The surface of the membrane is very dense, there are a large number of activation points on the membrane, bonded to a certain number of binding water, this water has lost the ability to solvent, the salt in the brine is not dissolved in it. The water molecules in the feed can form hydrogen bond with the activation point on the membrane under pressure, so that the activation point on the other bonded water down, the bonding of the bonded water and the following activation point, so that the point of the original bonding of the water down, this process continuously from the membrane to the lower layer, it is in this order diffusion, the water molecules from the surface of the membrane into the membrane, and finally from the bottom layer of the product water down to become solvent. Salt, on the other hand, is diffused from the membrane surface to the product water by cavity-type diffusion through the cavities between the polymer chains, but the model lacks more quantitative description of the mass transfer.
Donnan equilibrium model
Membrane for the fixed negative charge type, according to the principle of electroneutrality and the membrane and the solution of ionization degree balance, it is generally believed that with the help of exclusion of homoionic ability, the charged membrane can be used for desalination, generally only dilute solution, under pressure through the charged membrane, there is a more obvious desalination, with the increase of the concentration of the rate of desalination decreased rapidly. Divalent homoions are better removed than monovalent homoions, and monovalent homoions are better removed than divalent counterions. The theory is based on Donnan equilibrium to illustrate the desalination of charged membranes, but Donnan equilibrium is an equilibrium state, and for the mass transfer through charged membranes under pressure, it has not been possible to quantitatively describe the process from various aspects such as the membrane, the feed and the mass transfer process.
In addition to the above models, many scholars have also proposed not a small number of other models, such as the desalination center model, surface force - pore flow model, organic solute desalination mechanism, etc.