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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Chem.</journal-id>
<journal-title>Frontiers in Chemistry</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Chem.</abbrev-journal-title>
<issn pub-type="epub">2296-2646</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">864663</article-id>
<article-id pub-id-type="doi">10.3389/fchem.2022.864663</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Chemistry</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Carbon Dioxide Solubility in Nonionic Deep Eutectic Solvents Containing Phenolic Alcohols</article-title>
<alt-title alt-title-type="left-running-head">Alhadid et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">CO<sub>2</sub> Solubility in Phenolic DES</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Alhadid</surname>
<given-names>Ahmad</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<xref ref-type="fn" rid="fn1">
<sup>&#x2020;</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1655664/overview"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Safarov</surname>
<given-names>Javid</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<xref ref-type="fn" rid="fn1">
<sup>&#x2020;</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Mokrushina</surname>
<given-names>Liudmila</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>M&#xfc;ller</surname>
<given-names>Karsten</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Minceva</surname>
<given-names>Mirjana</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1295523/overview"/>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>Biothermodynamics, TUM School of Life Sciences</institution>, <institution>Technical University of Munich (TUM)</institution>, <addr-line>Freising</addr-line>, <country>Germany</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>Institute of Technical Thermodynamics</institution>, <institution>University of Rostock</institution>, <addr-line>Rostock</addr-line>, <country>Germany</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>Institute of Separation Science and Technology</institution>, <institution>Friedrich-Alexander-Universit&#xe4;t Erlangen-N&#xfc;rnberg (FAU)</institution>, <addr-line>Erlangen</addr-line>, <country>Germany</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/299618/overview">Manoj B. Gawande</ext-link>, Palacky University Olomouc, Czechia</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1089467/overview">Andrea Mezzetta</ext-link>, University of Pisa, Italy</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/650616/overview">Alessandro Triolo</ext-link>, Italian National Research Council, Italy</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Ahmad Alhadid, <email>ahmad.alhadid@tum.de</email>; Javid Safarov, <email>javid.safarov@uni-rostock.de</email>
</corresp>
<fn fn-type="equal" id="fn1">
<label>
<sup>&#x2020;</sup>
</label>
<p>These authors have contributed equally to this work and share first authorship</p>
</fn>
<fn fn-type="other">
<p>This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>22</day>
<month>03</month>
<year>2022</year>
</pub-date>
<pub-date pub-type="collection">
<year>2022</year>
</pub-date>
<volume>10</volume>
<elocation-id>864663</elocation-id>
<history>
<date date-type="received">
<day>28</day>
<month>01</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>08</day>
<month>03</month>
<year>2022</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2022 Alhadid, Safarov, Mokrushina, M&#xfc;ller and Minceva.</copyright-statement>
<copyright-year>2022</copyright-year>
<copyright-holder>Alhadid, Safarov, Mokrushina, M&#xfc;ller and Minceva</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<abstract>
<p>Deep eutectic solvents (DES) are a new class of green solvents that have shown unique properties in several process applications. This study evaluates nonionic DES containing phenolic alcohols as solvents for carbon dioxide (CO<sub>2</sub>) capture applications. Potential phenolic alcohols and the molar ratio between DES constituents were preselected for experimental investigations based on the conductor-like screening model for realistic solvation (COSMO-RS). CO<sub>2</sub> solubility was experimentally determined in two different DES, namely, L-menthol/thymol in 1:2 molar ratio and thymol/2,6-xylenol in 1:1 molar ratio, at various temperatures and pressures. CO<sub>2</sub> solubility in the studied systems was higher than that reported in the literature for ionic DES and ionic liquids. This study demonstrates that nonionic DES containing phenolic alcohols can be excellent, inexpensive, and simple solvents for CO<sub>2</sub> capture.</p>
</abstract>
<kwd-group>
<kwd>CO<sub>2</sub> capture</kwd>
<kwd>COSMO-RS</kwd>
<kwd>ionic liquids</kwd>
<kwd>hydrophobic deep eutectic solvents</kwd>
<kwd>green solvents</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>An increasing relevance has developed in removing carbon dioxide (CO<sub>2</sub>) from gas mixtures, such as flue gas or biogas. Absorption in liquid solvents, in addition to membrane and absorption-based processes, is a major technology in this field. For many years, aqueous solutions of amines have been used for CO<sub>2</sub> absorption. However, they suffer from some drawbacks, such as high vapor pressure, which causes evaporation during solvent regeneration. Recently, ionic liquids (IL) have drawn attention for CO<sub>2</sub> capture application because of their negligible vapor pressure (<xref ref-type="bibr" rid="B1">Albo et&#x20;al., 2010</xref>; <xref ref-type="bibr" rid="B31">Shiflett et&#x20;al., 2010</xref>). Furthermore, the properties of IL can be tailored to the requirements of the specific application by combining different cations and anions (<xref ref-type="bibr" rid="B15">Jork et&#x20;al., 2005</xref>). Although the hygroscopicity of IL can be overcome by using polymerized IL to prepare membranes for CO<sub>2</sub> capture applications (<xref ref-type="bibr" rid="B17">Kammakakam et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B12">Galiano et&#x20;al., 2021</xref>), the issues of IL cost, instability, and purity remain (<xref ref-type="bibr" rid="B34">Sowmiah et&#x20;al., 2009</xref>).</p>
