Name: steam distillation and low-boiling separation
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 54 s
Channel: JoVE

2つの混和性液体の理想的な混合物を沸騰するまで加熱すると、溶液は各成分の沸点間の温度で沸騰します。これらの液体の沸点が非常に異なる場合、混合物が沸騰し始めると、蒸気はより揮発性の高い成分の分子に富んでいます。この現象は、2つの混和性液体の混合物を加熱し、蒸気を凝縮して液体に戻して収集する単純蒸留を使用して混合物を分離するためによく使用されます。
揮発性の高い成分を多く含む蒸気を留出物として集めると、液相は揮発性の低い成分の分子を豊富に含みます。ただし、この手法では、溶液を少なくとも揮発性の高い化合物の沸点まで加熱し、多くの場合、それを超えて加熱する必要があります。
温度に敏感な有機化合物の場合、この高温により、有機分子が別のものに分解される可能性があります。では、これらの種類の化合物をどのように分離できるのでしょうか。まず、一歩下がってみよう。
凝縮相と平衡状態にある蒸気の圧力は蒸気圧と呼ばれることを思い出してください。液体の混合物の成分は、それぞれ独自の蒸気圧を持っており、これを分圧と呼んでいます。溶液の全蒸気圧が大気圧と等しいときに溶液が沸騰することがわかっています。全蒸気圧は、コンポーネントの分圧の合計に等しくなります。
混和性液体の混合物、つまり液体の任意の組み合わせが均質な溶液を形成する場合、分圧は、純粋な化合物の蒸気圧に溶液中のモル分率を掛けて計算されます。ただし、非混和性液体の不均一混合物、つまり液体が互いに不溶性である場合、分圧は単に純粋な化合物の蒸気圧です。
不均一混合物の各成分は、他の成分とは無関係に全蒸気圧に寄与するため、分圧の合計である全蒸気圧が大気圧と等しくなると、混合物は沸騰します。これは、全蒸気圧が温度とともに最も揮発性の高い成分よりもはるかに速く上昇するため、各コンポーネントの個々の沸点よりも低い温度で発生します。
この現象を利用して水蒸気蒸留を行い、高温で分解し、水に溶けない温度感受性有機化合物を不揮発性物質から分離します。水蒸気蒸留のセットアップは、プロセス全体で水を補充するための貯水池を追加する単純な蒸留セットアップと似ています。
混合物が沸騰すると、水と目的の有機化合物の両方が気化します。水と有機化合物の蒸気はコンデンサーに移動し、凝縮されて液体になり、収集されます。非混和性の液体は、その後分離されます。フラスコ内の混合物には、水と不揮発性物質のみが残ります。
このラボでは、オレンジの皮の不揮発性成分からエッセンシャルオイルを抽出するための水蒸気蒸留実験をセットアップして実行します。次に、液液抽出を使用して、水からエッセンシャルオイルを有機溶媒に抽出します。

