If you’ve ever administered albumin, you’ve undoubtedly seen the bubbles and foam while infusing this colloid solution. As the volume of albumin decreases in a fixed container (ie, glass bottle), a vacuum effect is generated that requires venting with special tubing to equalize pressure within the bottle and atmosphere. More air rushes into the bottle as more of the fluid is infused, so now you’ve got air mixing with an aqueous solution.
Like virtually all large proteins, albumin contains both hydrophilic and hydrophobic portions. This amphipathic duality makes it an excellent surfactant (a substance that decreases the surface tension of a liquid). Remember that 5% albumin has 5 grams of albumin in 100 cc of saline. Those albumin molecules adsorb to the air-water interface, decrease the surface tension, and allow foam to be generated/stabilized as the newly entrained air is dispersed within solution.
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