Evaluation of Molecular Behaviour of Priority of Water Pollutants with

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Evaluation of Molecular Behaviour of Priority of Water Pollutants with Ionic Liquids: COSMO based Approach M.S Vivek, R ANANTHARAJ, Sivshyamaprasad Shyam, and N. Mayuri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04089 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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Industrial & Engineering Chemistry Research

Evaluation of Molecular Behaviour of Priority of Water Pollutants with Ionic Liquids: COSMO based Approach. M.S.Vivek, R. Anantharaj*, S. Shyam, N. Mayuri *Department of Chemical engineering, SSN College of Engineering, Kalavakkam, Chennai-603110.

Abstract COSMO approach is a very efficient method to understand the molecular behaviour of different combination of ionic liquids with a priority of water pollutants by means of Hbond acceptor or donor at molecular level. COSMO is based on unimolecular quantum chemical calculations that provides the sigma profile of individual molecules. The sigma profiles of 50 cations, 28 anions and 46 water pollutants were generated and reported here. From this sigma profile, the activity coefficient at infinite dilution of all kind of water pollutants in a selected combination of cation and anion were predicted and analyzed. It is observed that, their type, nature and molecular structure of both IL’s and water pollutant has significant influence on the solubility of those pollutants. In future, the activity coefficient at infinite dilution can be used to predict both the phase composition and generate phase equilibria diagram for solvent extraction process.

Keywords: Cation, Anion, Water Pollutant, Sigma Profile, COSMO Approach.

*Author to whom all correspondence should be addressed E-mail : [email protected] Tel: 044-32909138-263 Fax: 044-32909138

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1. Introduction Conductor – like Screening Model (COSMO) presently is the most reliable and most widely accepted way of predicting sigma profile, sigma potential, activity coefficient at specific dilution and infinite dilution, enthalpy of mixing, volume of mixing, Gibbs free energy and gas solubility, etc. These predictions are of very high value for chemical and environmental engineers in the synthesis and design of chemical processes and plants. Since it does not require any prior experimental data and only requires the molecular structure of chemical species, the screening charge density is an extremely good and local descriptor of molecular surface polarity and other statistical thermodynamics parameters such as structural parameters (effective area and effective volume). The molecular surfaces interactions and H-bonding within the mixture can be expressed by their positive and negative screening charge density (i.e., polarization)

1-3.

The H-

bonding corresponds to a molecular surface contact of a very strongly positive polar surface region (donor) and a very strongly negative polar surface region (acceptor). In addition, the extra H-bonding energy arises from the penetration of the donor into the acceptor and vice versa. The sigma profiles provide a clear picture of molecular polarity and additional donors or acceptors which are required to build an efficient H-bonding with other species. The size and shape of the sigma profile of a specific chemical species can define its properties like vapour pressure, surface tension and boiling point. The molecular level interaction like intermolecular interaction or intra molecular interaction can be expressed by sigma profile or positive/negative screening charge density of contacting surface species. The sigma profiles are available only for 1432 chemicals species which contains only 10 elements like hydrogen, carbon, nitrogen, oxygen, phosphorus, sulphur, chlorine, bromine, and iodine

4-6.

The recent synthesis of chemical species or green solvent and

water pollutant has a complicated molecular structure with -OH, -SO3, -NO3 and several aromatic rings, alkyl groups, etc., Here, the removal of such a big size water pollutant using ionic liquids is a highly challenging task to the researchers around the world. In addition, there is no experimental data for these compounds in their mixtures neither from database nor from measurement at affordable costs. Therefore, the quantum chemical calculations is the most time-consuming task and low cost of applying COSMO approach to generate sigma profile is a better way and thereby predicting activity coefficient and other thermodynamic properties 7 – 20. 2 ACS Paragon Plus Environment

