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Protocol for Determining Foaming Ability and Foam Stability of Surfactant Solutions

When gas is dispersed in a liquid in the form of micro- to millimeter-sized bubbles, foam is formed — a gas/liquid dispersion system. Aqueous surfactant solutions are simple systems that can easily generate foam. The foaming ability of a surfactant refers to its capacity to produce foam instantly, while the foam stability describes the persistence or durability of the foam over time. Although surfactants are generally good foaming agents, they are not necessarily effective foam stabilizers.

The simplest method for foam preparation and characterization involves placing a surfactant aqueous solution of a specific concentration into a stoppered graduated cylinder. After sealing, the cylinder is shaken up and down a fixed number of times. The foam volume is then recorded, and its change over time is observed. The initial foam volume represents the foaming ability of the surfactant, whereas the foam volume after a given time reflects its foam stability.

👉 Aqueous surfactant solutions can easily generate foam. Common choices include non-ionic surfactants, which are known for their foamability and stability here.

Standard Method for Determining the Foaming and Foam Stability of Surfactant Aqueous Solutions

One of the standard methods for determining the foaming and foam stability of surfactant aqueous solutions is by using a Ross–Miles Foam Apparatus, as illustrated in Figure 1. Under constant temperature conditions, 50 mL of the surfactant solution is first placed into a jacketed glass column with an inner diameter of 50 mm, a height of 1000 mm, and graduated markings on the wall. Then, 200 mL of the same solution is poured down through a dropping tube (approximately 45 mm in outer diameter) from a height of 900 mm along the center of the column. The maximum foam height obtained is used to characterize the foaming ability, while the foam height after a specific period of time is used to characterize the foam stability.

Figure 1 Roche Foam Tester

Experimental Instruments and Reagents

Instruments: Ross–Miles foam analyzer, SHZ-82 constant-temperature water bath, analytical balance, volumetric flask, stoppered graduated cylinder, beakers, etc.

Reagents: Sodium linear alkylbenzene sulfonate (LAS), double-distilled water (conductivity: 7.8x10^-7S·cm^-1, prepared using a Millipore Synergy UV double-distillation system, USA). For more anionic surfactants, see here.

👉 For experiments requiring mild or versatile surfactants, amphoteric surfactants can be considered here.

Detailed Experimental Procedures

Method 1: Ross–Miles Foam Apparatus Procedure

1. Prepare a 1% (mass fraction, the same below) aqueous solution of LAS (linear alkylbenzene sulfonate). Be careful not to generate excessive foam. Keep the solution in a water bath at a constant temperature of (40 ± 1) °C for use.

2. Turn on the thermostatic water bath and maintain the water temperature in the jacketed tube of the Ross-Miles foam apparatus at (40 ± 1) °C.

3. Rinse the inner wall of the graduated cylinder with distilled water, then rinse thoroughly with the 1% LAS solution to ensure complete cleaning.

4. Pour the 1% LAS solution along the inner wall of the graduated cylinder up to the 50 mL mark. Fill the dropping tube with 200 mL of the 1% LAS solution and install it on top of the graduated cylinder. The outlet of the dropping tube should be positioned at the 900 mm mark, ensuring both tubes are vertical so that the surfactant solution drops along the central axis of the graduated cylinder.

5. Open the stopcock of the dropping tube to allow the surfactant solution to flow down. When the solution in the dropping tube has completely drained, immediately start the stopwatch and record the foam height H. After 5 minutes, record the foam height H again. Repeat the measurement three times and calculate the average value.

👉 Depending on your experiment, you may also use other types of surfactants, such as cationic surfactants here.

Method 2: Stoppered Graduated Cylinder Procedure

Pipette 20 mL of a 1% (w/w) aqueous solution of LAS into a 100 mL stoppered graduated cylinder. Secure the stopper tightly and shake the cylinder vigorously up and down 100 times within 1 minute. After shaking, place the cylinder on a flat surface, immediately start a stopwatch, and record the foam volume (including the solution volume). Record the foam volume again after 5 minutes. Repeat the measurement three times and calculate the average value.

Data Recording and Processing

Method 1: Determination of the Foaming and Foam Stability of Surfactants Using the Ross-Miles Foam Apparatus

Experimental Temperature:

Serial number	H0/mm	H5/mm
1
2
3
Average value

Method 2: Determination of the Foaming and Foam Stability of Surfactants Using the Stoppered Graduated Cylinder Method

Experimental Temperature:

Serial number	V0/mL	V5/mL
1
2
3
Average value

Frequently Asked Questions (FAQs)

What are the foaming and foam stabilization mechanisms of surfactants?

Foaming mechanism:

Surfactant molecules contain both hydrophilic and hydrophobic groups, allowing them to adsorb at the gas–liquid interface and reduce the liquid's surface tension.
When gas is introduced into the liquid (e.g., by stirring or aeration), surfactant molecules rapidly adsorb onto the surface of the bubbles, preventing them from bursting.

Result: A large number of bubbles are formed—this is foaming.

Foam stabilization mechanism:

The foam film formed by surfactants consists of a water layer sandwiched between surfactant molecular layers. Foam stability mainly depends on:

Interfacial film elasticity: The monomolecular film formed by surfactants can resist bubble film rupture.
Electrostatic or steric hindrance: Ionic or nonionic surfactants prevent foam coalescence through charge repulsion or steric effects.
Solution viscosity: A higher viscosity slows down liquid drainage from the foam, enhancing stability.

How to defoam? What are the requirements for defoamers?

Defoaming methods:

Physical methods: Stirring, vibration, heating, or using foam-breaking meshes.
Chemical methods: Adding defoamers (Defoamer / Antifoam).

Requirements for defoamers:

Must rapidly destroy the foam film or lower interfacial tension.
Must be compatible with the system and not affect the main reaction or product properties.
Should be effective at low concentrations.

Typical components of defoamers:

Hydrophobic particles or oils: Penetrate the foam film and cause bubble rupture.
Low surface tension liquids: Reduce foam film stability.

What factors influence foam stability?

Foam stability is affected by:

Type and concentration of surfactant: Proper concentration and suitable molecular structure enhance foam stability.

Liquid viscosity: Higher viscosity slows liquid drainage, increasing foam stability.

Solution composition:

Electrolytes: High ionic strength can destroy foam (electrolyte defoaming).
Polymers or colloids: Can enhance foam stability.

Bubble size: Smaller bubbles are more stable due to surface tension and pressure effects.

Temperature and pressure: Higher temperatures or pressure changes accelerate foam collapse.

What other substances, besides surfactants, can stabilize foam?

High-molecular-weight polymers: Such as gelatin, starch, and proteins (e.g., whey protein, ovalbumin).
Colloidal particles: Such as silica or clay particles (forming Pickering foams).
Protein–polysaccharide mixtures: Can form tough foam films.

These substances stabilize foam by increasing the elasticity of the interfacial film or by reducing liquid drainage.

What is the structure of foam?

Foam is a multiphase dispersed system with the following structural features:

Bubbles: The main body of the foam, spherical or polyhedral in shape.
Bubble film (thin liquid film): Formed by a liquid layer sandwiched between surfactant or colloidal layers, typically nanometers to micrometers thick.
Foam liquid: The liquid filling the spaces between bubbles, providing structural support.
Foam stability arises from: the elasticity of the bubble film, liquid viscosity, and the adsorption layer of surfactant molecules.