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Water Adsorption by Dry Soil and Its Thermodynamic Functions¹

ABSTRACT

The adsorption of increments of water vapor on Millville loam soil was studied in an air-free system. The resulting data included the amount of water that would condense on the soil at different relative humidities up to 90% at 14.8°, 24.8°, and 34.8°C. The temperature rise of the samples during adsorption was also recorded and the heat liberated during the reaction was calculated. Both integral and differential thermodynamic values were computed. A critical evaluation of the entropy function showed the integral values to be more useful than the differential values so far as describing the physical state of the system was concerned. In general, calculations of thermodynamic quantities based on the temperature dependence of the systems proved to be unreliable at low relative humidities due to changes in the available surface.

Specific conclusions drawn from the data suggest that when equilibrium relative humidity is < 80% in an air-free Millville loam system, the average entropy of the adsorbed film is a little greater than that of corresponding liquid water, but the average internal energy of the film is always less than that of liquid water, even though the heat content may become slightly greater as the relative humidity rises above 70%.

¹ This research was done in cooperation with the 12 western states in conjunction with the Soil and Water Conservation Research Division, ARS, USDA, through Western Regional Project W-68 and in part by contract No. DA-36-039-SC-80263, U. S. Army Signal Corps. Approved by the Director, Utah Agr. Exp. Sta., as Journal Paper 361.

² Soil Scientist, Soil and Water Conservation Research Division, ARS, USDA, now at the Irrigation Department, Univ. of Calif., Davis; Agricultural Missionary, the Lutheran Church—Missouri Synod; and Professor of Agronomy (Soil Physics), Utah State Univ., Logan, respectively.

Received for publication September 23, 1963. Accepted for publication January 30, 1964.