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Article

New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction

by
Denitsa Ivanova
1,
Apostol Apostolov
1,
Pencho Tuleshkov
2,
Christo Novakov
2,* and
Dragomir Yankov
1,*
1
Institute of Chemical Engineering, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
2
Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(7), 3564; https://doi.org/10.3390/app15073564
Submission received: 14 February 2025 / Revised: 18 March 2025 / Accepted: 19 March 2025 / Published: 25 March 2025
(This article belongs to the Special Issue Novel Extraction Methods and Applications)
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Abstract

:
In recent years, deep eutectic solvents (DESs) have attracted a lot of attention as a substitute for the current toxic organic solvents and can be applied in many chemical processes such as extraction and synthesis. The development of new deep eutectic solvents for use in the isolation of valuable biologically active substances with significant benefits for health, the environment, and others is being investigated with increasing scientific interest. Deep eutectic solvents were prepared using menthol as a hydrogen bond donor and different tertiary amines as hydrogen bond acceptors by varying the ratio of the two constituents. The DESs obtained were analyzed using densitometry, viscosimetry, IR, TGA, and DSC. The potential of the DESs for extraction and re-extraction was evaluated with a water solution of lactic acid. All the DESs obtained are suitable for the extraction of lactic acid. Deep eutectic solvents based on menthol and dioctyl amine (M/DOA 2:1), trioctyl amine (M/TOA 2:1), tridodecyl amine (M/TDDA 1:2), and trihexyl amine (M/THA 2:1) show highest results.

1. Introduction

In recent years, the demand for lactic acid (LA) has increased considerably as a consequence of the increasing use of LA in the production of polylactic acid and green solvents. The major problem in fermentative LA production is in product separation from fermentation broth and its purification. The classical approach for LA separation is the neutralization of the broth with an alkali reagent, usually Ca(OH)2 [1]. Precipitated calcium lactate undergoes additional treatment with sulphuric acid to obtain free LA, which is further subjected to purification. The process is complicated and does not assure the necessary purity. Other recovery techniques like membrane separation [2,3], ion exchange [4], liquid–liquid extraction [5,6,7], and aqueous two-phase systems [8] also have been applied for LA separation and purification. Among them, liquid–liquid extraction seems to be the most attractive because it is simple, low energy consuming, easily scaled up, and gives the possibility of in situ operation. Various tertiary amines or quaternary ammonium salts are preferred as extractants because of their selectivity and efficiency [9,10,11]. For increasing the solubility of the acid–amine complex and adjusting the physical properties of the extraction phase, the extractant is usually mixed with a diluent and/or modifier. The main drawback of using long-chain amine-based extractants is that they are harmful and toxic to the microorganisms usually used for fermentation. Deep eutectic solvents (DESs) represent a relatively new group of chemicals with very promising properties to be used as a substitute for high molecular weight aliphatic amines or quaternary ammonium salts in the extraction of organic acids. DES offers many properties, making them an excellent alternative to traditional extractants like low price, ease of preparation, low toxicity, biocompatibility and biodegradability, and possibility of tuning their properties by changing one or both components. The first publication describing the synthesis and properties of such compounds and their characteristic of having lower freezing points than their constituent compounds appeared in 2001 [12]. Later on, a huge number of papers described the synthesis and properties of various DES. The term DES was first introduced by Abbot et al. [13] describing the compounds formed between choline chloride and carboxylic acids. Now, for the characterization of DES, the following definition is accepted—a mixture of Lewis and Brønsted acids and bases that significantly reduce the freezing point compared with those of the components [14]. DESs are composed of two or more substances—one acting as a hydrogen bond acceptor (HBA) and the other as a hydrogen bond donor (HBD). Various tertiary amines or quaternary ammonium salts are most widely used as HBA and different alcohols, glycols, sugars, or carboxylic acid as HBD. Some chemicals, like menthol, thymol, and some organic acids, may serve as HBA or HBD depending on the second component. DES has broad applications in various fields [15]—metal processing (electrodeposition, electropolishing, and extraction), synthesis, extraction of valuable compounds, etc. In recent years, the number of papers devoted to the extraction of various compounds with DES have constantly increased. Different DESs were used for the extraction of natural bioactive compounds [16,17,18], phenolic compounds [19,20,21,22], carotenoids [23], essential oils [24], podophyllotoxins [25], antioxidants [26], etc. Recently, the interest in the application of DESs as extractants, diluents, or modifiers for the extraction of low molecular weight organic acids considerably increased. Different DESs were used in the separation of formic [27], acetic [28], citric [29], phthalic [30], lactic [31,32], nicotinic [33], palmitic [34,35], and various carboxylic acids [36,37]. Xu et al. [38] proposed an original use of DES for the separation of lactic acid—the in situ forming of DES using different lactams as HBA and LA as HBD.
The present work aims to synthesize and characterize new deep eutectic solvents formed between some aliphatic amines and menthol and to study and determine their ability in extraction and stripping of lactic acid

