Emulsifier

Emulsifiers are natural or synthetic surface-active substances (surfactants) that increase the compatibility of aqueous and non-aqueous liquids that are normally immiscible (e.g., water and oil), thereby reducing the energy required to form liquid-liquid mixtures (emulsions) and increasing their stability.

How do emulsifiers work?

Emulsifiers are substances that have an affinity for both oil and water—a property known as amphiphilicity. As a result, they reduce the interfacial tension between the hydrophobic and hydrophilic phases. This facilitates the formation of fine drops of one phase within the other phase, which is accompanied by an increase in interface area.

▶ See also our related glossary articles: emulsions, surfactants, and interfacial tension.

Where are emulsifiers used?

Emulsifiers are a key component of virtually any emulsion and are found in a wide variety of products and processes, primarily in the food industry, cosmetics, personal care, and pharmaceuticals, as well as in enhanced oil recovery (EOR) and chemical processes such as emulsion polymerization.

▶ For more information on the importance of emulsions in these areas, see the glossary article on emulsions.

Do emulsifiers occur naturally?

Many emulsions are formed using natural emulsifiers rather than artificially synthesized substances. Foods such as milk or egg yolk contain lecithin as a natural emulsifier. Other examples of native emulsifiers and emulsion stabilizers include beeswax and pectin, which is extracted from apples, among other sources.

What role do emulsifiers play in the food industry?

Natural emulsifiers, such as lecithin—which is usually derived from soybeans—are generally safe for human health and may even be essential for the human body. They therefore play an important role in the food industry in products such as margarine and other spreadable fats, dressings and sauces, mayonnaise, ice cream, and chocolate. In addition, synthetic emulsifiers such as DATEM or polysorbates, as well as modified natural substances such as carboxymethylcellulose (CMC), are used.

For the European market, emulsifiers approved for use in food are designated by E numbers, e.g., E 471 for mono- and diglycerides of fatty acids, which are the most commonly used emulsifiers by volume. Due to their good tolerability, these emulsifiers are also used in the cosmetics and pharmaceutical industries.

For the U.S. market, the Food and Drug Administration (FDA) lists approved emulsifiers in the GRAS (Generally Recognized As Safe) list, though without assigning identifying numbers.

What are the most important emulsifiers outside the consumer goods industry?

Emulsifiers are also used in other industrial sectors such as heavy industry, agrochemicals, construction, and lubricants. Here, the focus is not on physiological tolerability but on technically required properties such as chemical and thermal stability, as well as the ability to form stable emulsions as quickly and efficiently as possible. The following are a few examples:

• Alkylphenol ethoxylates (APEOs) and fatty alcohol ethoxylates (FAEOs) are used, for example, in paints and coatings, where they also act as dispersants to improve the distribution of powders and pigments in the liquid and prevent clumping. APEOs and FAEOs are also important for crop protection products, with the latter being preferred due to their better biodegradability. Active pesticide ingredients are often immiscible with water and must therefore be emulsified. 
• In emulsion polymerization, classic fatty acid soaps such as sodium or potassium oleate are often used; in addition, alkyl sulfates and sulfonates—such as sodium dodecyl sulfate (SDS, sodium lauryl sulfate), which is used almost universally in industry—are employed. 
• Cationic surfactants—that is, those with a positively charged hydrophilic group—are used in road construction and the building industry to form bitumen emulsions. The emulsion breaks on the negatively charged rock surface, and the bitumen, which has been wetted by the emulsifier, adheres to the rock. Cationic surfactants have a similar effect in hair conditioners and fabric softeners, where they adhere to negatively charged fibers.    
  The most important group of cationic emulsifiers consists of quaternary amines (“quats”).
• In the past, chemically very stable and highly surface-active fluorinated compounds such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) were also of great technical importance. However, these substances—now commonly referred to as “forever chemicals”—have been largely regulated and banned due to their extreme environmental persistence and health risks.

What are co-emulsifiers and emulsion stabilizers?

Surfactants alone are often insufficient to form and stabilize emulsions. Co-surfactants, such as fatty alcohols, are often used to further stabilize some emulsions. These substances form stabilizing films at the interfaces of the emulsified drops and prevent them from coalescing. Co-surfactants play a particularly important role in the formation of thermodynamically stable microemulsions—although these special multiphase mixtures are not emulsions in the strict sense of the term.

▶ See also our article on microemulsions.

How can the use of emulsifiers in formulations be optimized?

To assess the stability of emulsion-based formulations, sample aging tests are often conducted, giving an indication of shelf life. These tests provide reliable data, but can be very time consuming, especially for products that remain stable for days or more. Accelerated methods such as cyclic heating/cooling or centrifugation are often used to reduce testing times and draw conclusions about phase separation under real-world conditions.

Among the alternative testing methods, interfacial chemistry methods are particularly efficient because they can be performed quickly and easily and directly address the most important mechanism of emulsion formation: the reduction of interfacial tension between the dispersed droplet phase and the continuous phase. Depending on the research question, different methods are used.

Force tensiometry: the Du Noüy ring method and the Wilhelmy plate method

The ring and plate methods measure interfacial tension based on the wetting force on a measuring probe (ring- or plate-shaped) located at the phase boundary between two superimposed liquids. These are classic methods of tensiometry that have also been incorporated into many standards.

▶ Watch our video to see how the effectiveness of an emulsifier can be measured using the plate method.

Spinning drop method

In this method, a capillary filled with the continuous phase is rotated, and the change in shape of a drop introduced into it—its curvature or vertical diameter—is analyzed optically. This shape results from the equilibrium between centrifugal force and interfacial tension. The spinning-drop method can detect even very low interfacial tensions down to10^-6 mN/m and is therefore used in the targeted formulation of microemulsions, for which very low values are typical. An important application of this measurement technique is in enhanced oil recovery (EOR).

Pendant drop method

Like the spinning drop technique, this method analyzes the shape of a drop in a surrounding phase, thus simulating the dispersed and continuous phases of an emulsion. The interfacial tension is measured optically based on the curvature profile of a drop that is suspended from a needle or—depending on the direction of the density difference—floats upright in the surrounding liquid. The dynamic interfacial tension can also be measured by analyzing the temporal change to investigate the rate of interfacial formation.

Interfacial rheology 

This method is used for in-depth investigations of the dynamic behavior of emulsifiers and can be understood as an extension of the pendant-drop method. In this method, the drop does not hang statically from the needle but undergoes sinusoidal oscillations of its size. The interfacial elasticity and viscosity determined through this analysis provide information on how the interfacial tension changes as a function of the degree and rate of change in the drop’s size. Both of these results provide insight into emulsion stability, particularly during dynamic processes.

▶ For more information on this method, please also read our application reports on interfacial rheological investigations in the fields of EOR (AR276) and food (AR285).