It is a film with selective permeability. When a liquid or gas passes through a semi-permeable membrane, some components pass through, while others are retained. In fact, the semi-permeable membrane has permeability to any component, but the rate of permeation varies greatly. In the reverse osmosis process, the permeation rate of the solvent (water) is much greater than the solute dissolved in the water (salt). The semi-permeable membrane realizes the separation of solvent and solute, and obtains pure water and concentrated salt solution.
It is a natural process when the fluid crosses the semi-permeable membrane barrier. If a box of pure water is vertically divided into two parts with a semi-permeable membrane, the pure water is in contact with both sides of the ideal semi-permeable membrane at the same temperature and pressure, under such conditions there is no water flow across the semi-permeable membrane Occurs because the chemical potentials on both sides of the membrane are exactly equal. If a soluble salt is added to one side, the chemical potential of the salt solution is reduced. Pure water will permeate to the salt solution side, creating a permeate flow until the balance of chemical potential is re-established.
According to scientific terminology, there is a difference in ‘chemical potential’ (concentration difference of ions or dissolved molecules) on both sides of the semipermeable membrane, which is compensated for through the process of solution penetration. When the balance is re-established, a water level difference or static pressure difference is formed on both sides of the semi-permeable membrane. This pressure difference is the osmotic pressure. Osmotic pressure is the nature of the solution, depends on the concentration of the solution, and has nothing to do with the semi-permeable membrane.