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This paper studies at the sigma profile of different cations, anions and priority water pollutants. Ionic Liquids consist of aromatic or aliphatic cations and inorganic anions. The cations which are used are imidazolium, morpholinium, piperidinium, pyrrolidinium and pyridinium. Anions can be divided into four types; (i) smaller size anions: chlorine, bromine,

acetate,

nitrate,

tetracyano

borate,

thiocyanate,

tetrafluoroborate,

hexafluorophosphate; (ii) sulphate based anions: hydrogen sulphate, methyl sulphate, ethyl sulphate, octyl sulphate; (iii) Phosphate based anions: dimethyl phosphate, diethyl phosphate, dibutyl phosphate; (iv) larger size anions; bismethylsulphonylamide, bisoxaloborate,22- methoxyethoxyethylsulphate, decanoate, methylsulfonylacetamide, methylsulponate,

P-toulenesulphonate,

salicyclate,

trifluoroacetic

acid,

bis(trifluoromethane)sulfonimide,triflate, and tosylate. The molecular name, structure and COSMO surface of all studied species are reported in Table 1 and Table 2. The water pollutants are: 7 endocrine disrupts compounds, 6 chlorobenzenes and its alkyl derivatives, 7 polyaromatic hydrocarbons, 18 phenols and its alkyl derivatives and 8 different dyes. The molecular name, structure, and COSMO surface of all studied species are given in Table 3. 2. Background theory In COSMO approach, the cavity can be created over the molecules and it look like homogeneous medium. In this homogeneous medium, molecules should be treated as solvent. Figure 1. Illustrates the ideal solvation process in the COSMO approach. COSMO constructs the cavity within a perfect conductor, according to a specific set of rules and specific atomic radii. The molecules dipole and higher moments draw charge from the surrounding medium to the surface of the cavity to cancel out the electric field both inside the conductor and tangential to the surface. Hence, the induced surface charges in a discretized space can defined as 1;

tot  0  sol  Aq *

(1)

The above equation can be expressed in terms of the potential interactions between surface charges and it is a function of the cavity geometry. The surface-charge distribution in a finite dielectric solvent is well approximated by simple scaling of the surface charge distribution in a conductor. In this way, COSMO reduces the computational cost with a minimum loss of accuracy.



3. Computational details The sigma profile is the molecule – specific output from a series of calculations using DFT method in GAUSSIAN 03 package. The first step is to accurately draw the 3dimension (3D) view of each molecule using MOLDEN visualization package. The MOLDEN output file is used in GAUSSIAN03 for geometry optimization and thereby confirms its proper structure and its connectivity. Here, the geometry optimization has been done by HF/6-31G* as the potential route sections14,18. After geometry optimization step is completed in GAUSSIAN03, the output file is stored in the MOLDEN folder where the bond length and bond angles can easily be adjusted. The output file is more accuracy. 4. Results and discussion 4.1. Sigma profile of smaller size anions The sigma profile of chlorine, bromine, acetate, nitrate, tetacyanoborate, thiocyanate, tetrafluoroborate, hexafluorophosphate and hydrogen sulphate are presented in Figure 1. The two-vertical dashed line in Figure 1 are the locations of the cut off values for the Hbond donor (σ < - 0.008 e/Å2) and H-bond acceptor (σ > 0.008 e/Å2). The sigma profile of chlorine and bromine are rather narrow. But it’s smaller than hexafluorophosphate. Since chlorine, bromine and hexafluorophosphate anion has a peak at above the Hbonding cut off values (< 0.008 e/Å2) these three anions have the potential to form Hbonds with other species. In addition, chlorine, bromine and hexafluorophosphate anion have a very strong positive screening charge density when compared to others due to the negative partial charges of chlorine (Cl-), bromine (Br-) and fluorine (F-) atoms in their molecular structure. Therefore, these three anions may have additional H-bond formation with other species at molecular level. On the other hand, these three anions are highly symmetric structure. The symmetry of the sigma profile of potential anions can play a significant role on properties of ionic liquids and additional H-bond formation with neighbouring molecules. The sigma profile of tetrafluoroborate anion shows two different peaks above the H-bonding cut-off values due to the positive charges of bromine atom is not completely shared with negative charges of fluorine atom in their molecular structure. Since the sigma profile of tetrafluoroborate is asymmetric, it does not interact favourably with other species. But still can have minimum level of H-bond interaction with other species due to negative charges of fluorine atom in 4 ACS Paragon Plus Environment