2. Materials and Methods

2.1. DES Preparation

Deep eutectic solvents (DESs) were prepared using menthol as a hydrogen bond donor and different secondary and tertiary amines as hydrogen bond acceptors by varying the ratio of the two constituents. All the used chemicals are listed in Table 1.
The solvents are prepared by stirring constantly with a magnetic stirrer and at a constant temperature of 75 °C for 4 h. The DESs obtained were analyzed using densitometry, viscosimetry, IR, TGA, DSC, and liquid chromatography.
Various eutectic mixtures obtained in this study using menthol as a hydrogen bond acceptor and the set of secondary and tertiary amines of aliphatic hydrocarbon of different chain lengths as donors are listed in Table 2.

2.2. Fourier Transform Infrared Spectroscopy (FTIR)

Measurements were carried out using a Bruker Vector 22 apparatus (Ettlingen, Germany) spectrometer at room temperature with a wavenumber resolution of 1 cm−1 in the frequency range of 4000–400 cm−1. The silicon substrate coated with a liquid sample was made, which was then placed in the FTIR sample holder to be analyzed directly without dilution in KBr or a solvent. In order to achieve an acceptable resolution, for each sample, 32 scans were recorded at a spectral resolution of 4 cm−1, and five replica spectra were collected to evaluate reproducibility (OPUS v.6.5.9.2.). The instrumental error of the method is of the order of ±2 cm−1 (along X axis) and 1% of the measured absorbance (along Y axis). The smallest IR shifts in the analyzed DES, commented, are of the order of 15–20 cm−1 above the accuracy of the apparatus. A background spectrum was obtained for each experimental condition to eliminate the effect of the silicon surface.

2.3. Thermal Properties

Differential scanning calorimetry (DSC, TA Instrument Model DSC 120, New Castle, DE, USA) and thermogravimetric analysis (TGA, Perkin Elmer TGA 4000 analyzer, Waltham, MA, USA) were used to explore the thermal properties of the prepared eutectic mixtures. For TGA analyses, 10–15 mg of samples were heated in a range of 30–600 °C with a heating rate of 10 °C.min−1.
The liquid–solid phase transitions investigated by DSC were determined in a temperature range of −100°C–60 °C with a heating and cooling rate of 10 °C.min. Dry argon as a purge gas with a flow rate of 50 mL/min and 310 mL/min was used for cell and cooler purge, resp., to prevent condensation in the furnace. Indium and sapphire were used as standards for the calibration of the apparatus. The samples (10–15 mg) were transferred into the aluminum pans and hermetically sealed to prevent vaporization. The changes in the thermograms obtained by TGA and DSC methods are of the order of tens of degrees compared to the inaccuracy of the respective equipment (±1 °C along X axis and ±0.5% of the measured mass along Y axis for TGA and 0.5 °C along X axis and 0.5% of the measured absorption/release energy for DSC, respectively).