<p>Nevertheless, owing to the aforementioned problems of IL, researchers have focused on an alternative solvent class that has some similarities to IL, while avoiding some of their drawbacks. Deep eutectic solvents (DES) are eutectic mixtures with a large depression in the eutectic temperature obtained by mixing a hydrogen bond acceptor and donor. DES are a new class of designer solvents, which can also be prepared by simple mixing of natural and nontoxic components, usually referred to as Natural DES (NADES) (<xref ref-type="bibr" rid="B32">Smith et&#x20;al., 2014</xref>; <xref ref-type="bibr" rid="B38">van Osch et&#x20;al., 2020</xref>). Similar to IL, physicochemical properties of DES can be tuned by selecting their constituents and additionally molar ratios of those. Moreover, DES are easier and less expensive to prepare compared to IL. Therefore, more attention is directed to their use in several process applications, for example, in liquid&#x2013;liquid chromatography (<xref ref-type="bibr" rid="B28">Roehrer et&#x20;al., 2016</xref>; <xref ref-type="bibr" rid="B7">Bezold and Minceva, 2019</xref>; <xref ref-type="bibr" rid="B8">Cai and Qiu, 2019</xref>), extraction of bioactive compounds (<xref ref-type="bibr" rid="B16">Kalhor and Ghandi, 2019</xref>; <xref ref-type="bibr" rid="B22">Mako&#x15b; et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B23">Perna et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B27">Rodr&#xed;guez-Llorente et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B11">Fern&#xe1;ndez et&#x20;al., 2022</xref>), and crystallization (<xref ref-type="bibr" rid="B10">Emami and Shayanfar, 2020</xref>; <xref ref-type="bibr" rid="B13">Hall et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B14">Hamilton et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B24">Potticary et&#x20;al., 2020</xref>).</p>
<p>CO<sub>2</sub> capture is one application that can benefit from DES. Existing studies have demonstrated the potential of DES for CO<sub>2</sub> capture (<xref ref-type="bibr" rid="B20">Krishnan et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B33">Song et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B41">Wazeer et&#x20;al., 2021a</xref>; <xref ref-type="bibr" rid="B42">Wazeer et&#x20;al., 2021b</xref>). However, most studies investigating CO<sub>2</sub> capture in DES proposed using ionic DES, which has the same drawbacks as those related to IL, especially in terms of hygroscopicity. Recently, hydrophobic DES based on natural and inexpensive nonionic constituents has attracted much attention (<xref ref-type="bibr" rid="B37">van Osch et&#x20;al., 2019</xref>). Hydrophobic DES containing L-menthol possess outstanding properties, such as low viscosity and eutectic temperatures, especially when L-menthol is mixed with phenolic alcohols, such as thymol or carvacrol (<xref ref-type="bibr" rid="B3">Alhadid et&#x20;al., 2020a</xref>; <xref ref-type="bibr" rid="B4">Alhadid et&#x20;al., 2020b</xref>; <xref ref-type="bibr" rid="B2">Alhadid et&#x20;al., 2021a</xref>; <xref ref-type="bibr" rid="B5">Alhadid et&#x20;al., 2021b</xref>). Phenolic IL are good solvents for CO<sub>2</sub> capture applications (<xref ref-type="bibr" rid="B36">Vafaeezadeh et&#x20;al., 2015</xref>). Therefore, hydrophobic DES containing phenolic alcohols are assumed to be promising candidates for CO<sub>2</sub> capture applications.</p>
<p>Nevertheless, the large pool of substances that can form DES can make selecting the DES constituents challenging. Furthermore, the ratio between the constituents can be tuned, which is an additional degree of freedom during the selection of DES constituents. Therefore, a predictive screening method could noticeably assist in preselecting DES constituents for CO<sub>2</sub> capture applications. The conductor-like screening model for realistic solvation (COSMO-RS) is a predictive thermodynamic model based on quantum mechanics and statistical thermodynamics (<xref ref-type="bibr" rid="B18">Klamt, 1995</xref>; <xref ref-type="bibr" rid="B19">Klamt et&#x20;al., 1998</xref>; <xref ref-type="bibr" rid="B9">Eckert and Klamt, 2002</xref>). COSMO-RS successfully provides qualitative predictions for screening IL and ionic DES for gas capture applications (<xref ref-type="bibr" rid="B40">V&#xf6;lkl et&#x20;al., 2012</xref>; <xref ref-type="bibr" rid="B33">Song et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B21">Liu et&#x20;al., 2021</xref>; <xref ref-type="bibr" rid="B26">Qin et&#x20;al., 2021</xref>).</p>
<p>This study investigates nonionic DES containing phenolic alcohols as potential solvents for CO<sub>2</sub> capture. COSMO-RS was used to screen a list of possible DES containing L-menthol and phenolic alcohols to preselect those with the highest CO<sub>2</sub> solubility. Further, CO<sub>2</sub> solubility was experimentally investigated for selected DES systems at various temperatures and pressures using an isochoric method.</p>
</sec>
<sec sec-type="materials|methods" id="s2">
<title>Materials and Methods</title>
<sec id="s2-1">
<title>Eutectic Mixture Preparation</title>
<p>Pure components (L-menthol, purity &#x2265;99%, Sigma Aldrich; thymol, purity &#x2265;99%, Sigma Aldrich; 2,6-xylenol, purity 99%, Acros Organics), was mixed under continuous stirring and gentle heating until a clear homogenous liquid was formed. The water content of the prepared eutectic mixtures was measured in triplicate using Karl Fischer Coulometer (Hanna Instrument, United&#x20;States), and the results are shown in <xref ref-type="table" rid="T1">Table&#x20;1</xref>.</p>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>Prepared eutectic mixtures, their molar ratio, and water content.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Eutectic mixture</th>
