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Excess Molar Enthalpy of Binary Biazeotropic Ester–Carboxylic Acid systems

This study measured and correlated the excess molar enthalpies of three biazeotropic ester–carboxylic acid systems at distillation conditions to determine partial molar vaporization enthalpies, providing essential data for predicting azeotropic behavior under pressure changes for pressure swing distillation.

Original authors: Farid Bedretdinov, Valentina Raeva, Tatiana Chelyuskina, Nikita Tsvetov, Yuri Anufrikov, Irina Zvereva

Published 2026-09-29

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CC BY 4.0

Original authors: Farid Bedretdinov, Valentina Raeva, Tatiana Chelyuskina, Nikita Tsvetov, Yuri Anufrikov, Irina Zvereva

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of chemical engineering, separating mixtures is often a battle against nature's tendency to blend things together perfectly. Some liquids, when heated, form a special partnership called an azeotrope. In this state, the liquid and the vapor above it have the exact same composition, making it impossible to separate the components using a standard distillation column, no matter how tall or efficient it is. To break this stalemate, engineers often rely on pressure. By changing the pressure inside the column, they can shift the balance, causing the azeotrope to move or disappear, allowing the pure ingredients to finally be pulled apart. This technique, known as pressure swing distillation, is a powerful tool, but it requires precise knowledge of how much energy is needed to turn these specific mixtures into gas. Without accurate data on the heat required for this transformation, designing an efficient separation process is like trying to navigate a ship without a map.

A team of researchers from Russian institutions recently turned their attention to a particularly tricky group of mixtures: those containing esters and carboxylic acids. These are common industrial chemicals, often found in solvents and fragrances, but they form what is called a biazeotropic system. This means that under certain conditions, these mixtures don't just have one stubborn partnership; they can form two different azeotropes at the same time, one that boils at a lower temperature and another at a higher temperature. Predicting how these dual partnerships behave when pressure changes is notoriously difficult. The researchers set out to measure the exact amount of heat energy involved when these specific mixtures are heated, aiming to provide the missing data needed to predict how these complex systems would react to pressure changes.

The team focused on three specific pairs of chemicals: isobutyl acetate mixed with acetic acid, butyl propionate with propionic acid, and butyl butyrate with butyric acid. They conducted their experiments at a temperature of about 373 Kelvin, which corresponds to the conditions found in industrial distillation columns, and at standard atmospheric pressure. Using a highly sensitive instrument called a Calvet-type calorimeter, they measured the excess molar enthalpy. In simple terms, this is the extra heat energy absorbed or released when two liquids are mixed, beyond what would be expected if they were just sitting side by side without interacting. The researchers carefully mixed the liquids in a sealed chamber, breaking a membrane to combine them, and recorded the tiny heat flows that occurred during the process. They tested the mixtures across the entire range of concentrations, from a tiny drop of one chemical in a sea of the other, to the reverse.

The results revealed a distinct pattern in how these chemicals interact. For the first two systems, the heat measurements showed a curve that dipped slightly into negative values when the mixture was rich in acid, before rising to positive values as the amount of ester increased. This "S-shaped" curve indicates that the molecules are interacting in complex ways, sometimes releasing a small amount of energy and sometimes absorbing it. The third system, involving butyric acid, behaved differently, showing only positive heat values across the entire range, meaning the mixture consistently absorbed energy when the components were combined. By analyzing these heat measurements, the researchers were able to calculate the partial molar vaporization enthalpies. This is a specific value that tells us how much energy is required to turn a tiny amount of one specific component into vapor while it is still sitting inside the mixture.

A critical finding emerged from these calculations: in all three systems, the energy required to vaporize the ester component was consistently lower than the energy required to vaporize the carboxylic acid component. This difference is the key to understanding how pressure swing distillation would work for these mixtures. Because the acid always requires more energy to turn into gas than the ester does, the researchers determined that the behavior of the azeotropes follows a predictable rule. As the temperature rises, the low-boiling azeotrope becomes richer in acid, while the high-boiling azeotrope becomes richer in ester. This confirms that the complex internal interactions of these biazeotropic systems do not create unexpected surprises that would break the standard rules of separation.

Perhaps most importantly for industrial application, the study found that the difference in energy requirements between the two components was so significant that it overshadowed the smaller, more complex effects of mixing. This means that for the purpose of designing separation processes, engineers do not need to worry about the intricate, shifting details of the mixture's internal heat. Instead, they can safely use the known heat values of the pure, unmixed chemicals to plan their operations. The researchers concluded that their new data provides a solid foundation for predicting how these biazeotropic systems will evolve as pressure changes, offering a clear path forward for separating these difficult mixtures without the need for adding extra chemicals to force the separation.

Technical Summary: Excess Molar Enthalpy of Binary Biazeotropic Ester–Carboxylic Acid Systems

Problem Statement
The separation of binary and multicomponent azeotropic mixtures presents a significant thermodynamic challenge in industrial distillation, often requiring specialized methods such as pressure swing distillation (PSD) when components form pressure-sensitive azeotropes. The behavior of these azeotropes under varying pressure is governed by Vrevsky's law, which links the change in azeotropic composition to the difference in the partial molar vaporization enthalpies of the components ( $\bar{H}_1^V$ and $\bar{H}_2^V$ ).

