What is the primary thermodynamic factor dictating the minimum energy required for desalination?

The primary thermodynamic factor dictating the minimum energy required for desalination is the Gibbs free energy change associated with separating pure water from a saline solution. Gibbs free energy (G) represents the amount of energy available in a thermodynamic system to do useful work at a constant temperature and pressure. Desalination, by its nature, involves increasing the order of the system by concentrating salts in one stream (the brine) and producing pure water in another (the permeate). This increase in order reduces entropy (S), which is a measure of disorder or randomness in a system. To overcome this decrease in entropy, energy must be supplied to the system. The Gibbs free energy change (ΔG) is defined as ΔG = ΔH - TΔS, where ΔH is the change in enthalpy (heat content), T is the absolute temperature, and ΔS is the change in entropy. In desalination, ΔS is negative because order increases; therefore, to make ΔG negative (meaning the process is thermodynamically favorable), energy must be added to the system, thus increasing the enthalpy. The higher the salinity of the feed water, the greater the change in Gibbs free energy required to achieve separation, and therefore, the higher the minimum energy needed for desalination. For example, desalinating seawater with a higher total dissolved solids (TDS) concentration requires more energy input than desalinating brackish water with a lower TDS concentration because the change in entropy is more significant in the former case. While other factors such as inefficiencies in practical desalination processes increase the actual energy consumption far beyond this theoretical minimum, the Gibbs free energy change fundamentally sets the lower limit for energy input.