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Observed‐Predicted Shifts in Thermodynamic Functions of Water Adsorption by Smectite at Nanoscale
Author(s) -
Elprince Adel,
Saffan Mohamed,
Zabidi Mohammed
Publication year - 2015
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2015.03.0096
Subject(s) - clay minerals , adsorption , enthalpy , chemistry , dissolution , phase (matter) , nanoparticle , nano , bound water , chemical engineering , mineralogy , materials science , thermodynamics , nanotechnology , molecule , organic chemistry , physics , engineering
The unique properties of nanoparticles have attracted a great deal of soil chemists' and geochemists' interest. The objective of this study was to evaluate the integral enthalpy ( H ), entropy ( S ), and free energy ( F ) of water vapor adsorption on nano‐smectite surfaces. We used water adsorption isotherms at 28 and 35°C. The adsorbents were Na‐nano smectite of diameter 59 to 72 nm and Na‐ and Ca‐micro smectites of diameter 72 nm to 2 μm. The Na‐nano smectite was prepared using centrifugation under prevailed diffusion motion in centripetal field that directed nanoparticles to assemble into hydrogel phase. X‐ray diffraction (XRD) and selective dissolution analysis indicated separated smectite was the dominant phase. The means of Δ H were −8.6 ± 2.1, ‐11.6 ± 2.6, and ‐38.0 ± 7.4 kJ mol ‐1 for Na‐micro smectite, Ca‐micro smectites, and Na‐nano smectite, respectively, indicating Na‐nano smectite could function as effective thermal energy storage material and that bound water cannot be removed from Na‐nano smectite aggregates without coarsening. The corresponding means of Δ S were −9.9 ± 4.7, ‐18.0 ± 3.2, and ‐104.9 ± 16.2 J mol ‐1 k ‐1 , signifying ordered bound water cannot be disordered in Na‐nano smectite aggregates without coarsening. The corresponding means of Δ F were −5.7 ± 2.9, −6.2 ± 2.5, and −6.1 ± 2.9 (averaged −6.0 ± 4.8 kJ mol ‐1 ), indicating that Δ F values, contrary to Δ H and Δ S , did not show definite nano‐effect. These observed nano‐effects were successfully predicted using facility in scaling the magnitudes of Δ H , Δ S , and Δ F , as the length scale L shrunk. The high heat of water vapor adsorption by nano smectite and ability to hydrate and dehydrate makes soil effective in the storage of thermal energy.