Also un-ionized as a hyphen (which you used when writing it) and unionized doesn’t. Which is probably why you, and most chemists, would read it as unionized as well.
It does get legitimately used both ways. In a chemistry textbook, seeing it written as “unionized” is pretty common, and wiktionary says that the hyphenated form is predominantly used in contexts where it might be confused with “having a union” (which matches with my experience).
However, I still assert that it’s just not a word chemists use that much as there are other, less ambiguous synonyms available.
Also un-ionized as a hyphen (which you used when writing it) and unionized doesn’t. Which is probably why you, and most chemists, would read it as unionized as well.
It does get legitimately used both ways. In a chemistry textbook, seeing it written as “unionized” is pretty common, and wiktionary says that the hyphenated form is predominantly used in contexts where it might be confused with “having a union” (which matches with my experience).
However, I still assert that it’s just not a word chemists use that much as there are other, less ambiguous synonyms available.