2.4. Density and Viscosity Measurements

The density of each obtained DES sample was determined using Density Meter Excellence D4, Mettler-Toledo GmbH, Greifensee, Switzerland (accuracy–0.0001 g/sm3), and the viscosity was measured with rotational viscometer Reotest 2 with coaxial cylinders, VEB MLW Prüfgeräte-Werk Medingen, Sitz Freital, Dresden, Germany. The spindle used was N, giving the possibility to measure viscosity in the range of 1–20,000 cP in position I with an accuracy of 0.05%.

2.5. Extraction and Stripping of LA

With the formed DES, experiments were performed for extraction with an aqueous solution of lactic acid and stripping with sodium hydroxide solutions. The working solutions of lactic acid (10 g/L) were prepared from 90% L (+)-lactic acid. Because of the presence of dimers of the acid in concentrated lactic acid solutions (about 25% of the total concentration), a tenfold diluted solution was boiled under reflux for 8–10 h for dimers hydrolysis. The resulting solution, containing 100–120 g/L lactic acid, was used for the preparation of aqueous phases for the extraction studies. The experiments were carried out in 50 mL separatory funnels. Equal volumes (15 mL) of aqueous phase containing lactic acid and organic phase (DES) were shaken for 15 min at ambient temperature on the shaking machine IKA HS501 Digital (IKA Labortechnique, Wilmington, NC, USA). The extraction experiments were duplicated. Before the extraction experiments, DESs were contacted with distilled water (15 min) for equilibration, and after phase separation were used in the extraction and stripping experiments. After the stripping experiments, DESs were washed with 15 mL of distilled water before further use.

2.6. Lactic Acid Analysis

The concentration of LA in water phases after extraction and re-extraction was analyzed with an HPLC system composed of pump Smartline S-100, Knauer, refractometric detector—Perkin-Elmer LC-25RI, the column used was Aminex HPX-87H, Biorad (Hercules, CA, USA), 300 × 7.8 mm and specialized software EuroChom, v. 3.05 (Knauer, Berlin, Germany). As mobile phase, 0.1 N H2SO4 was used at flow 0,6 mL/min. Crystalline LA was used for the preparation of the standard solution. All the measurements were taken in triplicate. The standard deviation was 0.06 g/L for extraction and 0.64 g/L for striping. The concentration of lactic acid in the organic phase ([CLA]org) was calculated by subtracting the LA concentration in the water phase after extraction ([CLA]aq) from the initial LA concentration (C0). For comparison of the extraction ability of various DES, distribution coefficient (K) and extraction efficiency (E%) were calculated as follows:
K = C L A o r g C L A a q
E ( % ) = C 0 − [ C L A ] a q C 0 × 100

3. Results and Discussion

3.1. Fourier Transform Infrared Spectrophotometry (FTIR)

The formation of eutectic mixtures upon the direct mixing of the components in the liquid state is often attributed to intermolecular interactions leading to the formation of hydrogen bonds [39,40,41]. In order to clarify the interaction of menthol with amine upon their mixing, we conducted a series of FTIR experiments with the starting reagents—menthol and each of the amines—and then with their mixtures at different molar ratios.
The spectra of eutectic mixtures of menthol with the specific amine do not differ significantly. As an example, Figure 1 presents the overlaid spectra of menthol, TOA, and DES obtained at their different molar ratios.
In the FTIR spectrum of the menthol (blue line), the representative band corresponding to the hydroxyl group, at about 3250 cm−1, can be observed. In the region from 2800 cm−1 to about 2990 cm−1, stretching bands corresponding to aliphatic C-H groups are located, and below 1500 cm−1, C-H band stretching vibrations are placed. In the spectrum of TOA (red), at 1470 cm−1 CH2 and at 1380 cm−1 CH3 bending vibrations are located. At 1099 cm−1, a C-N stretch band can be observed. In the spectra of all the menthol/amine eutectic mixtures, the stretching vibrations of the O-H group are shifted significantly to 3340 cm−1 which can be attributed to the formation of hydrogen bond between the menthol hydroxyl group and the electron pair of the amino group of TOA acting as hydrogen bond donor. On the other hand, it is known that upon H-bond formation, C-H bending vibrations usually shift to higher frequencies. A similar effect is observed here, judging by a small but noticeable band shift from 1450 cm−1 to 1465 cm−1. In addition, the C-N stretching band at 1099 cm−1 slightly shifts to 1078 cm−1 and its intensity becomes weaker in the FTIR spectra of eutectic mixtures which also supports the assumption of the formation of complex compounds of menthol and amine due to the H-bonding of their functional groups.
The FTIR spectra of the M/TOA eutectic mixtures after extraction and subsequent re-extraction are shown in Figure 2. The overlapped spectra of the eutectic mixture, extract, and re-extract samples in the marked area around 3200–3300 cm−1 as well as in the inset scale-up region show no changes which testify to the stability of the newly formed complex compounds.

