Interactions between Polar and Non-polar molecules. The polar water molecules interact with each other, but not with the non-polar methane. Play with it.

The interactions between molecules is the basis for solubility, what dissolves in what. We all know oil and water don't mix. Another way to think about is that the non-polar lipid can't form bonds with the polar water molecules. The water molecules squeeze the lipid away by forming hydrogen bonds with each other.

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