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tetrafluoroborate anion. The sigma profile of nitrate anion is broad with small fluctuations, it is almost symmetric. On the positive side, nitrate shows a broad peak at about + 0.01 e/Å2 resulting from two lone pair of oxygen atom present in it and the positive charges of nitrogen atom can be shared charges equally with that two-oxygen atom in nitrate anion. Therefore, nitrate anion unable to form extra H- bond with other species. The sigma profile of hydrogen sulphate anion shows two peaks at 0.009 and 0.015 e/Å2 which is due to sulphur and oxygen atom present in it. Hence the hydrogen sulphate is asymmetric. Due to this asymmetry, hydrogen sulphate cannot interact favourably with itself because the oxygen is missing its appropriate polar partners and there is less polar surface on the hydrogen, which causes unfavourable interactions with alike polarities. This results in low boiling point, surface tension and high vapour pressure. Thiocyanate anion shows two peaks at above the H-bonding cut off values which indicates its ability to form H-bond with other species. Due to the asymmetric sigma profile of thiocyanate “does not like itself” and it has only H-bond acceptor tendency. Tetracyano borate anion has two peaks at above 0.01 due to nitrogen atom present in it. Since it has asymmetric structure and therefore it “does like itself”, explaining it’s relatively low boiling point and surface tension. The sigma profile of acetate is quite broad like water but it’s smaller than water. On the positive side, acetate shows a broad peak at above H-bonding cut off values (i.e. 0.008) which is caused by two oxygen atom presents in it, whereas on the negative side, the small broad peak at above 0.008 is present, resulting from three polar hydrogen atoms. These nine anions do not show any significant peak on the negative side, which means that all these nine anions are strong hydrogen bond acceptors and hydrophobic in nature (That is water unlike). The order of water unlike anions are as: chlorine > bromine > hexafluorophosphate> hydrogen sulphate > thiocyanate > nitrate > tetrafluoroborate > tetracyano borate > acetate. COSMO surface of anion as decreased as; 42.269 (hexafluorophosphate) > 28.491 (chlorine) > 25.557 (bromine) > 17.729 (hydrogen sulphate) > 13.709 (tetracyano borate) > 12.821 (thiocyanate) > 11.232 (nitrate) > 10.241 (tetrafluoroborate) > 8.734 (acetate). Moreover, all these anions may like small quantity of water, but it can be neglected. 4.2. Sigma profiles of sulphate based anions 5 ACS Paragon Plus Environment


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The sigma profile of methyl sulphate, ethyl sulphate and octyl sulphate shows highly asymmetric with two maxima at -0.001 and + 0.015 e/Å2, which corresponds to the SO3 and alkyl group (methyl, ethyl & octyl), respectively. Hence methyl, ethyl and octyl group forms a narrow peak at about at -0.001 e/Å2. Due to this asymmetry, sulphate based anions cannot interact favourably with itself because of sulphonyl oxygen is missing appropriately polar partners which causes unfavourable interactions with alike polarities. Therefore, sulphate based anion can only screen low charge densities on negative side, whereas positive side can screen high charge densities. In addition, it is capable to screen any stronger polarities efficiently. The COSMO surface is decreased as: 34.751 (octyl) > 7.535 (ethyl) >1.856 (methyl) and 16.523 Å2 for sulphate (SO3) which is less than that of sulphate (17.729 Å2) in hydrogen sulphate anion. 4.3 Sigma profiles of phosphate based anions Dimethyl phosphate, diethyl phosphate and dibutylphospahte two peak at -0.001 and 0.019 Å2, which corresponds to the dialkyl group and lone pairs of phosphonyl oxygen atoms. Since the carbons appear to be responsible for a neutral peak at σ = 0. Phosphate based anion has asymmetry and therefore it can only screen charge densities between 0.005 to 0.02 e/Å2. The COSMO surface is decreased as: 31.478 (dibutyl) > 15.706 (diethyl) >10.728 (dimethyl) and 9.352 Å2 for phosphate which is less than sulphate. 4.4. Sigma profiles of larger size anions Bimethylsulphonylamide, methylsulphonylacetamide,

