Properties of Dialkylcarbonate + 1-Alkanol Mixtures at the Vacuum

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Properties of Dialkylcarbonate + 1-Alkanol Mixtures at Vacuum Interface Mert Atilhan, and Santiago Aparicio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08952 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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The Journal of Physical Chemistry

Properties of Dialkylcarbonate + 1-Alkanol Mixtures at Vacuum Interface Mert Atilhan*a and Santiago Aparicio*b a

Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar b

Department of Chemistry, University of Burgos, 09001 Burgos, Spain

*

Corresponding authors:

Santiago Aparicio: e-mail [email protected] ; phone: +34 947 258062 Mert Atilhan: e-mail [email protected] ; phone: +974 4403 4142 ABSTRACT: The properties of solutions formed by dialkylcarbonate + 1-alkanol solutions are studied using classic molecular dynamics simulations. The effect of solutions composition, temperature and dialkylcarbonate type have been analysed. The structure at the interface was analysed and compared with the bulk liquid phases, with special attention to surface composition, molecular orientation and intermolecular forces. Evaporation rates were also calculated as a function of temperature. Reported results provide a detailed characterization of interfacial properties of the presented complex systems at the nanoscopic level for the first time.

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INTRODUCTION The behaviour of liquid mixtures in general is of great relevance both for practical and basic science purposes.1,2 The role of intermolecular forces on the physicochemical properties3 for mixed fluids and their change with mixture composition4 has a great relevance in process design for industrial operations involving complex liquid mixtures, which are ubiquitous in the chemical industry. Extensive efforts have been carried out for developing models for predicting properties of mixed fluids.5-8 The characterization and understanding of the behaviour of complex liquid mixtures at the molecular level has been done both from experimental and theoretical approaches for many different types of molecules, although the development of structure-property relationships is still difficult for many fluids such as those hydrogen bonding molecules.9,10 Besides the requirement of understanding the behaviour of complex liquid mixtures in bulk liquid phases there is also a compulsory need of understanding the properties of multicomponent liquid mixtures interfacial behaviour. The interfacial behaviour of liquid mixtures at gas / vacuum interfaces is of great relevance for many industrial operations11 where the interface plays a pivotal role,12,13 such as those involving phase equilibria, gas adsorption or absorption and heat or mass transfer operations. The knowledge of interfacial properties, and their molecular – level roots, would allow improving the design an efficiency of these technologies. In spite of this relevance studies the behaviour of liquid mixtures at gas/vacuum interfaces are scarce in the literature, being almost limited to measurements of surface tension at air interface14 together with thermodynamic characterization,15 but with a reduced number of studies with the objective of obtaining a detailed picture of interfaces at the nanoscopic level.16-21 The detailed characterization of complex liquid – gas/vacuum interfaces can be carried out through several experimental approaches,22-29 although being difficult, or through molecular simulations.30-34 Classic molecular dynamics simulations have proven to be useful for the characterization of complex liquid – gas/vacuum interfaces,35,36 providing detailed information about molecular arrangements at the interfaces allowing to infer the most relevant differences between structuring at the interfaces and those at bulk liquid phases. In previous studies, our group has studied the behaviour of complex liquids at vacuum or gas interfaces for complex fluids such as ionic liquids or deep eutectic solvents.3639

Following our systematic approach on the study of the properties of complex liquid 2


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mixtures at gas / vacuum interfaces, a study on the properties of dialkylcarbonate + 1alkanol mixtures at vacuum interface using molecular dynamics simulations (MD) is reported in this work. Carbonates are considered as green solvents,40-42 due to their null toxicity and biodegradability, 43-49 which make these compound suitable for developing many different clean and sustainable applications.50-52 In previous studies, we demonstrated the behaviour of dialkylcarbonate with n-alkanes53 or 1-alkanols54 as cosolvents showing the changes in fluids’ structuring and intermolecular interactions with mixture composition. In the case of dialkylcarbonate + 1-alkanol solutions, the role developed by the evolution of hydrogen bonding upon changing mixtures molar fraction showed a pivotal role on the values of physicochemical properties.54 Therefore, in order to understand the properties of dialkylcarbonate + 1-alkanol mixed fluids at the vacuum interfaces, mixtures of dimethylcarbonate (DMC) + 1-butanol (BUT) were studied in the full composition range at isothermal conditions (303 K) using molecular dynamics simulations. The effect of temperature on the interface structuring were also considered in the 293 - 353 K range and the effect of the type of dialkylcarbonate were also analysed by doing simulations also for systems involving diethylcarbonate (DEC). The reported results give a detailed picture at the nanoscopic level of these dialkylcarbonate + 1-alkanol complex mixtures at the vacuum interface.

