The R.O. membrane is semi-permeable with thin layer of annealed material supported on a more porous sub-structure. The thin skin is about 0.25 micron thick and has pore size in the 5 – 10 Angstrom range. The porous sub-structure is primarily to support the thin skin. The pore size of the skin limits transport to certain size molecules. Dissolved ions such as Na and Cl are about the same size as water molecules.
However, the charged ions seem to be repelled by the active portion of the membrane and water is attracted to it. So adsorbed water will block the passage and exclude ions. Under pressure attached water will be transferred through the pores.