bioxaloborate,22-methoxyethoxyethylsulpahte, methylsulphonate,

p-toluensulphonate,

decanoate, salicylate,

trifluoroaceticacid, bis(trifluoromethane)sulfonimide, triflate and tosylate are highly asymmetric when compared to other anions (Figure 2). These anions have relatively broad sigma profile ranging from -0.01 to 0.025 e/Å2, which corresponds to the positive hydrogen atom and the lone pairs of the negative oxygen atom, carbonyl oxygen, sulphonyl oxygen, phosphonyl oxygen, and other negative charge of hetero atoms. Therefore, the larger size anion can screen both low and large charge densities between 0.01 to 0.025 e/Å2. On the other hand, all are capable to screen any stronger polarities efficiently. Since there is no peak in the H-bonding donor area. Therefore, the selfinteraction is very unfavourable which means that it has relatively low boiling point and surface tension. It should be noted that the sigma profile of all kinds of anions are almost


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complementary in the region of misfit interactions which is indicating that they should mix quite favourably except chlorine and bromine anions. 4.5. Sigma profiles of imidazolium cations It can be observed that imidazolium cations have high asymmetry. The alkyl group from methyl to decyl forms a peak at about -0.001 and methyl group has a peak at -0.005 e/Å2, which indicating that Imidazolium cation shows more non-polar region (-0.008 < σ 36.513 (NoMIM) > 30.153 (OtMIM) > 27.074 (HpMIM) > 24.708 (HeMIM) > 20.849 (PeMIM) > 20 (MeMIM) > 19.616 (BuMIM) > 19.132 (EtMIM) > 17.65 (PrMIM) (Figure 3). On the other hand, the polar hydrogen atoms in the imidazolium cause negative screening charge density which indicating its ability to form H-bond with other species. Therefore, the combination of imidazolium with larger size anion can have multiple interactions with other species at molecular level. 4.6. Sigma profiles of morpholinium cations The sigma profile of morpholinium based cations are shown in Figure 4. From this figure 4, it can be observed that the shape and size of the sigma profile is same when compared to imidazolium cations which means that positive molecular polarities and misfit interaction is strongly depends on hydrogen atom in cations whereas negative molecular polarities are depends on hetero atom in anions. Since anion has high influence on ionic liquid properties. Since the maximum COSMO surface decreased as 36.674(4D4MMOR)

>31.884

(4N4MMOR)

>

26.935

(4O4MMOR)

>

23.164(4HP4MMOR) > 22.855 (4HE4MMOR) > 19.928(4PE4MMOR) > 17.155 (4B4MMOR) >12.441 (4PR4MMOR) > 9.138(4E4MMOR) >4.991(4M4MMOR) (Figure 4). 4.7. Sigma profiles of piperidinium cations The sigma profile of piperidinium cations are highly asymmetric. The polar hydrogen in alkyl group – form a peak at between -0.01 and +0.01 and another one peak from -face of the ring at above -0.01 which indicates that piperidinium cations has the ability to form additional H- bond without replacing an existing H-bond donor. Since the COSMO 7 ACS Paragon Plus Environment


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surface is decreased as; 41.447 (1D1MPIP) >34.184 (1N1PIP)>30.678 (1O1PIP) > 26.418 (1HP1PIP) >23.753 (1HX1PIP)>23.37 (1PE1PIP) > 21.373 (1B1PIP) > 17.907 (1P1PIP)>15.716 (1E1PIP)>9.578(1M1PIP) (Figure 5). 4.8. Sigma profiles of pyridinium cations The sigma profile of pyridinium based cations are highly asymmetric except methylpyridinium. Here methylpyridinium has a quite symmetric at H – bond donor region due to higher polar hydrogen and the - face of the ring.

