Study of solvation of substituted propylbenzene in ethanol-water solutions under subcritical conditions by molecular dynamics

J. of Supercritical Fluids 155 (2020) 104649

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Study of solvation of substituted propylbenzene in ethanol-water solutions under subcritical conditions by molecular dynamics Marina L. Antipova a , Valentina E. Petrenko a,∗ , Ekaterina G. Odintsova a , Tatyana V. Bogdan b a b

G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Akademicheskaya str., 1, Ivanovo, 153045, Russia Lomonosov Moscow State University, Faculty of Chemistry, GSP-1, 1-3, Leninskie Gory, Moscow, 119991, Russia

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Solutions • • • • •

of 2-methoxy-4-(2 hydroxypropyl)phenol in subcritical fluids have been simulated. Solvents were water and waterethanol mixtures at T = 573 K and P = 23 MPa. Distributions of components in the bulk and in the solvate shells have been researched. Tendencies to selective solvation have been analyzed. Solute molecules form stable longlived hydrogen bonds with water and ethanol. An increase in ethanol concentration leads to growing stability of the solvate complexes.

a r t i c l e

i n f o

Article history: Received 1 July 2019 Received in revised form 23 September 2019 Accepted 1 October 2019 Available online 18 October 2019 Keywords: Lignin Substituted propylbenzene Subcritical water Subcritical ethanol-water solutions Selective solvation Hydrogen bond Molecular dynamics

a b s t r a c t Solutions of 2-methoxy-4-(2 -hydroxypropyl)phenol (PrPh) in water and water-ethanol mixtures (0.020, 0.115, 0.285 mol fraction of ethanol) at temperature 573 K and densities corresponding to an experimental pressure of 23 MPa have been simulated by molecular dynamics. Distribution of components in the fluid bulk and in the PrPh solvate shell has been researched. Numbers and lifetimes of hydrogen bonds between the components have been determined, and tendencies to selective solvation have been analyzed. It has been shown that PrPh molecules form stable long-lived hydrogen bonds with water and ethanol. An increase in ethanol concentration leads to a decrease in the number of hydrogen bonds formed by PrPh and to an increase in their lifetime that indicates growing stability of the solvate complexes. The local mole fraction of ethanol around PrPh is lower than its average mole fraction in the fluid, i.e., the PrPh solvate shells mainly consist of water molecules. © 2019 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. E-mail address: [email protected] (V.E. Petrenko). https://doi.org/10.1016/j.supflu.2019.104649 0896-8446/© 2019 Elsevier B.V. All rights reserved.

Lignin is an irregular natural polymer whose monomeric units are substituted propylphenol derivatives. Due to the annual

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of lignin under subcritical conditions since it cannot describe the breakage of bonds during chemical reactions. However, studying the solvation of PrPh in water and water-ethanol mixtures under subcritical conditions provides information about the main ways of interaction of the components, which allows us to understand the processes occurring in the fluid medium. Mixtures of water and ethanol with ethanol mole fractions of 0.020, 0.115, 0.285 (corresponding mass fractions of ethanol 0.05, 0.25, and 0.50) at T = 573 K and densities corresponding to an experimental pressure of 23 MPa were taken as the solvent. The task was to research the distribution of components in the fluid volume and in the solvate shell of the solute, to obtain numerical parameters characterizing the composition and stability of the solvate structures, to analyze the trends to selective solvation, to determine the effect of the cosolvent concentration (ethanol).

2. Simulation details

Fig. 1. Molecule of PrPh. Designations of atoms.

