When did "zu Gunsten", "zu Liebe" etc. become "zugunsten", "zuliebe"? (In case they did)

Question

In this question we learned that one origin of new prepositions are adverbs or fixed prepositional phrases. These prepositions go initially with genitive (and then perhaps mutate into dative).

When I told to my German teacher this point against the extinction of the genitive, she replied that there is a fixed number of prepositions in a language, to which I replied that "language evolves" but I couldn't reply properly due to my lack of knowledge on the history of the German language.

Arguably, we can say that zugunsten, zuliebe are originated by zu Gunsten, zu Liebe, respectively. (Actually it seems that not only prepositions are origined that way, e.g. demzufolge).

Question: When were the new prepositions accepted as such? Is somewhere the process of "becoming preposition" documented?

Answer 1

A research on books.google.com shows that something happened at the start of the last century.
Look at the graphs, the word frequency in books changed around 1900-1905.

Maybe it goes back to the "German Orthographic Conference of 1901"? That's only a guess.

The Orthographic Conference of 1901 (also called the Berlin II Orthographic Conference) was a German spelling reform for the German Empire, it was largely based on the Prussian school spelling, but also based on the Orthographic Conference of 1876.

zugunsten vs. zu Gunsten

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zuliebe vs. zu Liebe

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Answer 2

This is from comments:

Without knowing an answer to your question I will say this.. the idea of "fixed" is not compatible with language. Language is ever changing. I think your teacher just didn't want to admit that she didn't really know what to say.

The process of regular words developing into function words is called grammaticalization. It's often difficult to observe the early stages, because there is not much visible going on; instead, the interpretation in writers' minds change.

Another good example is that zu Hause has also been accepted as zuhause since 1996 (see frustfrei-lernen.de/deutsch/…). At present, both versions are correct. And you're in the thick of it. And it's OK, since zuhause is more or less perceived as a common state while zu Hause with that zu-construction starts to sound somehow archaic like hoch zu Rosse.