas, Eη is the activation energy in J/mol, R is the gas constant in J/Kmol, and T is the temperature in K. Evaluated model parameters are given in Table 5. In general, activation energy follows the viscosity trend: higher viscosity means higher activation energy resulting from the stronger intermolecular forces in the DES [12].

1.2 Conductivity of DESs

The influence of temperature on the selected DESs’ conductivities is given in Fig. 13b; (Standard uncertainties u is: 1.0·10−3 S/m ≤ u(κ) ≤ 8.3·10−3 S/m). Conductivity is increased at higher temperatures due to the increased ion mobility at elevated temperatures. Electrical conductivity is strongly linked with viscosity, so the two groups of DESs mentioned in the previous section can also be observed. High viscosity DESs exhibit low conductivity. Both properties have been explained by the structure of DES and the hole theory [1]. As in the case of viscosity, salt and HBD influence the conductivity of DESs. The conductivity of choline chloride-based DESs is higher than in the case of potassium carbonate-based DESs with the same HBD. This can be attributed to higher HBD concentration in potassium carbonate-based DESs and consequently strong influence of ions and their interactions which resulted in lower overall mobility of charge carriers [4]. Formation of ion pairs or triplets and their aggregation reduce the number of free charge carriers thus lowering the conductivity. The structure of HBD influences the conductivity; the more complex the structure is, the lower conductivity is exhibited by DES, as can be seen for DES 1, DES 2, and DES 3. High conductivity is favorable, since it means that DES components are of high mobility. The higher the ionic mobility is, the better hydrodynamic conditions can be achieved and consequently higher mass transfer rates can be accomplished.

The conductivity of the selected DESs was correlated with an Arrhenius type equation:

$$ \kappa ={\kappa}_0\cdot \exp \left({E}_{\kappa }/ RT\right) $$
(6)

where κ is the conductivity in S/m, κ0 is the pre-exponential constant in S/m, Eκ is the activation energy in J/mol, R is the gas constant in J/Kmol, and T is the temperature in K. Evaluated model parameters are given in Table 5. Activation energy is higher for low conductivity DESs.

1.3 pH value of DESs

Based on the pH values of the prepared DESs, it can be seen that almost the entire scale of pH values is covered, Fig. 13c; (Standard uncertainties u is: 0.023 ≤ u(pH) ≤ 0.150). As expected, the type of salt and HBD strongly influence the pH value of DES [31, 58]. Acid HBDs (citric acid and malic acid) with choline chloride form highly acidic DESs (DES 1 and DES 2). Sugar-based DES is neutral, as reported previously by Hayyan et al., while alcohol-based DESs are neutral to slightly acidic [19]. Lower pH value of DES with ethylene glycol in comparison to DES with glycerol was also observed by Skulcova et al. [44]. Alcohols are acidic in nature due to their acidic hydrogen atom. According to Brauman and Blair, smaller ions are better stabilized by solvation, since the smaller alcoxide ion has a shorter radius of solvation, and consequently larger solvation energy which overcomes the stabilization that results from polarization of the charge [6]. Owing to slightly basic nature of urea, DES 6 is also slightly basic. DES 7 and DES 8 are basic due to the basicity of the salt. From the obtained results, it is obvious that both HBD and salt influence pH value of DESs.

Temperature slightly influences pH value of DESs. For highly acidic DESs, pH value is increased by increasing temperature to a small extent. For other examined DESs, pH value is slightly reduced with increasing temperature. The dependence of pH value on temperature was correlated with linear function:

$$ pH=\mathrm{a}\cdot T+\mathrm{b} $$
(7)

Model parameters are given in Table 5. For the majority of DESs, extremely low correlation was obtained even though experimentally obtained data followed the linear trend. Experimental data scattering can also be observed for previously published data [19]. The pH value of DES 1 was very low at all tested temperatures, so it is possible that low correlation of measured data resulted from high acidity.