METHODS Molecular dynamics (MD) simulations were done with the MDynaMix v.5.2 molecular modelling package.55 Force field parameterizations for the considered molecules, Figure 1, were reported in a previous work.54 Simulation boxes for x DMC + (1-x) BUT mixtures were built in the full composition range (in 0.1 mole fraction steps) containing 1000 total molecules. Periodic boundary conditions were used. Initial simulation boxes were built using Packmol program56 for each mixture composition. Low-density boxes (∼ 0.2 g cm-3) were initially built followed by a NPT equilibration at 303 K and 0.1 MPa and then several heating and quenching steps up to 500 K were done. After heating-quenching cycles, the boxes were additionally equilibrated at 303 K and 0.1 MPa in the NPT ensemble for 10 ns, the equilibration was assured through the constancy of total potential energy with simulation time. Several low density initial boxes were built for each system and no differences 3



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between the properties (density and potential energy) was inferred. Then, two vacuum slabs (200 Å long in the z-direction) were considered, one at each side of the liquid boxes (Figure S1, Supporting Information), for modelling liquid – vacuum interfaces. In this way, the interfaces are placed at xy plane with z-direction perpendicular to the interface. Simulations for the liquid – vacuum interfaces were carried out in the NVT ensemble with the temperature controlled using the Nose–Hoover thermostat and considering periodic boundary conditions. Simulations in the 293 - 353 K range were carried out. Simulations including vacuum interfaces were done for 20 ns, with the last 5 ns used for analysis. Coulombic interactions were considered with the Ewald method (15 Å cut-off radius).57 The equations of motion were solved according to the Tuckerman–Berne double time step algorithm (1 and 0.1 fs for long and short time steps)58. Lennard-Jones terms were treated according to the Lorentz-Berthelot mixing rules.

RESULTS AND DISCUSSION The analysis of interfacial properties of liquid mixtures is not a simple task, mainly raising from the difficulties for defining the exact location of the interface. Several approaches have been considered to circumvent this problem, such as the Identification of the Truly Interfacial Molecules (so-called ITIM method),20,32 although the most common approach in the literature defines the interfacial region as a slab parallel to the Gibbs dividing surface. The use of Gibbs dividing surface for defining the interface region has raised some doubts;32,36 however, it has led to a satisfactory description of the properties of many types or liquid mixtures in contact with gas/vacuum layers,16,36,59 and thus, it will be considered for the analysis along this work. In a first stage, the properties of pure solvents at vacuum interface were considered. The behavior of DMC molecules is almost equivalent for those placed in the bulk liquid phase and in DMC / vacuum interfaces, with no remarkable features at the interface, Figure 2a. On the contrary, the behavior of BUT showed remarkably more complex nature with an oscillatory behavior extending from the interface and attenuating on going to the bulk liquid phase, Figure 2b. Likewise, there is a densification at the BUT / vacuum interfaces. Up to our knowledge there are no published experimental or theoretical studies on the properties of BUT / vacuum interfaces, but there are some studies reported on methanol60 or ethanol61 / vacuum interfaces. Literature results for methanol and ethanol at vacuum interfaces did not 4


