Азеотропная смесь

Азеотропная смесь (от а — отрицательная частица, греческое zéō — киплю и tropē — поворот, изменение), нераздельно-кипящая смесь; однородная жидкая смесь, которая при перегонке не разделяется на фракции. Существование А. с. открыто в 1810 Дж. Дальтоном. Одни А. с. кипят при более высокой температуре, чем их отдельные компоненты, другие — при более низкой. Например, смесь из 95,57% C2H5OH (tkип. 78,5°С) и 4,43% H2O (tkип. 100°С) образует А. с., кипящую при 78,15°С. Напротив, А. с. из 69,2% HNO3 (tkип84°С) и 30,8% H2O кипит при 121,8°С. Из-за образования А. с. получить абсолютный спирт (100%-ный) или чистую азотную кислоту перегонкой их водных растворов невозможно. В таких случаях перегонку ведут с добавлением третьего компонента, изменяющего относительную летучесть первых двух. Около 50% жидких смесей промышленного значения образуют А. с. В связи с необходимостью разделения этих смесей на индивидуальные компоненты изучение А. с. имеет большое практическое значение.

АЗЕОТРОПНАЯ СМЕСЬ — однородная смесь двух жидкостей, состав к-рой не меняется при перегонке, т. е. содержание каждого компонента в парах остаёт­ся таким же, как и в жидкой смеси, до полного выки­пания её при постоянной температуре. Другими ело- йами А. с. при перегонке не разделяется на фракции и поэтому называется также нораздельнокипящей смесью. Существование А. с. открыл в 1810 англ. учё­ный Дж. Дальтон. Изучая температуры кипения вод­ных растворов азотной кислоты при атмосферном давлении, он нашёл, что раствор, содержащий 68% кислоты, обладает наиболее высокой точкой кипения (120°) и при перегонке не меняет своего состава; подобные же свойства имеет и соляная кислота, со­держащая ок. 19% хлористого водорода, кипящая ок. 111°. Из постоянства состава и температур ки­пения этих растворов в 1820 был сделан вывод, что их следует рассматривать как определённые соеди­нения состава 21IN03-3H20nНС1-8Л20.Ошибочность такого вывода показал в 1859 англ. учёный Роско, к-рый обнаружил, что состав жидкости, получаю­щейся при перегонке А. е., зависит от давления; напр. А. с. хлористого водорода и воды содержит 19% НС1 при перегонке под давлением 2 атм. и 20,2% НС1 нри перегонке под давлением 1 атм. Но только классич. исследования русского химика Д. П. Коновалова (1881—84) внесли полную ясность в вопрос об А. с. и окончательно показали, что нет никаких оснований считать А. с. определёнными химич. соединениями. Д. П. Коновалов открыл но­сящий его имя закон, со­гласно к-рому максимальному (минимальному) дав­лению пара или минимальной (максимальной) тем­пературе кипения в ряде смесей двух жидкостей отвечают пар и жидкость одинакового состава. В со­ответствии с этим различают А. е., кипящие при минимальной температуре, т. е.— ниже температуры кипения низкокитгящего компонента, и А. е., кипя­щие при максимальной температуре кипения, т. е.— выше температуры кипения высококинящего компо­нента. Примером первого случая может служить смесь 95,57% этилового спирта (г°Мп.78,50) и 4,43% воды (г°кмп.100°), к-рая кипит при 78,15°; примером второго случая — смесь 68% азотной кислоты (fKUn. 86°) и 32% воды (fr.vv. 100°), к-рая кипит при 120,5°. А. с. часто встречаются в практике. Образование А. с. воды со спиртом, воды с азотной кислотой свидетельствует о том, что получить абсолютный спирт  и 100%-ную азотную кислоту  пере­гонкой их водных растворов невозможно.

An azeotrope (/əˈziːətroʊp/ ə-zee-ə-trohp) is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simpledistillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture.

Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling mixtures. The wordazeotrope is derived from the Greek words ζέειν (boil) and τρόπος (state) combined with the prefix α- (no) to give the overall meaning, "no change on boiling.”

Azeotropic mixtures of pairs of compounds have been documented. (See Azeotrope (data)). Many azeotropes of three or more compounds are also known.

Each azeotrope has a characteristic boiling point. The boiling point temperature of an azeotrope is either less than the boiling point temperatures of any of its constituents (a positive azeotrope), or greater than the boiling point temperatures of any of its constituents (a negative azeotrope).

A well known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.2°C, which is lower than either of its constituents. Indeed 78.2°C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures or pressure maximum azeotropes.

An example of a negative azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water (by weight). Hydrogen chloride boils at −84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher than either of its constituents. The maximum temperature at which any hydrochloric acid solution can boil is 110°C. In general, a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called maximum boiling mixtures or pressure minimum azeotropes.