3.2. Thermogravimetric Analysis (TGA)

TGA is employed to study the primary reactions of the heated mixtures and to quantify their degradation. Traditionally, when TGA is performed, the weight loss due to the formation of thermal products is plotted as a function of temperature. The typical decomposition profile is plotted in Figure 3.
Figure 3a depicts the temperature dependence curves of the decomposition of the starting components—menthol, corresponding amine (DOA (a)), (TOA (b)), and of the eutectic mixtures before and after extraction and re-extraction.
The TGA curves of the thermal decomposition of menthol and amines differ significantly. As reported by other authors in the analysis of various eutectic mixtures, the latter show an increased resistance to thermal decomposition compared to that of the individual components [42]. The thermal durability is increased upon conversion to DES as both the inflection point and the endpoint of the thermograms are increased [29]. While the process of degradation of menthol starts as low as about 100 °C and ends at about 170 °C, the decomposition of amines starts over 200 °C and completes at least at 300 °C (DOA) or at much higher temperatures (TOA—300 °C; THA and TDDA—460 °C). The Tdeg and degree of degradation of eutectic mixtures are shown in Table 3.
Analyzing the thermal curves of menthol/amine eutectic mixtures, it is obvious that the process of their degradation proceeds at much higher temperatures compared to menthol degradation but lower than amine degradation. The mixtures of menthol and DOA or THA decompose gradually, which supports the assumption of the formation of menthol/amine complexes in their eutectic mixtures. The noticeable degradation in stages is observed during the process of the thermal degradation of mixtures of menthol, TOA, and TDDA (see Table 3—first step decomposition degree). It should be emphasized, however, that the decomposition temperature of the products in this first stage is higher than that of pure menthol, which implies the absence of unreacted menthol in mixtures with amine. The formation of more than one type of complex with the specified amines is also plausible. The observed differences in the degrees of degradation depending on the type of amine support this assumption that the number and length of the hydrocarbon chains affect the accessibility to the donor center and the manner of implementation of the hydrogen bonds in the complexes.
Last but not least, it should be pointed out that the degradation of extracted and re-extracted samples follows a similar course of thermal degradation, albeit at slightly lower temperatures, as seen from the data in Table 4.
Similarly to the thermal behavior of starting mixtures, the two-stage thermal decomposition of eutectic mixtures formed from menthol and TOA or TDDA is observed, and therefore, more than one type of menthol/amine complex is expected to be formed in extracted and re-extracted eutectic mixtures. The increased content of these products implies an occurrence of the process of partial disintegration of the complex resulting from the formation of hydrogen bonds between the donor (amine) and the acceptor (menthol) or the transformation of the type of complexes.