This relationship is particularly critical for biazeotropic systems, which exhibit both low-boiling and high-boiling azeotropes. In such systems, a single pressure change can have divergent effects on the compositions of the two azeotropes. However, obtaining precise values for partial molar vaporization enthalpies is difficult. While these values can theoretically be derived from excess molar enthalpies ( $H^E$ ), experimental calorimetric data for binary ester–carboxylic acid systems at distillation-relevant temperatures (approx. 373 K) are scarce. Consequently, researchers often rely on the assumption that the partial molar vaporization enthalpies of mixtures can be approximated by the vaporization enthalpies of the pure components, an assumption that requires experimental validation, especially for complex biazeotropic behaviors.

Methodology
To address this data gap, the authors experimentally measured the excess molar enthalpies ( $H^E$ ) for three specific biazeotropic systems over their full concentration ranges:

Isobutyl acetate – acetic acid
Butyl propionate – propionic acid
Butyl butyrate – butyric acid

The measurements were conducted at approximately 373 K (373.12–373.21 K) and atmospheric pressure (101.3 kPa), conditions corresponding to typical distillation operations. The study utilized a Setaram C80 Calvet-type heat-flow calorimeter equipped with membrane mixing cells. The experimental protocol involved:

Calibration: Using electrical calibration (Joule effect) to ensure accuracy.
Mixing: Employing a rotating mechanism to break a membrane separating two substances (ester and acid) within the cell, ensuring thorough mixing while minimizing mechanical heat artifacts.
Data Acquisition: Recording heat flow signals before and after mixing to determine the enthalpy of mixing. Corrections were applied for the heat of evaporation of the liquids.
Analysis: The experimental $H^E$ data were correlated using a fifth-order polynomial equation. From these correlations, partial molar excess enthalpies ( $\bar{H}_1^E, \bar{H}_2^E$ ) were calculated using the tangent line method. These were subsequently used to determine the partial molar vaporization enthalpies ( $\bar{H}_1^V, \bar{H}_2^V$ ) and the integral molar vaporization enthalpy ( $H^V$ ) of the mixtures.

Key Results
The study generated a comprehensive dataset of excess molar enthalpies for the three systems, characterized by the following observations:

Enthalpy Profiles:
- The isobutyl acetate–acetic acid and butyl propionate–propionic acid systems exhibited "S-shaped" $H^E$ curves, featuring regions of negative excess enthalpy in acid-rich compositions and positive values in ester-rich compositions.
- The butyl butyrate–butyric acid system displayed positive excess molar enthalpies across the entire concentration range.
Partial Molar Vaporization Enthalpies:
- Calculations confirmed that for all three systems, the partial molar vaporization enthalpy of the ester component ( $\bar{H}_1^V$ ) is consistently lower than that of the carboxylic acid component ( $\bar{H}_2^V$ ) across the entire composition range.
- The differences between the partial molar vaporization enthalpies ( $\bar{H}_1^V - \bar{H}_2^V$ ) were found to be significant (-2174, -4815, and -6709 J mol⁻¹, respectively), far exceeding the differences in partial molar excess enthalpies.
Additivity:
- Due to the dominance of the pure component vaporization enthalpies over the excess terms, the integral molar vaporization enthalpy of the mixtures ( $H^V$ ) was found to be nearly additive. The curves for $H^V$ calculated via the rigorous thermodynamic equation (incorporating excess terms) were virtually indistinguishable from those calculated using the simple additivity assumption (pure component values).

Significance and Claims
The paper posits that the obtained data provides a necessary foundation for predicting the evolution of biazeotropic systems under pressure changes. Specifically, the findings support the following conclusions:

Validation of Assumptions: The study demonstrates that for the investigated biazeotropic ester–acid systems, the replacement of partial molar vaporization enthalpies with the vaporization enthalpies of pure substances is a thermodynamically acceptable approximation. This simplifies the modeling of pressure swing distillation processes for these specific mixtures.
Predictive Capability: Based on the established ratio $\bar{H}_1^V < \bar{H}_2^V$ and Vrevsky's law, the authors predict that as temperature increases (or pressure decreases), the low-boiling azeotrope in these systems will become enriched with the acid component, while the high-boiling azeotrope will become enriched with the ether (ester) component.
Industrial Relevance: The precise values of excess molar enthalpies and the derived vaporization properties enable more accurate thermal balance calculations for distillation columns, facilitating the design of separation strategies for biazeotropic mixtures without the need for added entrainers.

In summary, the work bridges a gap in experimental thermodynamic data for biazeotropic systems at distillation temperatures, confirming that while the systems exhibit complex non-ideal mixing behavior (excess enthalpy), their vaporization enthalpy behavior remains sufficiently close to ideal additivity to allow for simplified process modeling.

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