Amphoteric surfactants

Amphoteric surfactants – characteristics and properties

Surfactants are a key group of chemical compounds with a wide range of applications – from advanced industrial and pharmaceutical processes to common products such as detergents and emulsifiers. Amphoteric surfactants are of particular importance in this class of compounds.

Characteristics

Amphoteric surfactants are characterised by the fact that they contain both positively and negatively charged groups within the same structure. This molecular structure leads to unique properties – high solubility, low irritation potential and resistance to complex systems.

The ionisation state of amphoteric surfactants varies depending on the pH of the solution in which they are found. In an acidic environment, the molecule acquires a positive charge and behaves like a cation, while in alkaline solutions, the negative charge dominates and the molecule behaves like an anion. In the area of the isoelectric point, amphoteric surfactants take the form of zwitterions.

The properties of amphoteric surfactants are closely related to the structure of the molecule. They depend primarily on the length of the hydrophobic hydrocarbon chain, the number of positively and negatively charged groups, and their relative positions.

The most important properties of this group of compounds are:

• High solubility in water,
• High surface activity,
• Low critical micelle concentration (CMC),
• High foam stability,
• Very good thickening properties,
• Gentle on the skin,
• High biodegradability.

Examples of amphoteric surfactants – a structural overview

Key chemical groups of amphoteric surfactants include:

Betaines: these are compounds that contain both a quaternary ammonium salt as the cationic moiety and a carboxylate as the anionic moiety. They constitute the most important group among amphoteric surfactants. Their behaviour varies depending on the pH of the environment. They are characterised by mild action and good cleaning properties. The PCC Group offers a wide range of betaine-based amphoteric surfactants from the ROKAmina series, e.g. ROKAmina K30B.

Amino acids: due to the fact that they contain two reactive groups in their molecule – an amino group and a carboxyl group – they have been recognised as raw materials for the production of amphoteric surfactants. They effectively reduce surface tension and form micellar systems at low concentrations. Given their natural origin, they are characterised by low toxicity and easy biodegradability.

Phospholipids: these are composed of two fatty acid molecules, an alcohol molecule and a phosphoric acid molecule. They are characterised by self-assembly into bilayer lipids. Phospholipids are examples of natural surfactants, owing to their high biocompatibility and biodegradability.

How can an amphoteric surfactant be effectively matched to an application?

Selecting the right surfactants is difficult due to the wide variety of options available.

The first step in selecting the right raw material for a formulation is to determine its intended use. Amphoteric surfactants are ideal for solutions requiring a gentle action, e.g. in cosmetics designed for children’s skincare. In products containing anionic surfactants, they can counteract their irritating effects. Betaines and phospholipids exhibit high wettability, good cleansing properties, as well as antistatic properties.

The most versatile group are betaine derivatives. They are characterised by very good solubility in aqueous solutions, regardless of acidity or alkalinity. They do not precipitate at the isoelectric point (which effectively reduces the effectiveness of amine-based amphoteric surfactants). They are also a good choice in products where the generation of a large amount of foam is desired.

The choice of surfactant is often based on the HLB value of the compound in question. In the case of amphoteric surfactants, the concept of hydrophilic-hydrophobic balance is more complex. These substances change their chemical properties depending on the pH of the solution. They reach their lowest HLB value at the isoelectric point – this leads to low solubility and a tendency to precipitate. However, it should be noted that most amphoteric surfactants achieve high HLB values, which makes them good emulsifiers and solubisers.

Amphoteric surfactants – how do they stand out from other surface-active compounds?

Amphoteric surfactants occupy a unique place in the detergent industry, mainly due to their mildness and good foaming properties. As a result, they are frequently used in formulations designed for sensitive skin, such as children’s shampoos or specialist personal care products.

Their exceptional mildness on the skin and mucous membranes makes them an excellent alternative to, for example, anionic surfactants. Although anions remain unrivalled in terms of cleaning power and their ability to remove the toughest dirt, their aggressive nature often causes irritation.

In terms of foam formation, these compounds rank in the middle of the pack. In this respect, they outperform cationic and non-ionic surfactants, which foam poorly or moderately. Although they are unable to generate foam as high and abundant as anionic compounds, their presence in the formulation allows for a stable and dense bubble structure, which is desirable in personal care products. Furthermore, they are stable in hard water, meaning that calcium and magnesium ions do not limit their surface activity.

Another important consideration in formulation is their compatibility with other groups of surfactants. Amphoteric surfactants readily combine with other surfactants, enhancing their performance.

Safety standards in the surfactant industry

The placing of surfactants on the market and their professional use in industrial and laboratory processes is strictly regulated by three key legal pillars: the REACH and CLP Regulations, and the Detergents Regulation.

The foundation of chemical safety is provided by the REACH and CLP regulations, which comprehensively define the rules for the registration, distribution and correct labelling of substances. In practice, this means the obligation to use standardised pictograms and hazard statements (H-statements), as well as the precise preparation of Safety Data Sheets.

In parallel with the general regulations, the introduction of amphoteric surfactants into specific sectors – such as cosmetics, food or agrochemicals – requires compliance with additional safety standards. These primarily relate to chemical purity and toxicological safety, but the biodegradability of amphoteric surfactants is equally important. A number of raw materials available in the PCC Group’s range bear the prestigious international ECOCERT label, which is the most important and widely recognised certificate of quality and environmental safety in Europe. Furthermore, they meet the biodegradability criteria set out in Regulation (EC) No 648/2004 on detergents.

In which areas do amphoteric surfactants play a key role?

Thanks to their unique properties, amphoteric surfactants are widely used in many industries.

Body care

These surfactants are particularly valued in body care products such as shampoos, hair conditioners and shower gels, where mildness and skin compatibility are important. They are characterised by their gentle cleansing properties. Products containing amphoteric surfactants are particularly recommended for use in cosmetics for sensitive skin, as well as in baby care products. Their antistatic effect is equally important. They reduce the static charge that builds up on hair and other surfaces, which is particularly important in shampoos and conditioners. They are also often used as auxiliary substances – they mitigate the effect of certain anionic surfactants.

Cosmetic formulations

The ability of amphoteric surfactants to concentrate at the phase boundary results from their amphiphilic structure. This is particularly important in the case of complex mixtures of ingredients with different chemical properties. Cosmetic formulations are an example of such mixtures. The use of amphoteric surfactants improves the stability of emulsions and the clarity of finished products, which translates into the functional properties of the finished product. In addition, amphoteric surfactants, in combination with anionic surfactants, show a positive, synergistic increase in foam volume and improved stability.

Cleaning products

One important area of application for amphoteric surfactants is in household cleaning products. They work well on various surfaces, such as glass, stainless steel, chrome fittings, stone countertops, etc. They have a low tendency to leave streaks. In addition to domestic use, they are ideal for products intended for professional cleaning and industrial and institutional washing.

Construction

Amphoteric surfactants play an important role in construction and materials engineering. They are often used as air-entraining admixtures for concrete. The introduction of small, stable air bubbles into the concrete mixture increases its durability and frost resistance. Amphoteric surfactants also act as stabilisers. They are also added in small quantities to asphalts and bituminous compounds.

Textiles

Another important industry that uses amphoteric surfactants is textiles. These compounds are a key component in, among other things, compositions for printing on fabrics and dyeing fibres. Thanks to their excellent surface properties, amphoteric surfactants support the process of dispersing, for example, poorly soluble ester salts in dyeing preparations. As a result, the salt does not precipitate and the emulsion remains stable during the process, which improves the quality of the final textile product.