thumb|upright=1.5|Vapour-liquid equilibrium of [[2-propanol/water showing positive azeotropic behaviour]]
An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Knowing an azeotrope's behavior is important for distillation.
Each azeotrope has a characteristic boiling point. The boiling point 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 of any of its constituents (a negative azeotrope). For both positive and negative azeotropes, it is not possible to separate the components by fractional distillation and azeotropic distillation is usually used instead.
For technical applications, the pressure-temperature-composition behavior of a mixture is the most important, but other important thermophysical properties are also strongly influenced by azeotropy, including the surface tension and transport properties.
Etymology
The term azeotrope is derived from the Greek words ζέειν (boil) and τρόπος (turning) with the prefix α- (no) to give the overall meaning, "no change on boiling". The term was coined in 1911 by English chemist John Wade and Richard William Merriman. Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.
Types
Positive azeotropes
thumb|left|Positive azeotrope – mixture of [[chloroform and methanol]]
thumb|Phase diagram of a positive azeotrope. Vertical axis is temperature, horizontal axis is composition
A solution that shows greater positive deviation from Raoult's law forms a minimum boiling azeotrope at a specific composition. 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. A well-known example of a positive azeotrope is an ethanol–water mixture (obtained by fermentation of sugars) consisting of 95.63% ethanol and 4.37% water (by mass), which boils at 78.2 °C. 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. Once this composition has been achieved, the liquid and vapour have the same composition, and no further separation occurs.
The boiling and recondensation of a mixture of two solvents are changes of chemical state; as such, they are best illustrated with a phase diagram. If the pressure is held constant, the two variable parameters are the temperature and the composition.
The adjacent diagram 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. as well as benzene and hexafluorobenzene.
Complex systems
thumb|Saddle azeotropic system Methanol/Acetone/Chloroform calculated with mod. [[UNIFAC]]
Some azeotropes fit into neither the positive nor negative categories. 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. Vertical axis is temperature, horizontal axis is composition. The dotted vertical line indicates the composition of the combined layers of the distillate whenever both layers are present in the original mixture]]
If the constituents of a mixture are completely miscible in all proportions with each other, the type of azeotrope is called a homogeneous azeotrope. Homogeneous azeotropes can be of the low-boiling or high-boiling azeotropic type. For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous solution.
If the components of a mixture are not completely miscible, an azeotrope can be found inside the miscibility gap. This type of azeotrope is called a heterogeneous azeotrope or heteroazeotrope. A heteroazeotropic distillation will have two liquid phases. Heterogeneous azeotropes are only known in combination with temperature-minimum azeotropic behavior. 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.
Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be zeotropic. Azeotropes are useful in separating zeotropic mixtures. An example is zeotropic acetic acid and water. It is very difficult to separate out pure acetic acid (boiling point: 118.1 °C): progressive distillations produce drier solutions, but each further distillation becomes less effective at removing the remaining water. Distilling the solution to dry acetic acid is therefore economically impractical. But ethyl acetate forms an azeotrope with water that boils at 70.4 °C. By adding ethyl acetate as an entrainer, it is possible to distill away the azeotrope and leave nearly pure acetic acid as the residue.
Number of constituents
thumb|Construction of the p-v-x diagram appropriate for an azeotrope
Azeotropes consisting of two constituents are called binary azeotropes such as diethyl ether (33%) / halothane (66%) a mixture once commonly used in anesthesia. Azeotropes consisting of three constituents are called ternary azeotropes, e.g. acetone / methanol / chloroform. Azeotropes of more than three constituents are also known.
Condition of existence
thumb|Total vapor pressure of mixtures as a function of composition at a chosen constant temperature
The condition relates activity coefficients in liquid phase to total pressure and the vapour pressures of pure components.
Azeotropes can form only when a mixture deviates from Raoult's law, the equality of compositions in liquid phase and vapor phases, in vapour-liquid equilibrium and Dalton's law the equality of pressures for total pressure being equal to the sum of the partial pressures in real mixtures.
In other words: Raoult's law predicts the vapor pressures of ideal mixtures as a function of composition ratio. More simply: per Raoult's law 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 affinity 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 the molecules in the mixture are sticking together more than in the pure constituents, they are more reluctant to escape the stuck-together liquid phase.
