The Search for Hydrophobic Deep Eutectic Solvents From Natural Materials.

[NNadir]

The paper I'll discuss in this post is this one: A Search for Natural Hydrophobic Deep Eutectic Solvents Based on Natural Components

Increasingly the quality of water supplies around the world is being degraded by waste materials represented not only by industrial practices, but also by fecal and agricultural waste products, as well as the results of this contamination, represented by eutrophic blooms like those that have destroyed the Mississippi Delta and are responsible for disasters like the microsystis blooms that left water supplies in Lake Erie highly toxic in 2015.

Potentially, one of the least energy intensive procedures for the removal of toxins from water is solvent extraction - with the important caveat the solvent in question not be toxic itself and not be very soluble in water, that is what chemists call "hydrophobic."

Also it is desirable to have such solvents for industrial processes, for example, the recovery of valuable materials from used nuclear fuels in order to provide sustainable energy. From my perspective, as a student and advocate of nuclear fuel reprocessing, the industrial process that has been in use for over 50 years - albeit with many modifications and tweaks - the PUREX process, depends on kerosene, an unsustainable product of the dangerous fossil fuel industry that is not sustainable. Therefore if solvent extraction continues to be used for nuclear fuel reprocessing, at the very least, sustainable hydrophobic solvents must be utilized. (I'm not really a fan of solvent extraction of actinides, but if we use solvent, we should do everything possible to divorce them from dangerous fossil fuels.)

In addition, to the extent that we can utilize carbon based materials without leaching them into the planetary atmosphere - our favorite waste dump as of 2019 - they are sequestered. If we use products obtained from atmospheric (or seawater or fresh water) carbon dioxide, we have removed carbon dioxide.

This is why this paper caught my eye, the caveat being that it is a lab scale process and is nowhere near pilot or industrial scale. (This is not a "we're saved" post.)

From the introduction to the paper:

In the near future, conventional solvents should be replaced by designer solvents to obey the 12 principles of Green Chemistry, introduced by Anastas and Warner.(1) In 2003 a class of designer solvents, called deep eutectic solvents (DESs), were reported that could obey these principles of Green Chemistry. The first DESs reported in the literature were composed of combinations of amides and choline chloride.(2) DESs consist of two or more components that liquify upon contact, which most likely is caused by entropy of mixing, hydrogen bonding and van der Waals interactions.(3,4) These physical interactions are supposed to induce a dramatic decrease in the melting temperature of the mixture, as opposed to the melting temperature of the pure components, by stabilizing the liquid configuration, inducing a liquid phase at room temperature.

DES research initially focused on hydrophilic DESs. In 2015 hydrophobic DESs were reported in the literature for the first time.(5,6) These were tested for the extraction of volatile fatty acids (VFAs) and biomolecules, such as caffeine and vanillin, from an aquatic environment.(5,6) Although the field of hydrophobic DESs is new, already quite some papers about their use were published. These include the removal of metal ions,(7?9) furfural and hydroxymethylfurfural by the use of membrane technology,(10) and pesticides from H2O.(11) Furthermore, hydrophobic DESs showed their potential for the capture of gases (CO2),(12,13) and their use for microextractions was investigated.(14,15) Moreover, the extraction of components from leaves using hydrophobic DESs was studied.(16,17)

The hydrophobic DESs currently presented in the literature are promising, especially application-wise, but several improvements are needed. The first improvement is the use of more natural components. In our first investigation on hydrophobic DESs quaternary ammonium salts were used,(6) that from an environmental point of view are not the best. It is the idea to overcome this by the use of natural components, terpenes. However, in the future also more detailed investigations on their sustainability and toxicity should be addressed with specific methods as stated in the literature,(18?20) even as these DESs based on natural components are generally accepted as environmentally friendly.(21,22)

Another improvement that we would like to introduce is testing the sustainability of these solvents from a chemical engineering point of view. If a hydrophobic solvent is too viscous or the density difference with water is too small phase separation will be difficult, which results in high energy demands. For ease of processability, the viscosity should be as low as possible, while the difference of the density between the DES and water should be as large as possible because a density difference enhances the macroscopic phase separation process to a large degree...

A eutectic is, of course, a mixture of two compounds that have a lower melting point than either of the pure compounds: The most familiar such eutectic is salt water, which is why we dump salt on our roads to maintain our car CULTure during ice and snow storms.

There are many eutectics known of various types. It's a fascinating area of study, and I love reading about eutectics.

