Examining Thermodynamic Changes During Soil Moisture Loss

Distribution constant, (K_c) is employed to account for desorption of water molecules from soil particles. When water molecules have strong affinity for soil particles, K_c and moisture desorption is small. Conversely, when water molecules have weak affinity for soil particles, K_c is large and moisture desorption is significant. By approximating the distribution constant, K_c to be equal to thermodynamic equilibrium constant and with the aid of standard equations, insight is obtained into physico-chemical factors responsible for changes in enthalpy, entropy and free energy at equilibrium soil moisture loss condition. Analysis of experimental results showed that moisture desorption from moist pulverized soils is endothermic process as energy is required to overcome physico-chemical and suction bonds between water molecules and surface of soil pores and hydrogen bonds between water molecules. The heat of desorption increases with reduction in environmental relative humidity (a_w) as water molecules occupy high binding energy sites at low environmental relative humidity. Moisture desorption is accompanied by increase in entropy as water molecules are disordered in vapor phase than in the liquid state. Trend of ∆G° values indicate that soil moisture loss becomes increasingly spontaneous (negative ∆G° values) with reduction in environmental relative humidity, which is supported by the highest K_c at least environmental relative humidity.