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開始行:
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Today I’d like to talk about microscopic structure of methanol-water mixture at high temperatures and high pressures.
-------------------------------------------------------------------------------------------------------------------------------------
This is outline of my presentation.
First, I will introduce the features of alcohol-water mixtures and objective of this study.
Second, I will show the evaluation method of local structural order and details of molecular dynamics simulation.
Next, I will show the calculation results.
Finally, I will show conclusions.
-------------------------------------------------------------------------------------------------------------------------------------
First, I’d like to talk about a features of alcohol-water mixture. アンフィフィリック
Alcohol-water mixtures are widely used for industrial application. rec・og・nize rékəgnὰɪz
And these are also scientifically attractive as models of amphiphilic aqueous solutions. ap・pli・ca・tion`æpləkéɪʃən
In addition, recently, alcohol-water mixtures of high temperatures and pressures have been recognized as functional solvents. sol・vent sάlv(ə)nt
-------------------------------------------------------------------------------------------------------------------------------------
Alcohol is amphiphilic, and alcohol-water mixtures include repulsive interaction and attractive interaction.
-------------------------------------------------------------------------------------------------------------------------------------
So, at high temperature and high pressures, this balance changes. al・co・hol ˈælkəh`ɔːl
As a result, unique physical properties and unique solvation effect are expected. re・pul・sive rɪpˈʌlsɪv
tem・per・a・ture témp(ə)rətʃ`ʊɚ
pres・sure préʃɚ
con・di・tion kəndíʃən
expected ɪksˈpɛktɪd
-------------------------------------------------------------------------------------------------------------------------------------
Density is one of the most important thermodynamic property.
Our group and several groups have measured the densities of alcohol-water mixtures at high temperatures and high pressures.
This slide shows the excess molar volumes of methanol-water mixtures at 573 K and 618 K.
Horizontal axis means methanol composition.
As you can see, the excess molar volumes are negative at under 573 K.
thèrmo・dynámic
ǽksis、アクシス
カンポジションkɑ̀mpəzíʃən com・po・si・tion
prop・er・ty prάpɚṭi
sev・er・al sév(ə)rəl/
-------------------------------------------------------------------------------------------------------------------------------------
However, our group have reported the excess molar volumes changed from negative to positive with increasing the methanol composition at 618 K.
I’d like to focus on this part.
-------------------------------------------------------------------------------------------------------------------------------------
This is an enlargement figure of excess molar volumes.
This zero crossing point composition increased with increasing pressure.
-------------------------------------------------------------------------------------------------------------------------------------
Here, consider the microscopic structure.
Basically, water molecules form 3 dimensional network structures like this.
-------------------------------------------------------------------------------------------------------------------------------------
In alcohol water mixture, alcohol molecules can enter the space of the 3 dimensional network.
As a result, excess molar volumes show negative values.
In this case apparently, the attractive interaction looks dominant.
---12--------------------------------------------------------------------------------------------------------------------------------
In general, the repulsive interaction is mainly determined by molecular size.
On the other hand, the attractive interaction is determined by various factors. ベアリアス
For example temperature, pressure or composition. プラバブリー
Probably, in these zero crossing points, the repulsive interaction and attractive interaction are balanced.
And, solution structures could change dramatically in this point.
However details are still unclear.
-------------------------------------------------------------------------------------------------------------------------------------
In this study, we thought that the water molecule structures around methanol are important to understand the physical properties of alcohol-water mixtures.
Especially, we predicted that forming of water 3 dimensional structure contributes to the attractive interaction.
-------------------------------------------------------------------------------------------------------------------------------------
So, we evaluated the water structure around methanol using local structure order.
And we discussed on why intermolecular interactions change from repulsive to attractive.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will explain how to evaluate the q value.
In this study, we adopted q value to evaluate the local structural order.
This is equation of q value.
The q value represent the tetrahedrality of water molecule.
The tetrahedrality of water molecules is important probe to understand the aqueous solution structure.
