Increasing water salinity reduces evaporation because dissolved salts lower the energy and vapor pressure of the water.
Salinity, the amount of dissolved salts in water, plays a significant role in regulating the rate at which water evaporates. According to research, including studies by Harbeck (1955), Lee (1927), and Salhotra et al. (1985), an increase in water salinity leads to a reduction in evaporation.
This phenomenon occurs due to the fundamental interaction between dissolved salt ions and water molecules. Here's the breakdown of the mechanism:
- Lowering Free Energy: When salts dissolve in water, the ions interact with the water molecules. These interactions require energy, effectively lowering the free energy of the water molecules themselves.
- Reducing Water Activity: The reduction in free energy corresponds directly to a reduction in water activity. Water activity is a measure of the "available" water in a system, representing the effective concentration of water molecules relative to pure water. Salinity ties up water molecules, making fewer available for evaporation.
- Decreasing Saturation Vapor Pressure: A lower water activity directly results in a lower saturation vapor pressure above the saline water surface at a given temperature. Saturation vapor pressure is the pressure exerted by water vapor when it is in thermodynamic equilibrium with its condensed phase (liquid water) in a closed system. Essentially, with lower water activity, fewer water molecules have the necessary energy to escape the liquid surface and become vapor.
Therefore, because the saturation vapor pressure is lower over saline water compared to fresh water at the same temperature, the driving force for evaporation (the difference between the saturation vapor pressure and the actual partial pressure of water vapor in the air) is reduced. This leads to a slower rate of evaporation.
In summary, dissolved salts inhibit evaporation by:
- Attracting water molecules (lowering free energy).
- Reducing the amount of 'available' water (decreasing water activity).
- Lowering the pressure of water vapor that can exist above the water surface (reducing saturation vapor pressure).
This effect is a key factor in understanding evaporation from various water bodies, from salty lakes to the oceans.