If the constituents of a mixture are not completely miscible an azeotrope can be found inside the miscibility gap. This type of azeotrope is called heterogeneous azeotrope. If the azeotropic composition is outside the miscibility gap or the constituents of the mixture are completely miscible the type of azeotrope is called a homogeneous azeotrope.

If two solvents can form a positive azeotrope, then distillation of any mixture of those constituents will result in the distillate being closer in composition to the azeotrope than the starting mixture. For example, if a 50/50 mixture of ethanol and water is distilled once, the distillate will be 80% ethanol and 20% water (see ethanol data page), which is closer to the azeotropic mixture than the original. Distilling the 80/20% mixture produces a distillate that is 87% ethanol and 13% water. Further repeated distillations will produce mixtures that are progressively closer to the azeotropic ratio of 95.5/4.5%. No number of distillations, however, will ever result in a distillate that exceeds the azeotropic ratio. Likewise when distilling a mixture of ethanol and water that is richer in ethanol than the azeotrope, the distillate (contrary to intuition) will be poorer in ethanol than the original but slightly richer than the azeotrope. This means the solution left behind will be richer in ethanol.

If two solvents can form a negative azeotrope, then distillation of any mixture of those constituents will result in the residue being closer in composition to the azeotrope than the original mixture. For example, if a hydrochloric acid solution contains less than 20.2% hydrogen chloride, boiling the mixture will leave behind a solution that is richer in hydrogen chloride than the original. If the solution initially contains more than 20.2% hydrogen chloride, then boiling will leave behind a solution that is poorer in hydrogen chloride than the original. Boiling of any hydrochloric acid solution long enough will cause the solution left behind to approach the azeotropic ratio.

The diagram on the right shows a positive azeotrope of hypothetical constituents, X and Y. The bottom trace illustrates the boiling temperature of various compositions. Below the bottom trace, only the liquid phase is in equilibrium. The top trace illustrates the vapor composition above the liquid at a given temperature. Above the top trace, only the vapor is in equilibrium. Between the two traces, liquid and vapor phases exist simultaneously in equilibrium: for example, heating a 25% X : 75% Y mixture to temperature AB would generate vapor of composition B over liquid of composition A. The azeotrope is the point on the diagram where the two curves touch. The horizontal and vertical steps show the path of repeated distillations. Point A is the boiling point of a nonazeotropic mixture. The vapor that separates at that temperature has composition B. The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A. The vapor is physically separated from the VLE (vapor-liquid equilibrium) system and is cooled to point C, where it condenses. The resulting liquid (point C) is now richer in X than it was at point A. If the collected liquid is boiled again, it progresses to point D, and so on. The stepwise progression shows how repeated distillation can never produce a distillate that is richer in constituent X than the azeotrope. Note that starting to the right of the azeotrope point results in the same stepwise process closing in on the azeotrope point from the other direction.

The diagram on the right shows a negative azeotrope of hypothetical constituents, X and Y. Again the bottom trace illustrates the boiling temperature at various compositions, and again, below the bottom trace the mixture must be entirely liquid phase. The top trace again illustrates the condensation temperature of various compositions, and again, above the top trace the mixture must be entirely vapor phase. The point, A, shown here is a boiling point with a composition chosen very near to the azeotrope. The vapor is collected at the same temperature at point B. That vapor is cooled, condensed, and collected at point C. Because this example is a negative azeotrope rather than a positive one, the distillate is farther from the azeotrope than the original liquid mixture at point A was. So the distillate is poorer in constituent X and richer in constituent Y than the original mixture. Because this process has removed a greater fraction of Y from the liquid than it had originally, the residue must be poorer in Y and richer in X after distillation than before.

If the point, A, had been chosen to the right of the azeotrope rather than to the left, the distillate at point C would be farther to the right than A, which is to say that the distillate would be richer in X and poorer in Y than the original mixture. So in this case too, the distillate moves away from the azeotrope and the residue moves toward it. This is characteristic of negative azeotropes. No amount of distillation, however, can make either the distillate or the residue arrive on the opposite side of the azeotrope from the original mixture. This is characteristic of all azeotropes.

The traces in the phase diagrams separate whenever the composition of the vapor differs from the composition of the liquid at the same temperature. Suppose the total composition were 50/50%. You could make this composition using 50% of 50/50% vapor and 50% of 50/50% liquid, but you could also make it from 83.33% of 45/55% vapor and 16.67% of 75%/25% liquid, as well as from many other combinations. The separation of the two traces represents the range of combinations of liquid and vapor that can make each total composition.

Alternatively, one can view the lower trace as the boundary for the region of the diagram in which liquids are in equilibrium, and the upper trace as the boundary of the region in which the vapor is in equilibrium. These two boundaries need not coincide. Indeed, the region between them is a no-man's-land: attempts to bring the system to the midpoint of line-segment AB will result in a mixture of liquid A and vapor B, but nothing at the midpoint.

