Thanks a lot for your fast response. I'm glad to hear my guess was right. I've been reading online courses, but even so I couldn't find any concrete answer to those two questions.
And sorry I should have mentioned earlier, I'm researching bronopol and its degradation products(2-bromoethanol/ bromonitromethane/ nitromethane/ 2-bromo-1,3-nitroethanol/ tris(hydroxymethane)nitromethyl), on a 25cm long c18 column (HPLC-DAD).
I know that by increasing one of the two solvents, compounds in a sample will have more retention with the mobile or stationairy phase(based on polarity in my case). I guess that increasing the amount of Milli-Q in my mobile phase, means that my compounds have less affinity with my mobile phase and more affinity with the stationairy phase, and therefor stay longer on the column.
I just found this strange, since bronopol and its degration products have a lot of polar functional groups, and therefor I didn't think that by increasing the polar solvent, the affinity with a non-polar stationary phase would increase.
It could be that I'm overthinking this, It's just that my deduction is contradicting itself, which results in me doubting myself.