Azeotropy: From a Distiller's Paradox to a Cornerstone of Chemical Engineering

Introduction: The Constant-Boiling Conundrum

In the vast landscape of physical chemistry and chemical engineering, few phenomena are as simultaneously confounding and fundamentally important as the azeotrope. An azeotrope is a liquid mixture of two or more components that, at a specific composition, exhibits a constant boiling point and whose vapor composition is identical to its liquid composition.¹ This unique characteristic means that the mixture's proportions cannot be altered or separated by simple distillation, a process that typically relies on the differential volatility of components.¹ When an azeotropic mixture is boiled, it vaporizes as if it were a single, pure substance, presenting a significant challenge to one of the oldest and most foundational separation techniques in chemistry.⁴

The term itself, formally introduced to the scientific lexicon in 1911 by English chemists John Wade and Richard William Merriman, perfectly encapsulates this behavior.⁵ It is derived from the Greek words

a- (no), zein (to boil), and tropos (to turn or change), literally meaning "no change on boiling".⁷ For this reason, azeotropes are also referred to in older texts as "constant boiling point mixtures".⁵

The existence of azeotropes is far from a mere academic curiosity; it is a physical manifestation of non-ideal molecular interactions that carries tangible, multi-billion-dollar consequences across global industries. The inability to easily distill ethanol beyond its 95.6% azeotropic point with water, for instance, is a defining constraint in the biofuel industry, necessitating advanced and costly purification technologies.⁹ Conversely, the very properties that make azeotropes a separation challenge can be deliberately engineered and exploited. This has led to the development of advanced materials such as high-performance, non-flammable cleaning fluids for the electronics and medical industries, as well as specialized refrigerant blends with precisely controlled thermodynamic properties.¹¹ The study of azeotropy, therefore, serves as a crucial bridge between the theoretical principles of thermodynamics and the practical, large-scale realities of industrial chemistry, driving both technological innovation and critical economic decisions.

The Thermodynamic Underpinnings of Azeotropic Behavior

The formation of an azeotrope is a direct consequence of the complex interplay of intermolecular forces within a non-ideal liquid mixture. To understand how and why they form, one must first grasp the principles of vapor-liquid equilibrium (VLE) and the ideal behavior against which azeotropes are defined.

Vapor-Liquid Equilibrium (VLE): The Foundation of Distillation

Distillation is predicated on a simple principle: when a liquid mixture is heated, the resulting vapor is typically richer in the more volatile component—that is, the component with the lower boiling point and higher vapor pressure.³ By repeatedly condensing this vapor and re-boiling it (a process known as fractional distillation), one can progressively enrich the concentration of the more volatile component, eventually achieving separation. This entire process is governed by the state of vapor-liquid equilibrium, where the rate of evaporation from the liquid equals the rate of condensation from the vapor. The VLE data for a given mixture, often visualized in phase diagrams, are the fundamental information required to design and operate any distillation process.¹³

Raoult's Law and the Ideal Solution Benchmark

In the 1880s, French chemist François-Marie Raoult formulated a law that describes the VLE of an "ideal solution".¹⁴ Raoult's Law states that the partial vapor pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the liquid phase.³ Mathematically, for a component

i:

Pi​=xi​Pisat​

where Pi​ is the partial vapor pressure of component i in the mixture, xi​ is its mole fraction in the liquid, and Pisat​ is the vapor pressure of pure component i.

An ideal solution is a theoretical construct where the intermolecular forces between unlike molecules (A-B) are assumed to be exactly the same as the average of the forces between like molecules (A-A and B-B).³ A mixture of benzene and toluene is a classic example that behaves nearly ideally.³ In such a system, the components do not interact in any special way, and their tendency to escape into the vapor phase is directly proportional to their concentration.

The Genesis of Azeotropes: Understanding Deviations from Ideality

Azeotropes can only form when a liquid mixture exhibits significant deviations from the behavior predicted by Raoult's Law.¹⁶ These deviations arise because, in real solutions, the A-B intermolecular forces are rarely identical to the A-A and B-B forces. This disparity in forces can either enhance or suppress the overall vapor pressure of the mixture, leading to two distinct types of azeotropes.

Positive Deviations and Minimum-Boiling Azeotropes

A positive deviation from Raoult's Law occurs when the adhesive forces between unlike molecules (A-B) are weaker than the cohesive forces between like molecules (A-A and B-B). In essence, the components "dislike" each other and have a greater tendency to escape the liquid phase than in an ideal solution. This mutual repulsion results in a total vapor pressure for the mixture that is higher than predicted by Raoult's Law.²

Because boiling point is the temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure, a higher vapor pressure means a lower boiling point. At a specific composition, this effect is maximized, resulting in a minimum-boiling azeotrope, where the mixture boils at a temperature lower than that of any of its pure components.¹

The most famous example is the ethanol-water mixture. Pure ethanol boils at 78.4°C and pure water at 100°C. However, at a composition of 95.6% ethanol and 4.4% water by mass, the mixture forms a minimum-boiling azeotrope that boils at 78.2°C, a temperature below that of either pure component.¹⁷

Negative Deviations and Maximum-Boiling Azeotropes

A negative deviation from Raoult's Law occurs when the adhesive forces between unlike molecules (A-B) are stronger than the cohesive forces between like molecules (A-A and B-B). This strong attraction, often due to forces like hydrogen bonding, holds the molecules more tightly within the liquid phase. Consequently, the molecules have a lower tendency to escape, and the mixture's total vapor pressure is lower than predicted by Raoult's Law.²

A lower vapor pressure requires a higher temperature to reach the boiling point. This leads to the formation of a maximum-boiling azeotrope, which boils at a temperature higher than that of any of its pure components.¹

A classic example is the nitric acid-water system. Pure nitric acid boils at 83°C and water at 100°C. At a composition of approximately 68% nitric acid and 32% water, the mixture forms a maximum-boiling azeotrope with a boiling point of 120.4°C.²

The type of azeotrope that forms, therefore, serves as a direct macroscopic indicator of the dominant microscopic intermolecular forces within the mixture. By simply observing whether the boiling point of the mixture is at a minimum or a maximum relative to its components, one can infer the nature of the molecular interactions—whether the unlike molecules weakly repel or strongly attract one another.

