xxviii vs twenty-eight

xxviii is not a noun while twenty-eight is a noun.

Word	Noun	Adjective	Verb	Adverb
xxviii	No	Yes	No	No
twenty-eight	Yes	Yes	No	No

As adjectives, xxviii and twenty-eight are synonyms defined as:

Other synonyms of xxviii include 28.

xxviii (noun)	twenty-eight (noun)
	the cardinal number that is the sum of twenty-seven and one

xxviii (adjective)	twenty-eight (adjective)
being eight more than twenty	being eight more than twenty