Defoaming Methods for Excessive Foam Caused by Increased Surfactant Concentration

When air enters a liquid phase, it barely dissolves in water and is split into countless tiny bubbles by external shear force, forming a heterogeneous foaming system. Foam formation greatly expands the gas-liquid contact area and increases the system’s free energy.

As surfactant concentration rises, the system’s surface tension drops sharply at first and levels off to an equilibrium value with further concentration growth. Experiments by Milles and Shedlovsky prove that abnormal minimum points on the surface tension curve mainly result from system impurities or interactions among different surfactants, and such tipping point refers to the Critical Micelle Concentration (CMC). Once the concentration hits the CMC, abundant surfactant molecules arrange compactly on the liquid surface to form a dense, gap-free unimolecular film, minimizing the overall surface tension. Lower surface tension reduces the free energy threshold for foam formation, making foaming much easier.

In industrial production, surfactants are generally dosed above the CMC to guarantee long-term storage stability of emulsions. Despite improving emulsion stability, excessive surfactants bring obvious downsides: besides lowering surface tension, surplus surfactants wrap entrapped air inside emulsions to form tough liquid films and build a bimolecular layer on the liquid surface, significantly increasing the difficulty of foam breakdown.

Foam is an aggregate of numerous bubbles, with gas as the dispersed phase and liquid the continuous phase. Gas inside bubbles can diffuse across adjacent bubbles or escape into ambient air, eventually leading to bubble coalescence and rupture.

Pure water or single-component surfactant solutions feature homogeneous compositions. Their formed liquid films lack elasticity, so the resulting foam is unstable and collapses spontaneously. Thermodynamic theories hold that foams generated from pure liquids are temporary and break down via liquid drainage from film walls.

Waterborne coatings contain water, polymer emulsifiers and various surface-active additives including dispersants, wetting agents and thickeners. These coexisting active substances readily trigger foaming and stabilize existing foam.

1. After foaming, intermolecular attraction drives surfactants to adsorb onto bubble films. Hydrophilic groups stretch into the external emulsion while hydrophobic groups point toward the air inside bubbles. Regular molecular arrangement at the gas-liquid interface produces elastic films that resist rupture. Gravity induces liquid drainage down the bubble film and partial film thinning. Two self-repair mechanisms prevent film collapse: Gibbs elastic contraction occurs when expanded film area raises local surface tension and generates tension gradients to shrink the film; the Marangoni effect enables adsorbed surfactant molecules to migrate from low-tension areas to high-tension zones for uniform surface tension. Jointly, the two effects maintain stable foam.

2. Ionic emulsifiers carry static charges on bubble films. Electrostatic repulsion between identically charged bubbles prevents small bubbles from merging into larger ones, hindering natural defoaming and stabilizing foam.

3. Long-chain emulsifiers create strong intermolecular attraction between molecular chains, granting bubble films excellent elasticity and mechanical strength to resist cracking and retard foam elimination.

4. Thickening agents are added to boost the viscosity of waterborne coatings. Higher viscosity impedes liquid flow between bubbles, slows film thinning and stops film rupture, which stabilizes foam and complicates defoaming.