A Taxonomy of Azeotropic Systems

To fully characterize azeotropic behavior, chemists and engineers use a classification system based on the number of components, the miscibility of the liquids, and the boiling point behavior.

Classification by Number of Components

• Binary Azeotropes: These are the most common and widely studied azeotropes, consisting of two components, such as the ethanol/water system.²⁰
• Ternary Azeotropes: These systems consist of three components. Their phase behavior is significantly more complex and can include unique phenomena like saddle azeotropes, where the boiling point is not an absolute minimum or maximum but is intermediate between the boiling points of the pure components.¹ A well-known example is a mixture of 30% acetone, 47% chloroform, and 23% methanol, which boils at 57.5°C.¹
• Quaternary and Higher-Order Azeotropes: Azeotropes with four or more components are also known, though they are less common and their behavior is exceptionally complex. Over 18,000 different azeotropic mixtures have been documented in scientific literature.²⁰

Classification by Phase Behavior (Miscibility)

• Homogeneous Azeotropes: In these systems, the components are completely miscible in all proportions, forming a single, uniform liquid phase. The ethanol/water azeotrope is a classic example of a homogeneous azeotrope.¹
• Heterogeneous Azeotropes (Heteroazeotropes): In these systems, the components are not fully miscible and, upon condensation, separate into two or more distinct liquid layers.¹ This property is particularly useful in certain separation processes, as the layers can be physically separated using a decanter after condensation.²³ Heterogeneous azeotropes are typically minimum-boiling. For example, a mixture of chloroform and water forms a heteroazeotrope; when boiled, the vapor condenses into two layers—one rich in water and the other rich in chloroform.¹

The following table provides a consolidated comparison of the key characteristics of minimum- and maximum-boiling azeotropes, summarizing the core thermodynamic principles that govern their formation and behavior.

Table 1: Comparison of Minimum- and Maximum-Boiling Azeotropes

A Historical Trajectory: Unraveling the Azeotropic Puzzle

The understanding of azeotropy is a story of scientific progress, tracing a clear path from a persistent practical anomaly observed by ancient distillers to a formalized, theoretically grounded concept that underpins modern chemical engineering.

Ancient Roots: The Long March of Distillation

The art of distillation is ancient, with archeological evidence of primitive stills found in Mesopotamia dating to around 3500 BCE.²⁴ Akkadian tablets from circa 1200 BCE describe distillation for perfumery.⁴ For millennia, the process was an empirical craft, refined by Greek philosophers, Alexandrian alchemists like Zosimos of Panopolis in the 3rd century CE, and later by Arab scientists who developed the alembic still.⁴ It was used to create perfumes, medicinal tinctures, and, most famously, potent alcoholic spirits, often called

aqua vitae or "water of life".²⁵ Throughout this long history, distillers of alcohol would have consistently encountered a practical barrier: the inability to produce ethanol purer than about 95% through simple boiling. This was a well-known, repeatable phenomenon—a puzzle without a name or a theoretical explanation.⁹

19th-Century Foundations: Early Observations of Constant-Boiling Mixtures

The scientific framework necessary to understand this puzzle began to form in the 19th century with the rise of physical chemistry. The work of French chemist François-Marie Raoult in the 1880s was particularly crucial. His formulation of Raoult's Law for ideal solutions provided the essential theoretical baseline, allowing scientists to quantitatively measure and describe the "non-ideal" behavior of real mixtures.¹⁴

Even before Raoult's work, other chemists had noted the peculiar behavior of certain mixtures. In the 1860s, the English chemist Henry Enfield Roscoe meticulously studied aqueous solutions of acids like perchloric, formic, and acetic acid. He observed that upon repeated distillation, these mixtures reached a fixed composition that then boiled at a constant temperature, a clear description of azeotropic behavior decades before the term was invented.²⁸

The Pivotal Work of Sydney Young: A Breakthrough in Fractional Distillation

The critical figure who connected these nascent theoretical ideas with industrial practice was the British chemist Sydney Young (1857-1937).²⁹ A pioneer in the purification of organic compounds, Young conducted exhaustive studies on fractional distillation, particularly for separating hydrocarbons from the burgeoning petroleum industry.³¹ He invented new, more efficient distillation apparatuses, known as still-heads, to improve separation.³¹

Young's most transformative discovery came in 1902. While investigating the boiling points of mixed liquids, he identified the existence of a ternary minimum-boiling azeotrope composed of benzene, water, and ethanol.²⁹ This was not merely an academic finding; it was the key to solving the centuries-old problem of ethanol purity. By adding benzene to the 95.6% ethanol-water azeotrope, a new, lower-boiling ternary azeotrope was formed, which could be distilled off, carrying the last traces of water with it. This discovery directly led to the first commercially viable process for producing absolute (anhydrous) ethanol, a major technological achievement that was quickly adopted by industry.²⁹

