The Propylene Oxide (PO) chain in EO/PO block co-polymers plays a vital role in the process of defoaming in aqueous systems, specifically in the foam destabilization stage. Here’s the role it plays:

1. Foam Film Displacement: The hydrophobic PO chain of the block co-polymer acts to displace the surfactant in the foam film once the defoamer has infiltrated it. The surfactant is what increases the stability of the foam by reducing the surface tension of the liquid, creating a boundary that helps maintain the foam’s structure. By displacing this surfactant, the PO chain causes the foam stability to decrease.

2. Foam Film Thinning: After displacing the surfactant, the PO chain aids in decreasing the thickness of the foam film. The foam film, otherwise known as the lamella, is the thin layer of liquid forming the wall of a foam cell or bubble. By thinning this film, the integrity of the foam cell structure is weakened, making it more susceptible to breaking.

3. Bubble Bridging & Destruction: Eventually, as the foam film continues to thin due to the action of the PO chain, ‘bridges’ or thin liquid sections are formed between adjacent bubbles. These bridges are unstable and rupture quickly, leading to the overall collapse of the foam.

4. Prevention of Foam Reformation: The PO chain promotes dewetting or the liquid’s ability to repel from the defoamer emulsion droplets on the foam film’s surface. This process is crucial in preventing the reformation of foam, as it stops the liquid in the system from coalescing and forming more foam.

Hence, the hydrophobic PO chain is one of the essential parts of the defoamer molecule. It is the primary factor that drives the foam disruption and ultimate bubble rupture dynamics, which result in effective foam control.