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show enrichments at the interfaces, on the contrary density profiles for these alkanols are smooth discarding restructuring at the interface.60,61 Results for BUT / vacuum show welldefined peaks at the interface, and the increase in the length of alkyl chain in 1-alkanols leads to ordering at the vacuum interface. Likewise, results in Figure 2b show that the head group of 1-alkanols, including the hydroxyl group and the neighbor methylene group occupy the same regions at the interface close to the vacuum layer whereas the alkyl tails are placed in inner interfacial regions close to the bulk liquid phase. Remarkable changes in the structure of bulk liquid phases are obtained upon DMC and BUT mixing,54 and thus, the structure of DMC + BUT mixtures at vacuum interfaces should be remarkable different to those for pure liquids reported in Figure 2, although these results also point to prevailing effect of BUT. Atomic density profiles are reported in Figure 3a, the comparison of these results with those in Figure 2 for pure fluids shows remarkable changes upon mixing. For DMC molecules at the vacuum interface, a non-smooth oscillatory behavior at the interface is inferred. Density profiles in Figure 3a show that DMC molecules are placed in inner regions of the interface, below a layer of BUT molecules. Likewise, the enrichment for BUT molecules at the interface is in contrast with the small density peaks for DMC. Regarding the arrangement of DMC molecules, the maxima for all the considered atoms appear at almost the same distances in the vicinity of vacuum interface, thus pointing to a parallel molecular arrangement (defined according to the vector joining carbon atoms in methyl groups of DMC placed parallel to the interface). Regarding BUT molecules, they are placed forming a layer in the vicinity of vacuum, Figure 3b. Results in Figure 2b showed that tail alkyl chains were placed in inner regions of the interface for pure BUT. On the other hand results in Figure 3a show that these alkyl chains are placed close to the vacuum layer for DMC + BUT mixtures, this rearrangement seem to be forced for placing DMC molecules in the inner layer immediately below the BUT layer. Moreover, density profiles for BUT in Figure 3a show that BUT molecules are skewed at the vacuum interface, with the vector joining the BUT head oxygen atom with the tail carbon at the methyl group tending to arrange perpendicular to the surface. The results in Figure 3c show that DMC molecules form islands separated by continuous regions of BUT molecules at vacuum interface, thus confirming the prevailing role of BUT molecules for determining the interfacial properties in DMC + BUT mixtures.

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Although density profiles reported in Figure 3 allow to infer the molecular orientations at the interface, this was quantified though the angle distribution as a function of mixture mole fraction reported in Figure 4. The arrangements of DMC molecules are clearly different to the ones for BUT molecules. The angle distribution for DMC molecules show a complex behavior with increasing DMC mole fraction in the mixtures, increasing mole fractions leads to a widening of the distribution showing that DMC molecules may develop different arrangements in the bulk liquid phase. At the interface regions, although results in Figure 3a showed a trend for developing parallel arrangements, distribution reported in Figure 4 show that although this arrangement is highly probable, other arrangements are also possible, confirming that the behavior of DMC molecules at the interface is similar to that in bulk regions. On the contrary, angle distributions for BUT at the vacuum interface show clearly the trend for developing skewed arrangements, as the hotspots in density distributions (for φ ∼ 150o) in the full composition range show. The temperature effect on molecular arrangements at the vacuum interface is reported in Figure S2 (Supporting Information) showing that both for DMC and BUT molecules the densification at the interface vanishes with increasing temperature, leading to interfaces with vacuum with properties equivalent to those in bulk regions. This is quantified by the comparison of mole fraction calculated at the interface region with that in bulk liquid, Figure 5. Two different values were used to define interface thickness around Gibbs dividing surface (r = 5 and 10 Å) and in both cases the concentration of DMC molecules at the interfacial region is lower than in bulk regions, in agreement with density profiles reported in Figure 3. Nevertheless, the DMC mole fraction approaches to the bulk value in a linear way with increasing temperature, showing that this heterogeneity almost vanishes for temperatures larger than 353 K. The behavior of molecules at vacuum interfaces has a relevant role on the kinetics of molecular evaporation, evaporation rates, and thus on the development of vapor - liquid equilibria, which can be analyzed at the molecular level from MD simulations reported in this work. Molecules are defined as belonging to the vapor phase when being placed in a region 10 Å above the Gibbs dividing surface, in agreement with density distributions reported in Figures 3 and S2 (Supporting Information). The results in Figures 6a and 6b show that BUT molecules are evaporated more quickly than DMC molecules, the number of BUT molecules in the vapor phase is roughly four times larger than of DMC molecules. The reason for this 6


