Pulmonary surfactant is produced in the lungs in order to facilitate breathing by increasing total lung capacity, and lung compliance. The human body produces diverse surfactants. Shown in red – choline and phosphate group black – glycerol green – monounsaturated fatty acid blue – saturated fatty acid. Phosphatidylcholine, found in lecithin, is a pervasive biological surfactant. Surface rheology can be characterized by the oscillating drop method or shear surface rheometers such as double-cone, double-ring or magnetic rod shear surface rheometer. The structure of surfactant layers can be studied by ellipsometry or X-ray reflectivity. surface tension as a function of time, can be obtained by the maximum bubble pressure apparatus ![]() Interfacial and surface tension can be characterized by classical methods such as theĭynamic surface tensions, i.e. The surface rheology of surfactant layers, including the elasticity and viscosity of the layer, play an important role in the stability of foams and emulsions.Ĭharacterization of interfaces and surfactant layers Such energy barriers can be due to steric or electrostatic repulsions. If such a barrier limits the adsorption rate, the dynamics are said to be ‘kinetically limited'. In some cases, there can exist an energetic barrier to adsorption or desorption of the surfactant. As the interface is created, the adsorption is limited by the diffusion of the surfactant to the interface. The dynamics of absorption depend on the diffusion coefficient of the surfactant. The dynamics of surfactant adsorption is of great importance for practical applications such as in foaming, emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The relation that links the surface tension and the surface excess is known as the Gibbs isotherm.ĭynamics of surfactants at interfaces Surfactants reduce the surface tension of water by adsorbing at the liquid-air interface. A measure of this is the hydrophilic-lipophilic balance (HLB). The shape of the aggregates depends on the chemical structure of the surfactants, namely the balance in size between the hydrophilic head and hydrophobic tail. Other types of aggregates can also be formed, such as spherical or cylindrical micelles or lipid bilayers. In the bulk aqueous phase, surfactants form aggregates, such as micelles, where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Other surfactants produced on a particularly large scale are linear alkylbenzene sulfonates (1.7 million tons/y), lignin sulfonates (600,000 tons/y), fatty alcohol ethoxylates (700,000 tons/y), and alkylphenol ethoxylates (500,000 tons/y). World production of surfactants is estimated at 15 million tons per year, of which about half are soaps. Hydrocarbon groups are usually lipophilic, for use in soaps and detergents, while fluorocarbon groups are lipophobic, for use in repelling stains or reducing surface tension. The hydrophobic tail may be either lipophilic ("oil-seeking") or lipophobic ("oil-avoiding") depending on its chemistry. ![]() The water-insoluble hydrophobic group may extend out of the bulk water phase into a non-water phase such as air or oil phase, while the water-soluble head group remains bound in the water phase. Surfactants diffuse in water and get adsorbed at interfaces between air and water, or at the interface between oil and water in the case where water is mixed with oil. As a result, a surfactant contains both a water-soluble component and a water-insoluble component. Surfactants are usually organic compounds that are akin to amphiphilic, which means that this molecule, being as double-agent, each contains a hydrophilic "water-seeking" group (the head), and a hydrophobic "water-avoiding" group (the tail). When the droplet is aprotic it is sometimes known as a reverse micelle. The compounds that coat a micelle are typically amphiphilic in nature, meaning that micelles may be stable either as droplets of aprotic solvents such as oil in water, or as protic solvents such as water in oil. This inhibits the oil droplets, the hydrophobic cores of micelles, from merging into fewer, larger droplets ("emulsion breaking") of the micelle. The polar "heads" of the surfactant molecules coating the micelle interact more strongly with water, so they form a hydrophilic outer layer that forms a barrier between micelles. ![]() Schematic diagram of a micelle – the lipophilic tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water.
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