<th align="center">Mole ratio</th>
<th align="center">Water content/ppm</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">L-menthol/thymol</td>
<td align="char" char=":">1:2</td>
<td align="char" char="plusmn">144.2&#x20;&#xb1; 2.1</td>
</tr>
<tr>
<td align="left">Thymol/2,6-xylenol</td>
<td align="char" char=":">1:1</td>
<td align="char" char="plusmn">108.4&#x20;&#xb1; 6.7</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
<sec id="s2-2">
<title>Carbon Dioxide Solubility Experiments</title>
<p>Before measurements, DES were degassed under vacuum for 48&#xa0;h at a temperature of <italic>T</italic>&#x20;&#x3d; 413.15&#xa0;K. After degassing, no mass loss was observed for the DES, indicating that there was no change in the stoichiometry between constituents and their negligible volatility under the experimental conditions. CO<sub>2</sub> from Westfalen AG, Germany, with a purity of 99.995% (quality 4.5), was used without further purification. The experiments were performed using a pressure-drop isochoric method at various temperatures and pressures. The apparatus and operational procedures of solubility measurements are described in detail in previous studies (<xref ref-type="bibr" rid="B30">Safarov et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B29">Safarov et&#x20;al., 2014</xref>). For the current measurements, the installation was used without modification. The temperature in the measuring cell was held constant (at <italic>T</italic>&#x20;&#x3d; 293.15&#x2013;323.15&#xa0;K) with an uncertainty of <inline-formula id="inf1">
<mml:math id="m1">
<mml:mrow>
<mml:mi>u</mml:mi>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mi>T</mml:mi>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> &#x3d; 0.030&#xa0;K. A pressure transducer with an accuracy of 0.1% was used to measure the pressure of CO<sub>2</sub> filled in the gas reservoir. The temperature inside the gas reservoir was measured with an uncertainty of <inline-formula id="inf2">
<mml:math id="m2">
<mml:mrow>
<mml:mi>u</mml:mi>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mi>T</mml:mi>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> &#x3d; 0.015&#xa0;K. The initial amount of CO<sub>2</sub> in the gas reservoir was determined from its pressure and temperature using the Span and Wagner equation of state (<xref ref-type="bibr" rid="B35">Span and Wagner, 1996</xref>).</p>
<p>To determine the concentration of CO<sub>2</sub> in the solution, liquid and gas densities under experimental temperature and pressure were required. The density of DES was measured using a density meter (Density meter Easy D40, Mettler-Toledo GmbH, Germany), and the results are indicated in <xref ref-type="table" rid="T2">Table&#x20;2</xref>. The mass of CO<sub>2</sub> in the gas phase was calculated using its density and volume. The gas volume in the cell was found by deducting the liquid volume from the total cell volume. The increase in the liquid volume because of dissolved CO<sub>2</sub> was neglected (<xref ref-type="bibr" rid="B30">Safarov et&#x20;al., 2013</xref>).</p>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>Density of eutectic mixtures measured in this work<xref ref-type="table-fn" rid="Tfn1">
<sup>a</sup>
</xref>.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">
<italic>T</italic>/K</th>
<th colspan="2" align="center">
<italic>&#x3c1;</italic>/kg m<sup>&#x2212;3</sup>
</th>
</tr>
<tr>
<th align="center">L-menthol/thymol (1:2)</th>
<th align="center">Thymol/2,6-xylenol (1:1)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">293.15</td>
<td align="char" char=".">947.6</td>
<td align="char" char=".">992.2</td>
</tr>
<tr>
<td align="left">303.15</td>
<td align="char" char=".">940.0</td>
<td align="char" char=".">983.9</td>
</tr>
<tr>
<td align="left">313.15</td>
<td align="char" char=".">932.5</td>
<td align="char" char=".">975.6</td>
</tr>
<tr>
<td align="left">323.15</td>
<td align="char" char=".">924.8</td>
<td align="char" char=".">967.2</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn1">
<label>a</label>
<p>Standard uncertainty <italic>u</italic>(<italic>&#x3c1;</italic>) &#x3d; 0.05&#xa0;kg&#xa0;m<sup>&#x2212;3</sup>
</p>
</fn>
</table-wrap-foot>
</table-wrap>
</sec>
<sec id="s2-3">
<title>Correlation of the Gas Solubility</title>
<p>Henry&#x2019;s law for a binary system (liquid &#x2b; CO<sub>2</sub>) for a non-ideal gas phase can be given as (<xref ref-type="bibr" rid="B25">Prausnitz and Shair, 1961</xref>)<disp-formula id="e1">
<mml:math id="m3">
<mml:mrow>
<mml:msub>
<mml:mi>y</mml:mi>
<mml:mrow>
<mml:mi>C</mml:mi>
<mml:msub>
<mml:mi>O</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>&#x22c5;</mml:mo>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mi>T</mml:mi>
<mml:mo>,</mml:mo>
<mml:mi>p</mml:mi>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
<mml:mo>&#x22c5;</mml:mo>
<mml:mi>p</mml:mi>
<mml:mo>&#x3d;</mml:mo>
<mml:msub>
<mml:mi>x</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>&#x22c5;</mml:mo>
<mml:msub>
<mml:mi>H</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
<label>(1)</label>
</disp-formula>where <inline-formula id="inf3">
<mml:math id="m4">
<mml:mrow>
<mml:msub>
<mml:mi>y</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> and <inline-formula id="inf4">
<mml:math id="m5">
<mml:mrow>
<mml:msub>
<mml:mi>x</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> are the mole fraction of CO<sub>2</sub> in the gas and liquid phases, respectively; <inline-formula id="inf5">
<mml:math id="m6">
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> is the fugacity coefficient of CO<sub>2</sub> in the gas phase, and <inline-formula id="inf6">
<mml:math id="m7">
<mml:mrow>
<mml:msub>