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behavior can be the prevailing presence of BUT molecules at the interface region reported in Figures 3 and 5, with DMC molecules placed in inner regions, thus hindering the evaporation of DMC. The number of molecules in the vapor phase follows an almost linear behavior with simulation time for all the studied temperatures and for both molecules in DMC + BUT mixtures, which allow to calculate the evaporation rates as a function of temperature. The temperature effect on the evaporation rate reported in Figure 6c follows a clearly non-linear behavior, especially for BUT molecules. The preferential evaporation of BUT molecules over DMC ones is clearly remarkable for large temperatures. The differences in intermolecular forces for those molecules placed in bulk liquids and those at interfaces are analyzed as a function of temperature and mixture composition in Figure 7. It should be remarked that all the forces are weaker for molecules at the interfaces, as the positive values of ΔE (values at the interface minus those at the bulk) show for any temperature and mixture composition. The bulk-surface differences for energies involving homoassociations (DMC-DMC and BUT-BUT) are lower than those for heteroassociations (DMC-BUT), which is justified considering the interface structuring involving a top layer rich in BUT and a lower layer rich in DMC. This interfacial arrangement allows the efficient development of homoassociations but hinders heteroassociations. Likewise, DMC – DMC interactions suffer less changes than BUT – BUT ones, which may be justified considering and end of aisle effect for BUT molecules (which do not have molecules for interacting above them at the interface). The evolution of ΔE with temperature reported in Figure 7 shows that although it increases linearly with temperature, the changes are almost negligible in the studied temperature range. The changes of ΔE with mixture composition are also almost negligible in the full composition range with the exception of data for DMC – BUT, which suddenly increases for xDMC > 0.5 pointing to weak DMC – BUT interactions at the interface for BUT – rich mixtures. As the regions closer to vacuum layer are richer in BUT whereas DMC molecules tend to occupy inner regions, only for DMC-rich mixtures BUT-DMC interactions at the interface start to be relevant which would justify the behavior reported in Figure 7b. The differences in intermolecular forces at the interface in comparison with bulk regions should lead also to different behavior for the dynamics of both regions, which is analyzed through the self – diffusion coefficients calculated from mean square displacements and reported in Figure 8. The calculation of self-diffusion coefficients for molecules at the interfaces was carried out considering molecules belonging to the 7



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interface as those with the z-coordinate of their center of mass standing in the zGDS ± 10 Å region. For these molecules, the self-diffusion coefficients were calculated from the mean square displacement. The results for self-diffusion coefficients, D, for equimolar mixtures as a function of temperature are reported in Figure 8a comparing values in bulk and interface regions. The ability of the proposed forcefield parameterization for predicting self-diffusion coefficients of neat dialkylcarbonates in bulk liquid phases was previously studied,53 showing reasonable agreement between predicted data and experiments. Nevertheless, there are not experimental data for comparison for the self-diffusion coefficients in dialkylcarbonate + alkanol mixtures neither in bulk nor interfacial regions. The behavior of D for molecules in the bulk liquid phase follows a close to linear behavior with increasing temperature (R2 = 0.99 both for BUT and DMC). Likewise, D are larger for DMC than for BUT molecules in the bulk regions, as it may be expected from the larger intermolecular interaction energies for BUT-BUT than for DMC-DMC. The behavior of D at vacuum interfaces is complex. First, D are lower for BUT than for DMC molecules at the interface as in bulk regions. Second, the comparison of D values for interface and bulk regions show that at low temperatures (T < 303 K) DMC and BUT molecules diffuse faster at the interface than in the bulk liquid, which would be in agreement with the weakening in intermolecular forces reported in Figure 7. For T > 303 K, D values are lower at the interface than in the bulk regions. This can be justified considering that although intermolecular interaction energies are also weaker at the interface than in the bulk for these temperatures, with increased temperature the mobility induced by thermal motion is larger in the bulk than at the interface, in which, although mobility is also improved with increasing temperature, the presence of a vacuum layer above and the difficulties for leaving the interface region for moving to lower layers, because of the different molecular structuring, hinders molecular mobility in the direction perpendicular to the interface. Third, the effect of mixture composition on D is reported in Figure 8b showing an almost parallel evolution with mole fraction for molecules in the bulk and interface region. The molecular dynamics simulation allowed the calculation of surface tension, σ, as a function of pressure and temperature, which was obtained from the corresponding pressure tensors, Figure S3 (Supporting Information).62 The procedure for calculating surface tension from molecular dynamics simulations was validated by comparison between the simulated values and those experimental data from the literature (BUT and DMC; 19.5 and 29.9 mN m8