3.3. Differential Scanning Calorimetry (DSC)

In order to confirm the eutectic nature of the menthol/amine mixtures, the melting point of the formulated mixtures was determined by recording the DSC measurements. The solid−liquid phase transition was measured from −90 °C to 50 °C with a heating and cooling rate of 10 °C/min.
The selected DSC traces of the menthol: amine(s) (DOA and TDDA) eutectics are shown in Figure 4. The mass fraction of menthol in the mixture with DOA increases as the data are viewed from top to bottom. Menthol, DOA, and TDDA show clear endothermic peaks in the region of 35–42 °C [43,44], 11–12 °C [45], and 15–16 °C [46], respectively, as their melting temperatures. The thermograms of the various mixtures clearly indicate the formation of binary (or even triple) eutectic mixtures. Multiple thermal transition peaks are expected while heating a mixture of menthol and a suitable conformer to obtain a eutectic solvent. On the other hand, eutectic formation, irrespective of the stoichiometry, is a process accompanied by heat generation. In general, the thermal behavior of the systems studied is marked by transitions between crystalline and amorphous phases which in turn are dependent on the composition and reflect the complex molecular ordering that the systems can adopt [47]. In each of them, the melting points are significantly lower than that of menthol or amine used and hence, qualify the definition of eutectic solvent. The transitions that occur at temperatures that are significantly lower than those observed for the pure menthol and amine compounds suggest the strong interactions that exist between the components. Upon heating, the pre-melting appears sequentially as a broad convolution of several endothermic peaks between about −25 °C to 5 °C, followed up by the final melting from about 8 °C to 22 °C depending on the type of amine and its quantity in the eutectic mixture.
During cooling from the liquid state at 50 °C down to −90 °C, mixtures exhibit a sharp single crystallization (in the case of M/DOA 2:1) at −15.73 °C in the case of M/DOA 2:1 or more than one peak (M/DOA 1:1, 1:2 or M/TDDA 1:1, respectively) occurring at a temperature range from 8.81 °C to −20.46 °C.
The suggested formation of eutectic mixtures as a result of the hydrogen bonding between the hydroxyl group in menthol and amino group in amine proposed in the discussion of the FTIR results is also consistent with the DSC results. It can also be noted the relatively smaller decrease in the value of the melting temperature of the mixture of menthol and TDDA, where the larger number and longer chain of hydrocarbon substituents in the amine obviously hinder the formation of hydrogen bonds, thus reducing their strength due to spatially difficult access to the donor center.
The prediction in the TGA part of the discussion formation of more than one type of complex with the specified amines is also plausible. The observed differences in the DSC thermograms depending on the type of amine and its quantitative content in the eutectic mixture support the assumption that the number and length of the hydrocarbon chains affect the accessibility to the donor center and the manner of implementation and strength of the hydrogen bonds formed in the complexes.

3.4. Density and Viscosity Measurements

In general, the density of the DES decreased with increasing the temperature from 20 to 60 °C for all the studied DES. The increase in the menthol quantity in the composition of DES led to an increase in the density, while an increase in the amine content led to a decrease in the density. Figure 5 shows changes in the density with temperature for M/DOA and M/TDDA as examples.
As can be seen in Figure 5a, there is a small increase in the density values of the used DESs (after extraction and stripping). This increase could be attributed to the presence of a small amount of LA after stripping and washing.
The results from the experimental viscosity measurements of M/DOA DES are presented in Figure 6. Increasing the menthol content led to an increase in the viscosity, while the increase in the DOA content led to a decrease in the viscosity.