<mml:mi>H</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> is Henry&#x2019;s constant. Due to the negligible vapor pressure of the studied eutectic mixtures at studied temperatures (<xref ref-type="bibr" rid="B43">Xin et&#x20;al., 2021</xref>) (see also <xref ref-type="sec" rid="s10">Supplementary Table S1</xref> in <xref ref-type="sec" rid="s10">Supplementary Material</xref> for calculations), the gas phase can be assumed as pure CO<sub>2</sub>, i.e.,&#x20;<inline-formula id="inf7">
<mml:math id="m8">
<mml:mrow>
<mml:msub>
<mml:mi>y</mml:mi>
<mml:mrow>
<mml:mi>C</mml:mi>
<mml:msub>
<mml:mi>O</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>&#x3d;</mml:mo>
<mml:mn>1</mml:mn>
</mml:mrow>
</mml:math>
</inline-formula> (see also <xref ref-type="sec" rid="s10">Supplementary Table S1</xref> in <xref ref-type="sec" rid="s10">Supplementary Material</xref> for calculations). Henry&#x2019;s constant can be defined using the following equation.<disp-formula id="e2">
<mml:math id="m9">
<mml:mrow>
<mml:msub>
<mml:mi>H</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>&#x3d;</mml:mo>
<mml:munder>
<mml:mrow>
<mml:mi>lim</mml:mi>
</mml:mrow>
<mml:mrow>
<mml:mi>p</mml:mi>
<mml:mo>&#x2192;</mml:mo>
<mml:mo>&#xa0;</mml:mo>
<mml:mn>0</mml:mn>
</mml:mrow>
</mml:munder>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mrow>
<mml:mfrac>
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:mi>T</mml:mi>
<mml:mo>,</mml:mo>
<mml:mi>p</mml:mi>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
<mml:mo>&#x22c5;</mml:mo>
<mml:mi>p</mml:mi>
</mml:mrow>
<mml:mrow>
<mml:msub>
<mml:mi>x</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
<mml:mo>]</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
<label>(2)</label>
</disp-formula>
</p>
</sec>
<sec id="s2-4">
<title>COSMO-RS Calculations</title>
<p>The solubility of CO<sub>2</sub> in eutectic mixtures was screened a priori by evaluating CO<sub>2</sub> activity coefficients at infinite dilution. The activity coefficients of CO<sub>2</sub> at infinite dilution in different pure constituents and eutectic mixtures were calculated using the COSMO-RS model (BIOVIA COSMOtherm X19, Dassault Syst&#xe8;mes) and BP_TZVP_19. ctd parameters. Molecular conformations of components were obtained using BIOVIA COSMOconf 17 (Dassault Syst&#xe8;mes). The geometry optimization and screening charge density were determined by density functional theory calculations using BP86 functional and def-TZVP basis set by Turbomole version 6.6 (TURBOMOLE GmbH).</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>Results and Discussion</title>
<sec id="s3-1">
<title>Screening With COSMO-RS</title>
<p>To enable the usage of a DES in CO<sub>2</sub> capture applications, the DES should 1) be liquid in the appropriate temperature range, i.e.,&#x20;approximately room temperature; and 2) be a good solvent for CO<sub>2</sub>. A number of nonionic DES containing phenolic alcohols can satisfy these two criteria. The phenolic alcohols considered in this study are phenol, methylphenols (cresols), dimethylphenols (xylenols), trimethylphenols, and two natural phenolic terpenes, thymol (2-isopropyl-5-methylphenol) and carvacrol (5-isopropyl-2-methylphenol). Phenolic alcohols with higher molecular weights or dihydroxy benzenes were not considered because they are not expected to form liquid DES at room temperature because of their high melting temperature and enthalpy (<xref ref-type="bibr" rid="B6">Alhadid et&#x20;al., 2019</xref>). When phenolic alcohols are mixed with L-menthol, a significant negative deviation from the ideal behavior is expected (<xref ref-type="bibr" rid="B4">Alhadid et&#x20;al., 2020b</xref>; <xref ref-type="bibr" rid="B2">Alhadid et&#x20;al., 2021a</xref>). The negative deviation from the ideal behavior results in the formation of DES with a sufficiently low melting temperature. Thus, it is desirable to have L-menthol as a DES constituent. First, the activity coefficients of CO<sub>2</sub> at infinite dilution were calculated using COSMO-RS in pure constituents at 293.15&#xa0;K because the physicochemical properties of pure constituents influence the physicochemical properties of DES (<xref ref-type="bibr" rid="B5">Alhadid et&#x20;al., 2021b</xref>), and the results are shown in <xref ref-type="fig" rid="F1">Figure&#x20;1</xref>. As COSMO-RS is based on quantum mechanical calculations, the calculated activity coefficients implicitly include the atomistic rationalization and quantify the intermolecular interactions between CO<sub>2</sub> and the constituents in the liquid phase. The low CO<sub>2</sub> activity coefficient values indicate strong intermolecular interactions between CO<sub>2</sub> and the constituents and, accordingly, high CO<sub>2</sub> solubility. The calculated CO<sub>2</sub> activity coefficients in all phenolic alcohols are lower than in L-menthol (cyclohexyl alcohol) (<xref ref-type="fig" rid="F1">Figure&#x20;1</xref>), proving that CO<sub>2</sub> solubility is relatively high in phenolic alcohols.</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Activity coefficients at infinite dilution of carbon dioxide in pure L-menthol and various phenolic alcohols at 293.15&#xa0;K calculated by COSMO-RS.</p>
</caption>
<graphic xlink:href="fchem-10-864663-g001.tif"/>
</fig>
<p>
<xref ref-type="fig" rid="F1">Figure&#x20;1</xref> shows that the limiting activity coefficients decrease with the addition of methyl groups to phenolic alcohols. The general order of the CO<sub>2</sub> activity coefficients in phenolic alcohols is phenol &#x3e; methylphenol (cresol) &#x3e; dimethylphenol (xylenol) &#x3e; trimethylphenols. Furthermore, compared to thymol and carvacrol, substituting a methyl group with an isopropyl group, i.e.,&#x20;2,5-xylenol, decreases CO<sub>2</sub> limiting activity coefficients. By comparing different isomers, the limiting activity coefficients are lower in isomers with methyl groups at position 2, i.e.,&#x20;close to the hydroxyl group. Furthermore, the lowest values of CO<sub>2</sub> activity coefficients are observed in dimethyl and trimethyl isomers with a methyl group on the two and six positions, respectively. The CO<sub>2</sub> activity coefficient in thymol is lower than that in carvacrol, as the isopropyl group is nearer to the hydroxyl group in thymol than to carvacrol. Therefore, the structure of the phenolic alcohol influences CO<sub>2</sub> solubility. The four marked phenolic alcohols with the lowest limiting activity coefficient values in CO<sub>2</sub> were considered potential DES constituents for further screening.</p>