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1

, respectively, from MD, and 23.6363 and 27.764 and mN m-1, respectively, experimental

from literature, all at 303 K). Calculated surface tension for DMC + BUT mixtures decreases in a linear trend with increasing temperature, whereas the variation with mixture composition is clearly non-linear with negative deviations from linearity, Δσ, reaching a minimum at equimolar composition. Negative Δσ rise from the enrichment in BUT molecules at the interfacial region in comparison with bulk regions, and thus, leading to lower surface tension values as may be inferred from values for pure fluids. This is confirmed by the density profiles reported in Figure 9a as a function of mixture composition in which the enrichment in BUT molecules at the interface region is clearly inferred in the full composition range, even for highly diluted mixtures, thus decreasing surface tension in the mixtures. On the contrary, for DMC molecules the behavior of interface regions is very similar to the bulk phases, Figure 9b, and thus the properties of DMC + BUT / vacuum interfaces are heavily dependent on the trend of BUT molecules for occupying regions in the vicinity of the vacuum layer. The analysis of the mixture composition at the interface shows the enrichment in BUT molecules in the whole composition range, Figure 10, following a clearly non-linear trend with larger deviations at equimolar mixtures, which is in agreement with the behavior of surface tension reported in Figure S3 (Supporting Information). This enrichment in BUT molecules extends for a large region around the Gibbs dividing surface, even when considering a region 10 Å wide around the Gibbs dividing surface the compositions are lower than in the bulk region, although the effects are more remarkable for an interface region of ± 5 Å thickness. Regarding the charge distribution at the interface, results in Figure S4 (Supporting Information) show a region slightly positively charged above the Gibbs dividing surface and negatively charged below it, which rise from the atomic distribution of BUT molecules in that region reported in Figures 2 and 3; nevertheless, these values are very low and additional features cannot be inferred from them. The mixture composition effect on evaporation rates is reported in Figure 11 in which the preferential evaporation of BUT molecules is maintained in the full composition range. Both for BUT and DMC molecules evaporation rates decrease with increasing amount of the cosolvent in the mixtures but only for mixtures with composition larger than 0.8 DMC mole fraction rates for BUT are lower than for DMC, in agreement with results in Figures 9 and 10. In the last stage of the research, the effect of increasing alkyl chains in dialkylcarbonate were analyzed. It should be remarked that pure DMC is denser than pure 9



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DEC,54 and thus DEC-containing mixtures are less dense than those with DMC, which is showed in the number density profiles reported in Figure S5 (Supporting Information). Likewise, the results in Figure S5 (Supporting Information) shows that on going from DMC to diethylcarbonate (DEC) + BUT mixtures the structuring of dialkylcarbonate at the interface is even more similar to the bulk regions: the small peaks appearing for DMC at the interface vanish for DEC. Regarding the behavior of BUT molecules, it is not affected by the longer alkyl chains in DEC in comparison with DMC-containing mixtures, the density peaks at the vacuum interface is maintained together with the trend of BUT molecules to occupy regions close to the vacuum layer and of dialkylcarbonate to stay in inner regions close to the bulk liquid phase. The most remarkable effect of the presence of DEC molecules is the decrease in evaporation rates when compared with mixtures with DMC, both for BUT and the especially for the dialkylcarbonate, Figure 12. This may be justified considering that increasing the number of methyl groups in the dialkylcarbonate increases the number of Lennard-Jones contacts