1911: The Christening of a Concept by Wade and Merriman

With the phenomenon now well-documented and industrially relevant, the final step was its formal christening. In a 1911 paper published in the Journal of the Chemical Society, Transactions, titled "Influence of Water on the Boiling Point of Ethyl Alcohol at Pressures above and Below the Atmospheric Pressure," chemists John Wade and Richard William Merriman presented their systematic study of the ethanol-water system.⁶ It was within this work that they proposed the term

azeotrope to replace the unwieldy description "mixtures having a minimum (or maximum) boiling point," thereby giving the phenomenon its enduring modern name.²⁸

The Age of Data: Systematic Compilation and Characterization

Following its formal definition, the scientific community began the immense task of identifying and cataloging the vast number of azeotropic systems. A key figure in this effort was Lee Herbert Horsley of the Dow Chemical Company. Beginning in the 1950s, Horsley compiled comprehensive tables of binary and ternary azeotropes, which were published by the American Chemical Society as the Azeotropic Data series.³⁸ These volumes became indispensable references for chemical engineers designing separation processes. This tradition of data compilation continues today with the creation of large digital databases, such as the Dortmund Data Bank (DDB), which provide the critical VLE and azeotropic data needed for modern computer-aided process simulation and design.⁴¹

The Engineering Imperative: Separating the Inseparable

The defining feature of an azeotrope—that its liquid and vapor phases have identical compositions—makes it impossible to separate via simple distillation.¹ This fundamental challenge has spurred the development of a suite of sophisticated "enhanced" separation techniques, each designed to circumvent the azeotropic barrier.

Enhanced Distillation Techniques

1. Azeotropic Distillation

This technique involves the deliberate addition of a third component, known as an entrainer, to the mixture. The entrainer is carefully chosen to form a new, low-boiling heterogeneous azeotrope with one or both of the original components.²³ The classic example remains Sydney Young's method for dehydrating ethanol. Benzene (or a safer modern alternative like cyclohexane or toluene) is added as the entrainer to the ethanol-water mixture.²³ This forms a new ternary azeotrope (benzene-ethanol-water) that boils at a lower temperature than the original binary azeotrope. This new azeotrope is distilled off as the overhead product. Because it is heterogeneous, upon condensation it separates into two immiscible liquid layers: an organic layer rich in the entrainer and an aqueous layer. The entrainer-rich layer is recycled back into the distillation column, while the aqueous layer is processed further to recover any remaining components.²³

2. Extractive Distillation

Extractive distillation also involves adding a third component, but in this case, it is a high-boiling, relatively non-volatile solvent that does not form a new azeotrope with the original components.² The solvent's role is to interact differently with the components of the azeotropic mixture, thereby altering their relative volatilities.⁴⁶ For example, in the ethanol-water system, a solvent like ethylene glycol can be added. The glycol forms strong hydrogen bonds with water, effectively "holding on" to the water molecules and reducing their volatility. This allows the more volatile ethanol to be distilled off as a pure overhead product. The bottom product, a mixture of the solvent and water, is then fed to a second distillation column where the solvent is easily recovered and recycled due to its high boiling point.⁴⁷

3. Pressure-Swing Distillation (PSD)

This elegant method requires no third component and instead exploits the fact that for many mixtures, the azeotropic composition is sensitive to pressure.²³ The process typically uses two distillation columns operating at different pressures (e.g., one at atmospheric pressure and one at a higher pressure).⁴⁹ The feed is sent to the first (low-pressure) column, where one pure component is removed from the bottom. The overhead product, which has a composition near the azeotrope at that pressure, is then fed to the second (high-pressure) column. Because the azeotropic point shifts with pressure, this feed is no longer azeotropic in the second column. This allows the other pure component to be separated as the bottom product of the second column, while its near-azeotropic overhead is recycled back to the first column.⁵¹ While PSD avoids the cost and complexity of handling an entrainer, it is only applicable to pressure-sensitive azeotropes and typically involves higher capital costs for the two columns.²³

Non-Distillation Methods

4. Adsorptive Separation with Molecular Sieves

A widely used industrial method, particularly for final ethanol dehydration, involves molecular sieves. These are crystalline materials, typically zeolites, containing a network of uniform pores of molecular dimensions.⁵² For drying ethanol, a 3Å (0.3 nm) molecular sieve is used. The pores are large enough to allow small water molecules to enter and be adsorbed, but they are too small for the larger ethanol molecules to pass through.¹⁰ A stream of near-azeotropic ethanol vapor is passed through a bed of these sieves, which selectively trap the water, allowing anhydrous ethanol (>99.5% purity) to exit. The process is run in a cyclic fashion, with one bed adsorbing water while a second, saturated bed is regenerated by heating under vacuum or purging with an inert gas to drive off the captured water.¹⁰

5. Membrane-Based Separations: Pervaporation and Vapor Permeation

Emerging as highly energy-efficient alternatives, membrane-based processes like pervaporation and vapor permeation use a dense, non-porous membrane that exhibits preferential permeability to one component of the mixture.⁵⁵ The separation is driven not by VLE but by a solution-diffusion mechanism: components from the feed adsorb onto the membrane surface, diffuse through it at different rates, and desorb as a vapor on the other side, which is kept at a low pressure (vacuum).⁵⁶ In

pervaporation, the feed is a liquid, while in vapor permeation, the feed is a vapor, a configuration often coupled with a primary distillation column to minimize energy consumption.⁵⁸ These technologies are particularly effective for breaking azeotropes and can achieve very high purity levels with significantly lower energy input compared to distillation.⁵⁵

The choice among these techniques depends on factors such as the specific chemical system, the required product purity, energy costs, and capital investment, with each offering a unique solution to the azeotropic challenge.