<p>Further, potential DES containing L-menthol with 2,6-xylenol, thymol, 2,3,6-trimethylphenol, and 2,4,6-trimethylphenol were screened using COSMO-RS. The calculated activity coefficients of CO<sub>2</sub> at infinite dilution in five different DES and ratios between the constituents at 293.15&#xa0;K are shown in <xref ref-type="fig" rid="F2">Figure&#x20;2</xref>. CO<sub>2</sub> activity coefficients in L-menthol-based DES at any molar ratio are in the order L-menthol:thymol (MTH) &#x3e; L-menthol:2,6-xylenol (M26X) &#x3e; L-menthol:2,4,6-trimethylphenol (M246) &#x3e; L-menthol:2,3,6-trimethylphenol (M236) (<xref ref-type="fig" rid="F2">Figure&#x20;2</xref>), which is consistent with the order of CO<sub>2</sub> activity coefficients in the pure phenolic alcohols present in the DES (see <xref ref-type="fig" rid="F1">Figure&#x20;1</xref>). Moreover, increasing the molar ratio of the phenolic alcohol to L-menthol decreases CO<sub>2</sub> activity coefficients. Therefore, it is logical to select L-menthol-based DES containing trimethylphenols in a 1:2 ratio between the constituents as potential solvents for CO<sub>2</sub> capture. However, melting properties of pure constituents influence the melting temperature of the DES (<xref ref-type="bibr" rid="B6">Alhadid et&#x20;al., 2019</xref>). M236 and M246 in 1:2 ratio are solid at room temperature, which is attributed to the high melting temperature of 2,3,6- and 2,4,6- trimethylphenols (T<sub>m</sub> &#x3d; 331.2 and 342.15 K, respectively) (<xref ref-type="bibr" rid="B39">Verevkin, 1999</xref>). MTH and M26X in 1:2 ratio are liquid at room temperature (<xref ref-type="bibr" rid="B2">Alhadid et&#x20;al., 2021a</xref>), which indicates that both can be considered for further experimental investigation. CO<sub>2</sub> activity coefficients in MTH and M26X at 1:2 ratio are of similar values (<xref ref-type="fig" rid="F2">Figure&#x20;2</xref>). However, MTH is considered a better option because of the low toxicity of thymol compared to 2,6-xylenol. Therefore, MTH was chosen for measurements.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Activity coefficients at infinite dilution of carbon dioxide in selected eutectic mixtures at 293.15&#xa0;K calculated by COSMO-RS.</p>
</caption>
<graphic xlink:href="fchem-10-864663-g002.tif"/>
</fig>
<p>Next, eutectic mixtures containing two phenolic alcohols were considered. These eutectic mixtures are expected to show ideal solution behavior with no significant decrease in the melting temperature of the mixture relative to pure constituents (<xref ref-type="bibr" rid="B6">Alhadid et&#x20;al., 2019</xref>). For such mixtures, the melting temperature of the DES at any ratio between constituents can be obtained from the solid&#x2013;liquid phase diagram based on the pure constituent melting properties. A brief explanation of solid&#x2013;liquid equilibrium calculations is given in the <xref ref-type="sec" rid="s10">Supplementary Material</xref>. Based on the ideal solution model calculations, thymol: 2,6-xylenol (T26X) eutectic system should form a liquid mixture at room temperature. The calculated eutectic composition and temperature for T26X are x<sub>e,thymol</sub> &#x3d; 0.46, and T<sub>e</sub> &#x3d; 292.8 K, respectively (see <xref ref-type="sec" rid="s10">Supplementary Material</xref> for details about the calculations). Altering the ratio between constituents in T26X does not influence CO<sub>2</sub> activity coefficients (<xref ref-type="fig" rid="F2">Figure&#x20;2</xref>), in contrast to what is observed in L-menthol-based DES. Thus, the molar ratio close to the eutectic ratio of the T26X system (&#x223c;1:1 ratio) was selected to ensure that the mixture is liquid at room temperature. Eventually, the two DES, MTH in 1:2 ratio, and T26X in 1:1 ratio were selected for the solubilty measurements.</p>
</sec>
<sec id="s3-2">
<title>Experimental Solubility</title>
<p>CO<sub>2</sub> solubility in the two DES was measured based on the pressure-drop isochoric method at four different pressures (&#x223c;4, 3, 2, and 1&#xa0;MPa) and four different temperatures (293.15, 303.15, 313.15, and 323.15&#xa0;K). The CO<sub>2</sub> solubility in weight percentage <inline-formula id="inf8">
<mml:math id="m10">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>w</mml:mi>
<mml:mrow>
<mml:mi>C</mml:mi>
<mml:msub>
<mml:mi>O</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> and mole fraction <inline-formula id="inf9">
<mml:math id="m11">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>x</mml:mi>
<mml:mrow>
<mml:mi>C</mml:mi>
<mml:msub>
<mml:mi>O</mml:mi>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula>, as well as its fugacity coefficient <inline-formula id="inf10">
<mml:math id="m12">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mrow>
<mml:mtext>CO</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> calculated using Span and Wagner equation of state for the MTH and T26X system are shown in <xref ref-type="table" rid="T3">Tables 3</xref>, <xref ref-type="table" rid="T4">4</xref>, respectively. The values of Henry&#x2019;s constant at different temperatures calculated using <xref ref-type="disp-formula" rid="e2">Eq. 2</xref> are shown in <xref ref-type="table" rid="T5">Table&#x20;5</xref>. As one would expect, the solubility of CO<sub>2</sub> in the two studied systems decreases as temperature increases.</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>Experimental carbon dioxide (CO<sub>2</sub>) solubility (both in weight percent <inline-formula id="inf11">
