Titan does seem to be latitudinally stratified. Equatorial dunes, mid-lat stuff, empty lakes, and filled lakes, in the Northern hemisphere. Sorta-kinda similar but a little bit drier in the South.
The term for the Titan's mid-latitudes is officially the "Titan's mid-latitudes", and unofficially "the Blandlands".
"Desolate plains" has also been used.
There is no solid (!) evidence that Titan's lakes are frozen. Titan is just a tad too warm, even at the poles. There is a very small difference between the equatorial temperatures (95 K) and the polar temperatures (91 K). But even that is just enough to mess (!) with the expected equilibrium ratio of nitrogen/methane/ethane/propane in the lake fluids.
If the lakes are in equilibrium with the atmosphere (which may not be the case), polar lake fluids would have a different equilibrium concentration than temperate lakes.
One estimate of these values is in Cordier et al., 2009. (freely available here, check out table 3 in the text:
http://iopscience.iop.org/1538-4357/707/2/..._707_2_L128.pdf.
Local humidity can also be a driver and affect evaporation rates thus composition. At equilibrium, humidity (actual vapor pressure vs. saturation vapor pressure) is 100%. If the equatorial areas are drier, they could affect evaporation rates and the mix of lake composition.
Methane is most volatile (nitrogen dissolves into methane, so it gets affected, too.)
Ethane is not volatile (but could evaporate on a long enough time-scale. We are talking multi-year climate timescale.)
Propane is seriously not volatile (Myr timescale????)
1-Butene is not predicted to be produced by atmospheric chemistry models to any appreciable extent. But if a puddle of that material were present, it would not evaporate, even on geological timescales.