and

thus

increasing

the

intermolecular

interaction

energy

between

dialkylcarbonate molecules (DEC-DEC is 33 % larger than DMC-DMC for dialkylcarbonate molecules at the interface of dialkylcarbonate + BUT equimolar mixtures at 303 K) and also increasing dialkylcarbonate – BUT interaction energies (7 %), which results in large decrease of evaporation rates for DEC when compared with DMC and lower decrease for BUT on going from DMC to DEC molecules. Therefore, the increase in the dialkylcarbonate does not lead to remarkable changes in the structuring at the vacuum interfaces beyond the increase of van der Waals forces but without molecular reorganization.

CONCLUSIONS The structure of dialkylcarbonate + 1-alkanol / vacuum interfaces has been analyzed through molecular dynamics simulations as a function of mixture composition and temperature. The reported results show structuring at the interface completely different to the bulk liquid regions, with a trend of 1-alkanol molecules to develop a highly dense layer in the vicinity of the vacuum region confining dialkylcarbonate molecules to inner regions closer to the bulk fluid. This molecular arrangement at the interface is maintained in the full composition range and in the wide temperature range studied and is the molecular reason of the nonlinear behavior with mixture composition of relevant surface properties such as surface tension. Likewise, the heterogeneity at the interface determines the mechanism and kinetics 10


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of evaporation although the thermodynamics of the process was not characterized in the present work. The reported results show that large effects can be produced on liquid mixtures properties by the development of a vacuum interface, and it will be extended in future works to other families of molecules for rationalization purposes.

ACKNOWLEDGEMENT This work was funded by Ministerio de Economía y Competitividad (Spain, project CTQ201340476-R) and Junta de Castilla y León (Spain, project BU324U14). The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) for providing supercomputing facilities. The statements made herein are solely the responsibility of the authors.

SUPPORTING INFORMATION Figure S1 (model simulation box); Figure S2 (temperature effect on number density profiles); Figure S3 (surface tension); Figure S4 (charge density profiles); Figure S5 (comparison of number density profiles for DMC and DEC containing mixtures).

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Figure Captions.

Figure 1. Molecules and atom labelling used along this work.

Figure 2. Number density profiles of relevant atoms in (a) DMC and (b) BUT, in DMC / vacuum or BUT / vacuum interfaces at 303 K. z stands for the coordinate perpendicular to the vacuum interface, and zGDS for the coordinate of the Gibbs dividing surface corresponding to the top layer in contact with vacuum. Atoms labeling as in Figure 1. Figure 3. (a) Number density profiles for relevant atoms in x DMC + (1-x) BUT (x = 0.5) / vacuum interface at 303 K. Panels (b) and (c) show snapshots perpendicular and on the interface with vacuum, respectively. z stands for the coordinate perpendicular to the corresponding interfaces, and zGDS for the coordinate of the Gibbs dividing surface. Density profiles in panel a are obtained as averages in the 4 to 5 ns simulation range. Atoms labeling in panel a as in Figure 1. Color code in panels b and c: (blue) DMC and (red) BUT. Continuous lines in panels b and c show limits of periodic boundary conditions used in the simulations; vertical dashed lines in panel b show relevant positions. Figure 4. Two-dimensional density distribution of angles formed between a vector perpendicular to the interface and vector ܸ1 (vector joining C2 atoms in DMC, Figure 1) or vector ܸ2 (vector joining O3 and C4, methyl, atoms in BUT, Figure 1), for x DMC + (1-x) BUT / vacuum interface at 303 K. z stands for the coordinate perpendicular to the corresponding interfaces, and zGDS for the coordinate of the Gibbs dividing surface (for the top interface). Figure 5. Comparison between DMC mole fraction in the bulk liquid phase, xbulk, and in the interfacial region defined as zGDS ± r, xinterface, in x DMC + (1-x) BUT (xbulk = 0.5) as a function of temperature, T. z stands for the coordinate perpendicular to the corresponding interfaces, and zGDS for the coordinate of the Gibbs dividing surface. xbulk is showed as a dashed line.