Therefore,

Methylpyridinium can interact with others without replacing an existing H-bond donor. Since pyridinium based cation shows a favourable ideal interaction with other species through electrostatic or misfit and H-bonding. The COSMO surface of pyridinium is decreased as: 44.04 (DePy) > 38.388 (NoPy) > 38.345 (OtPy) >32.108 (HpPy) > 28.57 (HePy) > 26.919 (PePy) > 24.248 (BuPy) >19.694 (EtPy) 17.869 > (MePy) >16.663 (PrPy) (Figure 6). 4.9. Sigma profiles of pyrrolidinium cations The sigma profile of pyrrolidinium cations are highly asymmetric. It shows three peaks at above zero due to polarization by oxygen and nitrogen and the  face of the ring. Due to this asymmetry, pyrrolidinium cation cannot interact favourably with itself because  - electron cloud density is completely delocalized around their structure. In addition, the non -polar surface higher than other organic solvent and therefore it can be act as hydrophobic nature whereas the small negative charged segment were also appeared. COSMO surface of pyrrolidinium is decreased as: 41.447 (DeMePyrro) >36.544 (PeMePyrro) >31.921 (OtMePyrro)>28.629 (HpMePyrro) >25.175 (HeMePyrro) > 17.09 (PeMePyrro) > 16.423 (BuMePyrro) > 13.713 (PrMePyrro) > 4.461 (EtMePyrro) >3.595 (MeMePyrro) (Figure 7). 4.10. Sigma profiles of endocrine disrupts compounds The sigma profile of endocrine disrupts compounds are highly asymmetric. The donor peak of polarization of hydrogen atom, the hydroxy group hydrogen and the  - face of the ring is visible which highly broader than all kind of cations. Whereas an acceptor peak of the chlorine is visible, this is slightly less broad than all kind of anions. Therefore, the smaller size cation with larger size anion combination can have a significant role on separation of all kind of EDCs from their resources without affecting 8 ACS Paragon Plus Environment

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their properties at atomic and molecular level. Since the COSMO surface of EDCs decreased as; 46.161(DDT) > 27.974(VO) > 27.667(17-alpha ethynyl estradiol) > 26.089 (Diethylstilbestrol), 26.165(Estrone) >24.109(Estradiol)>24.096(Estriol) (Figure 8). 4.11. Sigma profiles of chlorobenzene and its alkyl derivatives The sigma profile of all chlorobenzene has the same shape with slightly negative screening charge density at above -0.01 which is caused by polar hydrogen atom and the  face of the ring. Hence, the influence of chlorine atom present in chlorobenzene derivatives is negligible because the negative charge of chlorine is completely shared with its benzene ring and therefore the peak of all chlorobenzene derivatives slightly shifted towards negative screening charge density at above -0.01. Since all these chlorobenzene derivatives have more non-polar region which means that all are hydrophobic nature and therefore it is not completely miscible in water. Due to the lake of donors, chlorobenzene derivatives do not want more acceptor functionality and therefore only very strong acceptors are able to undergo efficient H-bonding in it. In addition, such an asymmetric shape of the sigma profile indicates an unfavourable electrostatic interaction of chlorobenzene derivatives with itself. On the other hand, there is no peak in the hydrogen bonding donor which indicating it’s not like itself. Since the COSMO surface of chlorobenzene and its alkyl derivatives decreased as; 37.839 (6Cbe) >32.556(5Cbe) > 27.453 (4Cbe) > 23.116 (3Cbe) > 20.454(2Cbe) > 17.708 (1Cbe) (Figure 9). 4.12. Sigma profiles of polyaromatic hydrocarbons The sigma profile of all poly aromatic hydrocarbons shows a peak in between 0.01 and 0.01 which is caused by the  face of the ring and it’s confirmed that there is no intensity in both H-bonding regions that is ±0.01. Only pyrene has slightly positive and negative screening charge density at above ±0.01 and others has very small positive screening charge density at above -0.01 which means that very stronger acceptor can form Hbonding in polyaromatic hydrocarbons and possible for structural orientation through - stacking. Since the COSMO surface of polyaromatic hydrocarbons decreased as; 36.177