<mml:math id="m13">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">w</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> and in mole fraction <inline-formula id="inf12">
<mml:math id="m14">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">x</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>) in the L-menthol/thymol (MTH) eutectic system and calculated CO<sub>2</sub> fugacity coefficient <inline-formula id="inf13">
<mml:math id="m15">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> at various temperatures T and pressures p.<xref ref-type="table-fn" rid="Tfn2">
<sup>a</sup>
</xref>
</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">
<italic>T</italic>/K</th>
<th align="center">
<italic>p</italic>/MPa</th>
<th align="center">
<inline-formula id="inf14">
<mml:math id="m16">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">w</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>,</mml:mo>
</mml:mrow>
</mml:math>
</inline-formula> %</th>
<th align="center">
<inline-formula id="inf15">
<mml:math id="m17">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">x</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> /Mole fraction</th>
<th align="center">
<inline-formula id="inf16">
<mml:math id="m18">
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>
<xref ref-type="table-fn" rid="Tfn3">
<sup>b</sup>
</xref>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">323.11</td>
<td align="char" char=".">4.1939</td>
<td align="char" char=".">11.410</td>
<td align="char" char=".">0.3082</td>
<td align="char" char=".">0.8453</td>
</tr>
<tr>
<td align="left">313.12</td>
<td align="char" char=".">4.1543</td>
<td align="char" char=".">12.090</td>
<td align="char" char=".">0.3224</td>
<td align="char" char=".">0.8295</td>
</tr>
<tr>
<td align="left">303.18</td>
<td align="char" char=".">4.0967</td>
<td align="char" char=".">13.409</td>
<td align="char" char=".">0.3488</td>
<td align="char" char=".">0.8121</td>
</tr>
<tr>
<td align="left">293.15</td>
<td align="char" char=".">4.0201</td>
<td align="char" char=".">15.769</td>
<td align="char" char=".">0.3931</td>
<td align="char" char=".">0.7931</td>
</tr>
<tr>
<td align="left">323.15</td>
<td align="char" char=".">3.5059</td>
<td align="char" char=".">9.290</td>
<td align="char" char=".">0.2616</td>
<td align="char" char=".">0.8701</td>
</tr>
<tr>
<td align="left">313.16</td>
<td align="char" char=".">3.4703</td>
<td align="char" char=".">10.173</td>
<td align="char" char=".">0.2815</td>
<td align="char" char=".">0.8570</td>
</tr>
<tr>
<td align="left">303.13</td>
<td align="char" char=".">3.4247</td>
<td align="char" char=".">11.780</td>
<td align="char" char=".">0.3159</td>
<td align="char" char=".">0.8426</td>
</tr>
<tr>
<td align="left">293.13</td>
<td align="char" char=".">3.3691</td>
<td align="char" char=".">13.335</td>
<td align="char" char=".">0.3474</td>
<td align="char" char=".">0.8265</td>
</tr>
<tr>
<td align="left">323.14</td>
<td align="char" char=".">2.5790</td>
<td align="char" char=".">7.090</td>
<td align="char" char=".">0.2088</td>
<td align="char" char=".">0.9039</td>
</tr>
<tr>
<td align="left">313.18</td>
<td align="char" char=".">2.5541</td>
<td align="char" char=".">7.615</td>
<td align="char" char=".">0.2219</td>
<td align="char" char=".">0.8942</td>
</tr>
<tr>
<td align="left">303.13</td>
<td align="char" char=".">2.5229</td>
<td align="char" char=".">8.347</td>
<td align="char" char=".">0.2395</td>
<td align="char" char=".">0.8835</td>
</tr>
<tr>
<td align="left">293.14</td>
<td align="char" char=".">2.4860</td>
<td align="char" char=".">9.297</td>
<td align="char" char=".">0.2617</td>
<td align="char" char=".">0.8716</td>
</tr>
<tr>
<td align="left">323.02</td>
<td align="char" char=".">1.6819</td>
<td align="char" char=".">4.644</td>
<td align="char" char=".">0.1442</td>
<td align="char" char=".">0.9369</td>
</tr>
<tr>
<td align="left">313.18</td>
<td align="char" char=".">1.6662</td>
<td align="char" char=".">5.027</td>
<td align="char" char=".">0.1547</td>
<td align="char" char=".">0.9305</td>
</tr>
<tr>
<td align="left">302.98</td>
<td align="char" char=".">1.6478</td>
<td align="char" char=".">5.436</td>
<td align="char" char=".">0.1659</td>
<td align="char" char=".">0.9235</td>
</tr>
<tr>
<td align="left">293.15</td>
<td align="char" char=".">1.6281</td>
<td align="char" char=".">5.937</td>
<td align="char" char=".">0.1792</td>
<td align="char" char=".">0.9155</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn2">
<label>a</label>
<p>Standard uncertainties <italic>u</italic> are: <italic>u</italic>(<italic>T</italic>) &#x3d; 0.030 K, <italic>u</italic>(<italic>p</italic>) &#x3d; 0.1%, <italic>u</italic>(<italic>w</italic>) &#x3d; 0.01&#xa0;wt%, and <italic>u</italic>(<italic>x</italic>) &#x3d; 0.0001&#xa0;mol fraction.</p>
</fn>
<fn id="Tfn3">
<label>b</label>
<p>Calculated using the Span and Wagner equation of state (<xref ref-type="bibr" rid="B35">Span and Wagner, 1996</xref>) implemented in ThermoFluids (Springer, V. 1.0).</p>
</fn>
</table-wrap-foot>
</table-wrap>
<table-wrap id="T4" position="float">
<label>TABLE 4</label>
<caption>
<p>Experimental carbon dioxide (CO<sub>2</sub>) solubility (both in weight percent <inline-formula id="inf17">
<mml:math id="m19">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">w</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> and in mole fraction <inline-formula id="inf18">
<mml:math id="m20">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">x</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>) in the thymol/2,6-xylenol (T26X) eutectic system and CO<sub>2</sub> calculated fugacity coefficient <inline-formula id="inf19">
<mml:math id="m21">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> at various temperatures T and pressures p.<xref ref-type="table-fn" rid="Tfn4">
<sup>a</sup>
</xref>
</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">
<italic>T</italic>/K</th>