A "deep eutectic" is a eutectic that has a temperature roughly at or considerably lower than "room temperature," 25C, while other eutectics exist at higher temperatures.

For example a eutectic forms between neptunium and plutonium, which melts at 570C when compared to the melting points of the pure metals, 638C for pure plutonium, and 640C for pure neptunium. Plutonium and iron form a eutectic that melts at 428 C, compared to the melting point of pure iron, which is 1538 C.

We could list thousands of examples of eutectics.

The eutectics here are all organic compounds, and all, more or less, are natural products or can be obtained from natural products in a facile fashion.

From the text:

The following components were used as DES constituents in this work: decanoic acid (DecA), dodecanoic acid (DodE), menthol (Men), thymol (Thy), 1-tetradecanol (1-tdc), 1,2-decanediol (1,2-dcd), 1-10-decanediol (1,10-dcd), cholesterol (Chol), trans-1,2-cyclohexanediol (1,2-chd), 1-napthol (1-Nap), atropine (Atr), tyramine (Tyr), tryptamine (tryp), lidocaine (Lid), cyclohexanecarboxyaldehyde (Chcd), caffeine (Caf) and coumarin (Cou). Some components were used as hydrogen bond donors (HBDs), while others were used as hydrogen bond acceptors (HBAs). A few of these components can both donate and accept hydrogen bonds. In the literature some of the combinations with lidocaine were previously presented in the literature as eutectic mixtures.(23?25) More recently a debate has started on the definition of DESs, specifically on the deepness in melting point depression, and models were developed for predicting their phase diagram.(26?32) Because there are still debates on the definition of DESs in the literature, for now we consider all the presented combinations as DESs.

For the purposes of their experiments, they evaluated the extraction of riboflavin (Vitamin B12) from water - a difficult extraction - using various deep eutectic solvents prepared from these compounds.

After rejecting some possible deep eutectics prepared from this list because they tended to crystallize on storage, some promising systems were evaluated for thermal stability.



The caption:

Figure 1. Thermograms of the DESs Men:Lid (2:1), DecA:Men (1:2), 1-tdc:Men (1:2), 1,2-dcd:Thy (1:2) and DecA:Men (1:1). The x-axis shows an increase in temperature [K], while the y-axis shows the loss in weight [%].

The caption:

Figure 2. Thermograms of the DESs Thy:Men (1:2), Thy:Men (1:1), Thy:Cou (1:1), Thy:Cou (2:1) The x-axis shows an increase in temperature [K], while the y-axis shows the loss in weight [%].

(The boiling point of water is 373K.)

The proton nuclear magnetic resonance (NMR) spectrum of one deep eutectic:




The caption:

Figure 3. 1H NMR of the DES Thy:Cou in a 2:1 molar ratio

A remark: Thymol is a natural product that is responsible for the pleasant taste and odor of thyme. It is worth noting that it is similar in structure to the dangerous fossil fuel derivative cumene, inasmuch as it is an isopropyl benzene, which is utilized industrially to make phenol and acetone (nail polish remover). It is certainly possible to synthesize thymol from dangerous fossil fuels, but this would be defeating the purpose of banning dangerous fossil fuels, even if it would reduce their overall toxicity. I am certainly no expert in thymol sourcing, but it's doubtful it could be obtained sustainably from thyme, since if you've ever grown a thyme plant, they're not all that bulky. A better route to thymol might, however be from the digestion and processing of lignins, the "other" constituent of wood (and grain plant stalks) besides cellulose.

This however, is research, not industrial practice.

The 13C NMR of the same DES:



The caption:

Figure 4. 13C NMR of the DES Thy:Cou in a 2:1 molar ratio.

The criteria for the viability of these deep eutectics is that they show low solubility in water, and that they have reasonable viscosities.

While some exhibit low solubility in water, water is not entirely insoluble in them. Thus it is important to understand whether they react with water and are degraded in the process. This is important for their sustainability with respect to reuse.

The following spectra shows that in this case, they are not:



The caption:

Figure 5. 1H NMR of the DES Thy:Cou in a 2:1 molar ratio after mixing with H2O.

The caption:

Figure 5. 1H NMR of the DES Thy:Cou in a 2:1 molar ratio after mixing with H2O.

The caption:

Figure 6. 13C NMR of the DES Thy:Cou in a 2:1 molar ratio after mixing with H2O.

A number of other physical traits are examined in the paper to identify promising mixtures.

