I see no noticeable differences between them. So can we use them interchangeably ? If so, which one do you see more often and tend to use when you mean "not allowing a liquid to pass through".
I've only seen "impermeable" used in a technical sense, Jeffsmiles. "Impervious" has the same technical meaning, but is used figuratively as well, as in definition 2: impervious to argument.
See the meanings of "repellent" as an adjective. These meanings are not possible for "impermeable" and "impervious". Even if only the scientific meanings are contemplated, there are differences. "Repellent" can mean resistant to penetration by a substance, while "impermeable" says that penetration is impossible.
Impermeable is (probably) most commonly a technical term in biological sciences to describe membranes. Usually it is specified what the material is impermeable to, for example impermeable to water or impermeable to sodium. I haven't really seen the word used outside of biological sciences.