Figure 6. (a,b) Number of molecules evaporated, N, for x DMC + (1-x) BUT (xbulk = 0.5) / vacuum interface as a function of temperature. Linear fits are showed for guiding purposes as dashed lines. Panel c shows the variation of the slopes of linear fits, (dN / dt), with temperature. Evaporated molecules are defined as those with z > (zGDS + 10 Å), where zGDS stands for the coordinate of the Gibbs dividing surface. Linear fits are showed for guiding purposes as dashed lines.

Figure 7. Differences between intermolecular interaction energies, ΔE, for molecules at the vacuum interface and those in the bulk liquid phases for (a) x DMC + (1-x) BUT (x = 0.5) as a function of temperature or (b) x DMC + (1-x) BUT at 303 K as a function of mole fraction. Interface region defined as zGDS ± r, with r = 10 Å, where zGDS stands for the coordinate of the Gibbs dividing surface.

12


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Figure 8. Self-diffusion coefficients, D, for molecules at the vacuum interface and those in the bulk liquid phases for (a) x DMC + (1-x) BUT (x = 0.5) as a function of temperature or (b) x DMC + (1-x) BUT at 303 K as a function of mole fraction. Interface region defined as zGDS ± r, with r = 10 Å, where zGDS stands for the coordinate of the Gibbs dividing surface.

Figure 9. Mixture composition effect on number density profiles of (a) O3 atom in BUT and (b) C1 atom in DMC, in x DMC + (1-x) BUT / vacuum interface. z stands for the coordinate perpendicular to the corresponding interfaces, and zGDS for the coordinate of the Gibbs dividing surface. Density profiles are obtained as averages in the 4 to 5 ns simulation range. Atoms labeling as in Figure 1.

Figure 10. Comparison between DMC mole fraction in the bulk liquid phase, xbulk, and in the interfacial region defined as zGDS ± r, xinterface, in x DMC + (1-x) BUT at 303 K (x = xbulk). z stands for the coordinate perpendicular to the corresponding interfaces, and zGDS for the coordinate of the Gibbs dividing surface.

Figure 11. Number of molecules evaporated, N, for x DMC + (1-x) BUT / vacuum interface at 303 K. Linear fits are showed for guiding purposes as dashed lines. Panel c shows the variation of the slopes of linear fits, (dN / dt), with temperature. Evaporated molecules are defined as those with z > (zGDS + 10 Å), where zGDS stands for the coordinate of the Gibbs dividing surface. Linear fits are showed for guiding purposes as dashed lines.

Figure 12. Number of molecules evaporated, N, for x DMC + (1-x) BUT / vacuum (x = 0.5) and x DEC + (1-x) BUT / vacuum (x = 0.5) interfaces at 303 K. Linear fits are showed for guiding purposes as dashed lines. Numbers -1

inside the Figure show the variation of the slopes of linear fits, (dN / dt, ns ), with temperature. Evaporated molecules are defined as those with z > (zGDS + 10 Å), where zGDS stands for the coordinate of the Gibbs dividing surface. Linear fits are showed for guiding purposes as dashed lines.

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O2

C3,H2

O

H1,C2

C2,H1

H3C

CH3

O C1 O O1

R

HO Hb,O3

O1 DMC

R = – (CH2)n – CH3

O2

O H1,C3

H3C

C4,H2

C2,H1

O C1 O O1

C2,H1 C3,H1

CH3

O1

n = 2 (1-butanol)

DEC

Figure 1.

14


BUT

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0.008

(a) DMC

0.008

C2 O1 C1 O2

0.004

0.002

0

0 -50 -25 0 z - zGDS / Å

C4 C3 O3 HB

0.004

0.002

-75

(b) BUT

0.006 ρ / Å-3

0.006 ρ / Å-3

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25

-75

-50 -25 0 z - zGDS / Å

Figure 2.