3.5. Extraction and Stripping of Lactic Acid

Tests with all the synthesized DESs were performed to determine their ability to extract LA from an aqueous solution. A total of 15 mL of DES was mixed with aqueous LA solution (10 g/L) in a ratio of 1:1. After phase separation, the concentration in the water phase was measured, and this, in the organic phase, was calculated as a difference between the initial and final LA concentration in the aqueous phase. The values of the extraction efficiency and distribution coefficient were calculated for all 12 DESs and the results obtained are presented in Figure 7.
As can be seen from the figure, the extraction efficiency varied from 43% (for M/THA 1:2) to 86% (for M/DOA 1:2). In general, the best results were achieved with the DES formed by DOA and menthol—from 82 to 86% depending on the ratio. Other DES also showed efficiency over 80%—M/TOA 2:1 and M/THA 2:1. The values of the distribution coefficient varied from 0.8 to 6.2 depending on the amine and component ratio. Best values (about six) were achieved with M/DOA 2:1 and M/THA 2:1. Most values of K are between 1.5 and 4.5. These values are close to those reported by Kyuchoukov and Yankov [9] for LA extraction with different long-chain tertiary amines. Demmelmayer et al. [31] reported extraction efficiency of LA of about 30%, 70% for acetic acid, and 90% for oxalic acid when using thymol–menthol-based DES as a modifier for acid extraction from sweet sorghum silage press juice with TOA. Similar results were reported also for other extractants like Aliquat, TOPO, and TBP for the same system [32]. Şahin and Kurtulbaş [28] reported an increase of up to four times in the extraction of acetic acid with DES composed of glycerol and quaternary ammonium salt. An extraction efficiency of over 90% was achieved by Toprakçı et al. [48] in the extraction of 2,4-dichlorophenoxyacetic acid with DES menthol–formic acid as a solvent. Rivero et al. [37] investigated the extraction of adipic, levulinic, and succinic acids with DES based on TOPO. However, the extraction with pure TOPO was better. Xu et al. [38], applying a new approach for the in situ formation of DES, achieved 99% separation of LA from fermentation broth. Lalikoglu [27] used DES formed by menthol and nonanoic acid, decanoic acid, and dodecanoic acid for formic acid extraction. When using DES, the extraction efficiency is about 10–13%, but using DES as diluents for TOA the efficiency increased to 90%. Baş et al. [29] investigated the extraction of citric with DES formed of menthol and TBP. Physical extraction with the studied DES lead to an extraction efficiency of about 38%, while E% increased to 90% when DES were used as a solvent for TOA. Liu et al. [36] studied the extraction of lactic, acetic, and succinic acids with DES formed by amides and geraniol. The values of the distribution coefficient varied from 0.38 to 2.85 depending on DES composition and extracted acid, while extraction efficiency changed between 40 and 80% for LA, 60 and 90% for AA, and 75 and 95% for SA. The DES can be recycled and used in about 15 cycles. A simulation of a continuous extractive column showed about 99% of the LA extraction yield using amide-based hydrophobic DESs (volume ratio of 2.0 and nine stages). In another paper [49] the authors reported similar results for the extraction of lactic malic and tartaric acids with the same DES. Van Osch et al. [50] investigated various DES formed by decanoic acid and quaternary ammonium slats for the extraction of acetic, propionic, and butyric acids. The extraction efficiencies of the acids that the studied hydrophobic DESs perform better than extraction with TOA. The values of E% varied between 25 and 38% for AA, 45 and 76% for PA, and 74 and 92% for BA. The extraction efficiencies increase with the increasing chain length of the quaternary ammonium salt. Aşçı and Lalikoglu [51] investigated different DESs composed of TOPO and menthol for the extraction of seven carboxylic acids. The distribution coefficient varied from 0.19 and 3.76 and extraction efficiency from about 16 to 79%. The best results (3.76 and 79%) were obtained for propionic acid. The values of lactic acid are 0.48 and 32.4, respectively. Gautam and Datta [33] used DES formed of menthol and tri octyl phosphine oxide for the extraction of nicotinic acid. The highest distribution coefficient of 7.8 and extraction efficiency of 88.33% were observed with 0.01 M acid concentration. From the presented data it is clearly seen that the results obtained with DES menthol and DOA are among the best reported in the literature for the extraction of lactic acid with DES.
The stripping of loaded DES was performed with NaOH. The efficiency of striping was between 70 and 95%. The second strip can separate additionally only about 1–2%. The best results for stripping are presented in Table 5. Liu et al. [36] reported stripping efficiency between 35 and 75% for lactic, acetic, and succinic acid with DES formed by amides and geraniol.