From the conclusion of the paper:

In this work a series of new, hydrophobic DESs based on natural components were reported. From 507 combinations of two solid components, 17 became a liquid at room temperature, which were further assessed for their sustainability via four criteria. These criteria are based on the use of the hydrophobic DESs as extractants and include a viscosity below 100 mPa·s, a density that should be rather different than the density of the water phase (50 kg·m–3) a limited pH change of the water phase upon mixing with water and a low amount of DES that transfers to the water phase.

More than 10 DESs follow the viscosity criterion below 100 mPa·s. Regarding the density, the criterion was set at a density difference between the DES and water as large as possible (? ? 50 kg·m–3).

The hydrophobic DESs Deca:Men (1:1), DecA:Men (1:2), Men:Lid (2:1), Thy:Cou (2:1), Thy:Men (1:1), Thy:Cou (1:1), Thy:Men (1:2) and 1-tdc:Men (1:2) satisfy this criterion.

Furthermore, the criterion of a limited pH change (between 6 and 8) of the water phase coexisting with the DES showed that the hydrophobic DESs DecA:Lid (2:1), DecA:Atr (2:1), Thy:Cou (2:1), Thy:Men (1:1), Thy:Cou (1:1), Thy:Men (1:2), 1-tdc:Men (1:2) and Atr:Thy (1:2) have a negligible pH change. The amount of organics that transfers to the water phase was comparable for all developed hydrophobic DESs, except for DecA:Lid (2:1), DecA:Atr (2:1) and Atr:Thy (1:2), which had considerably higher TOC values.

In general, the newly developed DESs Thy:Cou (2:1), Thy:Men (1:1), Thy:Cou (1:1), Thy:Men (1:2) and 1-tdc:Men (1:2) satisfied all four criteria. Therefore, these hydrophobic DESs may be considered as relatively sustainable, hydrophobic designer solvents. These DESs were used for the removal of riboflavin from an aqueous environment. All new hydrophobic DESs showed moderate to high extraction yields. The highest extraction efficiency of riboflavin, 81.1%, was achieved with the hydrophobic DES DecA:Lid (2:1).

Cool paper I think.

I hope you're having a pleasant Sunday afternoon.

[EarnestPutz]

1. The last thing I read about "eutectic salts" was about their use in...

....heat storage for solar farms.

[NNadir]

2. Oh. Would that be from Amory Lovins' 1976 announcement that we'd all have solar storage...

...tanks with molten salts in our suburban backyards by the year from 2000?

Industrial and residential solar energy remains, as of 2019, trivial, with the possible exception of the use of a technology called "windows." This late in the game, it's not even worth discussing, although many such discussions, albeit of little worth, take place anyway.

As for salts:

There are literally hundreds of thousands of scientific publications on low melting salts, and only a tiny fraction of them are involved with the solar schemes that have been spectacularly unsuccessful at addressing climate change, despite endless and frankly, delusional, pop enthusiasm for them.

Of these hundreds of thousands of scientific papers about "molten salts," but not all of them are formally eutectics.

Many of these refer to the modern organic or partially organic salts known as "ionic liquids," a topic I follow quite closely, in particularly for their use in metal extraction and electrochemistry. Many of these are hydrophobic but conduct electricity quite well anyway.

[EarnestPutz]

3. Oh. Delusional. Got it. Thanks.

Is that Eleanor Roosevelt's picture? She was a nice lady.

[NNadir]

4. Yes.

On both scores.

Belief in the "solar will save us" scheme is delusional. As I noted below, this morning, after the expenditures of trillion dollar quantities on solar energy schemes, we set a new record for dangerous fossil fuel waste concentrations in the atmosphere, over 412 ppm.

New Record Weekly High For CO2 Measurements at Mauna Loa.

That may not be popular, but it is reality. I'm very much into reality.

The picture is indeed of Ms. Roosevelt, who I regard as the greatest Democrat ever, because of her concern for marginalized people, which she summarized, while working as the US UN ambassador, by pushing through the Universal Declaration of Human Rights in 1948.

Universal Declaration of Human Rights

Regrettably the declaration is honored more in breach than in practice, but like Thomas Jefferson's "All men are created equal" concept, of which he himself was a notable scoffer in practice, I believe, as Lincoln believed of Jefferson's statement, it is an ideal to which humanity should aspire.

Ms. Roosevelt, unlike Mr. Jefferson, lived her ideals. She was a freedom rider in Mississippi during the early 1960's, shortly before she died, an old woman at the time.

We will never see the like again.