The adjacent diagram 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. In general solely mixtures of chemically similar solvents, such as n-hexane with n-heptane, form nearly ideal mixtures that come close to obeying Raoult's law. 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. On the other hand, if two solvents can form a positive azeotrope, then distillation of any mixture of those constituents will result in the residue away from the composition at the azeotrope than the original mixture. For example, if a 50/50 mixture of ethanol and water is distilled once, the distillate will be 80% ethanol and 20% water, which is closer to the azeotropic mixture than the original, which means the solution left behind will be poorer in ethanol. 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 numbers of distillations 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 still richer than the azeotrope.
Distillation is one of the primary tools that chemists and chemical engineers use to separate mixtures into their constituents. Because distillation cannot separate the constituents of an azeotrope, the separation of azeotropic mixtures (also called azeotrope breaking) is a topic of considerable interest. A more compelling reason for believing that azeotropes are not compounds is, as discussed in the last section, that the composition of an azeotrope can be affected by pressure. Contrast that with a true compound, carbon dioxide for example, which is two moles of oxygen for each mole of carbon no matter what pressure the gas is observed at. That azeotropic composition can be affected by pressure suggests a means by which such a mixture can be separated.
Pressure swing distillation
thumb|Azeotrope composition shift due to pressure swing
A hypothetical azeotrope of constituents X and Y is shown in the adjacent diagram. Two sets of curves on a phase diagram one at an arbitrarily chosen low pressure and another at an arbitrarily chosen, but higher, pressure. The composition of the azeotrope is substantially different between the high- and low-pressure plots: higher in X for the high-pressure system. The goal is to separate X in as high a concentration as possible starting from point A. At the low pressure, it is possible by progressive distillation to reach a distillate at the point, B, which is on the same side of the azeotrope as A. Successive distillation steps near the azeotropic composition exhibit very little difference in boiling temperature. If this distillate is now exposed to the high pressure, it boils at point C. From C, by progressive distillation it is possible to reach a distillate at the point D, which is on the same side of the high-pressure azeotrope as C. If that distillate is then exposed again to the low pressure, it boils at point E, which is on the opposite side of the low-pressure azeotrope to A. So, by means of the pressure swing, it is possible to cross over the low-pressure azeotrope.
When the solution is boiled at point E, the distillate is poorer in X than the residue at point E. This means that the residue is richer in X than the distillate at point E. Indeed, progressive distillation can produce a residue as rich in X as is required.
In summary:
- Low-pressure rectification (A to B)
- High-pressure rectification (C to D)
- Low-pressure stripping (E to target purity)
- Rectification: the distillate, or "tops", is retained and exhibits an increasingly lower boiling point.
- Stripping: the residue, or "bottoms", is retained and exhibits an increasingly higher boiling point.
A mixture of 5% water with 95% tetrahydrofuran is an example of an azeotrope that can be economically separated using a pressure swing: a swing in this case between 1 atm and 8 atm. By contrast the composition of the water to ethanol azeotrope discussed earlier is not affected enough by pressure to be easily separated using pressure swings
Azeotropic distillation
Other methods of separation involve introducing an additional agent, called an entrainer, that will affect the volatility of one of the azeotrope constituents more than another. When an entrainer is added to a binary azeotrope to form a ternary azeotrope, and the resulting mixture distilled, the method is called azeotropic distillation. The best known example is adding benzene or cyclohexane to the water/ethanol azeotrope. With cyclohexane as the entrainer, the ternary azeotrope is 7% water, 17% ethanol, and 76% cyclohexane, and boils at 62.1 °C. Just enough cyclohexane is added to the water/ethanol azeotrope to engage all of the water into the ternary azeotrope. When the mixture is then boiled, the azeotrope vaporizes leaving a residue composed almost entirely of the excess ethanol. Anhydrous calcium chloride is used as a desiccant for drying a wide variety of solvents since it is inexpensive and does not react with most nonaqueous solvents except simple primary alcohols. Chloroform is an example of a solvent that can be effectively dried using calcium chloride.