3.6. Consecutive Extraction of Lactic Acid with M/DOA 1:2 DES

In order to investigate the stability of the used DES, five consecutive cycles of extraction and stripping were performed with DES M/DOA 2:1. The same portion of DES was used for extraction after stripping and washing. The results are presented in Figure 8.
It is seen from the figure that the DES is stable, and the distribution coefficient increases from the first to the fourth cycle. It can be explained with additional sites for LA molecule attachment from already extracted acid. The possible mechanism should be like the formation of extractant–acid complexes with more than one acid molecule in the ordinary extraction with aliphatic amines. The decrease in the value of the distribution coefficient for the fifth cycle can be attributed to exhausting the possible sites for extraction, already occupied by the unextracted acid, remaining in the DES after stripping. The values of extraction efficiency are between 85 and 90%, almost identical to that obtained in the single extraction. Gautam and Datta [33] reported using DES for five successive cycles without regeneration and further regeneration with 1 N NaOH.

4. Conclusions

Twelve DES composed of menthol and secondary and tertiary amines were synthesized and characterized to be used for the lactic acid extraction from an aqueous solution. The best results were obtained with DES formed by menthol and DOA in a 2:1 ratio—distribution coefficient of 6.2 and extraction efficiency of 86%. The one loaded with LA DES can be successfully stripped with NaOH. The obtained DESs are stable and can be used in at least five consecutive cycles of extraction and stripping without changes in efficacy. The results obtained are among the best reported for LA extraction employing different DESs. These preliminary results are a good base for the further investigation of using DES in the extraction of lactic acid from real fermentation broth.

Author Contributions

Conceptualization, D.Y. and C.N.; methodology, D.Y. and C.N.; software, P.T.; validation, D.Y. and C.N.; formal analysis, D.I., A.A. and P.T.; investigation, D.I., A.A. and P.T.; resources, D.Y.; data curation, P.T.; writing—original draft preparation, D.Y. and C.N.; writing—review and editing, D.Y., C.N. and P.T.; visualization, D.I. and D.Y.; supervision, D.Y.; project administration, D.Y. and C.N.; funding acquisition, D.Y. and C.N. All authors have read and agreed to the published version of the manuscript.