Table 2: Overview of Modern Azeotrope Separation Techniques

Azeotropes in the Modern World: Applications and Innovations

While the challenge of separating azeotropes has driven significant innovation, the unique properties of azeotropes are now deliberately harnessed in a wide range of modern applications. This reflects a paradigm shift in chemical engineering, where azeotropy has evolved from being solely a "problem" to be overcome into a "property" to be engineered for creating advanced materials with tailored performance.

Industrial Workhorses: Biofuel, Solvents, and Chemicals

• Biofuel Production: The dehydration of ethanol to produce fuel-grade ethanol (>99.5% purity) remains one of the largest-scale industrial processes dictated by azeotropic behavior. While distillation can only concentrate fermented ethanol to the 95.6% azeotrope, technologies like molecular sieve adsorption or membrane pervaporation are employed to remove the remaining water, a critical step for ensuring the fuel mixes properly with gasoline and does not cause engine problems.⁶³
• Solvent Purification and Recycling: In the pharmaceutical and fine chemical industries, azeotropic and extractive distillation are indispensable tools for recovering and recycling high-value solvents from reaction mixtures. This not only reduces the cost of purchasing new solvents but also minimizes hazardous waste streams, aligning with principles of green chemistry.⁶⁴
• Chemical Synthesis: The constant boiling point of an azeotrope can be used to control reaction temperatures with high precision. Furthermore, azeotropic distillation is used in certain reactions, such as esterifications, to remove a product (e.g., water) as it is formed. By continuously removing water as an azeotrope, the reaction equilibrium is shifted towards the product side, driving the reaction to completion.²

Specialized Roles: Cleaning, Degreasing, and Refrigeration

• Precision Cleaning Fluids: Azeotropic and near-azeotropic blends are intentionally formulated to serve as high-performance cleaning solvents. These fluids are used in vapor degreasers for critical cleaning of sensitive components in the electronics, aerospace, and medical device industries.⁶⁶ The key advantage is their compositional stability; because the blend evaporates and condenses without changing its proportions, the cleaning performance remains consistent throughout the operational life of the solvent in the degreaser.¹¹ Perhaps most importantly, this principle allows for the creation of powerful,
  non-flammable cleaners by blending a highly effective but flammable solvent with a non-flammable component to form a stable, safe azeotrope.⁶⁸
• Refrigeration and HVAC: In the field of refrigeration, refrigerant blends are classified based on their boiling behavior. Azeotropic refrigerants (e.g., R-502) are mixtures that behave like a single-component fluid, evaporating and condensing at a constant temperature. In contrast, zeotropic refrigerants (e.g., R-407C) have components that boil at different temperatures, resulting in a phenomenon known as "temperature glide".¹² During a phase change, the composition of a zeotropic blend can shift, a process called "fractionation." This distinction is critical for HVAC technicians and system designers, as it affects equipment design, charging procedures, and performance diagnostics.¹² The choice between an azeotropic or zeotropic blend is a deliberate engineering decision based on the desired heat transfer characteristics for a specific application.

Predictive Science: Computational Modeling of Azeotropic Systems

Modern process design relies heavily on computational tools to predict the behavior of chemical mixtures, thereby reducing the need for costly and time-consuming laboratory experiments. Thermodynamic activity coefficient models such as UNIQUAC (Universal Quasi-Chemical) and UNIFAC (UNIQUAC Functional-group Activity Coefficients) are powerful methods used to predict VLE and the formation of azeotropes.¹³ The UNIFAC model, in particular, is a group-contribution method that estimates the properties of a mixture based on the functional groups present in its constituent molecules, allowing for the prediction of behavior for systems where no experimental data exists.¹³ These models are the computational engines inside process simulation software that allow engineers to screen for potential azeotropes and design effective separation strategies.

The Future of Azeotrope Separation: A Green Chemistry Perspective

The evolution of methods for separating azeotropes provides a compelling case study in the maturation of green chemistry principles. The journey from using highly toxic solvents to developing benign-by-design alternatives reflects a growing commitment to sustainability across the entire chemical lifecycle.

The Push for Sustainable Solvents

The historical workhorse for azeotropic distillation, particularly for ethanol dehydration, was benzene.²³ While chemically effective, benzene is a known carcinogen, and its use poses significant health and environmental risks.²³ This has driven a decades-long search for safer, more sustainable entrainers and solvents. The goal of modern green chemistry is not just to reduce emissions but to consider the entire lifecycle of a chemical: its sourcing, synthesis, toxicity, and ultimate fate in the environment.

Ionic Liquids (ILs) as Designer Entrainers

One of the first major steps beyond traditional organic solvents was the investigation of ionic liquids (ILs). ILs are salts with melting points below 100°C, composed of a large organic cation and an organic or inorganic anion.⁷¹ Their most attractive feature is a virtually negligible vapor pressure, which means they do not evaporate and contribute to air pollution.⁷³ This property aligns perfectly with an early green chemistry focus on pollution prevention. Furthermore, the properties of ILs can be finely tuned by changing the cation-anion pair, earning them the moniker "designer solvents".⁷¹ This allows for the creation of ILs with high selectivity for separating specific azeotropic mixtures in processes like extractive distillation.⁷³ However, as the principles of green chemistry evolved, the focus expanded beyond just volatility. Many ILs are expensive to synthesize, can be toxic to aquatic life, and are not readily biodegradable, raising concerns about their overall lifecycle impact.⁷⁵

