Others have suggested that vapor pressure is the driving force for osmosis, they were slapped in the face also. Osmotic pressure is approximately 140 times greater than vapor pressure.
We asked the question what is vapor pressure?
Frankel (1943) suggested that liquids and solids can be represented as crystal lattice structures with molecules at the lattice points. Liquids have a weaker bond than solids. He also proposed that high-energy molecules near the surface escape the molecular bonds as vapor.
We considered condensation of a gas at the surface of a solid or liquid. The speed distribution of a kinetic theory gas is predicted by Maxwell-Boltzmann’s equation. As energy is removed from the gas, slower molecules condense to form liquids or solids. We proposed that faster molecules do not condense but remain as free gas molecules. To check this hypothesis, we compared the total kinetic energy of the high-speed gas molecules (S greater than Scutoff) to the published vapor pressure energy. We looked at more than 30 materials and found agreement between the Kinetic energy and the Vapor Pressure energy for Scutoff approximately 3 times the SRMS (root mean square speed).
From this molecular model of a condensing gas, we propose that:
- Free high-speed molecules exist in both solids and liquids.
- These free molecules cause the vapor pressure of the solids and liquids.
- Osmosis flow results from the net flow of these free molecules across a membrane.
- Small gas (water) molecules can pass through the membrane, larger sugar molecules can’t.
- Net flow proceeds from a high vapor pressure region to a lower vapor pressure region.
We note that kinetic energy is in the form of energy per unit mass and that pressure represents energy per unit volume. We used a dimensionless form of the energy expressions for comparison.