Is it generally easier for small non-polar substances to pass through the cell membrane by simple diffusion or is it easier for small polar substances to?
I always thought polar substances had a bit more trouble, as the interior of the phospholipid bilayer is non-polar?
Non-polar substances can easily diffuse through the membrane and small polar molecules can as well but larger polar (hydrophilic) and charged molecules require protein channels.
The interior of the phospholipid is non-polar however the phosphate heads are not but since the membrane is porous, water molecules and certain other substances can diffuse through.
Note that the factors which affect diffusion are size, charge and polarity.