Deep Eutectic Solvents (DESs): A New Class of Green Solvents

More recently, deep eutectic solvents (DESs) have emerged as an even more promising class of green solvents.⁷⁸ A DES is a mixture of two or more components, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), which, when mixed at a particular molar ratio, form a eutectic with a melting point significantly lower than that of the individual components.⁷⁹

DESs share the key advantages of ILs, such as low volatility and tunable properties, but they are often synthesized from inexpensive, abundant, and biodegradable starting materials like choline chloride (a component of the vitamin B complex), urea, sugars, and organic acids.⁷⁹ This "benign-by-design" approach represents a more holistic application of green chemistry principles. Research has shown DESs to be highly effective entrainers for separating azeotropes in both extractive distillation and liquid-liquid extraction, often outperforming traditional solvents.⁸¹ While challenges remain, such as their typically high viscosity and the need for more comprehensive toxicological data, DESs represent the current frontier in the quest for truly sustainable separation technologies.⁷⁵

Conclusion: An Enduring and Evolving Concept

The journey of the azeotrope is a microcosm of the advancement of chemical science itself. It began as an empirical frustration for ancient distillers, a practical limit that defied simple explanation. Through the rigorous theoretical work of 19th-century physical chemists and the ingenious experimentalism of pioneers like Sydney Young, this anomaly was transformed into a well-defined phenomenon, complete with a name, a theoretical foundation, and a catalog of known examples.

This understanding turned a challenge into an opportunity. The engineering imperative to separate these "inseparable" mixtures drove the invention of a host of sophisticated technologies—from pressure-swing and extractive distillation to molecular sieves and advanced membranes—that are now mainstays of the modern chemical industry. Simultaneously, the unique constant-boiling property of azeotropes has been harnessed, allowing engineers to design advanced fluids with precisely tailored characteristics for applications ranging from safety-conscious cleaning solvents to high-performance refrigerants.

Today, the study of azeotropy continues to evolve, guided by the principles of sustainability and green chemistry. The ongoing development of novel separation agents like ionic liquids and deep eutectic solvents, coupled with the power of predictive computational modeling, promises a future of more efficient, economical, and environmentally benign industrial processes. From a simple paradox to a cornerstone of modern technology, the azeotrope remains a vibrant and essential concept, critical to shaping the future of chemical engineering.

Visual Timeline of Key Milestones in Azeotropic Research

• c. 1200 BCE: Early textual evidence of distillation for perfumery is found on Akkadian tablets in Mesopotamia.⁴
• c. 3rd Century CE: Alchemists in Alexandria, such as Zosimos of Panopolis, develop and document early distillation apparatus, including the alembic still.⁴
• 1860s: British chemist Henry Enfield Roscoe studies aqueous solutions of acids and provides the first scientific descriptions of mixtures that boil at a constant temperature and composition.²⁸
• 1887: French chemist François-Marie Raoult publishes his law describing the vapor pressure of ideal solutions, establishing the theoretical baseline for identifying and quantifying non-ideal behavior that leads to azeotropy.¹⁵
• 1902: Sydney Young discovers the ternary minimum-boiling azeotrope of benzene-water-ethanol, leading to the first industrial process for producing anhydrous (absolute) alcohol.²⁹
• 1911: John Wade and Richard William Merriman formally coin the term "azeotrope" in a paper published in the Journal of the Chemical Society, Transactions, giving the phenomenon its modern name.⁵
• 1952: The American Chemical Society publishes the first edition of Azeotropic Data, a comprehensive compilation by L.H. Horsley of the Dow Chemical Company that becomes an essential reference for engineers.³⁹
• 1970s: Development and widespread application of computational thermodynamic models like UNIQUAC and UNIFAC, allowing for the prediction of azeotropic behavior without extensive experimentation.¹³
• 1980s-Present: Membrane separation processes, such as pervaporation and vapor permeation, are developed and commercialized as highly energy-efficient technologies for breaking azeotropes, particularly in solvent dehydration.⁵⁶
• 2000s-Present: Intensive research focuses on green chemistry alternatives to traditional toxic entrainers. Ionic Liquids (ILs) and, more recently, Deep Eutectic Solvents (DESs) are investigated as effective, recyclable, and environmentally benign agents for azeotrope separation.⁷³

References

1. Wade, J., & Merriman, R. W. (1911). CIV.—Influence of water on the boiling point of ethyl alcohol at pressures above and below the atmospheric pressure. Journal of the Chemical Society, Transactions, 99, 997–1011. https://doi.org/10.1039/CT9119900997
2. Young, S. (1902). XXXV.—The relative efficiency and usefulness of various forms of still-head for fractional distillation, with a description of some new forms possessing special advantages. Journal of the Chemical Society, Transactions, 75, 679–709.
3. Roscoe, H. E. (1861). On the composition of the aqueous acids of constant boiling point. Chemical Society Quarterly Journal, 13, 146–164.
4. Horsley, L. H. (1973). Azeotropic Data—III. American Chemical Society.
5. Raoult, F. M. (1887). Loi générale des tensions de vapeur des dissolvants. Comptes Rendus, 104, 1430–1433.
6. Wisniak, J. (2011). Sidney Young. Educación Química, 22(2), 168-177.
7. Lazzaroni, M. J., Bush, D., & Bruno, T. J. (2009). Study of azeotropic mixtures with the advanced distillation curve approach. Fluid Phase Equilibria, 282(2), 93-102.
8. Kiss, A. A., & Suszwalak, D. J. P. C. (2012). Enhanced bioethanol dehydration by extractive and azeotropic distillation in dividing-wall columns. Separation and Purification Technology, 86, 70–78.
9. Ferreira, O., Gonzalez-Miquel, M., & Rodriguez, O. (2013). Deep eutectic solvents as extraction media for azeotropic mixtures. Green Chemistry, 15(1), 233-241.
10. Huang, H., Ramaswamy, S., Tschirner, U. W., & Ramarao, B. V. (2008). A review of separation technologies in current and future biorefineries. Separation and Purification Technology, 62(1), 1–21.
11. Vane, L. M. (2019). A review of pervaporation and vapor permeation for hydrophilic solvent drying. Membranes, 9(3), 39.
12. Lei, Z., Chen, B., & Ding, Z. (2002). Special distillation processes. Elsevier.
13. Li, Q., et al. (2025). Membranes for the pervaporation of solvent azeotropes: from molecular to process design. Chemical Engineering Journal, 515(4), 163728.
14. Shah, M. S., et al. (2024). Insight into separation of azeotrope in wastewater to achieve cleaner production by extractive distillation and pressure-swing distillation based on phase equilibrium. Separation and Purification Technology, 353, 127461.
15. Abbott, A. P., Capper, G., Davies, D. L., Rasheed, R. K., & Tambyrajah, V. (2003). Novel solvent properties of choline chloride/urea mixtures. Chemical Communications, (1), 70–71.

