Foaming agents

Foaming agents, also known as simply foamers, are additives that create voluminous foams in the base medium of water through mechanical mixing or the introduction of gas. Foaming agents are surface-active substances (surfactants). In addition, surface-active proteins in foodstuffs often serve as naturally occurring or added foaming agents, e.g., in beer.

In materials engineering, propellants that generate foam through gas release or evaporation are also referred to as foaming agents. This glossary on interfacial chemistry is dedicated exclusively to foam-generating surfactants. 

Why are foaming agents needed?

Foam is specifically generated in a wide variety of products for various purposes:

• Cleaning: In washing and cleaning foams, the cleansing and conditioning active ingredients are finely dispersed and, due to the large internal surface area, ensure intensive and uniform interfacial contact. Therefore, foaming agents are included in many personal care and cosmetic formulations. In foam cleaning, they are the central component, for example in automatic car washes or in carpet foams.
• Firefighting: In firefighting foams, multiple effects are utilized, with varying emphasis and foam characteristics depending on the fire class and application. First, foam has a cooling effect, as the water it contains extracts energy from the burning material as it evaporates. Furthermore, it forms a barrier between the fuel and the air, which is particularly useful in liquid fires. Particularly voluminous foams are used to flood cavities in order to displace oxygen. Finally, foam acts as an insulator and protects against radiant heat.
• Food industry: Foam plays a key role in the sensory experience of many foods, with texture and moisture content being particularly important factors. Essentially, natural foaming agents—typically proteins—act as surfactants to create foam. These are often naturally present, such as in beer or sparkling wine, but are also frequently included in the formulations of industrial food products.
• Foam flotation: This separation process is used, for example, in ore processing or to clean pulp (deinking) in paper recycling. Air is blown through an aqueous slurry. The hydrophobic particles—or those rendered hydrophobic by special substances (collectors)—adhere to the gas bubbles and rise to the surface with them, while hydrophilic particles remain in the suspension. The foaming agent contained in the mixture creates a foam layer on the surface, which is regularly skimmed off.
• Oil production: Foaming agents are frequently used in Enhanced Oil Recovery (EOR) methods, particularly in gas flooding. In this process, foam is pumped into the reservoir or formed there to channel the gas flow.

How do foaming surfactants work?

In pure water, air bubbles are unstable because water has a high surface tension (SFT) and tends to minimize the air-water interface. Surfactants lower the SFT, thereby reducing the energy required to generate many small bubbles. They thus promote foam formation and the degree of surface tension reduction additionally influences bubble size.

▶ Read more about the structure and function of surfactants in the glossary article Surfactant.

An even more important mechanism is that surfactant molecules adsorb at the air-water interface and form stabilizing surfactant bilayers at the air-water-air transition within the lamellae of a foam. Their lifetime can vary greatly; foaming agents are generally characterized by the formation of relatively stable lamellae and slowly decaying foams.

Formulations designed to produce foam often contain additional foam stabilizers, which are frequently surfactants as well.

How is the effectiveness of foaming agents measured?

Depending on the application, different properties of the resulting foams are relevant. Various complementary methods can be considered for the specific problems at hand.

Foam volume and foam capacity

One of the most important criteria is the amount of foam produced from a given volume of liquid. However, the optimum is not always the largest possible foam volume—for example, foaming surfactants are used in toothpaste, but an excessive foam volume would be perceived as unpleasant.

At KRÜSS Instruments, foam analyses are typically performed by reproducible foaming in a gas flow or by stirring followed by optical foam height detection. One of the resulting parameters from height measurement is called foam capacity—the amount of foam produced relative to the original liquid volume. The measurements provide guidance for the targeted use of foaming agents in a formulation.

Automatic, optical foam height measurements can also be performed in accordance with ASTM D 1173 (Ross-Miles), whereby a specified volume of liquid is poured from a reservoir at a defined height into a predetermined liquid volume.

Foam formation rate
A key term in the evaluation of foam-forming agents is “flash foam.” This evaluation criterion indicates how quickly a large amount of foam can be generated, which is important, for example, in firefighting foam. KRÜSS uses its proprietary Foam Flash method for this purpose, which operates with short agitation cycles and, through the gradual increase in foam volume, enables clear differentiation between various highly foaming samples.

Foam stability

There is also a wide range of requirements regarding foam stability. While food foam should remain stable over time, firefighting foams must break down after a certain period because foam left at the fire scene would be difficult to remove. Foam height analyses, using flexible time control, can capture the decay curves of foams ranging from short-lived to highly stable and provide parameters such as the decay half-life. The criteria mentioned below—liquid content and foam structure—are also relevant for assessing the stability of long-lasting foams, as structural changes and drainage of the foam can be observed long before the foam volume measurably decreases.

Liquid content and drainage

When measuring foam height, the height of the liquid interface below the foam is also recorded, allowing the amount of liquid bound in the foam to be determined—also over time—to characterize drainage behavior. For many products, such as personal care items, the moisture content of the foam is closely related to the sensory experience—as well as to the mouthfeel in food products. In firefighting, the classification into light foam, medium foam, and heavy foam is directly related to the water content, which determines the throw distance and extinguishing effectiveness.

Foam structure

Like moisture content, the desired bubble size of generated foams depends on the application and product requirements. The foam structure analysis developed by KRÜSS measures the average bubble size of a foam and the statistical bubble size distribution—both as a snapshot at the end of foam formation and as a time-series record of structural decay.

▶ See also our application reports AR279 on toothpaste foam and AR302 on the foaming behavior of plant-based milks.

Measurements at high pressures and temperatures for EOR

In foam-assisted flooding processes for enhanced oil recovery (EOR), the foam is exposed to the pressure and temperature conditions of the reservoir and behaves differently than under ambient conditions. HP/HT analyses of foamability, as well as the structure and stability of the foam, allow the reservoir conditions to be simulated in order to optimize the use of foaming agents.