Anionic surfactants are amongst the most widely used surfactants globally. They have excellent cleaning and foaming properties due their negatively charged hydrophilic group when dissolved in water. In this article, we will explore some key properties of it and discuss their major applications.

Chemical Structure
They contain an anionic, or negatively charged, hydrophilic group attached to a hydrophobic tail. Common anionic head groups include carboxylates, sulfates and sulfonates. The hydrophobic tail is typically a straight-chained or branched saturated or unsaturated alkyl group ranging from C10 to C18 in length.

Some of the most widely used are sodium lauryl sulfate (SLS), linear alkylbenzene sulfonate (LAS) and alpha-olefin sulfonate (AOS). SLS has a sulfate headgroup attached to a C12 tail. LAS has an aromatic sulfonate head attached to a C10-C13 tail. AOS has an internal olefin sulfonate headgroup attached to a C14-C16 tail.

Physicochemical Properties
The negatively charged hydrophilic group and nonpolar hydrophobic tail give anionic surfactants their distinctive amphiphilic character. In aqueous solutions, their molecules self-assemble into micelles above the critical micelle concentration (CMC).

The CMC depends on factors like temperature, electrolyte concentration and structural features. Typical CMC values range from 0.1-1 g/L. At the CMC, further addition of surfactant molecules does not significantly reduce surface tension.

Anionic Surfactants are highly soluble in water compared to nonionic surfactants. They can form stable foams and have good wetting properties. However, they are inactivated to some extent by hardness ions like calcium and magnesium through soap scum formation.

Cleaning Applications
Due to their excellent detergency and solubilizing abilities, they find widespread use in cleaning products such as laundry and dish detergents. In laundry detergents, anionic surfactants work by suspending oily and particulate soils to facilitate their removal in the wash cycle.

Commonly used anionic surfactants are LAS, AOS and particularly SLS, which provides good grease removal. In automatic dishwashing formulations, SLS and more recently LAS are workhorse surfactants that help solubilize and suspend food residues. Their good foaming abilities are advantageous for several cleaning steps.

Personal Care Applications
They are also core ingredients in personal care products. In hair shampoos and body washes, SLS and LAS function as primary surfactants. They have good detergency to gently lift away dirt, oil and other impurities from hair and skin during washing.

In toothpastes, SLS acts as a surfactant as well as an abrasive agent to foam and help remove plaque and food debris. Its foaming properties are advantageous for delivering active ingredients. They are also used in liquid hand soaps and shower gels where mildness is important.

Other Industrial Uses
Beyond cleaning products, they find niche applications in various other industries owing to their versatility. In petroleum recovery processes, they are employed as viscosity reducers, dispersants and emulsifiers.

In agrochemical formulations, they serve to wet leaf surfaces, improve coverage and penetration of active ingredients into plant tissues. They are also utilized as emulsifiers or dispersants in pesticides, herbicides and fungicides.

The textile industry leverages they as leveling, softening and anti-static agents. In paints and coatings, they are added as wetting agents and dispersants. Anionic polymers produced via sulphonation of polyolefins are important rheology modifiers.

Toxicity Considerations
With sustained usage, some anionic surfactants like SLS have been reported to cause irritation to skin and mucous membranes at high concentrations. Their alkaline nature may also damage eyes on contact. To mitigate toxicity, milder surfactants like LAS are being increasingly adopted in personal care products.

Manufacturers strictly adhere to regulations on surfactant and preservative levels in formulations. Risk of systemic toxicity is minimal as most anionic surfactants are not readily absorbed into the body due to their hydrophilic head groups. Their breakdown products are also considered to pose low environmental hazard.

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