A Review on Molecular Interactions in Binary Liquid Mixtures Investigated by Ultrasonic Studies

Abstract

Ultrasonic techniques is one of the most reliable and non-destructive methods for the investigation of molecular interactions in liquid mixtures. The propagation of ultrasonic waves through liquids is strongly influenced by intermolecular forces, making ultrasonic measurements valuable for understanding the structural and thermodynamic behavior of binary liquid mixtures. By measurement of ultrasonic velocity with density and viscosity measurements, several acoustical and thermodynamic parameters—including adiabatic compressibility, intermolecular free length, free volume, internal pressure, acoustic impedance, relaxation strength, and excess properties—can be evaluated. These parameters providesvaluable information about the nature and strength of molecular interactions such as hydrogen bonding, dipole–dipole interactions, ion–dipole interactions, charge-transfer complexes, and dispersion forces. Numerous studies have demonstrated that deviations in acoustical parameters from ideal behavior reveal the extent of molecular association or dissociation in liquid mixtures. This review gives an idea about the theoretical principles of ultrasonic wave propagation, discusses the determination of important acoustical parameters, reviews ultrasonic investigations of binary liquid mixtures, and highlights applications of ultrasonic studies in chemistry and industrial processes. The article also discusses recent developments and future prospects for ultrasonic characterization of molecular interactions.

Introduction

This review discusses the use of ultrasonic studies to investigate molecular interactions in binary liquid mixtures. Ultrasonic techniques provide a simple, accurate, rapid, and non-destructive method for understanding intermolecular forces that influence the physical and chemical behavior of liquid systems. These studies are important in fields such as chemistry, pharmaceuticals, petrochemicals, food science, and materials engineering.

Background

Binary liquid mixtures, composed of two different chemical components, are widely used to study molecular interactions because their composition can be varied continuously. Understanding these interactions is essential for explaining phenomena such as:

• Solvation
• Diffusion
• Chemical reactivity
• Phase equilibrium
• Structural organization of liquids

Ultrasonic waves (frequencies above 20 kHz) travel through liquids, and their velocity depends on the liquid's elasticity, density, and molecular arrangement. Changes in intermolecular forces alter these properties, making ultrasonic velocity a useful indicator of molecular interactions.

Principle of Ultrasonic Studies

Ultrasonic waves propagate through liquids as longitudinal pressure waves. Their velocity depends on wavelength and frequency and is governed by the Newton–Laplace equation. Measurements of:

• Ultrasonic velocity
• Density
• Viscosity

allow researchers to calculate several acoustical and thermodynamic parameters that reveal the strength and nature of molecular interactions.

Important Acoustical Parameters

The review explains several key parameters used to characterize molecular interactions:

• Adiabatic Compressibility: Indicates how easily a liquid can be compressed. Lower compressibility signifies stronger intermolecular attractions and closer molecular packing.
• Intermolecular Free Length: Represents the average distance between molecules. A decrease in free length indicates stronger molecular association.
• Acoustic Impedance: Depends on both density and ultrasonic velocity. Higher impedance reflects stronger intermolecular interactions.
• Free Volume: Measures the space available for molecular movement. Strong interactions reduce free volume.
• Internal Pressure: Reflects cohesive forces within the liquid; higher values indicate stronger hydrogen bonding or dipole interactions.

Excess Acoustical and Thermodynamic Parameters

Excess properties measure deviations from ideal mixing behavior and provide more sensitive information about molecular interactions. Common excess parameters include:

• Excess ultrasonic velocity
• Excess adiabatic compressibility
• Excess intermolecular free length
• Excess molar volume
• Excess acoustic impedance
• Excess viscosity
• Excess Gibbs free energy of activation

Negative excess values generally indicate strong attractive interactions such as hydrogen bonding, while positive values suggest weaker interactions or structural disruption.

Types of Molecular Interactions

Ultrasonic studies can distinguish several intermolecular forces, including:

• Hydrogen bonding
• Dipole–dipole interactions
• Dipole-induced dipole interactions
• London dispersion forces
• Charge-transfer complex formation

The variation of acoustical parameters with concentration and temperature helps identify the dominant interaction mechanisms.

Applications

Ultrasonic investigations have wide-ranging applications:

• Chemical Industry: Solvent selection, reaction monitoring, and solvent–solute interaction studies.
• Pharmaceutical Sciences: Drug solubility, formulation stability, and compatibility of excipients.
• Petrochemical Industry: Quality assessment of fuels, lubricants, and petroleum products.
• Food and Beverage Industry: Non-destructive evaluation of edible oils, dairy products, juices, and alcoholic beverages.
• Materials Science: Characterization of polymers, ionic liquids, nanofluids, and advanced functional materials.

Recent Advances

Recent technological developments have improved the accuracy and scope of ultrasonic studies:

• High-precision digital ultrasonic interferometers operating between 1–10 MHz.
• Automated digital density meters, thermostats, and viscometers for reliable measurements.
• Integration with spectroscopic techniques such as FTIR, Raman spectroscopy, NMR, and UV–Visible spectroscopy to identify specific molecular interactions.
• Use of molecular dynamics simulations and quantum chemical calculations to correlate experimental results with microscopic molecular structures.
• Expanded applications to ionic liquids, deep eutectic solvents, biofuels, nanofluids, and environmentally friendly ("green") solvents.

Combining experimental and computational approaches provides a more comprehensive understanding of liquid structure and molecular behavior.

Future Perspectives

The review highlights several future research directions:

• Investigation of multicomponent liquid mixtures with industrial relevance.
• Studies on green solvents and sustainable chemistry.
• Characterization of pharmaceutical formulations.
• Research on nanofluids and colloidal systems.
• Integration of ultrasonic techniques with artificial intelligence and machine learning for predicting thermodynamic properties.
• Simultaneous use of ultrasonic, spectroscopic, and computational methods for molecular-level analysis.
• Standardization of experimental procedures and reporting to improve reproducibility across laboratories.

Conclusion

Ultrasonic studies have become one of the most effective methods for investigating molecular interactions in binary liquid mixtures. Measurements of ultrasonic velocity and associated parameters together, allow the calculation of important acoustical and thermodynamic parameters. These parameters provide valuable information regarding the strength and nature of intermolecular forces, including hydrogen bonding, dipole–dipole interactions, dipole-induced dipole interactions, and London dispersion forces.The variations in the values of excess acoustical properties has proven particularly useful for distinguishing ideal and non-ideal mixing behavior. Negative excess compressibility and free length generally indicate strong attractive interactions and molecular association, whereas positive deviations often reflect weaker interactions or structural disruption. Studies of binary mixtures containing alcohols, ketones, amides, sulfoxides, aromatic hydrocarbons, and water have demonstrated the versatility of ultrasonic techniques in characterizing liquid structure over a wide range of compositions and temperatures. Recent advances in instrumentation, molecular simulation, and complementary spectroscopic techniques have expanded the scope of ultrasonic investigations. As new solvent systems, pharmaceutical formulations, ionic liquids, and sustainable chemical processes continue to emerge, ultrasonic characterization is expected to play an increasingly important role in understanding molecular organization and predicting physicochemical behavior.

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Copyright

Copyright © 2026 Pravin J. Ganjare. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.