Theory of Foams, Emulsions, Surfaces and Interfaces
Surfactants
Surfactants (surface-active molecules) are amphiphilic compounds composed of a hydrophilic head and a hydrophobic tail.
They adsorb at interfaces between two immiscible fluids, such as air/water or oil/water, reducing surface or interfacial tension. Their main uses include wetting, emulsifying, dispersing, foaming, and detergency.
They are classified into four major groups:
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Anionic surfactants (negatively charged): e.g., soaps, sulfates, sulfonates.
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Cationic surfactants (positively charged): e.g., quaternary ammonium compounds.
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Amphoteric surfactants: carry both positive and negative charges depending on pH
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Nonionic surfactants: uncharged molecules...
Surface tension, Theory and characterization technics
Surface / interfacial tension is the force existing at the interface between two immiscible fluids. It arises from imbalanced molecular attraction forces at the interface and in the bulk.
Surface / interfacial tension (γ) is the force per unit length acting along an interface expressed in N·m⁻¹.
Understanding and quantifying surface and interfacial tension is essential in fields involving foams, emulsions, coatings, detergency, pharmaceuticals, and material science, where control of interfacial phenomena dictates stability and performance.
Among the main methods to measure dynamic surface or interfacial tension; the Pendant/Rising Drop Method measures the droplet profile under gravity by image analysis to determine γ. It is accurate, versatile, and suitable for dynamic studies (adsorption kinetics)...
Dynamics of Surfactant Adsorption at Air/Water interface
The adsorption of surfactants at the air–water interface governs the reduction of surface tension over time, a process known as dynamic surface tension (DST). When a new surface is formed, surfactant molecules diffuse from the bulk solution to the interface, adsorb, and reorganize until equilibrium is reached.
Two main mechanisms describe this process:
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Diffusion-controlled adsorption, where diffusion is the rate-limiting step.
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Mixed diffusion–kinetic adsorption, where molecular barriers (steric hindrance, micelle stability, or surface crowding) slow the transfer of molecules to the interface...
Liquid Foams
Liquid foams are dispersions of gas bubbles in a liquid phase, whose unique structural, mechanical, and interfacial properties make them valuable in many industries.
Their behavior depends on the liquid fraction, bubble size distribution, and degree of order or polydispersity. Their stability and lifetime are limited by drainage, Ostwald ripening, coalescence, and evaporation, which progressively thin films and alter bubble size distribution.
Surfactants play a central role by reducing surface tension and stabilizing films via adsorption and Gibbs–Marangoni effects. Foamability is maximized near the critical micelle concentration (CMC), where interfacial elasticity is optimal.
Ultimately, understanding these multiscale mechanisms , from nanometric films to macroscopic foam flow, is crucial for optimizing formulation, predicting foam lifetime, and controlling foaming behavior...
Liquid Foams characterization
Liquid Foam characterization focuses on parameters such as foam height or volume, liquid fraction, bubble size, and polydispersity, as well as time-dependent properties like foamability, foam stability, and liquid drainage.
Basic methods include the Ross–Miles test, which measures foam height and decay over time under standard conditions, and the Bikerman method, which assesses foam formation under continuous gas injection to derive a foaming index reflecting dynamic stability.
Advanced instrumentation, such as the FOAMSCAN™ analyzer, integrates image analysis and conductance measurements to evaluate both macroscopic parameters (foam volume, liquid fraction, drainage rate) and microscopic structure (bubble size distribution, shape, and polydispersity). These methods allow multiscale analysis of foam generation, growth, and aging, helping researchers to understand, predict, and optimize foam behavior, ensuring reproducible performance across diverse industrial and scientific applications...