Sonia Gay

2022-06-15

The book says that oil spreads over water due to the greater surface tension of water as compared to oil, so the comparatively stronger water film stretches the oil surface and makes it spread... But if this is the case, doesn't that mean that the oil layer will go on spreading indefinitely(even if it gets one molecule thick)until it covers the whole surface, because at any point the surface tension of water is still greater than oil.... But this does not happen, oil spreads only until it makes a particular contact angle with the water surface.
So is the explanation given in the book a bit faulty or am I getting the whole thing wrong? Someone please help...

ejigaboo8y

If the surface tensions are such that ${\gamma }_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r},\mathrm{a}\mathrm{i}\mathrm{r}}>{\gamma }_{\mathrm{o}\mathrm{i}\mathrm{l},\mathrm{a}\mathrm{i}\mathrm{r}}+{\gamma }_{\mathrm{o}\mathrm{i}\mathrm{l},\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$ then here is no solution to the (simplified for a flat water surface) contact angle equation
${\gamma }_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r},\mathrm{a}\mathrm{i}\mathrm{r}}={\gamma }_{\mathrm{o}\mathrm{i}\mathrm{l},\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}+{\gamma }_{\mathrm{o}\mathrm{i}\mathrm{l},\mathrm{a}\mathrm{i}\mathrm{r}}\mathrm{cos}{\theta }_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}}$
In that case the oil will spread until it is a layer one molecule thick -- a Langmuir–Blodgett film. This is the case of olive oil on water, where it was used by Lord Rayleigh to determine the size of the oleic acid molecule..

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