renewal, ligno-containing materials are the most common natural material with a practically inexhaustible resource: every year the plant world produces about 170 million tons of lignocellulose [1]. Due to its availability and abundance of active functional groups in its structure, lignin is ideally suited as a raw material for the production of various chemical substances: phenols, alcohols, ethers, acids, hydrocarbons, as well as new composite materials [2–4]. However, no more than 5% of all lignocellulosic materials are currently used [1]. The development of methods for processing lignin is hampered by the irregular structure and complex variable composition of the natural polymer, which depends on the type of raw material (wood grade, type of agricultural crop, etc.). One of the promising ways of processing natural lignin is its depolymerization under supercritical conditions, and the use of organo-aqueous media allows, in this case, to adjust the effect on the polymer framework and increase the selectivity of the yield of the target products [4–6]. However, the strong dependence of the reaction products on the type of raw material, external conditions, the nature and concentration of solvents makes it difficult to find patterns [7,8]. In this respect, it seems promising to use a computer experiment, which allows seeing the characteristics of the reaction medium at the molecular level, to trace changes in the structure depending on external conditions, concentrations of substances and the nature of cosolvents. To study the process of lignin solvation under subcritical conditions, the 2-methoxy-4-(2 -hydroxypropyl)phenol (PrPh) molecule was chosen (Fig. 1). The 4-propyl-substituted phenol molecule is the most common model of the lignin structural unit. PrPh is one of the derivatives of guaiacyl propane, containing methoxy groups in the ortho-position to the hydroxyl group. Therefore, its molecule can serve as a model of the structural unit of both coniferous and hardwood lignin (as it is well known, coniferous lignins are guaiacil, and hardwood lignins are guaiacyl-syringyl [9,10]). In the present work, PrPh was simulated in water and water-alcoholic media under subcritical conditions using the method of classical molecular dynamics (MD). The method of classical MD is not very informative, as applied to the processes of chemical transformation

MD simulations were carried out using a GPU-accelerated Gromacs-5.0.7 software package [11]. Molecular graphics and visualization were performed using VMD 1.8.6 [12]. The MD simulations were carried out for the NVT ensemble. The temperature of T = 573 K was kept constant using a Nose-Hoover thermostat [13,14] with the coupling constant ␶ = 0.1 ps. Periodic boundary conditions were applied to all the three directions of the simulated cubic box. The Verlet algorithm [15] was adopted to integrate the equations of motion. The modified Ewald summation method [16,17] was used to account for the corrections of the long-range electrostatic interactions with a cutoff radius of 1.5 nm, which was also the cutoff value for the Van-der-Waals interactions. All bond length constraints were implemented using the LINCS algorithm [18]. A model molecule of the solute PrPh was built on the basis of the structure of 2-methoxy-4-propylphenol [19]. The optimization of the molecular structure of PrPh was carried out within the framework of the density functional theory using the B3LYP functional [20,21] and the 6–31 G + + (d, p) basis set. All calculations were performed using the Gaussian 09 software package [22]. The absence of imaginary vibration frequencies confirmed the stationary nature of the structure obtained. Natural charges on atoms were obtained from NBO analysis. A “partially non-rigid” PrPh model was used: the hydroxypropyl group changed both the valence and torsion angles, the methoxy and hydroxy groups changed position with a change in the torsion angle. The geometry of the molecule and the interaction parameters for this model corresponded to the force field OPLS-AA [23]. Potential TIP4P was selected for water, a rigid model with parameters from the force field OPLS-AA was used for ethanol [23]. An analysis of the literature has shown that studies of the thermodynamic and bulk properties of water-ethanol mixtures were carried out in limited intervals of density/pressure, temperature, and composition. The experimental data on the density of the mixture in the sub- and supercritical regions are very few [24–27]. Therefore, the required density values at 573 K and 23 MPa were calculated on the basis of experimental data [27] using polynomial equations. The confidence level was 0.9983. The estimated fluid density was 0.612 g/cm3 at 0.285 mol fraction of ethanol, 0.708 g/cm3 at 0.115 mol fraction of ethanol, 0.737 g/cm3 at 0.020 mol fraction of ethanol and 0.740 g/cm3 in the absence of ethanol. Four cubic cells with periodic boundary conditions containing 5832 water molecules were created. Ethanol molecules (1662 at 0.285 mol fraction of ethanol, 671 at 0.115 mol fraction of ethanol, 116 at 0.020 mol fraction of ethanol) were added to three cells, the fourth cell did not contain ethanol. The edge lengths of the cells were adjusted in accordance with a given density of the system and

