Osmosis

Unilateral diffusion processes are summarized under the term osmosis. In order to better understand this process, we first look at two vessels containing a solvent. In vessel 2, a substance was also dissolved in the solvent. Since the liquid surfaces should be the same at the beginning, there is more solvent in vessel 1 than in vessel 2 at the beginning. Both vessels should now be connected to each other via a special, semi-permeable membrane that lets the particles of the solvent through, but not the particles of the dissolved substance. Then you notice that part of the solvent from vessel 1 migrates into vessel 2 and the liquid level rises there. This happens until the same number of solvent particles are in both vessels.

When the solvent flows into the vessel 2, a pressure is created there which is directed against the further inflowing solvent. This pressure becomes negative osmotic pressure. If both sides have the same number of solvent particles, the osmotic pressure is at a maximum and the diffusion of the particles comes to a standstill. In principle, the particles in the solvent can be treated like particles of an ideal gas if the dilution is sufficiently high ($c < 1\,\mathrm{mol/l}$), since their distance from one another is large and interaction processes can be neglected. The Dutch chemist Jacobus Henricus van 't Hoff found this connection as early as 1887 and established the law named after: \begin{equation} \pi V = niRT \end{equation} Here $i$ stands for the Van 't Hoff factor, which indicates how many particles a molecule dissociates into in the solvent. In the case of glucose, for example, this factor is 1, but in the case of sodium chloride (NaCl), $i=2$ must be used, since it breaks down into $Na^+$ and $Cl^-$ ions when it is dissolved. In principle, the equation is identical to the general gas equation, with the concentration here given by the ratio of the amount of substance to volume \begin{equation} c = \frac{n}{V} \end{equation} is specified and the pressure is denoted by $\pi$. Thus, the Van 't Hoff equation can also be used as \begin{equation} \boxed{\pi = ciRT} \end{equation} to be written. The principle of osmosis is also vital for the survival of human cells. The cell walls consist of semipermeable membranes, which allow certain substances to get inside the cell. In this way, depending on the type of membrane, the supply of food or messenger substances can be controlled, for example.

If a pressure is artificially created in vessel 2, which acts on the solvent, then this migrates back into vessel 1 and leaves the dissolved substances behind in vessel 2. This process is accordingly called reverse osmosis. There are commercially available machines that use this principle to provide filtered water that contains a significantly lower concentration of germs and chemical substances such as lime or nitrite.