Water has to be pushed through a semipermeable membrane that blocks the salt and other impurities from going through.
Similarly the diffusion of sols through "semipermeable" membranes is affected by lyotrope influence.
semipermeable sem·i·per·me·a·ble (sěm'ē-pûr'mē-ə-bəl, sěm'ī-)
Allowing passage of certain, especially small, molecules or ions but acting as a barrier to others. Used of biological and synthetic membranes.