1.4 Refractive index of DESs

Refractive index of a solvent enables indirect measurement of concentration if the calibration curve is known. For a given compound, its value depends on concentration and temperature.

The influence of temperature on the examined DESs is presented in Fig. 13d; (Standard uncertainty u is: 3.3·10−5 ≤ u(nD) ≤ 3.7·10−4). At higher temperatures, refractive index is reduced. As for other measured properties, the type of HBD and HBA influence the refractive index of DES. The effect of HBD can be established by comparison of DESs based on the same salt. For choline chloride-based DESs with the same molar ratio, DES 4 and DES 6, solvent with urea as HBD exhibits higher refractive index, since the refractive index of urea is higher. Similarly, for DESs based on potassium carbonate, the refractive index of solvent with glycerol is higher, due to the higher refractive index of glycerol in comparison to ethylene glycol. Even though salt/HBD molar ratio in DES 7 and DES 8 is not the same, refractive indices for potassium carbonate/glycerol with molar ratio 1:10 range from 1.4850 to 1.4771 in the temperature range from 293 to 333 K [13] and these values are higher than for DES 8. The lowest values were obtained for DESs with ethylene glycol as HBD. The effect of salt on refractive index can be analyzed by comparing refractive indices of DES 4 to DES 7, and DES 5 to DES 8. Having in mind that choline chloride-based DESs have lower concentration of HBD than potassium carbonate-based DESs prepared in this article and published data for potassium carbonate DESs [13], it can be concluded that choline chloride-based DESs have lower refractive indices. The dependence of refractive index on temperature was correlated with linear function:

$$ {n}_D=\mathrm{a}\cdot T+\mathrm{b} $$
(8)

Evaluated model parameters are given in Table 5.

1.5 Polarity of DESs

The polarity of a solvent strongly influences its ability to dissolve solutes. Measured polarities of selected DESs and some selected solvents are presented in Fig. 14. It was not possible to measure the polarities of DES 3 and DES 7, since these two solvents were insoluble in reagent solution. The polarities of DESs are slightly lower than water. The highest ET(30) were obtained for acidic DESs (solvents with malic and citric acid: DES 1 and DES 2), followed by choline chloride-based DESs with glycerol, urea, and ethylene glycol, while the lowest polarity was observed for DES 8, based on potassium carbonate. The polarity of DES is usually explained by hydrogen bonding ability of HBDs. Generally speaking, the polarity of functional groups increases in the following order: amide > acid > alcohol. DES 2 exhibits lower polarity than DES 1 due to the higher number of carbons in carboxylic acid. The other possible explanation is that smaller dicarboxylic acid forms stronger hydrogen bonds with choline chloride and in dimers than tricarboxylic acid. Besides that, molar ratio HBA/HBD is higher in DES 2, resulting in formation of a quite different hydrogen bond network. Hydrogen bonds are divided between one molecule of HBD and two molecules of HBA. Glycerol is more polar than ethylene glycol due to the additional hydroxyl group, so DES 4 exhibits higher polarity than DES 5. The polarity of DES with urea as HBD is between DES 4 and DES 5. Even though polarity of amide group is higher, interactions between HBA and HBD are different. Glycerol has three hydroxyl groups so it can form stronger bonds than urea. All HBDs, as well as choline chloride, can accept and donate hydrogen bonds, so it is not surprising that DES 8 has the lowest polarity, since potassium carbonate can only accept hydrogen bonds on three oxygens.

Fig. 14
figure 14

Molar transition energy of examined DESs

Full size image

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Petračić, A., Sander, A. & Vuković, J.P. Deep eutectic solvents for deacidification of waste biodiesel feedstocks: an experimental study. Biomass Conv. Bioref. 12 (Suppl 1), 3–23 (2022). https://doi.org/10.1007/s13399-021-01511-z

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s13399-021-01511-z

Keywords

Search

Navigation