I'm trying to work out some really basic biochemical details for cold-world aliens whose bodily fluids are based on liquid methane rather than water. I figure they have to be hydrogen breathers, because you won't get free oxygen on a world with methane oceans! But that means that their metabolism would produce water and ammonia as waste products- and both of those are very solid at liquid methane temperatures.
Additionally, although the solubility of water in liquid methane is apparently unexpectedly high according to Nature, it's still extremely low on an absolute scale. It seems this would be a bit of a problem for anything larger than a microbe that needs to expel metabolic wastes- or for "plants" that need to consume water and ammonia to get oxygen and nitrogen supplies for synthesis.
So, how might a methane-based alien go about transporting waste water and ammonia in its blood? I imagine special transport molecules would be required, but is there something relatively small and simple that could act like a reverse-soap to dissolve these polar solids in methane, or would it necessary require something large and highly specialized, like the hemoglobin we use to transport oxygen?