M.L. Antipova, V.E. Petrenko, E.G. Odintsova et al. / J. of Supercritical Fluids 155 (2020) 104649 Table 1 Average numbers of HBs water — water nHB (w—w), ethanol — ethanol nHB (et—et) и ethanol — water nHB (et—w). Mol fraction of ethanol

0

0.020

0.115

0.285

nHB (w—w) nHB (et—et) nHB (et—w)

1.07 — —

1.02 0.01 1.28

0.91 0.06 1.09

0.67 0.13 0.84

the number of molecules in the system. Energy minimization and balancing of the systems were performed for 0.5 ns. The prepared solutions were poured into cells containing eight PrPh molecules. After minimizing the energy, the equilibration of the systems was performed for 0.5 ns then the data for the subsequent analysis were collected during 1 ns with the step 0.1 ps. 3. Results and discussion Solvation of an organic compound containing methoxyl and hydroxyl groups in water and water-ethanol media occurs through the formation of hydrogen bonds (HBs). Table 1 shows the average numbers of HBs water — water nHB (w—w), ethanol — ethanol nHB (et—et) и ethanol — water nHB (et—w). Table 2 presents the average numbers of HBs formed by the PrPh molecule with water (w) and ethanol (et): nHB (PrPh—w) and nHB (PrPh—et), as well as the average numbers of HBs formed by individual oxygen and hydrogen atoms of PrPh (the symbols of atoms correspond to Fig. 1). HBs were calculated using the criterion: rOO ≤0.35 nm, rOH ≤ 0.26 nm, ∠(H—O···O) ≤ 30◦ . Under the conditions considered by us (T = 573 K, the density corresponding to the experimental pressure is 23 MPa) the fluid is in the subcritical state. For water, in particular, this state is characterized by a violation of the tetrahedral mesh of HBs [28–30]. Under conditions of higher pressures (30—100 MPa), the average number of HBs per water molecule is estimated as 1.3—1.6 according to different experimental and calculated data [31–33]. At a relatively low pressure of 23 MPa, the destruction of HBs is faster, and, as it can be seen from Table 1, the average number of HBs water — water nHB (w—w) is 1.07 in a fluid containing no ethanol. The average number of HBs ethanol — ethanol nHB (et—et) is very small and is only 0.13 even at a mole fraction of ethanol of 0.285. The average number of HBs formed by ethanol with water nHB (et—w) is much more: from 0.84 to 1.28. It should also be noted that the total average number of HBs formed by ethanol nHB (et—et) + nHB (et—w) decreases from 1.29 to 0.97 with the increasing mole fraction of ethanol from 0.020 to 0.285. At the same time, a decrease in the average number of HBs water — water is also observed. That is, the total amount of HBs in the fluid decreases with increasing ethanol mole fraction. This is also observed for HBs involving solute: the total average number of HBs formed by the molecule PrPh with water and ethanol nHB (PrPh—w) + nHB (PrPh—et) decreases from 3.89 to 3.51 with the increasing mole fraction of ethanol from 0.020 to 0.285 (Table 2). The largest number of HBs is formed through the hydrogen H1 of the hydroxyl group connected with the benzene ring, the oxygen O2 of the methoxyl group and the oxygen O3 of the hydroxyl group connected with the alkyl radical. To estimate the stability of HBs, we calculated their average lifeC and intermittent  I time. The calculation of the continuous HB HB average lifetime HBs [34] was carried out from the autocorrelation function (ACF) CHB (t) of the parameter Sij (t) of the existence of HB between molecules i and



CHB (t) =



Sij (0)Sij (t)





S2ij (0)

(1)

where the parameter Sij (t) = 1 if criterion of HB between molecules i and j was executed at the initial moment of time, is executed at

3

the current time t, and the duration of the periods of violation of the criterion during the time interval from 0 to t did not exceed the predetermined value t*; otherwise Sij (t) = 0. ACFs of the continuous existence of HB CCHB (t) are the result when t* = 0, ACF of intermittent existence of HB CIHB (t) are the result when t*=∞. Their integration allows to obtain the average lifetimes of HB:

∞ C HB

∞ CCHB

= 0

(t) dt ,

I HB

CIHB (t) dt

=

(2)