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Works cited

1. Azeotrope - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Azeotrope
2. Azeotropic Mixture: Introduction, Types, Separation and Applications. - Allen, accessed August 7, 2025, https://allen.in/jee/chemistry/azeotropic-mixture
3. Azeotropes - Chemistry LibreTexts, accessed August 7, 2025, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Solutions_and_Mixtures/Nonideal_Solutions/Azeotropes
4. Distillation - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Distillation
5. en.wikipedia.org, accessed August 7, 2025, https://en.wikipedia.org/wiki/Azeotrope#:~:text=The%20term%20was%20coined%20in,texts)%20constant%20boiling%20point%20mixtures.
6. What Is an Azeotrope? Definition and Examples - Science Notes, accessed August 7, 2025, https://sciencenotes.org/what-is-an-azeotrope-definition-and-examples/
7. en.wikipedia.org, accessed August 7, 2025, https://en.wikipedia.org/wiki/Azeotrope#:~:text=9%20External%20links-,Etymology,Wade%20and%20Richard%20William%20Merriman.
8. azeotrope - Wiktionary, the free dictionary, accessed August 7, 2025, https://en.wiktionary.org/wiki/azeotrope
9. The History Of How Ethanol Was First Isolated | Kimia, accessed August 7, 2025, https://kimia.co.uk/blog/the-history-of-how-ethanol-was-first-isolated/
10. Molecular Sieves - N.B.Oler, accessed August 7, 2025, https://www.nboler.com/blogs/news/molecular-sieves
11. Azeotrope | Electrical Cleaning Products | MicroCare, accessed August 7, 2025, https://www.microcare.com/en-US/Resources/Resource-Center/FAQs/What-is-an-Azeotrope-Why-Are-They-Important
12. Azeotrope vs. Zeotropic Refrigerants: Understanding Key ..., accessed August 7, 2025, https://hvacknowitall.com/blog/azeotrope-refrigerants-vs-zeoptrope
13. VLE Prediction of Azeotropic Systems using UNIQUAC and UNIFAC Models and Experimental Validation using Othmer VLE - MAK HILL Publications, accessed August 7, 2025, https://makhillpublications.co/files/published-files/mak-rjas/2018/3-216-224.pdf
14. Raoult's Law - Chemistry LibreTexts, accessed August 7, 2025, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Solutions_and_Mixtures/Ideal_Solutions/Changes_In_Vapor_Pressure_Raoult's_Law
15. Raoult's law - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Raoult%27s_law
16. en.wikipedia.org, accessed August 7, 2025, https://en.wikipedia.org/wiki/Azeotrope#:~:text=Azeotropes%20can%20form%20only%20when,partial%20pressures%20in%20real%20mixtures.
17. What is meant by azeotropic? - Quora, accessed August 7, 2025, https://www.quora.com/What-is-meant-by-azeotropic
18. What are differences between minimum boiling azeotropes and maximum bo - Doubtnut, accessed August 7, 2025, https://www.doubtnut.com/qna/141185925
19. Azeotropes: Definition, Example, Types & Application | AESL - Aakash, accessed August 7, 2025, https://www.aakash.ac.in/important-concepts/chemistry/azeotropes
20. Azeotrope - New World Encyclopedia, accessed August 7, 2025, https://www.newworldencyclopedia.org/entry/Azeotrope
21. minimum boiling | maximum boiling Azeotropes | examples - YouTube, accessed August 7, 2025, https://www.youtube.com/watch?v=LdAImpMb7Ys
22. Azeotropic Mixture - BYJU'S, accessed August 7, 2025, https://byjus.com/jee/azeotropic-mixture/
23. Azeotropic distillation - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Azeotropic_distillation
24. (PDF) Distillation – from Bronze Age till today - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/publication/260392019_Distillation_-_from_Bronze_Age_till_today
25. History of Alcohol Distillation | Iberian Coppers Lda, accessed August 7, 2025, https://www.copper-alembic.com/en/page/history-of-alcohol-distillation
26. DISTILLING HISTORY - Montgomery Distillery, accessed August 7, 2025, https://www.montgomerydistillery.com/distilling-history
27. History of Azeotrope (azeotropic mixtures) - Chemistry Stack Exchange, accessed August 7, 2025, https://chemistry.stackexchange.com/questions/180112/history-of-azeotrope-azeotropic-mixtures
28. Henry Enfield Roscoe | Educación Química - Elsevier, accessed August 7, 2025, https://www.elsevier.es/es-revista-educacion-quimica-78-articulo-henry-enfield-roscoe-S0187893X16300039
29. (PDF) Sidney Young - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/publication/236232560_Sidney_Young
30. Sidney Young - SciELO México, accessed August 7, 2025, https://www.scielo.org.mx/pdf/eq/v22n2/v22n2a11.pdf
31. Sydney Young (1857-1937) - ACS Publications, accessed August 7, 2025, https://pubs.acs.org/doi/pdf/10.1021/ed022p211
32. Sidney Young | Educación Química - Elsevier, accessed August 7, 2025, https://www.elsevier.es/es-revista-educacion-quimica-78-articulo-sidney-young-S0187893X18301307