In each of the examples discussed so far the constituents have been miscible in all proportions with each other. For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous solution. There are pairs of solvents for which this is not the case. For example, if equal volumes of chloroform (water solubility 0.8 g/100 ml at 20°C) and water are shaken together and then left to stand, the liquid will separate into two layers. Analysis of the layers shows that the top layer is mostly water with a small amount of chloroform dissolved in it, and the bottom layer is mostly chloroform with a small amount of water dissolved in it. If the two layers are heated together, the system of layers will boil at 53.3°C, which is lower than either the boiling point of chloroform (61.2°C) or the boiling point of water (100°C). The vapor will consist of 97.0% chloroform and 3.0% water regardless of how much of each liquid layer is present (provided both layers are indeed present). If the vapor is re-condensed, the layers will reform in the condensate, and will do so in a fixed ratio, which in this case is 4.4% of the volume in the top layer and 95.6% in the bottom layer. Such a system of solvents is known as a heteroazeotrope. The diagram illustrates how the various phases of a heteroazeotrope are related.

Heteroazeotropes are always minimum boiling mixtures.

Raoult's law predicts the vapor pressures of ideal mixtures as a function of composition ratio. In general only mixtures of chemically similar solvents, such as n-hexane with n-heptane, form nearly ideal mixtures that come close to obeying Raoult's law. Solvent combinations that can form azeotropes are always nonideal, and as such they deviate from Raoult's law.

The diagram on the right illustrates total vapor pressure of three hypothetical mixtures of constituents, X, and Y. The temperature throughout the plot is assumed to be constant.

The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture. The top trace illustrates a nonideal mixture that has a positive deviation from Raoult's law, where the total combined vapor pressure of constituents, X and Y, is greater than what is predicted by Raoult's law. The top trace deviates sufficiently that there is a point on the curve where its tangent is horizontal. Whenever a mixture has a positive deviation and has a point at which the tangent is horizontal, the composition at that point is a positive azeotrope. At that point the total vapor pressure is at a maximum. Likewise the bottom trace illustrates a nonideal mixture that has a negative deviation from Raoult's law, and at the composition where tangent to the trace is horizontal there is a negative azeotrope. This is also the point where total vapor pressure is minimum.

For both the top and bottom traces, the temperature point of the azeotrope is the constant temperature chosen for the graph. If the ambient pressure is controlled to be equal to the total vapor pressure at the azeotropic mixture, then the mixture will boil at this fixed temperature.

Vapor pressure of both pure liquids as well as mixtures is a sensitive function of temperature. As a rule, vapor pressure of a liquid increases nearly exponentially as a function of temperature. If the graph were replotted for a different fixed temperature, then the total vapor pressure at the azeotropic composition will certainly change, but it is also possible that the composition at which the azeotrope occurs will change. This implies that the composition of an azeotrope is affected by the pressure chosen at which to boil the mixture. Ordinarily distillation is done at atmospheric pressure, but with proper equipment it is possible to carry out distillation at a wide variety of pressures, both above and below atmospheric pressure.

Azeotropes can only form when a mixture deviates from Raoult's law. Raoult's law applies when the molecules of the constituents stick to each other to the same degree as they do to themselves. For example, if the constituents are X and Y, then X sticks to Y with roughly equal energy as X does with X and Y does with Y. A positive deviation from Raoult's law results when the constituents have a disaffinity for each other – that is X sticks to X and Y to Y better than X sticks to Y. Because this results in the mixture having less total sticking together of the molecules than the pure constituents, they more readily escape from the stuck-together phase, which is to say the liquid phase, and into the vapor phase. When X sticks to Y more aggressively than X does to X and Y does to Y, the result is a negative deviation from Raoult's law. In this case because there is more sticking together of the molecules in the mixture than in the pure constituents, they are more reluctant to escape the stuck-together liquid phase.

When the deviation is great enough to cause a maximum or minimum in the vapor pressure versus composition function, it is a mathematical consequence that at that point, the vapor will have the same composition as the liquid, and so an azeotrope is the result.

The rules for positive and negative azeotropes apply to all the examples discussed so far. But there are some examples that don't fit into the categories of positive or negative azeotropes. The best known of these is the ternary azeotrope formed by 30% acetone, 47% chloroform, and 23% methanol, which boils at 57.5°C. Each pair of these constituents forms a binary azeotrope, but chloroform/methanol and acetone/methanol both form positive azeotropes while chloroform/acetone forms a negative azeotrope. The resulting ternary azeotrope is neither positive nor negative. Its boiling point falls between the boiling points of acetone and chloroform, so it is neither a maximum nor a minimum boiling point. This type of system is called a saddle azeotrope.[2]Only systems of three or more constituents can form saddle azeotropes.

A rare type of complex binary azeotrope is one where the boiling point and condensation point curves touch at two points in the phase diagram. Such a system is called a double azeotrope, and will have two azeotropic compositions and boiling points. An example is water and N-methylethylenediamine.