0

When the HB criterion is violated, the molecules do not always leave the coordination spheres of each other. It is likely that they will move closer then again to the distance given by the criterion. However, according to the definition of a continuous HB lifetime, any violation of the criterion will be regarded as an HB breakage. C characterize to a greater Therefore, continuous HB lifetimes HB degree not the true duration of the existence of HBs, but a local restructuring of the nearest environment, associated with the oscilI provide latory motion of molecules. Intermittent HB lifetimes HB a more accurate picture of the true duration of the existence of HBs [35,36]. Table 3 shows the continuous and intermittent lifetimes of HBs PrPh — water, PrPh — ethanol, water — water, ethanol — ethanol, and ethanol — water. As you can see, with an increase in the fraction of ethanol in the fluid, continuous HB lifetimes change very little, while intermittent HB lifetimes increase significantly. That is, the amount of HBs in the fluid, as we see from Tables 1 and 2, decreases with increasing ethanol concentration, but the stability of HBs also increases. The results obtained are related to the dynamics of molecules: the heavier components (ethanol and, to a greater extent, PrPh) are less mobile and consequently form longer-lived hydrogen bonds. HBs with ethanol are more long-lived than HBs with water. The difference between the lifetimes of HBs ethanol — ethanol and water — water is small. The life span of HBs ethanol — water is about average compared to them. At the same time, lifetimes of HBs formed by PrPh with water and ethanol are significantly longer. Intermittent lifetimes of HBs PrPh — water and PrPh — ethanol are about 2.5 times more than HB lifetimes water — water and ethanol — ethanol, respectively. The difference for continuous HB lifetimes is not so great: less than 2 times, but also suggests that HBs with the participation of a solute are more durable than HBs between solvent molecules. Atom-atom radial distribution functions (RDFs) g(r) water — water, ethanol — ethanol, and ethanol — water are shown in Fig. 2. As you can see, the peak height in all cases decreases with decreasing ethanol concentration. This indicates the stabilizing effect of ethanol on intermolecular interactions in the fluid (this is also indicated by the previously noted decrease in the average lifetime of HBs with decreasing ethanol concentration). As it was repeatedly noted in the works of Yu. E. Gorbaty [28,29], signs of tetrahedral molecular packing, which are typical for standard conditions, disappear in the subcritical region with increasing water temperature and approaching the critical isotherm. This phenomenon was noted at a high pressure of 100 MPa. Our simulation was carried out under conditions corresponding to a significantly lower pressure of 23 MPa. And we see that the so-called tetrahedricity peaks on RDFs Ow Ow (in the region of 0.45 nm) are completely absent. Peaks are observed on RDFs Ow Hw at distances less than 0.26 nm (the range corresponding to the HB criterion), but their heights are small. That is, HBs water — water are formed in the fluid, but their number is very small (as confirmed by the data from Table 1), and the hydrogen-bonded associates of water molecules consist of a small number of molecules and are linear, not tetrahedral. The peaks of HBs on RDFs Oet Het are very small, the peaks of

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M.L. Antipova, V.E. Petrenko, E.G. Odintsova et al. / J. of Supercritical Fluids 155 (2020) 104649

Table 2 Average numbers of HBs formed by molecule of PrPh with water nHB (PrPh—w) and ethanol nHB (PrPh—et). Average numbers of HBs formed by individual atoms of PrPh: H1, O1, O2, H3, O3 with oxygen and hydrogen atoms of water and ethanol. Molfraction of ethanol

0

0.020

Partners

PrPh—w

PrPh—w

PrPh—et

PrPh—w

PrPh—et

PrPh—w

PrPh—et

nHB H1 O1 O2 H3 O3

3.90 0.95 0.64 0.86 0.59 0.86

3.83 0.93 0.63 0.85 0.57 0.85

0.06 0.02 0.01 0.01 0.01 0.01

3.44 0.84 0.56 0.79 0.49 0.76

0.32 0.09 0.05 0.06 0.06 0.06

2.75 0.64 0.46 0.67 0.36 0.62

0.76 0.25 0.10 0.15 0.11 0.15

0.115

0.285

Table 3 C I and intermittent HB average lifetimes of HBs MHPP — water (MHPP—w), MHPP — ethanol (MHPP—et), water — water (w—w), ethanol — ethanol (et—et) и Continuous HB ethanol — water (et—w). Mole fraction of ethanol