(Chrysene)

29.261(Anthracene)

> >

32.728(Triphenylene) 27.291

>

(Phenanthrene)>

30.581(Fluoranthene) 23.668

22.489(Acenaphthylene) >19.49 (Naphthalene)> 16.504 (Pyrene) (Figure 10).


>

(Fluorene)


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4.13. Sigma profile of phenol and its alkyl derivatives The phenol and its alkyl derivatives show two different sigma profiles within hydrogen cut off values which is caused by alkyl group and chlorine atom and hydroxy – hydrogen. Since the sigma profile of simple phenolic compounds shows more positive screening density at above -0.01, while on the other side, an acceptor peak of the oxygen is visible which slightly less positive screening charge density at 0.01. Due to lack of acceptor and strong donors, simple phenol and its derivatives are required more acceptor functionality and less donor functionality. Therefore, the smaller size cations with larger size anion combination of ionic liquids can build efficient H-bonding in all phenolic compounds. Since the COSMO surface of phenol and its alkyl derivatives decreased as 50.156(Depth)>

41.655(OcPh)>40.716(NoPh)>36.846(HpPh)>

32.032(HePh)

>30.137(PCP)>27.122(PePh)>23.146 (4BUPh)>23.114(246TCPh)>19.212(PrPh)>16.071(4CMPh)>15.663(2CP)>15.476(24D NPh) >14.446(Eph)> 12.354(NPh)>10.896(MPh)> 10.298(2NPh) >10.292(Phenol) (Figure 11). 4.14. Sigma profiles of selective acid, base and reactive dyes The sigma profile of all dyes has highly asymmetric. On the negative side dyes shows a broad peak at above -0.01 resulting from the more than two  face of the ring, while on the positive side, the same broad peak at +0.01 is present, resulting from the lone pair of the oxygen, sulphur, and nitrogen atoms, SO3 group, N = N groups. Due to strong donors, all dyes required more acceptor functionality than donor functionality. Therefore, additional donors and acceptors are desired, and they can undergo an H-bond interaction without replacing an existing H-bond donor and acceptor at molecular level. In addition, the smaller size cation with larger size anion combination is better for an effective separation of dyes from their resources. Since the COSMO surface of selective acid, base and reactive dyes decreased as 41.373(RB5G) > 36.41(AB25) > 28.653(BO) >28.228(AR14) >25.245(BR9) > 24.372(RO107) > 22.718(BB9)>20.207(A07) (Figure 12). 4.15 Activity coefficient at infinite dilution of water pollutants The generated COSMO file is used to predict the activity coefficient at infinite dilution of all the studied water pollutant in selected cations from imidazolium, pyridinium, piperidinium and morpholinum with tetrafluoroborate [BF4], hexafluorophosphate [PF6] 10 ACS Paragon Plus Environment

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and bis (fluorosulfonyl) imide [NtF2] IL’s. From these prediction, it is noted that, their type, nature and molecular structure of both IL’s and water pollutant has significant influence on the solubility of water pollutants. However, butyl- and hexyl- alkyl chain length of cation with [BF4], [PF6] and [NtF2] shows activity coefficient at infinite dilution lesser than one (i.e.infinite γ

Evaluation of Molecular Behaviour of Priority of Water Pollutants with

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