<th align="center">
<italic>p</italic>/MPa</th>
<th align="center">
<inline-formula id="inf20">
<mml:math id="m22">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">w</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
<mml:mo>,</mml:mo>
</mml:mrow>
</mml:math>
</inline-formula> %</th>
<th align="center">
<inline-formula id="inf21">
<mml:math id="m23">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">x</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> /Mole fraction</th>
<th align="center">
<inline-formula id="inf22">
<mml:math id="m24">
<mml:mrow>
<mml:msub>
<mml:mi>&#x3d5;</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>
<xref ref-type="table-fn" rid="Tfn5">
<sup>b</sup>
</xref>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">322.90</td>
<td align="char" char=".">4.0853</td>
<td align="char" char=".">11.549</td>
<td align="char" char=".">0.2878</td>
<td align="char" char=".">0.8492</td>
</tr>
<tr>
<td align="left">313.13</td>
<td align="char" char=".">4.0441</td>
<td align="char" char=".">12.563</td>
<td align="char" char=".">0.3078</td>
<td align="char" char=".">0.8339</td>
</tr>
<tr>
<td align="left">303.17</td>
<td align="char" char=".">3.9840</td>
<td align="char" char=".">14.856</td>
<td align="char" char=".">0.3506</td>
<td align="char" char=".">0.8172</td>
</tr>
<tr>
<td align="left">293.22</td>
<td align="char" char=".">3.9057</td>
<td align="char" char=".">17.093</td>
<td align="char" char=".">0.3895</td>
<td align="char" char=".">0.7990</td>
</tr>
<tr>
<td align="left">323.14</td>
<td align="char" char=".">3.3380</td>
<td align="char" char=".">9.512</td>
<td align="char" char=".">0.2455</td>
<td align="char" char=".">0.8762</td>
</tr>
<tr>
<td align="left">313.15</td>
<td align="char" char=".">3.3029</td>
<td align="char" char=".">10.417</td>
<td align="char" char=".">0.2646</td>
<td align="char" char=".">0.8638</td>
</tr>
<tr>
<td align="left">303.13</td>
<td align="char" char=".">3.2563</td>
<td align="char" char=".">11.456</td>
<td align="char" char=".">0.2859</td>
<td align="char" char=".">0.8502</td>
</tr>
<tr>
<td align="left">293.14</td>
<td align="char" char=".">3.1988</td>
<td align="char" char=".">13.073</td>
<td align="char" char=".">0.3176</td>
<td align="char" char=".">0.8352</td>
</tr>
<tr>
<td align="left">323.13</td>
<td align="char" char=".">2.6988</td>
<td align="char" char=".">7.477</td>
<td align="char" char=".">0.2000</td>
<td align="char" char=".">0.8995</td>
</tr>
<tr>
<td align="left">313.15</td>
<td align="char" char=".">2.6696</td>
<td align="char" char=".">8.022</td>
<td align="char" char=".">0.2125</td>
<td align="char" char=".">0.8895</td>
</tr>
<tr>
<td align="left">303.16</td>
<td align="char" char=".">2.6344</td>
<td align="char" char=".">9.205</td>
<td align="char" char=".">0.2388</td>
<td align="char" char=".">0.8784</td>
</tr>
<tr>
<td align="left">293.15</td>
<td align="char" char=".">2.5932</td>
<td align="char" char=".">10.284</td>
<td align="char" char=".">0.2619</td>
<td align="char" char=".">0.8661</td>
</tr>
<tr>
<td align="left">323.14</td>
<td align="char" char=".">1.6670</td>
<td align="char" char=".">4.462</td>
<td align="char" char=".">0.1263</td>
<td align="char" char=".">0.9375</td>
</tr>
<tr>
<td align="left">313.15</td>
<td align="char" char=".">1.6500</td>
<td align="char" char=".">4.909</td>
<td align="char" char=".">0.1377</td>
<td align="char" char=".">0.9313</td>
</tr>
<tr>
<td align="left">303.16</td>
<td align="char" char=".">1.6300</td>
<td align="char" char=".">5.398</td>
<td align="char" char=".">0.1501</td>
<td align="char" char=".">0.9243</td>
</tr>
<tr>
<td align="left">293.15</td>
<td align="char" char=".">1.6070</td>
<td align="char" char=".">6.057</td>
<td align="char" char=".">0.1663</td>
<td align="char" char=".">0.9166</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn4">
<label>a</label>
<p>Standard uncertainties <italic>u</italic> are: <italic>u</italic>(<italic>T</italic>) &#x3d; 0.030 K, <italic>u</italic>(<italic>p</italic>) &#x3d; 0.1%, <italic>u</italic>(<italic>w</italic>) &#x3d; 0.01&#xa0;wt%, and <italic>u</italic>(<italic>x</italic>) &#x3d; 0.0001&#xa0;mol fraction.</p>
</fn>
<fn id="Tfn5">
<label>b</label>
<p>Calculated using the Span and Wagner equation of state (<xref ref-type="bibr" rid="B35">Span and Wagner, 1996</xref>) implemented in ThermoFluids (Springer, V. 1.0).</p>
</fn>
</table-wrap-foot>
</table-wrap>
<table-wrap id="T5" position="float">
<label>TABLE 5</label>
<caption>
<p>Calculated carbon dioxide Henry&#x2019;s constant <inline-formula id="inf23">
<mml:math id="m25">
<mml:mrow>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">H</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mo>)</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
</inline-formula> in the studied systems.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">T/K</th>
<th colspan="2" align="center">
<inline-formula id="inf24">
<mml:math id="m26">
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="italic">H</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">C</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">2</mml:mi>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula> /MPa</th>
</tr>
<tr>
<th align="center">L-menthol/thymol (1:2)</th>
<th align="center">Thymol/2,6-xylenol (1:1)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">293.15</td>
<td align="char" char="plusmn">8.52&#x20;&#xb1; 0.13</td>
<td align="char" char="plusmn">9.45&#x20;&#xb1; 0.32</td>
</tr>
<tr>
<td align="left">303.15</td>
<td align="char" char="plusmn">8.98&#x20;&#xb1; 0.13</td>
<td align="char" char="plusmn">10.45&#x20;&#xb1; 0.23</td>
</tr>
<tr>
<td align="left">313.15</td>
<td align="char" char="plusmn">9.61&#x20;&#xb1; 0.25</td>
<td align="char" char="plusmn">11.46&#x20;&#xb1; 0.15</td>
</tr>
<tr>
<td align="left">323.15</td>
<td align="char" char="plusmn">10.51&#x20;&#xb1; 0.30</td>