0

Lifetime

C HB ,

w—w et—et et—w MHPP—w MHPP—et

0.17 — — 0.30 —

0.020 ps

I HB ,

0.51 — — 1.23 —

ps

C HB ,

ps

0.17 0.24 0.21 0.30 0.37

0.115 I HB ,

C HB ,

ps

0.53 0.65 0.56 1.29 1.50

0.18 0.24 0.21 0.31 0.37

ps

0.285 I HB ,

ps

0.62 0.69 0.64 1.48 1.61

C HB , ps

I HB , ps

0.19 0.25 0.22 0.33 0.40

0.71 0.75 0.72 1.78 2.19

Atom-atom (H O and OH ) RDFs PrPh — water and PrPh — ethanol are shown in Figs. 3 and 4. The designations of atoms correspond to Fig. 1. The ratio of the first peaks located at rOH ≤ 0.26 nm corresponds to the average numbers of HBs formed by individual atoms of the solute: the highest peaks are observed on RDFs H1Ow and H1Oet ; O2Hw and O2Het ; O3Hw and O3Het . The first peaks on RDFs H3Ow and H3Oet are significantly lower since the water and ethanol molecules are mainly concentrated not near H3 atom itself, but near oxygen O3 associated with H3. RDFs O1Hw and O1Het behave similarly: their first peaks are low since the solvent and cosolvent are localized not at the O1 atom, but at the hydrogen H1. The higher the ethanol concentration in the fluid, the higher the peaks of RDFs, both PrPh — water and PrPh — ethanol. It is possible to estimate whether the compound is selectively solvated by one of the components of the mixed solvent using the local mole fraction distributions xloc (R) [37,38] of the fluid components. The calculation of the local mole fraction of water and ethanol around PrPh was carried out according to the ratios: xw(PrPh) (R) =

Nw(PrPh) (R) Nw(PrPh) (R) + Net(PrPh) (R) + NPrPh(PrPh) (R)

(3)

and xet(PrPh) (R) =

Fig. 2. Atom-atom RDFs g(r) water — water: Ow Ow (a) and Ow Hw (b); ethanol — ethanol: Oet Het (c); ethanol — water: Oet Hw (d) and HetOw (e). Mole fraction of ethanol: 1 — 0.285, 2 — 0.115, 3 — 0.020, 4 — 0.

the second coordination sphere are practically absent (small “hangers” are observed in their place). That is, ethanol is distributed in the fluid medium and its molecules do not show a tendency to self-association as a whole, although they can form HBs between themselves. Moreover, they are solvated by water molecules, as it follows from the behavior of RDFs Oet Hw and Het Ow . Ethanol molecules preferably form HBs with water, rather than with other ethanol molecules (that is also confirmed by the data from Table 1) and, therefore, ethanol is distributed in the fluid bulk, rather than grouped into clusters.

Net(PrPh) (R) Nw(PrPh) (R) + Net(PrPh) (R) + NPrPh(PrPh) (R)

(4)

where R is the distance from the center of mass of the PrPh molecule, Nw(PrPh) (R), Net(PrPh) (R) and NPrPh(PrPh) (R) are the numbers of water molecules, ethanol, and PrPh, respectively, whose centers of mass are located at a distance r ≤ R from the center of mass of the PrPh molecule. In the same way, it is possible to estimate the degree of ethanol self-aggregation from the distribution of the local mole fraction of ethanol — ethanol: xet(et) (R) =

Net(et) (R) Nw(et) (R) + Net(et) (R) + NPrPh(et) (R)

(5)

where R is the distance from the center of mass of the ethanol molecule, Nw(et) (R), Net(et) (R) and NPrPh(et) (R) are the numbers of water molecules, ethanol, and PrPh, respectively, whose centers of mass are located at a distance r ≤ R from the center of mass of the ethanol molecule.

