Thermodynamic Functions for Water-Nanosmectite System.

The unique properties of nanoparticles have attracted a great deal of soil and environmental scientists’ interest. The objective of this study was to investigate a possible nano-effect on the thermodynamic functions of water-smectite system. Using water vapor adsorption isotherms at 28 and 35^o C (fitted to BET equation) we computed integral entropy ΔS = S-S_L, enthalpy ΔH, and free energy ΔF changes relative to bulk liquid water (L). The adsorbents were Na-smectites of effective diameters (49 to 55 nm) and (< 2 μm) and Ca-smectite (< 2 μm). The Na-nanosmectite was prepared using centrifugation procedure where the centripetal force directed nanoparticles to assemble into gel phase. The ΔS, ΔH, and ΔF values were plotted against water surface coverage and were found negative for all the three adsorbents indicating that the system lost entropy, heat, and free energy upon transfer of H₂O molecules from bulk water into adsorbed state. There was good agreement between calorimetric data in literature and our values of ΔS and ΔH for the Na- and Ca-smectites (< 2 μm). The magnitudes of ΔH for the Na-nanosmectite were 3 and 4 times the values for the Ca- and Na-smectites (< 2 μm), respectively, indicating that Na-nanosmectite could function as an effective heat storage. Values of ΔS for the Na-nanosmectite were 6 and 11 times the values for the Ca- and Na-smectites (< 2 μm), respectively. These results indicated a pronounced nano-effect on the magnitude of ΔS and ΔH. However, the ΔF values did not show a definite nano-effect. These observed nano-effects were explained using facility in scaling the magnitudes of ΔH, ΔS, and ΔF, as the length scale shrinks.