Defoamer Series – How foams are generated and the additives that break them down.

Very simply put, foam is formed by the entrapment or encapsulation of air, or a gas, in a surface stable film of water.  As a rule, pure liquids will not produce foam.  This is best illustrated by taking a straw and trying to blow bubbles in a glass of water.  Nothing happens.  Now, add a drop of dish detergent or some other surface tension altering contaminant to that same glass and things get out of hand very quickly.

Depending on the application, the presence of foam can be considered a good thing.  For example, in a sanitation environment, foams can improve cleaning activities by enhancing the scouring effect of a solution or by increasing its contact time to allow for better penetration and overall performance.  In a different operational setting, like in a manufacturing plant or perhaps a WWT facility, foams can be problematic.  Some of the more common operational issues tied to the presence of foam are listed below.

• Product quality and or off-spec materials.
• Cavitation of pumps and other relevant processing equipment.
• Inhibited liquid-solid separation that will promote the carryover of contaminants from a treatment step to downstream unit operations.
• Limit or adversely affect the operational performance of vital instrumentation
• Promote the overflow and loss of hazardous, value-added, and or waste materials.
• Promote health and safety hazards local to the plant or within the surrounding area.
• Promote damage to existing or adjacent equipment

For part 1 of this series on defoamers, let’s look at the science of how foams are generated and some of the additives that can be used to break them down.

Bubble Formation and Foam Development

Air, or some other gas, can be introduced to the bulk liquid of an industrial processes using a variety of methods.  Some of which include, chemical reactions, mechanical aeration, agitation and or pumping.  When this takes place, the expectation is that the bubbles that form will rise and burst as they breach the surface.

When an air bubble breaches the surface, a small film of water will encapsulate it.  This film is called the “lamella”.  As the bubble continues to rise, the water within the lamella will drain back toward the surrounding liquid.  In doing so, the lamella wall will continue to thin until its structural integrity can no longer be maintained.  This is illustrated in the diagram provided below.

There are typically two conditions that will interfere with the natural rise and eventual collapse of a bubble; “air entrainment” and the presence of a “foaming agent”.  Alone or in combination, both can contribute to bubble stability and the eventual development of unwanted surface foam.

Air entrainment is identified as the distribution and retention of small, evenly dispersed bubbles within a bulk fluid.  This can be brought on by an increase in viscosity or excessive solids content in the water.  When this takes place, the bulk fluid will have a tendency to swell and contribute to significant liquid losses.  It can also promote cavitation within pumping equipment or impede settling within any sedimentation-based processes (clarifiers, decanters, etc.).  To alleviate operational issues tied air entrainment, small bubbles must coalesce to form larger ones so they can quickly rise to the surface.  In some cases, mechanical mixing can promote coalescence however the addition of a chemically based foam control additive is often the best solution.

Foaming agents are considered surface-active chemicals that will prevent the immediate collapse of any bubbles that form and are trying to breach the water surface.  If the bubbles are unable to collapse, a head of foam will develop.  This is called surface foam.

All foaming agents (surface-active chemicals) have a hydrophilic (water soluble) and hydrophobic (non-water soluble) head.  As a result, these chemistries will preferentially migrate to the air/water interface of an entrained air bubble or at the surface of the bulk fluid.  Once there, the foaming agents will align themselves so the hydrophobic head migrates to the air side while the hydrophilic head remains in the water.  In this orientation, the hydrophilic ends form weak bonds with adjacent water molecules which in turn restrict flow, increase surface tension and elasticity within the lamella and prevent the thinning that is needed to promote bubble collapse.  End result, highly stable and flexible structures that are more likely to deform than break down as they accumulate.  An example of how structural stability that is brought on by a foaming agent is illustrated in the diagram below.

Within industry, there are a number of foaming agents that can contribute to the formation of stable foams.  Some examples include, proteins, lignans, the saponification of fats and oils, by-products derived from bacterial activity, detergents, cleaners, and a variety of commercial surfactants (stabilizers).  If any of these are present, uncontrollable foam production can be a concern.

The Mechanism of Defoaming

To break down a stable bubble, two mechanisms are employed.

• Bubble Wall Thinning
• Piercing Effect

Bubble Wall Thinning is accomplished via the addition of chemistries that will destabilize the weak bonds being formed by the foaming agents.  These additives are polar in nature and have both hydrophilic and hydrophobic components.  Unlike foaming agents, these additives align themselves in a horizontal fashion at the air/water interface.  When this happens, everything gets spread out, the bubble starts to enlarge, there is a net improvement in outward water flow, the lamella starts to thin and there is a loss of structural integrity.  End result, the bubble(s) will burst.

The effect of Bubble Wall Thinning via the addition of a defoamer additive can be seen in the diagram below.

The defoaming action described above is the same for both surface foam and air entrainment related issues.  The only difference is that for air entrainment, the spreading and enlargement of bubbles will take place beneath the surface.  When this happens, the bubbles become more buoyant, their upward velocities intensify, and their ability to collapse as they breach the surface will increase significantly.

The Piercing Effect is best defined as the addition of colloidal solids that will adhere to a bubble’s surface and create weak points that will thin the film locally or breakthrough it entirely to promote collapse.  These weak points will also promote the coalescence of smaller bubbles into larger ones as they interact and collide with each other.  Each time larger bubbles are formed, the resultant structures are much less stable and more likely to collapse.  The figures below provide a visual representation of what takes place when colloidal solids are introduced.

In an entrained air environment, colloidal defoamer additives will promote the coalescence of smaller bubbles into larger ones while beneath the surface.  Similarly, this will increase their rise rates and promote collapse as they breach the surface.

Antifoam and Defoamer Additives

The words antifoam and defoamer are often used interchangeably as they typically contain the same actives and employ the same mechanisms needed to promote bubble collapse.  The only real difference is the distinction of where they are applied.  Antifoams are typically added as a preventative measure and fed upstream of a problem area to prevent the formation and accumulation of foam.  Defoamers are more often applied as a corrective measure to “knock down” foam that has already accumulated and is causing problems.

While many of these chemistries are designed to supress foam using one mechanism, there are a number of additives that can be blended to employ both.  The table below provides a list of defoamer additives that are available for use and the foam destruction mechanisms that are employed.

The decision to use one over another is a difficult choice and will depend on a variety of factors.  Some of which include.

• Compatibility of the product actives with the process and any materials in the water.
• Product efficacy and its ability to promote good “initial knockdown” and “sustained” foam suppression.
• The use of product actives that are “fully approved” or have a “limited use” designation by regulatory bodies (food production)
• Product availability
• Product pricing and relative treatment costs

While the decision of use one additive over another can be daunting, Aquasan is well versed in the application of defoamer chemistries and can support your facility’s operational needs.  Applied as a “contingency additive” or as a “continuous application”, Aquasan has a diverse portfolio of products that can quickly knock down foams and maintain control.

At Aquasan, your success is how we measure ours!