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Surfactants: properties, types, applications

This chemical characteristic allows them to be located in a water-air interface. Its main action at the interface is the reduction of surface tension. Some surfactants have the property of being detergents, so they are capable of removing dirt from surfaces and keeping them in suspension in the aqueous medium.

The salts that make up soap bars are examples of surfactants that are quite common in daily life.

Surfactants are capable of keeping lipids, hydrophobic substances, suspended in an aqueous medium by forming emulsions or micelles. This property facilitates the intestinal absorption of lipids.

Properties of surfactants

Reduction of surface tension

Surfactants, because they are amphiphilic or amphipathic compounds, have a polar and a non-polar region. This property allows them to be located at the air-water interface, the polar or hydrophilic part being in contact with the water. Meanwhile, the non-polar part is oriented towards the air.

The surfactant molecules at the air-water interface act to displace the water molecules found at this interface.

Therefore, by reducing the number of water molecules, the surface tension caused by the attraction that these molecules exert from within the liquid on the water molecules found on its surface also decreases.

Increased fat solubility

Surfactants are able to orient their molecules to form spherical structures, known as micelles. In these structures, surfactants orient their polar part or head towards the water, while the non-polar part remains confined inside.

The interaction of the polar part of the surfactants with the water keeps the micelles suspended in the water. And in turn, micelles can encapsulate substances that are not soluble in water, such as fats, thus increasing their solubilities.

Surfactants are capable of removing substances deposited on them from solid surfaces. A set of properties are involved in this process, such as wetting, dispersion, emulsion and foam formation.

Detergents facilitate the interaction of water with the surface of solid bodies, an essential process for cleaning the surface. At the same time, through the action of detergents, the particles removed during washing are kept dispersed in the water.

This is achieved through the formation of emulsions that prevents the fatty material from staying in suspension and not accumulating again on the surfaces of the solids. The foam does not intervene in the cleaning process itself, but it facilitates the removal of particles attached to the bubbles.

Types of surfactants

There are four different types of surfactants depending on the polarity of the head or globular structure of the surfactants: nonionic, anionic, cationic and zwitterionic.

Non ionic

These surfactants lack ionic dissociable groups at their head, representing 45% of total industrial production. Its hydrophilic group (head) has molecules of alcohol, phenol, ether, ester or amide, which do not dissociate into ions in aqueous solution.

These surfactants include the following:

– (Triton-X-100). It acts as a wetting agent and in coatings.

– (Nonoxynol-9). Spermicidal action.

Anionic

They represent 50% of the industrial production of surfactants. The hydrophilic head has anionic groups such as sulfonates, phosphates, sulfates and carboxylates, associated with cations such as sodium or potassium. Surfactants in this group include the following:

-Sodium dioctyl sulfosuccinate (Pentex 99). It is used as a wetting agent, as well as in coatings and toothpaste.

-Linear alkylbenzene sulfonates (Calsoft). They are used as laundry and dish detergents.

-Sodium lauryl ether sulfate (Texapan). It is used in shampoos and bath products.

Cationic

The surfactant head dissociates in aqueous solution into an amphiphilic cation and an anion, most often a halogen. They can present quaternary ammonium cations, such as cetyl trimethylammonium bromide (CTAB) and cetyl trimethylammonium chloride (CTAC).

Benzalkonium chloride (BAC) and cetylpyridinium chloride (CPC) are surfactants that alter the cell membranes of bacteria and the coatings of viruses.

Zwitterionics or amphoteric

They have cationic and anionic groups attached to the same molecule.

The anionic part is variable, appearing as sulfonates, as in sulphataine (CHAPS) 3 – [(3-cholamidopropyl) dimethylammonium-1-propanesulfonate], and betaine known as cocamidopropylbetaine.

Meanwhile, the cationic part is made up of primary, secondary, tertiary amines or quaternary ammonium cations.

Surfactant applications

Cleaning and personal care

Mechanism to remove dirt from the skin with a surfactant

They are used as laundry and dish washing detergents, as well as wetting, emulsifying and sudsing agents. Surfactants are also used in personal care in the form of cosmetics, shampoos, shower gels, hair conditioners, etc.

In addition, surfactants are used in making toothpaste and cleaning contact lenses.

Industrial

Surfactants are present in many foods, and are also used in leather processing and photography. They can be deposited on metal surfaces, thus giving them protection against corrosion.

Surfactants are used to promote the flow of oil through porous rocks and in the flotation of minerals or other solid particles. They also increase the wettability of the mineral particles, while the foaming agent allows the particles to bond with the bubble and float.

Surfactants help uniform penetration of dyes onto fabrics during the dyeing process.

They intervene in the formation of latex particles, creating stabilizing micelles of the monomers before polymerization. Polymerization occurs when polymerization initiators reach the micelles to induce the monomer molecules to polymerize to form the latex particles.

Surfactants also act in the production of paints, adhesives, inks, disinfectants, insecticides and herbicides.

Medicinal

Surfactants are used in the washing and disinfection of wounds. They are also used to impregnate cotton pads and bandages to enhance the absorption of medicinal solutions. In addition, they are used in the application of medicinal lotions on the surface of the skin and mucous membranes.

Surfactants are used to improve transcutaneous absorption of drugs by altering the stratum corneum and denaturation of keratin, thus increasing drug penetration.

Biological function

One surfactant in particular, dipalmitoylphosphatadylcholine, intervenes in respiration, promoting the process of alveolar distension that occurs during inspiration. The alveoli are covered by a layer of water, and therefore, there is a surface tension that opposes alveolar distension.

Surfactant reduces alveolar surface tension, facilitating air inspiration. Surfactants begin to be synthesized between weeks 24 and 30 of fetal gestation, so premature infants who have not developed their ability to synthesize surfactants should receive treatments.

Surfactants are involved in the intestinal absorption of lipids. Bile salts: cholate and deoxycholate, for example, are surfactants secreted by the gallbladder, which form micelles with the degradation products of triacylglycerides, thus favoring their absorption in the duodenum.

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