There is nothing we love more than a good soapy lather when washing dishes.
What causes the suds, and why do we associate then with cleaning?
Well, those are two separate questions. We’ll answer the first one today and the second one tomorrow.
Firstly, soap doesn’t make suds, it simply stabilises them. If you squirt some detergent into a sink full of water, it doesn’t instantly suds up – you have to agitate it to cause some suds. Actually, what you are doing is aerating it. The agitation that you do mixes air in with the water, and it is these air bubbles that ultimately become the suds.
Without a detergent, the air bubbles simply float to the surface and pop. Or, to put it another way, air bubbles don’t like being in the water, and the water doesn’t like having them there. The reason is, that water is a polar liquid and the interface with air is unstable. Therefore, bubbles are a lot worse because where there are bubbles there are lots and lots of interfaces.
So the air and water try to minimise the contact they have with each other by separating out into two separate phases with a single interface (the surface of the water) between them.
But with a detergent in there, things are different. The detergent is a surface active agent (surfactant), and essentially it stabilises the interface. So when you have made a bubble with your aeration, the bubble is now quite happy, as since surfactants love surfaces, they will coat themselves on both the inside and outside of each bubble. In doing so, they balance the surface charges on the water molecules, and remove any need for the water to minimise its surface area.
So are suds a good indicator of cleaning power? Well, the answer is both yes and no. Tune in tomorrow for the answer.