two solutions molecules act

According to Fick's law a solute in a solvent will move from a higher to a lower concentration. The simple formulae is a simple differential F=D(dC/dx) where D is the diffusion coefficient for a particular solute in a reticular solvent. Semi-permeable membranes are able to reverse this process, but like all things to perform this bit of magic you gotta feed Maxwell's Demon, e.g. energy is required.

If you read a little further, you'll find that the sources for semi-permeable membranes come from flora and fauna and have a relative small unit throughput because they tend to be fragile. Moreover, one size doesn't fit all (the membranes are specific for the solute and solvent).

Hope this is some help.

Rap

Osmosis refers solely to the movement of water. The water moves from a region of high salt concentration to a region of low salt concentration (all else being equal). This conforms to Fick, though -- water is at a lower concentration in the region where salt is at a high concentration, there being less room there for water.