Osmosis

One-sided diffusion processes are collectively referred to as osmosis. To better understand this process, let us first consider two containers filled with a solvent. In container 2, an additional substance has been dissolved in the solvent. Since the liquid levels are initially the same, container 1 must therefore contain more solvent than container 2.

Now suppose the two containers are connected by a special semipermeable membrane that allows the solvent particles to pass through, but not the particles of the dissolved substance. It is then observed that part of the solvent migrates from container 1 into container 2, causing the liquid level in container 2 to rise. This continues until both containers contain the same number of solvent particles.

When the solvent flows into container 2, a pressure arises there which opposes further inflow of solvent. This pressure is called the osmotic pressure. When both sides contain the same number of solvent particles, the osmotic pressure is maximal and the diffusion of particles comes to a halt.

At sufficiently low concentrations (\(c < 1\) mol/l), the particles in the solvent can essentially be treated as particles of an ideal gas, since their spacing is large and interaction processes can be neglected. The Dutch chemist Jacobus Henricus van ’t Hoff discovered this relationship in 1887 and formulated the law named after him:

$$
\pi V = n i R T
$$

Here, \(i\) denotes the van ’t Hoff factor, which specifies into how many particles a molecule dissociates in the solvent. For glucose, this factor is 1. For sodium chloride (NaCl), however, one must use \(i = 2\), since it dissociates into \(Na^+\) and \(Cl^-\) ions.

The equation is essentially identical to the ideal gas law, except that the concentration is expressed as the ratio of amount of substance to volume:

$$
c = \frac{n}{V}
$$

and the pressure is denoted by \(\pi\). Thus, the van ’t Hoff equation can also be written as:

$$
\pi = c i R T
$$

The principle of osmosis is also vital for human cells. Cell walls consist of semipermeable membranes, which allow certain substances to enter the cell interior. In this way, depending on the type of membrane, the supply of nutrients or signaling substances can be controlled.

If an artificial pressure is applied in container 2 that acts on the solvent, then the solvent flows back into container 1, leaving the dissolved substances behind in container 2. This process is called reverse osmosis. Commercially available devices use this principle to provide filtered water with a significantly reduced concentration of pathogens and chemical substances such as lime or nitrite.