-------------------------------------------------------------------------------------------------------------------------------------
First, we focused on one water molecule
-------------------------------------------------------------------------------------------------------------------------------------
and picked out nearest four neighbor water molecules.
-------------------------------------------------------------------------------------------------------------------------------------
Next, we calculated the angle between the oxygen atoms.
-------------------------------------------------------------------------------------------------------------------------------------
and we calculate the angle in all combinations.
-------------------------------------------------------------------------------------------------------------------------------------
If water molecules form tetrahedral structure, the q value becomes 1.
-------------------------------------------------------------------------------------------------------------------------------------
This figure shows the q value of water calculated by Errington using molecular dynamics simulation.
In this figure, there are two peaks around q = 0.7 and 0.5.
-------------------------------------------------------------------------------------------------------------------------------------
This peak represents the tetrahedral structure of water molecules.
-------------------------------------------------------------------------------------------------------------------------------------
On the other hand, this peak represents the water molecules exiting inside of the tetrahedral structure.
-------------------------------------------------------------------------------------------------------------------------------------
Computational details are shown in this slide. カンピュテーショナル
The characteristic point of this molecular dynamics simulation is this.
Number density was set to the experimentally determined densities.
As a result of this, we can reflect actual volumetric behavior to molecular dynamics simulation.
-------------------------------------------------------------------------------------------------------------------------------------
For the molecular models, we used 3site flexible models based on SPC and OPLS model.
-------------------------------------------------------------------------------------------------------------------------------------
As you know, the methanol molecule consists of hydroxyl group and methyl group. ハイドロキシル メソル
The hydration structures around each functional group are different. マァラァキュゥール
So, we divided the space around the methanol molecule into two regions like this. デファレント
We defined this region as the hydroxyl group side.
And this region as the methyl group side.
We calculated the q values of water molecules in these regions.
-------------------------------------------------------------------------------------------------------------------------------------
In this slide, I will show the radial distribution functions in these regions.
The conditions are 298 K and 0.1 MPa. カァンディション
This side is hydroxyl group side.
And this side is methyl group side.
As you can see, the position of the peak does not show composition dependence in both radial distribution functions.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will show the temperature dependency of the radial distribution function.
The conditions are 20 MPa and composition 0.1. コンパリスン
For comparison, I showed the result of ambient.
The position of peak does not also show temperature dependence.
-----------------------------------------------------------------------------------------------------------------------------------
So, based on the peaks of the radial distribution function, we divided the space around methanol into 4 sections like this.
And, we calculated q value in each section.
---30--------------------------------------------------------------------------------------------------------------------------------
In this slide I will show the q values at ambient and composition 0.1.
Except for section 1 and section 2 of hydroxyl group side, a shoulder peak was observed around the q value of 0.7.
This shoulder peaks represent the tetrahedral structure of water molecules.
However this shoulder peaks were not observed in section 1 and 2 of hydroxyl group side.
This is probably because the hydroxyl group form hydrogen bonding with the nearest water molecules.
So, in section 1 and 2 of hydroxyl group side, water molecule cannot form the tetrahedral structure.
We can see the large difference between the methyl group side and the hydroxyl group side in section 2.
So, I’d like to focus on the section 2 in following slide.
-------------------------------------------------------------------------------------------------------------------------------------
The q values of all compositions in section 2 are shown in this slide.
Focusing on the methyl group side, we can see the shoulders at around q = 0.7 at low methanol compositions.
These peaks reflect the tetrahedral structures of water molecules at low methanol composition.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will show the result at 618 K and 20 MPa.
For comparison, the results at ambient were also shown.
The shoulder peaks at around q = 0.7 disappeared at all methanol compositions.
This represents that the tetrahedral structure of water molecule might disappear at 618 K – 20 MPa.
-------------------------------------------------------------------------------------------------------------------------------------
We considered the possibility that we cannot evaluate the local structural order with q value at high temperature and high pressure.
-------------------------------------------------------------------------------------------------------------------------------------
At high temperatures, molecular motions are large.
