Protein Skimming and Surface Tension

[Dripper_CS]

Hi @Randy Holmes-Farley (and Reef2Reef community),

Conventional wisdom is that protein skimmers work well in seawater because seawater has higher surface tension than freshwater.

This line of thought assumes that foams are easier to form when surface tension is high.

However, the fact is that foams are easier to form when surface tension is lower. That’s how surfactants like soap work, by reducing, not increasing, surface tension. See, e.g., https://web.mit.edu/nnf/education/wettability/bubbles.html

So why seawater, with its higher surface tension, foams more readily than freshwater, especially in protein skimmers? What factors make this possible despite the apparent paradox? Thanks for your insights!

 

[Dan_P]

Hi @Randy Holmes-Farley (and Reef2Reef community),

Conventional wisdom is that protein skimmers work well in seawater because seawater has higher surface tension than freshwater.

This line of thought assumes that foams are easier to form when surface tension is high.

However, the fact is that foams are easier to form when surface tension is lower. That’s how surfactants like soap work, by reducing, not increasing, surface tension. See, e.g., https://web.mit.edu/nnf/education/wettability/bubbles.html

So why seawater, with its higher surface tension, foams more readily than freshwater, especially in protein skimmers? What factors make this possible despite the apparent paradox? Thanks for your insights!

Is the surface tension of seawater very different from freshwater? Though I don’t recall where the notion that seawater foams better than freshwater came from.

 

[Miami Reef]

Source by Randy Holmes-Farley (2024).


“Skimmers do not work nearly as well in fresh water for a variety of reasons. From an article of mine:

Why does skimming work better in saltwater than in freshwater?
There are two fundamental reasons that skimming is more effective in seawater than in freshwater. One is the reduced solubility of organics, especially hydrophobic ones. Because many organics are less soluble in saltwater than in fresh, they are more easily squeezed out of it to an air/water interface, and collected as foam. This is the basis for the well-known salting-out effect of proteins. Quoting from a basic biochemistry text: "At sufficiently high ionic strength a protein may be almost completely precipitated from solution, an effect called salting-out."

A second reason for less efficient skimming of freshwater relates to bubble formation and coalescence. It turns out that air bubbled into seawater forms smaller bubbles than if the same device bubbled into freshwater.1-4 The possible reasons for this have been discussed in the scientific literature, but the exact reason is not universally agreed upon.

Despite the fact that skimmers usually produce larger bubbles in freshwater, and that organics are often more soluble in freshwater, it is not impossible to skim freshwater. Rivers from certain areas of the northeastern United States sometimes have foam on them, which comes from tree sap and other natural organics that enter the water. They have a low solubility in water, and are easily collected as foam in a natural skimming action.”

Source:
https://www.reef2reef.com/threads/skimmer-not-working-in-low-salinity-tank.1086987/post-13125632

 

[Randy Holmes-Farley Verified Community Expert]

If you want any additional discussion or clarification of my article above, I’m happy to do so.