I personaly don't think its a split personality case, (unless is somehow linked to the rain) but considering how this chapter presented it, I almost discarded it.
I believe it was almost as if Riko had... Predicted this kind of development. I mean, not in a "known it all" kind of a way, but like, perhaps she had know some personal information regarding the top members of the research department(a department that she clearly want to join) and somehow found suspicious that Momo didn't seem interested in "usual relationships" as her peers were.
Riko is shown to be manipulative, very inteligent, controlled and smart - therefore, she could easily made a good guess based on evidence and decided to thrown some "bait" for her. Who knows, she might have intended to catch another woman and ended up getting Momo instead, and decided to just roll with it.
Hm... Writting this down does seem kind of silly, but it is a manga - and I have already seen quite the stretch in my suspension of disbelief even in the most realistic ones - so, I won't discard this until more development is shown(if that ever is going to happen. At this point, I'm starting to think this is just turning into a pseudoHentai).
Another possibility is that Riko doesnt really "feel" anything(or at least, never did before). Like, emotions. She can get basically feel primal things like rage, annoyance, satisfaction, pleasure(including sexual attraction/orientation), boredom... But not something complex and troublesome like love.
Maybe she decided that she could at least enjoy sex with women without dealing with the troublesome parts while she still "young and beautiful", - and getting paid for it - while she is in college. Is not like most of her colleagues(whom statiscally are mostly male) would ever known about it.
...And that is how Momo arrived in her life. I'm sure, she must have been both surprised by her presence and excited at the prospect of using her.
Hm... That last possibility I've just wrote is more likely to be the case, even if Riko does actually feel something like everyone else.
Who knows. I wish the characters would have more development than merely sex. I mean, the scenes are really good, and we do get to see a few things here and there on how Riko is intentionally/acidentally pushing Momo to change herself during said scenes, but still.. Its kind of dissapointing.
Regardless, thanks for translating this chapter.