As a result, many water molecules can exist at inside space of the tetrahedral structures temporarily.テンポラリリー
-------------------------------------------------------------------------------------------------------------------------------------
In this case, the peak of q-value becomes broad even if the tetrahedral structures exit.
So, we removed the effect of these molecules
-------------------------------------------------------------------------------------------------------------------------------------
We picked out water molecule by limiting the intermolecular distance.
This figure is radial distribution function between oxygen and oxygen of water molecules.
Based on this radial distribution function, we set the limiting intermolecular distance to 2.8 Å ± 5 %.
-------------------------------------------------------------------------------------------------------------------------------------
This slide shows the modified q value at 618 K and 20 MPa and 30 MPa.
The peaks at around q = 0.6 are observed at low xm condition.
This indicates that the tetrahedral structures of water molecule remain somewhat.
In addition, q values are almost same in hydroxyl group side and methyl group side.
-------------------------------------------------------------------------------------------------------------------------------------
Focusing on the composition dependence of the peak, these peaks disappeared at composition 0.3 or 0.4 and at 20MPa.
On the other hand, these peaks disappeared at composition 0.4 or 0.6 and at 30MPa.
-------------------------------------------------------------------------------------------------------------------------------------
This figure is excess molar volumes shown in introduction.
This q values contain arbitrary elements because we decided the limiting intermolecular distance to 2.8 Å ± 5 %.
アービトラリ
So, the threshold compositions are not directly related. スレッショウルド
However, I think that these characteristic trend are important to understand the volumetric behaviors.
-The tetrahedral structures remain even at high temperatures.
-The tetrahedral structures remains until higher methanol composition at higher pressure.
-------------------------------------------------------------------------------------------------------------------------------------
In other words, at low methanol composition, water molecules form tetrahedral structures and the attractive interaction is dominant.
On the other hand, at high methanol composition, water molecules cannot form tetrahedral structures and the repulsive interaction is dominant.
These could be reflected in the excess molar volumes at high temperatures and high pressures.
終了行:
-------------------------------------------------------------------------------------------------------------------------------------
Today I’d like to talk about microscopic structure of methanol-water mixture at high temperatures and high pressures.
-------------------------------------------------------------------------------------------------------------------------------------
This is outline of my presentation.
First, I will introduce the features of alcohol-water mixtures and objective of this study.
Second, I will show the evaluation method of local structural order and details of molecular dynamics simulation.
Next, I will show the calculation results.
Finally, I will show conclusions.
-------------------------------------------------------------------------------------------------------------------------------------
First, I’d like to talk about a features of alcohol-water mixture. アンフィフィリック
Alcohol-water mixtures are widely used for industrial application. rec・og・nize rékəgnὰɪz
And these are also scientifically attractive as models of amphiphilic aqueous solutions. ap・pli・ca・tion`æpləkéɪʃən
In addition, recently, alcohol-water mixtures of high temperatures and pressures have been recognized as functional solvents. sol・vent sάlv(ə)nt
-------------------------------------------------------------------------------------------------------------------------------------
Alcohol is amphiphilic, and alcohol-water mixtures include repulsive interaction and attractive interaction.
-------------------------------------------------------------------------------------------------------------------------------------
So, at high temperature and high pressures, this balance changes. al・co・hol ˈælkəh`ɔːl
As a result, unique physical properties and unique solvation effect are expected. re・pul・sive rɪpˈʌlsɪv
tem・per・a・ture témp(ə)rətʃ`ʊɚ
pres・sure préʃɚ
con・di・tion kəndíʃən
expected ɪksˈpɛktɪd
-------------------------------------------------------------------------------------------------------------------------------------
Density is one of the most important thermodynamic property.
Our group and several groups have measured the densities of alcohol-water mixtures at high temperatures and high pressures.
This slide shows the excess molar volumes of methanol-water mixtures at 573 K and 618 K.
Horizontal axis means methanol composition.
