How can polar molecules pass through a membrane?
1 Answer
Polar molecules can have somewhat stronger intermolecular forces than non-polar molecules, but as liquids they can also flow through a membrane if the molecular size is small in comparison to the pore size of the membrane.
The most important factor in determining whether or not a liquid can flow through a membrane is the size of the molecules, not their polarity.
Polarity is important in determining whether a liquid will separate into distinct phases (like oil and water). This happens because subtle differences in intermolecular interactions can allow a system to lower its overall free energy if huge numbers of molecules are involved (e.g., a mole).
By comparison, the interaction energy of a relatively small number of molecules at the surface of a membrane can have very little influence on molecular behavior unless the interactions are very strong (i.e., if the molecules can't fit through the pores).
