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

当两种互溶液体的理想混合物被加热至沸腾时，该溶液的沸点介于各组分的沸点之间。如果这两种液体的沸点相差较大，则混合物开始沸腾时，蒸气中富含挥发性较高组分的分子。这一现象常用于通过简单蒸馏来分离混合物，即加热两种互溶液体的混合物，将产生的蒸气冷凝为液体并收集。
随着富含易挥发组分的蒸气被收集为馏出液，液相中则逐渐富集难挥发组分的分子。然而，该技术要求将溶液加热至至少易挥发组分的沸点，通常还需超过该温度。
对于温度敏感的有机化合物而言，这种高温可能导致有机分子分解为其他物质。那么，我们该如何分离这类化合物呢？首先，让我们退一步思考。
请记住，与凝聚相处于平衡状态的蒸气所具有的压力称为蒸气压。液体混合物中的各组分各自具有自身的蒸气压，我们称之为分压。我们知道，当溶液的总蒸气压等于大气压时，溶液就会沸腾。总蒸气压等于各组分分压之和。
对于互溶液体的混合物，即任意比例的液体组合均能形成均相溶液的情况，其分压由各纯组分的蒸气压与其在溶液中的摩尔分数的乘积计算得出。然而，对于不互溶液体的非均相混合物，即液体彼此不溶的情况，其分压即为各纯组分的蒸气压。
由于非均相混合物中的各组分对总蒸气压的贡献彼此独立，因此当总蒸气压（即各组分分压之和）等于大气压时，混合物即发生沸腾。该过程发生的温度低于各组分各自的沸点，因为总蒸气压随温度升高的增长速率远高于即使是最易挥发组分单独存在时的预期增长速率。
我们可以利用这一现象进行水蒸气蒸馏，该方法用于从非挥发性物质中分离对温度敏感、在高温下易分解且不溶于水的有机化合物。水蒸气蒸馏装置与简单蒸馏装置类似，但增加了一个水储罐，以便在蒸馏过程中持续补充水分。
当混合物沸腾时，水和目标有机化合物同时汽化。水蒸气和有机化合物蒸气进入冷凝器，冷凝为液体后被收集。随后将不相混溶的液体分离。烧瓶中的混合物仅剩下水和非挥发性物质。
在本实验中，您将搭建并实施蒸汽蒸馏实验，以从橙皮的非挥发性组分中提取精油。随后，您将使用液-液萃取法将精油从水中萃取至有机溶剂中。

水蒸气蒸馏

Steam Distillation

<h4>水蒸气蒸馏</h4>
蒸馏是一种利用不混溶混合物低沸点特性的分离技术。该方法主要用于从非挥发性污染物中分离对温度敏感的有机分子。该有机分子必须与水不混溶。
在蒸汽蒸馏过程中，将不混溶的混合物加热至沸腾，使水和挥发性有机化合物同时发生蒸馏。这意味着气态混合物上升进入冷凝器，冷凝器将蒸气转化为液体，以便收集。与简单蒸馏不同，蒸汽蒸馏通过一个储水装置在整个过程中不断向加热的混合物中补充水分。不混溶的有机组分随水缓慢蒸馏出来，而非挥发性组分则留在加热的混合物中。一旦有机组分被蒸馏出来，便可利用液-液萃取法将其从水中分离。 
<h4>混合物的蒸气压</h4>
对于形成均相溶液的互溶混合物，根据拉乌尔定律，各组分的蒸气压取决于该纯组分的蒸气压及其在液相混合物中的摩尔分数。
pA = pA*xA
其中 pA 是互溶液体混合物中某一液体组分的蒸气压，pA* 是该纯液体的蒸气压，xA 是该液体在混合物中的摩尔分数，等于 nA/nt。nA 是混合物中该单一液体的物质的量（摩尔数），nt 是混合物中所有液体的总物质的量（总摩尔数）。
互溶液体混合物上方的总蒸气压等于其中各组分的分蒸气压之和，这被称为道尔顿定律。液体的蒸气压随温度升高而增大，因为更多的分子获得足够的动能从液相逸出进入气相。在含有两种液体的互溶混合物中，总压可表示为：
P = pA + pB
其中 pA 和 pB 分别为混合物上方液体 A 和液体 B 的蒸气压。P 是混合物上方两种液体的总蒸气压。联立这些方程可描述溶液的总蒸气压与各组分摩尔分数之间的关系：
P = pA*xA + pB*xB
在不相溶的混合物中，各组分形成非均相混合物，每种组分的蒸气压均有所贡献 独立地 对总蒸气压的贡献。因此，总蒸气压等于各纯组分蒸气压之和。在由两种液体组成的不混溶混合物中，总压定义为第一种液体的蒸气压与第二种液体的蒸气压之和。
P = pA* + pB*
<h4>不混溶混合物的沸点</h4>
当液体被加热时，其蒸气压随之升高。混合物中的每种组分都有各自的沸点。在互溶液体的混合物中，沸腾发生在各组分液体沸点之间的某一温度。
对于不混溶的混合物，沸腾发生在远低于各组分沸点的温度。由于每种组分独立贡献蒸气压，因此只需较少热量即可使总蒸气压达到大气压。
例如，考虑苯和水形成的不混溶混合物。在常压下，苯的沸点为 80.1 °C，水的沸点为 100 °C。当混合溶液的总蒸气压达到 760 mm Hg（常压）时，溶液开始沸腾。在 69.3 °C 时，水的蒸气压为 227 mm Hg，苯的蒸气压为 533 mm Hg，两者之和正好达到沸腾所需的 760 mm Hg。这一温度远低于任一纯组分的沸点。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). 化学与化学反应性。加利福尼亚州贝尔蒙特：Brooks/Cole, Cengage Learning。</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). 化学：物质的分子本质与变化。马萨诸塞州波士顿：McGraw Hill。</li>
</ol>

水蒸气蒸馏  
水蒸气蒸馏是一种利用不混溶混合物低沸点特性的分离技术。该方法主要……

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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.

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Steam Distillation and Low-Boiling Separation

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