As you can see, the excess molar volumes are negative at under 573 K.
thèrmo・dynámic
ǽksis、アクシス
カンポジションkɑ̀mpəzíʃən com・po・si・tion
prop・er・ty prάpɚṭi
sev・er・al sév(ə)rəl/
-------------------------------------------------------------------------------------------------------------------------------------
However, our group have reported the excess molar volumes changed from negative to positive with increasing the methanol composition at 618 K.
I’d like to focus on this part.
-------------------------------------------------------------------------------------------------------------------------------------
This is an enlargement figure of excess molar volumes.
This zero crossing point composition increased with increasing pressure.
-------------------------------------------------------------------------------------------------------------------------------------
Here, consider the microscopic structure.
Basically, water molecules form 3 dimensional network structures like this.
-------------------------------------------------------------------------------------------------------------------------------------
In alcohol water mixture, alcohol molecules can enter the space of the 3 dimensional network.
As a result, excess molar volumes show negative values.
In this case apparently, the attractive interaction looks dominant.
---12--------------------------------------------------------------------------------------------------------------------------------
In general, the repulsive interaction is mainly determined by molecular size.
On the other hand, the attractive interaction is determined by various factors. ベアリアス
For example temperature, pressure or composition. プラバブリー
Probably, in these zero crossing points, the repulsive interaction and attractive interaction are balanced.
And, solution structures could change dramatically in this point.
However details are still unclear.
-------------------------------------------------------------------------------------------------------------------------------------
In this study, we thought that the water molecule structures around methanol are important to understand the physical properties of alcohol-water mixtures.
Especially, we predicted that forming of water 3 dimensional structure contributes to the attractive interaction.
-------------------------------------------------------------------------------------------------------------------------------------
So, we evaluated the water structure around methanol using local structure order.
And we discussed on why intermolecular interactions change from repulsive to attractive.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will explain how to evaluate the q value.
In this study, we adopted q value to evaluate the local structural order.
This is equation of q value.
The q value represent the tetrahedrality of water molecule.
The tetrahedrality of water molecules is important probe to understand the aqueous solution structure.
-------------------------------------------------------------------------------------------------------------------------------------
First, we focused on one water molecule
-------------------------------------------------------------------------------------------------------------------------------------
and picked out nearest four neighbor water molecules.
-------------------------------------------------------------------------------------------------------------------------------------
Next, we calculated the angle between the oxygen atoms.
-------------------------------------------------------------------------------------------------------------------------------------
and we calculate the angle in all combinations.
-------------------------------------------------------------------------------------------------------------------------------------
If water molecules form tetrahedral structure, the q value becomes 1.
-------------------------------------------------------------------------------------------------------------------------------------
This figure shows the q value of water calculated by Errington using molecular dynamics simulation.
In this figure, there are two peaks around q = 0.7 and 0.5.
-------------------------------------------------------------------------------------------------------------------------------------
This peak represents the tetrahedral structure of water molecules.
-------------------------------------------------------------------------------------------------------------------------------------
On the other hand, this peak represents the water molecules exiting inside of the tetrahedral structure.
-------------------------------------------------------------------------------------------------------------------------------------
Computational details are shown in this slide. カンピュテーショナル
The characteristic point of this molecular dynamics simulation is this.
Number density was set to the experimentally determined densities.
As a result of this, we can reflect actual volumetric behavior to molecular dynamics simulation.
-------------------------------------------------------------------------------------------------------------------------------------
For the molecular models, we used 3site flexible models based on SPC and OPLS model.
-------------------------------------------------------------------------------------------------------------------------------------
As you know, the methanol molecule consists of hydroxyl group and methyl group. ハイドロキシル メソル
The hydration structures around each functional group are different. マァラァキュゥール
So, we divided the space around the methanol molecule into two regions like this. デファレント
We defined this region as the hydroxyl group side.
And this region as the methyl group side.
We calculated the q values of water molecules in these regions.
-------------------------------------------------------------------------------------------------------------------------------------
In this slide, I will show the radial distribution functions in these regions.
The conditions are 298 K and 0.1 MPa. カァンディション
This side is hydroxyl group side.
And this side is methyl group side.