水蒸気蒸留

Steam Distillation

<h4>水蒸気蒸留 </h4>
水蒸気蒸留は、非混和性混合物の低沸点特性を利用した分離技術です。主に、温度に敏感な有機分子を不揮発性汚染物質から分離するために使用されます。有機分子は水と混和しない必要があります。
水蒸気蒸留では、非混和性混合物が沸騰するまで加熱され、水と揮発性有機化合物の両方が蒸留されます。これは、ガス混合物がコンデンサーに上向きに移動し、コンデンサーが蒸気を凝縮して液体に集めることを意味します。単純な蒸留とは対照的に、水蒸気蒸留は、プロセス全体で加熱された混合物に水を補充するために貯水池を使用します。非混和性の有機成分は水とともにゆっくりと蒸留されますが、不揮発性成分は加熱された混合物に残ります。有機成分が蒸留されたら、液液抽出を使用して水から分離できます。
<h4>混合物の蒸気圧</h4>
均質な溶液を形成する混和性混合物の場合、各成分の蒸気圧は、ラウルトの法則に従って、純粋な成分の蒸気圧と液体混合物中のそのモル分率に依存します。
pA = pA*xA
ここで、pAは混和性液体混合物中の1つの液体成分の蒸気圧、pA*は純粋な液体の蒸気圧、xAは混合物中のその液体のモル分率であり、これはnA/ntに等しい。 ntは、混合物中のすべての液体の合計モル数です。
混和性液体混合物を超える全蒸気圧は、その中の各成分の部分蒸気圧の合計に等しく、これはダルトンの法則として知られています。液体の蒸気圧は、より多くの分子が運動エネルギーを獲得して液相から気相に逃げるにつれて、温度とともに増加します。2つの液体を含む混和性混合物では、全圧は次のように説明できます。
P = pA + pB
ここで、p AおよびpBは、混合物の上の液体A、液体Bのそれぞれ蒸気圧です。Pは、混合物より上の両方の液体の合計蒸気圧です。方程式を組み合わせると、溶液の全蒸気圧と個々の成分のモル分率との関係が表れます。
P = pA*xA + pB*xB
非混和性混合物では、成分が不均一な混合物を形成するため、各成分の蒸気圧は全蒸気圧に独立して寄与します。したがって、全蒸気圧は、個々の純粋な蒸気圧の合計に等しくなります。2つの液体からなる非混和性混合物では、全圧は最初の液体の蒸気圧と2番目の液体の蒸気圧の合計として定義されます。
P = pA* + pB*
<h4>非混和性混合物の沸点</h4>
液体が加熱されると、蒸気圧が上昇する。混合物中の各成分は、それ自身の沸点を有する。混和性液体の混合物では、構成液体の沸点間の温度で沸騰が起こります。
非混和性混合物の場合、沸騰は個々の成分の沸点よりもはるかに低い温度で発生します。個々のコンポーネントが独立して寄与するため、全蒸気圧を大気圧まで上昇させるために必要な熱が少なくて済みます。
たとえば、ベンゼンと水の非混和性混合物を考えてみましょう。ベンゼンの常圧での沸点は80.1°C、常圧での水の沸点は100°Cです。全蒸気圧が760mmHg(通常の大気圧)に達すると、溶液が沸騰します。69.3°Cでは、水の蒸気圧は227 mm Hg、ベンゼンの蒸気圧は533 mm Hgで、これは合計で沸騰に必要な760 mm Hgに相当します。これは、個々のコンポーネントの沸点をはるかに下回っています。
<h4>参照</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012).化学および化学反応性。カリフォルニア州ベルモント:ブルックス/コール、Cengage Learning。</li>
	<li style="font-size:17px">Silderberg, M.S. (2009).化学:物質と変化の分子的性質。マサチューセッツ州ボストン:マグロウヒル。</li>
</ol>

水蒸気蒸留
水蒸気蒸留は、非混和性混合物の低沸点特性を利用した分離技術です。それは主に...

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JoVE Lab Manual

Chemistry

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steam distillation and low-boiling separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations.

In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser, where it cools and becomes a liquid that can be collected. Unlike simple distillation, this setup uses a water reservoir to keep replacing water in the heated mixture during the process.

The organic component is carried over slowly with the water, while the non-volatile component stays behind in the heated flask. After distillation, the organic product can be separated from the water by liquid-liquid extraction. This makes steam distillation useful for isolating sensitive organic materials from unwanted impurities.

The same low-boiling behavior can be explained by vapor pressure. For a miscible mixture, each liquid follows Raoult’s law, so its vapor pressure depends on the pure liquid vapor pressure and its mole fraction. The mole fraction is the number of moles of one liquid divided by the total number of moles in the mixture. The total vapor pressure above a miscible mixture follows Dalton’s law and equals the sum of the partial vapor pressures.

As temperature rises, vapor pressure increases because more molecules gain enough kinetic energy to escape into the gas phase. In a two-liquid miscible mixture, the total pressure can be written as P = pA + pB, or as P = pA*xA + pB*xB when Raoult’s law is included. In an immiscible mixture, the components act independently, so the total pressure is the sum of the vapor pressure of the first liquid and the vapor pressure of the second liquid.

This independent vapor-pressure behavior is why immiscible mixtures boil at lower temperatures than either pure liquid. In a miscible mixture, boiling usually happens between the boiling points of the components. In an immiscible mixture, less heat is needed to reach atmospheric pressure, so the boiling point drops well below the boiling points of the individual liquids.

A benzene-water mixture shows this effect clearly. Benzene boils at 80.1 °C, and water boils at 100 °C at normal atmospheric pressure. The mixture boils when the total vapor pressure reaches 760 mm Hg. At 69.3 °C, water contributes 227 mm Hg and benzene contributes 533 mm Hg, which adds up to 760 mm Hg. That means the mixture boils at a temperature below either pure component.

Steam Distillation and Low-Boiling Separation

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入門化学ラボコースの指導者と学生のための、包括的でカリキュラムに焦点を当てたビデオ。

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Learn how steam distillation separates temperature-sensitive organic compounds from water-insoluble mixtures using vapor pressure and low boiling points.

Steam Distillation and Low-Boiling Separation | JoVE K12

水蒸気蒸留 を教える準備を、この 4 分間の教員向けガイドで行いましょう。JoVE Lab Manual の Chemistry ラボの主要な準備手順を解説します。

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Steam Distillation and Low-Boiling Separation

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