33. Distillation Tube | National Museum of American History, accessed August 7, 2025, https://americanhistory.si.edu/collections/object/nmah_1822
34. Azeotropes as Powerful Tool for Waste Minimization in Industry and Chemical Processes, accessed August 7, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7698242/
35. CIV.—Influence of water on the boiling point of ethyl alcohol at pressures above and below the atmospheric pressure - Journal of the Chemical Society, Transactions (RSC Publishing) DOI:10.1039/CT9119900997, accessed August 7, 2025, https://pubs.rsc.org/en/content/articlehtml/1911/ct/ct9119900997
36. Azeotrope Definition and Examples - ThoughtCo, accessed August 7, 2025, https://www.thoughtco.com/definition-of-azeotrope-605826
37. AMAND BINEAU - Redalyc, accessed August 7, 2025, https://www.redalyc.org/journal/1816/181669387007/html/
38. Azeotropic data-II - New York University Abu Dhabi, accessed August 7, 2025, https://search.abudhabi.library.nyu.edu/discovery/fulldisplay?vid=01NYU_AD%3AAD&docid=alma990056062790107871&context=L
39. Azeotropic data - Catalog Record - HathiTrust Digital Library, accessed August 7, 2025, https://catalog.hathitrust.org/Record/001114128
40. Table of Azeotropes and Nonazeotropes - Advances in Chemistry (ACS Publications), accessed August 7, 2025, https://pubs.acs.org/doi/10.1021/ba-1952-0006.ch001
41. AZD – Azeotropic and Zeotropic Data in Non-Electrolyte Mixtures - DDBST, accessed August 7, 2025, https://www.ddbst.com/ddb-azd.html
42. Azeotropic distillation | PPTX - SlideShare, accessed August 7, 2025, https://www.slideshare.net/slideshow/azeotropic-distillation-249538338/249538338
43. How can a Mixture of Ethanol and Water be Separated by Azeotropic Distillation? - BYJU'S, accessed August 7, 2025, https://byjus.com/chemistry/azeotropic-distillation/
44. Azeotropic Distillation Process In Detail | Binary Separation Technique - Chemical Tweak, accessed August 7, 2025, https://chemicaltweak.com/azeotropic-distillation-process-binary-separation-technique/
45. Extractive distillation: An advanced separation technique - Processing Magazine, accessed August 7, 2025, https://www.processingmagazine.com/home/article/55261249/extractive-distillation-an-advanced-separation-technique
46. separation of ethanol-water azeotrope mixtures using extractive distillation method, accessed August 7, 2025, https://www.researchgate.net/publication/375729628_SEPARATION_OF_ETHANOL-WATER_AZEOTROPE_MIXTURES_USING_EXTRACTIVE_DISTILLATION_METHOD
47. Extractive distillation - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Extractive_distillation
48. Process flow diagram of extractive distillation section Automatic... - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/figure/Process-flow-diagram-of-extractive-distillation-section-Automatic-control-of-the_fig3_45523244
49. Pressure swing distillationn | PPTX - SlideShare, accessed August 7, 2025, https://www.slideshare.net/slideshow/pressure-swing-distillationn/58783971
50. Overview of Pressure-Swing Distillation Process for Separation of Azeotropic Mixture - FAMT, accessed August 7, 2025, https://famt.ac.in/NAAC_Documents/aqar/2018-19/Criteria3/3.3.3/3.3.3_CHEM_NGK_2018-19_3.pdf
51. Pressure Swing Distillation Process | PDF - Scribd, accessed August 7, 2025, https://www.scribd.com/document/234347140/Pressure-Swing-Distillation-Process
52. Molecular Sieves - Sigma-Aldrich, accessed August 7, 2025, https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/chemistry-and-synthesis/reaction-design-and-optimization/molecular-sieves
53. Molecular sieve - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Molecular_sieve
54. Molecular Sieve for Ethanol Dehydration, accessed August 7, 2025, https://www.molecularsievedesiccants.com/molecular-sieve-for-ethanol-dehydration
55. Recent Trends in Azeotropic Mixture Separation: A Comprehensive Review - MDPI, accessed August 7, 2025, https://www.mdpi.com/2673-4591/67/1/56
56. Pervaporation as a Successful Tool in the Treatment of Industrial ..., accessed August 7, 2025, https://www.mdpi.com/2073-4360/14/8/1604
57. Review: Membrane Materials for the Removal of Water from Industrial Solvents by Pervaporation and Vapor Permeation, accessed August 7, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC6436640/
58. PERVAPORATION & VAPOR PERMEATION ... - Pervatech, accessed August 7, 2025, https://pervatech.com/app/uploads/2024/02/White-paper-pervaporation-vapor-permeation-processes-v2024-02-05.pdf
59. INDUSTRIAL EXPERIENCE WITH HYBRID DISTILLATION – PERVAPORATION OR VAPOR PERMEATION APPLICATIONS, accessed August 7, 2025, https://skoge.folk.ntnu.no/prost/proceedings/distillation06/CD-proceedings/paper059.pdf