As you can see, the position of the peak does not show composition dependence in both radial distribution functions.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will show the temperature dependency of the radial distribution function.
The conditions are 20 MPa and composition 0.1. コンパリスン
For comparison, I showed the result of ambient.
The position of peak does not also show temperature dependence.
-----------------------------------------------------------------------------------------------------------------------------------
So, based on the peaks of the radial distribution function, we divided the space around methanol into 4 sections like this.
And, we calculated q value in each section.
---30--------------------------------------------------------------------------------------------------------------------------------
In this slide I will show the q values at ambient and composition 0.1.
Except for section 1 and section 2 of hydroxyl group side, a shoulder peak was observed around the q value of 0.7.
This shoulder peaks represent the tetrahedral structure of water molecules.
However this shoulder peaks were not observed in section 1 and 2 of hydroxyl group side.
This is probably because the hydroxyl group form hydrogen bonding with the nearest water molecules.
So, in section 1 and 2 of hydroxyl group side, water molecule cannot form the tetrahedral structure.
We can see the large difference between the methyl group side and the hydroxyl group side in section 2.
So, I’d like to focus on the section 2 in following slide.
-------------------------------------------------------------------------------------------------------------------------------------
The q values of all compositions in section 2 are shown in this slide.
Focusing on the methyl group side, we can see the shoulders at around q = 0.7 at low methanol compositions.
These peaks reflect the tetrahedral structures of water molecules at low methanol composition.
-------------------------------------------------------------------------------------------------------------------------------------
Next, I will show the result at 618 K and 20 MPa.
For comparison, the results at ambient were also shown.
The shoulder peaks at around q = 0.7 disappeared at all methanol compositions.
This represents that the tetrahedral structure of water molecule might disappear at 618 K – 20 MPa.
-------------------------------------------------------------------------------------------------------------------------------------
We considered the possibility that we cannot evaluate the local structural order with q value at high temperature and high pressure.
-------------------------------------------------------------------------------------------------------------------------------------
At high temperatures, molecular motions are large.
As a result, many water molecules can exist at inside space of the tetrahedral structures temporarily.テンポラリリー
-------------------------------------------------------------------------------------------------------------------------------------
In this case, the peak of q-value becomes broad even if the tetrahedral structures exit.
So, we removed the effect of these molecules
-------------------------------------------------------------------------------------------------------------------------------------
We picked out water molecule by limiting the intermolecular distance.
This figure is radial distribution function between oxygen and oxygen of water molecules.
Based on this radial distribution function, we set the limiting intermolecular distance to 2.8 Å ± 5 %.
-------------------------------------------------------------------------------------------------------------------------------------
This slide shows the modified q value at 618 K and 20 MPa and 30 MPa.
The peaks at around q = 0.6 are observed at low xm condition.
This indicates that the tetrahedral structures of water molecule remain somewhat.
In addition, q values are almost same in hydroxyl group side and methyl group side.
-------------------------------------------------------------------------------------------------------------------------------------
Focusing on the composition dependence of the peak, these peaks disappeared at composition 0.3 or 0.4 and at 20MPa.
On the other hand, these peaks disappeared at composition 0.4 or 0.6 and at 30MPa.
-------------------------------------------------------------------------------------------------------------------------------------
This figure is excess molar volumes shown in introduction.
This q values contain arbitrary elements because we decided the limiting intermolecular distance to 2.8 Å ± 5 %.
アービトラリ
So, the threshold compositions are not directly related. スレッショウルド
However, I think that these characteristic trend are important to understand the volumetric behaviors.
-The tetrahedral structures remain even at high temperatures.
-The tetrahedral structures remains until higher methanol composition at higher pressure.
-------------------------------------------------------------------------------------------------------------------------------------
In other words, at low methanol composition, water molecules form tetrahedral structures and the attractive interaction is dominant.
On the other hand, at high methanol composition, water molecules cannot form tetrahedral structures and the repulsive interaction is dominant.
These could be reflected in the excess molar volumes at high temperatures and high pressures.
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