IMO - the right explanation is something like this below. it was a long time ago that I study these things and my bad memory force me to use AI in order to update both my memory and knowledge
The foam you get in a skimmer at 35 psu is a result of many different physical laws.
In general - 35 psu has a too high ion concentration to provide good foam formation for most substances that are not amphiphilic (bipolar—one part loves water and the other hates water). However - with these amphiphilic substances (proteins is amphiphilic) - then it's a different matter. The high ion concentration together with the tiny bubbles in a skimmer force amphiphilic substances to the air/water surface of the bubbles. (thermodynamic driving force / "salting-out") Upon reaching this interface , they undergo surface denaturation, whereby the hydrophobic end attaches to the gas phase at the interface and the hydrophilic end attaches to the water. What happens next is that the amphiphilic substances form a rigid, viscoelastic network (intermolecular cross-linking/gel formation) that stabilizes the foam which builds up and is transferred into the skimmer cup.
What could disrupt this foam formation and cause the foam to collapse?
My memory played a bit of a trick on me when I stated that the interfering substances acted primarily by lowering surface tension—that is only part of what happens.
Since the foam structure at 35 psu is entirely dependent on this rigid, viscoelastic network—formed as a result of the surface denaturation of the amphiphilic substances—this matrix can collapse via distinct chemical and physical pathways.
Since oils and fats have lower surface tension, they can easily penetrate the pre-existing film of amphiphilic substances (the spreading trajectory).
The oil forms local “bridges” within the bubble wall. The oil molecules do not form continuous, cross-linked intermolecular networks. The surrounding capillary pressure causes water molecules to be rapidly drawn away from the oil surface—a process known as dewetting. Immediate thinning and a mechanical rupture of the bubble wall occur.
Furthermore, the hydrophobic pockets of the amphiphilic substances can bind directly to oils and fats, thereby impairing their ability to undergo surface denaturation.
Because detergent molecules are significantly smaller and diffuse more readily, they move faster toward the air-water interface, thereby physically displacing the amphiphilic substances (competitive adsorption).
Within the water column itself, detergent monomers encapsulate the amphiphilic substances. This masks the non-polar regions of these substances, neutralizes the "salting-out effect," and eliminates the thermodynamic driving force that causes the amphiphilic substances to migrate toward the air bubbles (macromolecular solubilization).
In freshwater with low calcium and magnesium ion concentrations—"soft" water—detergents form their own foam, but this detergent layer cannot withstand shear forces at the interface. In "hard" water and saltwater, a capillary suction pump forms where three bubbles meet; this drains water from the foam structure (Young-Laplace Law), causing the foam column to collapse once the robust network formed by the amphiphilic substances has been removed. In saltwater with a salinity of 35 psu (in the absence of detergents), it is precisely this strong network formed by the amphiphilic substances that counteracts these capillary suction forces.
Summary
Oil, grease, and cleaning agents lower the surface tension locally in different ways, but the result is the same—the foam column collapses.
You can test this yourself: take one or two fish food pellets, lift the skimmer lid, and drop them into the foam column.
It is important to note that I compiled the text above using my existing knowledge and updated it through a discussion with an AI agent (Google). I have also use the term amphiphilic substances since substances other than proteins—which are bipolar—may be present in a reef aquarium. It is worth noting that neither my existing knowledge nor my updated information confirms the common belief among aquarists that non-bipolar substances—such as bacteria or unspecified uncharged organic compounds—are effectively removed by a protein skimmer.
Sincerely Lasse
