Chemico-osmotic flow may be defined as the movement of water molecules down the chemical potential gradient of the water. This flow can be either advective or diffusive. Since the chemical potential of free water can be lowered by the addition of solutes, chemico-osmotic flow can occur when two aqueous electrolyte solutions of differing concentrations are separated by a semipermeable membrane. The membrane properties of compact clays are mainly due to electrical restrictions on the movement of the solute ions through the narrow pore spaces between negatively-charged clay mineral surfaces. The chemical potential of water can also be lowered by the adsorption of molecules onto solid surfaces.
Heat is given off by the exothermic hydration reaction and the water undergoes a change in enthalpy.
If two clay powders are exposed to atmospheres of differing relative humidity and then brought together, there will be a flow of water down the gradient of chemical potential between them. Water molecules will flow from thick adsorption films towards thin adsorption films. This is a second form of chemico-osmotic flow. In water-saturated systems, the transient movement of water is accompanied by local volume changes. If the volume increases the process is known as swelling.