M.L. Antipova, V.E. Petrenko, E.G. Odintsova et al. / J. of Supercritical Fluids 155 (2020) 104649

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Fig. 3. Atom-atom RDFs g(r) PrPh — water: H1Ow (a) and H3Ow (c); PrPh — ethanol: H1Oet (b) and H3Oet (d). Mole fraction of ethanol: 1 — 0.285, 2 — 0.115, 3 — 0.020, 4 — 0.

Fig. 4. Atom-atom RDFs g(r) PrPh — water: O1Hw (a), O2Hw (c) and O3Hw (e); PrPh — ethanol: O1Het (b), O2Het (d) and O3Het (f). Mole fraction of ethanol: 1 — 0.285, 2 — 0.115, 3 — 0.020, 4 — 0.

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Fig. 6. PMF W(r) PrPh — water (a) and PrPh — ethanol (b). Mole fraction of ethanol: 1 — 0.285, 2 — 0.115, 3 — 0.020, 4 — 0.

Fig. 5. The distributions of the local mole fraction of the water xw(PrPh) (R) (a) and ethanol xet(PrPh) (R) (b) around the PrPh, and the local mole fraction of the ethanol around the ethanol xet(et) (R). Average mole fraction of ethanol in fluid: 1 — 0.285, 2 — 0.115, 3 — 0.020 (the dotted lines correspond to the values).

The distributions of the local mole fraction xw(PrPh) (R), xet(PrPh) (R) and xet(et) (R) are presented in Fig. 5. As we can see, there are no peaks in the distributions of water and ethanol around the solute (Fig. 5a, b). However, the mole fraction of water around PrPh is increased compared to the average mole fraction in the fluid, and the mole fraction of ethanol, respectively, is reduced. The character of the distributions xet(et) (R) (Fig. 5c) suggests that ethanol is almost evenly distributed in the fluid and does not tend to self-association. The observed peaks are so small in height that they can only be the result of random, rather than preferable, interaction of ethanol molecules in a collision in the fluid bulk. To estimate the change in free energy under the influence of the concentrations of the fluid components, the potential mean force (PMF) was calculated, see Fig. 6. As it can be seen, the free energy of PrPh — water interaction decreases, and the free energy of PrPh — ethanol interaction increases with growth of ethanol mole fraction. The minimum free energy is at 0.54 nm in the case of PrPh — water interactions and at 0.60 nm in the case of PrPh — ethanol ones. Small “shoulders” on the distributions of the local mole fractions xw(PrPh) (R) and xet(PrPh) (R) (Fig. 5a, b) correspond to the region of ¨ quite noticeable at 0.285 minimum free energy. These s¨ houldersare and 0.115 mol fraction of ethanol and are almost indistinguishable at 0.020 one. RDFs center of mass — center of mass PrPh — water and PrPh — ethanol are shown at Fig. 7. The peak positions on these RDFs coincide with the minima of free energy. Accordingly to the chosen force field the effective radii r␴ of molecules in MD simulation are r␴ (w) = 0.194 nm for water r␴ (et) = 0.254 nm for ethanol, and r␴ (PrPh) = 0.379 nm for PrPh (the latter the value is an average value, since a flexible potential model was used for PrPh, and the intrinsic molecular volume and therefore the effective radius changed during the simulation). Summing the effective radii, we

Fig. 7. Center of mass — center of mass RDFs gc.m. —c.m. (r) PrPh — water (a) and PrPh — ethanol (b). Mole fraction of ethanol: 1 — 0.285, 2 — 0.115, 3 — 0.020, 4 — 0.

obtain: r␴ (PrPh) + r␴ (w) = 0.573 nm и r␴ (PrPh) + r␴ (et) = 0.633 nm. The fact that the peaks at the corresponding center of mass — center of mass RDFs and free energy minima are located at distances shorter than the sum of the effective molecular radii is explained by the branched structure of PrPh: water and ethanol molecules that form HBs with hydroxyl and methoxy substituents are inside the PrPh intrinsic molecular volume. 4. Conclusion As it was possible to establish as a result of our study of PrPh solutions in subcritical water-ethanol mixtures at relatively low pressure, water does not show any signs of tetrahedral packaging, but in principle, the number of HBs between water molecules is