60. Potential of Pervaporation and Vapor Separation with Water Selective Membranes for an Optimized Production of Biofuels—A Review - MDPI, accessed August 7, 2025, https://www.mdpi.com/2073-4344/7/6/187
61. Hybrid Separation Scheme for Azeotropic Mixtures – Sustainable Design Methodology - Aidic, accessed August 7, 2025, https://www.aidic.it/cet/18/69/107.pdf
62. Process Simulation of the Separation of Aqueous Acetonitrile Solution by Pressure Swing Distillation - MDPI, accessed August 7, 2025, https://www.mdpi.com/2227-9717/7/7/409
63. Ethanol Dehydration | ZEOCHEM, accessed August 7, 2025, https://zeochem.com/our-applications/ethanol-dehydration/
64. Dive Into Azeotropic Distillation: Essential Techniques - GWSI, accessed August 7, 2025, http://gwsionline.com/azeotrope-distillation-techniques/
65. allen.in, accessed August 7, 2025, https://allen.in/jee/chemistry/azeotropic-mixture#:~:text=Solvent%20Recycling%3A,recover%20solvents%20from%20reaction%20mixtures.
66. What is an azeotrope? - Best Technology, accessed August 7, 2025, https://www.besttechnologyinc.com/faq/what-is-an-azeotrope/
67. Why Azeotropes Matter in Vapor Degreasing for Precision Cleaning - Best Technology, accessed August 7, 2025, https://www.besttechnologyinc.com/ultrasonic-cleaning-systems/azeotrope-vapor-degreasing-importance/
68. What Are the Benefits of Azeotropes for Precision Cleaning? - MicroCare, accessed August 7, 2025, https://www.microcare.com/en-US/Resources/Resource-Center/FAQs/What-Are-the-Benefits-of-Azeotropes-for-Precision
69. Zeotropic mixture - Wikipedia, accessed August 7, 2025, https://en.wikipedia.org/wiki/Zeotropic_mixture
70. VLE Prediction of Azeotropic Systems using UNIQUAC and UNIFAC ..., accessed August 7, 2025, https://www.researchgate.net/publication/337154515_VLE_Prediction_of_Azeotropic_Systems_using_UNIQUAC_and_UNIFAC_Models_and_Experimental_Validation_using_Othmer_VLE_Still
71. Recent Applications of Ionic Liquids in Separation Technology - MDPI, accessed August 7, 2025, https://www.mdpi.com/1420-3049/15/4/2405
72. Recovery and purification of ionic liquids from solutions: a review - PMC - PubMed Central, accessed August 7, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC9086388/
73. (PDF) Applications of Ionic Liquids in Azeotropic Mixtures Separations, accessed August 7, 2025, https://www.researchgate.net/publication/221905811_Applications_of_Ionic_Liquids_in_Azeotropic_Mixtures_Separations
74. Design and use of ionic liquids in separation processes for azeotropic mixtures, accessed August 7, 2025, https://digital.car.chula.ac.th/chulaetd/73411/
75. Deep Eutectic Solvents - ACS Publications - American Chemical Society, accessed August 7, 2025, https://pubs.acs.org/doi/full/10.1021/bk-2025-1504.ch001
76. Techno-Economic Analysis of Recycled Ionic Liquid Solvent Used in a Model Colloidal Platinum Nanoparticle Synthesis - National Renewable Energy Laboratory, accessed August 7, 2025, https://research-hub.nrel.gov/en/publications/techno-economic-analysis-of-recycled-ionic-liquid-solvent-used-in
77. Techno-Economic Analysis of Recycled Ionic Liquid Solvent Used in a Model Colloidal Platinum Nanoparticle Synthesis - OSTI, accessed August 7, 2025, https://www.osti.gov/servlets/purl/1764929
78. Deep eutectic solvents: Preparation, properties, and food applications - PMC, accessed August 7, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC11015381/
79. Separation of Benzene-Cyclohexane Azeotropes Via Extractive Distillation Using Deep Eutectic Solvents as Entrainers - MDPI, accessed August 7, 2025, https://www.mdpi.com/2227-9717/9/2/336
80. Deep Eutectic Solvents: Overcoming XXI Century Challenges - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/publication/329665901_Deep_Eutectic_Solvents_Overcoming_XXI_Century_Challenges
81. Deep eutectic solvents as extraction media for azeotropic mixtures - Green Chemistry (RSC Publishing) DOI:10.1039/C3GC37030E, accessed August 7, 2025, https://pubs.rsc.org/en/content/articlehtml/2013/gc/c3gc37030e
82. Separation of Ethanol Azeotropic Mixture Using Deep Eutectic Solvents in Liquid- Liquid Extraction Process | Request PDF - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/publication/352096215_Separation_of_Ethanol_Azeotropic_Mixture_Using_Deep_Eutectic_Solvents_in_Liquid-_Liquid_Extraction_Process
83. Development and Challenges of Deep Eutectic Solvents for Cathode Recycling of End-of-Life Lithium-Ion Batteries | Request PDF - ResearchGate, accessed August 7, 2025, https://www.researchgate.net/publication/369016336_Development_and_Challenges_of_Deep_Eutectic_Solvents_for_Cathode_Recycling_of_End-of-Life_Lithium-Ion_Batteries