Theory of Foams, Emulsions, Surfaces and Interfaces
Surfactants Theory
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:
-
Anionic surfactants (negatively charged): e.g., soaps, sulfates, sulfonates.
-
Cationic surfactants (positively charged): e.g., quaternary ammonium compounds.
-
Amphoteric surfactants: carry both positive and negative charges depending on pH
-
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:
-
Diffusion-controlled adsorption, where diffusion is the rate-limiting step.
-
Mixed diffusion–kinetic adsorption, where molecular barriers (steric hindrance, micelle stability, or surface crowding) slow the transfer of molecules to the interface...