15


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3.

16


Page 16 of 29

Page 17 of 29

DMC (

)

x = 0.0

x = 0.1

φ / deg

x = 0.3

x = 0.5

x = 0.7

x = 0.9

x = 1.0

180

180

180

180

180

180

150

150

150

150

150

150

120

120

120

120

120

120

90

90

90

90

90

90

60

60

60

60

60

60

30

30

30

30

30

30

0 -50 -40 -30 -20 -10 0 10

180

180

0 -50 -40 -30 -20 -10 0 10

180

0 -50 -40 -30 -20 -10 0 10

180

0 -50 -40 -30 -20 -10 0 10

180

0 -50 -40 -30 -20 -10 0 10

180

0 -50 -40 -30 -20 -10 0 10

1

high 0.9

150

150

150

150

150

150

120

120

120

120

120

120

90

90

90

90

90

90

60

60

60

60

60

60

30

30

30

30

30

30

60

0.8

)

50

BUT (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7 40

0.6 0.5

30 0.4 0.3

20

0.2 10 0.1

0 -50 -40 -30 -20 -10 0 10

0 -50 -40 -30 -20 -10 0 10

0 -50 -40 -30 -20 -10 0 10

0 -50 -40 -30 -20 -10 0 10

0 -50 -40 -30 -20 -10 0 10

z-zGDS / Å

Figure 4.

17


0 -50 -40 -30 -20 -10 0 10

low

0 0

0.5

1


0.6

r=5 Å r=10 Å

0.5 xinterface

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

0.4

0.3 280

300

320 T/K

340

Figure 5.

18


360

Page 19 of 29

250

(a) BUT

353 K 343 K 333 K 323 K 313 K 303 K 293 K

200 N

150 100 50 0 50

(b) DMC

40 N

30 20 10 0 0

1

2

3

t / ns

80 (dN / dt) / ns-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BUT DMC

60 40 20

(c)

0 280

300

320 T/K

340

Figure 6.

19


360


5

16

DMC-DMC DMC-BUT BUT-BUT

(a)

(b)

12 ∆E / kJ mol-1

4 ∆E / kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3 2

8 4

1

0

0

-4 280

300

Page 20 of 29

320 T/K

340

360

0

0.2

0.4

0.6 x

Figure 7.

20


0.8

1

Page 21 of 29

3

1.2 DMC @ bulk DMC @ interface BUT @ bulk BUT @ interface

(a)

(b)

1 109 D / m2 s-1

2.5 109 D / m2 s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1.5

0.8

0.6

1 0.5

0.4 280

300

320 T/K

340

360

0

0.2

0.4

0.6 x

Figure 8.

21


0.8

1


0.008

(a) O3 (BUT)

ρ / Å-3

0.006 0.004

x=0.0 x=0.1 x=0.3 x=0.5 x=0.7 x=0.9 x=1.0

0.002 0 0.008

(b) C1 (DMC)

0.006 ρ / Å-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

0.004 0.002 0 -20

-10 0 z - zGDS / Å

Figure 9.

22


10

Page 23 of 29

1

r=5 Å r=10 Å

0.8

xinterface

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8

xbulk

Figure 10.

23


1


40

(a) BUT

N

30 20 10 0 40

(b) DMC

x=0.0 x=0.1 x=0.3 x=0.5 x=0.7 x=0.9 x=1.0

N

30 20 10 0 0

1

2

3

t / ns

16 (dN / dt) / ns-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

BUT DMC

12 8 4

(c)

0 0

0.2

0.4

0.6

0.8

x Figure 11.

24


1

Page 25 of 29

20

DMC in DMC + BUT DEC in DEC + BUT BUT in DMC + BUT BUT in DEC + BUT

15 N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.83 4.04

10

2.22

5 0.48

0 0

1

2 t / ns

Figure 12.

25


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Graphic Table of Contents

29


Properties of Dialkylcarbonate + 1-Alkanol Mixtures at the Vacuum

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