From chemical studies we know that molecules of oils and fats are longer than they are wide and that one end is attracted to water, If a drop of oil is placed on a water surface it floats and spreads out to form a thin layer with the molecules standing upright like a piece of carpet pile. When the spreading stops it is reasonable to assume that the layer is one molecule thick.

A suitable substance is oleic acid prepared as a 0.5% solution inof methanol. When this is added to water the methanol partly evaporates and partly dissolves in the water leaving a film of oleic acid.

Assuming the film to be a flat cylinder of thicknesswhereis the length of a molecule then

Hence the length of a molecule

Typically