Funding

Research equipment of Distributed Research Infrastructures INFRAMAT, part of Bulgarian National Roadmap for Research Infrastructures, supported by Bulgarian Ministry of Education and Science under Grant Agreement DO1-322/30.11.2023 and of “Energy storage and hydrogen energetics (ESHER)”, approved by DCM No 354/29.08.2017 under Grant Agreement DO1-349/13.12.2023 was used in this investigation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 03564 g001
Figure 1. A plot of the transmittance vs. wave numbers of the menthol (blue), TOA (red), and M/TOA eutectic mixtures at 2:1 (green), 1:1 (magenta), and 1:2 (cyan) molar ratio. The inset shows a scale-up in the region below 1150 cm−1.
Figure 1. A plot of the transmittance vs. wave numbers of the menthol (blue), TOA (red), and M/TOA eutectic mixtures at 2:1 (green), 1:1 (magenta), and 1:2 (cyan) molar ratio. The inset shows a scale-up in the region below 1150 cm−1.
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Figure 2. FTIR spectra of the M/TOA eutectic mixtures (blue), after extraction (red) and re-extraction (magenta). The inset shows a scale-up in the region below 1500 cm−1.
Figure 2. FTIR spectra of the M/TOA eutectic mixtures (blue), after extraction (red) and re-extraction (magenta). The inset shows a scale-up in the region below 1500 cm−1.
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Figure 3. TGA decomposition profile of compounds (gray—menthol, blue—amine (DOA (a); TOA (b))), eutectic mixture (red), after extraction (green), and after re-extraction (black).
Figure 3. TGA decomposition profile of compounds (gray—menthol, blue—amine (DOA (a); TOA (b))), eutectic mixture (red), after extraction (green), and after re-extraction (black).
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Figure 4. DSC thermograms of menthol/amine: (a)—M/DOA 1:2, (b)—M/DOA 1:1, (c)—M/DOA 2:1, and (d) M/TDDA 1:1 mixtures.
Figure 4. DSC thermograms of menthol/amine: (a)—M/DOA 1:2, (b)—M/DOA 1:1, (c)—M/DOA 2:1, and (d) M/TDDA 1:1 mixtures.
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Figure 5. Densities of M/DOA (a) and M/TDDA (b) DES as a function of temperature.
Figure 5. Densities of M/DOA (a) and M/TDDA (b) DES as a function of temperature.
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Figure 6. Experimental viscosity measurements of M:DOA DES.
Figure 6. Experimental viscosity measurements of M:DOA DES.
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Figure 7. LA extraction with DES: (a) extraction efficiency; (b) distribution coefficient.
Figure 7. LA extraction with DES: (a) extraction efficiency; (b) distribution coefficient.
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Figure 8. Distribution coefficient (a) and extraction efficiency (b) during consecutive extraction and stripping cycles.
Figure 8. Distribution coefficient (a) and extraction efficiency (b) during consecutive extraction and stripping cycles.
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Table 1. List of chemicals used in the experiments.
Table 1. List of chemicals used in the experiments.
NameAbbreviationSupplierPurity
L (+) Lactic acidLAAcros Organics, Oslo, Norway≥90%
Crystalline L-(+) Lactic acidLAThermo Scientific, Waltham, MA, USA≥98%
L-MentholMSigma-Aldrich, Burlington, MA, USA≥99%
Dioctyl amineDOAFluka, Buchs, Switzerland≥97%
Trihexyl amineTHAJanssen Chemica, Beerse, Belgium≥95%
Trioctyl amineTOAThermo Scientific, Waltham, MA, USA≥95%
Tridodecyl amineTDDAMerck, Rahway, NJ, USA≥95%
Table 2. Eutectic mixtures and molar ratio of menthol and amines.
Table 2. Eutectic mixtures and molar ratio of menthol and amines.
Mixture ComponentsMolar Ratio
M/DOA1:1, 1:2, 2:1
M/TOA1:1, 1:2, 2:1
M/THA1:1, 1:2, 2:1
M/TDDA1:1, 1:2, 2:1
Table 3. Thermal properties of compounds and some eutectic mixtures.
Table 3. Thermal properties of compounds and some eutectic mixtures.
Pure CompoundT °CDESTdeg (°C)First Step Decomposition Degree (%)
L-Menthol175M/DOA 1:1280-
DOA305M/DOA 1:2290-
TOA365M/TOA 1:236026.5%
THA465M/THA 1:1275-
TDDA470M/THA 1:2290-
M/TDDA 1:146030%
M/TDDA 1:246018%
Table 4. Tdeg and decomposition degree after extraction and re-extraction.
Table 4. Tdeg and decomposition degree after extraction and re-extraction.
Eutectic Mixtures (Ratio)First Step Tdeg (°C)Decomposition Degree (%)Second Step Tdeg (°C)Decomposition Degree (%)
M/DOA 1:2 ex--275100
M/DOA 1:2 re-ex--275100
M/TOA 1:1 ex2205034050
M/TOA 1:1 re-ex2205034050
M/THA 1:1 ex--275100
M/THA 1:1 re-ex--275100
M/TDDA 1:2 ex2304145059
Table 5. Striping of DES loaded with LA.
Table 5. Striping of DES loaded with LA.
DESRatioFirst Stripping, %First and Second Stripping, %
M/DOA2:19597
M/THA1:28686
M/THA2:19495
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Ivanova, D.; Apostolov, A.; Tuleshkov, P.; Novakov, C.; Yankov, D. New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction. Appl. Sci. 2025, 15, 3564. https://doi.org/10.3390/app15073564

AMA Style

Ivanova D, Apostolov A, Tuleshkov P, Novakov C, Yankov D. New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction. Applied Sciences. 2025; 15(7):3564. https://doi.org/10.3390/app15073564

Chicago/Turabian Style

Ivanova, Denitsa, Apostol Apostolov, Pencho Tuleshkov, Christo Novakov, and Dragomir Yankov. 2025. "New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction" Applied Sciences 15, no. 7: 3564. https://doi.org/10.3390/app15073564

APA Style

Ivanova, D., Apostolov, A., Tuleshkov, P., Novakov, C., & Yankov, D. (2025). New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction. Applied Sciences, 15(7), 3564. https://doi.org/10.3390/app15073564

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