How good as solvents are the fluids thought to exist on Titan? Do they ionise compounds as water does?
Well liquid water is thought to exist on Titan… clicky clicky
I’m not sure which fluids you’re referring to that are “thought to exist” on Titan; it’s been confirmed that Titan contains a lot of methane and ethane gas in the atmosphere and hydrocarbon bodies of liquid that don’t freeze even at the poles, so is there another type of liquid besides those that you’d be alluding to? A lot of credit is given to Titan as being very similar to Earth in its infancy, but it’s just too far away from the sun and too cold to have the kind of liquid water necessary as a solvent for the building blocks of life (as we know it). I’m kinda curious about the solvency of liquid methane and ethane myself, and wish I could answer it for you but the answer isn’t easily available in decipherable layman’s terms … why do you want to know, anyway? Trying to do a Miller-Urey experiment in your garage? 
Did you miss the whole liquid water layer thing on the diagram?
The short answer is no, I did not, but on the other hand, I already knew a bit about Titan and I don’t need to read the Wiki article to know about the potential subsurface water. The long answer is that even though Quartz’s post was very brief, I was assuming he wanted to know about Titan’s surface liquids (of which water is not a member because of its very high melting point compared to methanol or ethanol), not a subsurface layer of water sandwiched between two layers of pressurized ice. The change in barometric pressure and proximity to Titan’s core has quite a lot to do with why that layer of water is there rather than on the surface.
Our search for water on other atmospheric worlds is a priority because it is the ultimate solvent and it fosters the development of complex molecules better than anything else we know. The question posed in the OP has a complicated answer best explained by someone with a background in chemistry, but suffice it to say, all the liquids generally discussed on Titan (water, methane, ethane) are simlarly-related “polar protic” solvents despite their different melting points.
I guess in answer to the OP, you could just check the wiki entry on solvents, though I doubt you’ll find much about how to easily compare the solvency of different liquids … which I guess might explain why they came here. We got any chemists here?
Yes, it’s the surface chemistry in which I’m interested.