Every now and then I have a glass full of ice cubes and tonic soda. After the soda is gone, the ice cubes are left and I eat the ice cubes one-by-one. I have noticed that sometimes the outer layer of the ice cubes are really bitter. It seems that somehow the bittering agent "prefers" to be stuck to the ice rather than being suspended in soda water. Has anyone else noticed it? Does anyone have an idea how could it be?

PS: Sorry for the boring recipe - ice cubes and tonic soda. I have been sober since 2008 and I keep the interesting stuff out of my drinks.