M.L. Antipova, V.E. Petrenko, E.G. Odintsova et al. / J. of Supercritical Fluids 155 (2020) 104649

small (less than 1.07). Ethanol does not tend to self-association. PrPh, in contrast, is characterized by the formation of HBs with both components of the mixed solvent. The number of HBs formed by PrPh with water and ethanol ranges from 3.51 to 3.90 (it decreases with increasing concentration of ethanol in the fluid), and these HBs have a long lifetime. If we estimate the fraction of HBs with ethanol in the total number of HBs formed by PrPh (Table 2), then it will be less than the average mole fraction of ethanol in the fluid. The local mole fraction of molecules of ethanol and water in the immediate environment of PrPh also differs from their average mole fraction in the fluid. The constructed distributions show that PrPh is predominantly solvated by water. It can be assumed that the PrPh solvation shell contains only those ethanol molecules that form HBs with PrPh, and the rest of the solvation shell consists of water molecules, both forming, and not forming HBs with PrPh. Solvate complexes formed by PrPh with water and ethanol behave as stable structures in the fluid and exist for a longer time than hydrogen bonded associates of water and ethanol. An increase in the lifetime of HBs is observed with an increase in the concentration of ethanol in the fluid, that is, an additional stabilization of the hydrogen bonded solvate complexes occurs. 5. Author agreement “Study of solvation of substituted propylbenzene in ethanolwater solutions under subcritical conditions by molecular dynamics” This manuscript is the authors’ original work and hasn’t received prior publication and isn’t under consideration for publication elsewhere. All authors approved the manuscript and this submission. Declaration of Competing Interest None. Acknowledgements This research was supported by the Russian Foundation for Basic Research (grant No. 18-29-06072). Calculations have been performed by means of MVS-100 K supercomputer resources of the Joint Supercomputer Center of the Russian Academy of Sciences. References [1] Y. Zhang, H. He, Y. Liu, Y. Wang, F. Huo, M. Fan, H. Adidharma, X. Li, S. Zhang, Recent progress in theoretical and computational studies on the utilization of lignocellulosic materials, Green Chem. 21 (2019) 9–35. [2] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [3] H. de Lasa, E. Salaices, J. Mazumder, R. Lucky, Catalytic steam gasification of biomass: catalysts, thermodynamics and kinetics, Chem. Rev. 111 (2011) 5404–5433. [4] X. Wang, J. Zhou, H. Li, G. Sun, Depolymerization of lignin with supercritical fluids: a review, Adv. Mater. Res. 821-822 (2013) 1126–1134. [5] C. Nitsos, U. Rova, P. Christakopoulos, Organosolv fractionation of softwood biomass for biofuel and biorefinery applications, Energies 11 (1-23) (2018) 50. [6] J. Dominguez-Robles, T. Tamminen, T. Liitia, M.S. Peresin, A. Rodríguez, A.-S. Jaaskelainen, Aqueous acetone fractionation of kraft, organosolv and soda lignins, Int. J. Biol. Macromol. 106 (2018) 979–987. [7] Y.-Y. Bai, L.-P. Xiao, Z.-J. Shi, R.-C. Sun, Structural variation of bamboo lignin before and after ethanol organosolv pretreatment, Int. J. Mol. Sci. 14 (2013) 21394–21413. [8] X. Chen, H. Li, S. Sun, X. Cao, R. Sun, Effect of hydrothermal pretreatment on the structural changes of alkaline ethanol lignin from wheat straw, Sci. Rep. (1−9) (2016), 39354. [9] T.L. Highley, Influence of type and amount of lignin on decay by Coriolus versicolor, Can. J. For. Res. 12 (1982) 435–438. [10] R.A. Blanchette, T. Nilsson, G. Daniel, A. Abad, Biological degradation of wood, Archaeological Wood 225 (1989) 141–174.

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