<td align="char" char="plusmn">12.52&#x20;&#xb1; 0.17</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>Further, the CO<sub>2</sub> solubility in the studied DES was compared with that in some ionic DES and IL reported in the literature. The comparison was made in terms of molality, i.e.,&#x20;moles of CO<sub>2</sub> absorbed per mass of solvent. The results are shown in <xref ref-type="fig" rid="F3">Figure&#x20;3</xref>. CO<sub>2</sub> solubility at 303.15&#xa0;K in two ionic DES, namely, choline chloride (ChCl)/urea and ChCl/ethylene glycol in 1:2 molar ratio, is compared with the two DES from this study in <xref ref-type="fig" rid="F3">Figure&#x20;3A</xref>. As seen, CO<sub>2</sub> solubility is significantly higher in nonionic DES than in ionic DES, especially at high pressures (<xref ref-type="fig" rid="F3">Figure&#x20;3A</xref>). The CO<sub>2</sub> solubility in MTH and T26X is also higher than in the two IL (BMIM) (BF4) and (BMIM) (TfO), as shown in <xref ref-type="fig" rid="F3">Figure&#x20;3B</xref>. In addition to good CO<sub>2</sub> solubility, nonionic DES are more stable, less hygroscopic, and less expensive than IL and ionic&#x20;DES.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Carbon dioxide solubility <bold>(A)</bold> in L-menthol/thymol (MTH), thymol/2,6-xylenol (T26X), choline chloride (ChCl)/urea (<xref ref-type="bibr" rid="B45">Leron et al., 2013</xref>), and ChCl/ethylene glycol (<xref ref-type="bibr" rid="B44">Leron and Li, 2013</xref>) at 303.15&#xa0;K <bold>(B)</bold> and in MTH, T26X (BIMI) (BF4), and (BMIM) (TfO) (<xref ref-type="bibr" rid="B46">Aki et&#x20;al., 2004</xref>) at 313.15&#xa0;K.</p>
</caption>
<graphic xlink:href="fchem-10-864663-g003.tif"/>
</fig>
<p>The solvent capacity to absorb CO<sub>2</sub> is not the only selection criterion to consider when selecting a solvent for CO<sub>2</sub> capture applications. The temperature dependence of CO<sub>2</sub> solubility is critical as well because CO<sub>2</sub> should be absorbed with high solubility and desorbed from the solvent at a higher temperature for solvent regeneration. Thus, a strong temperature dependence is required for CO<sub>2</sub> solubility to reduce the energy demand for desorption. The temperature dependence of CO<sub>2</sub> solubility in the two DES being studied is shown in <xref ref-type="fig" rid="F4">Figure&#x20;4</xref>. The T26X system shows a slightly higher CO<sub>2</sub> solubility than the MTH system. Furthermore, the T26X system has a stronger temperature dependence. Therefore, the T26X system can be identified as a very promising candidate for CO<sub>2</sub> capture applications.</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>Temperature dependence of carbon dioxide solubility at medium pressure &#x223c;2.6&#xa0;MPa in L-menthol/thymol (MTH) and thymol/2,6-xylenol (T26X).</p>
</caption>
<graphic xlink:href="fchem-10-864663-g004.tif"/>
</fig>
</sec>
</sec>
<sec sec-type="conclusion" id="s4">
<title>Conclusion</title>
<p>This study examines the use of nonionic DES for CO<sub>2</sub> capture applications. DES were designed to contain phenolic alcohols for improving CO<sub>2</sub> solubility and L-menthol to decrease the melting temperature of the DES. COSMO-RS was used to preselect the DES constituents from a pool of possible phenolic alcohols and to tune the molar ratio between the constituents. It was found that the structure of phenolic alcohols can influence CO<sub>2</sub> solubility. Furthermore, increasing the phenolic alcohol molar content in the DES can enhance the CO<sub>2</sub> solubility. However, the selection of the constituents and their molar ratio was restricted by the melting temperature of the DES. The COSMO-RS screening results identified two potential DES: MTH in 1:2 molar ratio and T26X in 1:1 molar&#x20;ratio.</p>
<p>In the two preselected DES, the CO<sub>2</sub> solubility was studied experimentally using a pressure-drop isochoric method at various temperatures and pressures. The high experimentally determined CO<sub>2</sub> solubility in the two DES validated the COSMO-RS preselection, demonstrating the model&#x2019;s advantage as a screening tool. CO<sub>2</sub> solubility in the DES proposed in this study is significantly higher than in ionic DES and IL proposed in the literature. Furthermore, the temperature dependence of CO<sub>2</sub> solubility in DES proves their suitability for CO<sub>2</sub> capture applications. The high solubility of CO<sub>2</sub> at lower temperatures and its high decrease at higher temperatures make the two DES promising candidates for CO<sub>2</sub> capture. This study shows that simple and widely available organic substances can be used to form novel solvents with unique properties.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/<xref ref-type="sec" rid="s10">Supplementary Material</xref>, further inquiries can be directed to the corresponding authors.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>Conceptualization: AA; Investigation: AA and JS; Formal Analysis: AA, JS, and LM; Writing&#x2013;Original Draft: AA and JS; Writing&#x2013;Review and Editing: LM, KM, and MM; Supervision: KM and&#x20;MM.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This work was supported by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program.</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
<sec id="s10">
<title>Supplementary Material</title>
<p>The Supplementary Material for this article can be found online at: <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/fchem.2022.864663/full#supplementary-material">https://www.frontiersin.org/articles/10.3389/fchem.2022.864663/full&#x23;supplementary-material</ext-link>
</p>
<supplementary-material xlink:href="Table1.DOCX" id="SM1" mimetype="application/DOCX" xmlns:xlink="http://www.w3.org/1999/xlink"/>
<supplementary-material xlink:href="DataSheet1.DOCX" id="SM2" mimetype="application/DOCX" xmlns:xlink="http://www.w3.org/1999/xlink"/>
</sec>
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