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Name: Jake Status: student Grade: 9-12 Country: USA Date: Winter 2012-2013

I am doing a science fair project to examine hydrophobic properties of household products. The project has went well. But, I have an issue that I cannot explain. Non-stick Cooking Spray changes properties after about 30 seconds. For the 1st few seconds (maybe 30) a drop of water appears to be repelled by a non-stick cooking spray coated surface, but, after that the water droplet breaks down to a much larger surface angle (hope that makes sense). How do I explain what is happening?

Without knowing it you have chosen a topic that seems straightforward but in fact is quite challenging. I do not have a complete picture of the exactly what your experimental arrangement is, so my response is only partial. First, what substrate are you spraying the “Cooking Spray” on? – a glass plate, or a non-stick fry pan? Second, are you aware that “non­sticking Cooking Spray is not a pure chemical compound? It is a formulated product that, in addition to the cooking oil, contains surface active agents (surfactants). The function of these additives, which may only be present at less than 1% of the product, is to make the oil phase (the cooking spray) and the water more compatible. They in fact are called “wetting” agents, because they coat the surfaces with a thin layer (often only a molecular film thick)! These compounds are “bi polar”. Their composition contains a hydrophobic composition at one end, and a hydrophilic composition at the other end. There are hundreds, maybe even thousands, of such materials in the commercial industry. Their specific function is to “solubilize” hydrophilic and hydrophobic molecules. Usually these additives coat the surface of a substrate, or couple the surfaces of two phases of very different polarity.

So what I am thinking is that when you first put (you did not say how?) water on the oil (or the other way around), it takes some time for the surfactants, which I am assuming are part of the formulated Cooking Spray, to migrate between the two phases – hydrophobic (water hating) and hydrophilic (water loving). When that happens the two phases are able to mix.

This whole area of surfactants and their properties is not simple experimentally, because it only takes a very small amount of the coupling agent to “solubilize” the phases that normally would be insoluble.

Vince Calder

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