J. Adv. Agric. Res. (Fac. Agric. Saba Basha) _______________________________________________________________________________ 524 _______________________________________________________________________________ Vol. 18 (3), 2013 The Surface Free Energy of Soil Colloids M. D. Zabidi † , M. M. Saffan Ŧ and A. M. Elprince † † Department of Soils and Agricultural Chemistry, Alexandria University, Alexandria, Egypt, and ‡ Department of Soils and Water, Kafr El-Shaikh University, Egypt. Corresponding author: A. M. Elprince, e-mail: aelprince@gmail.com _______________________________________________________________________ ABSTRACT: Water repellency, floatability, hydrophobicity, swelling, wetting processes, water adsorption, and structure of adsorbed water are examples of interactions at interfaces. A property of solids involved in these interactions is the surface free energy (tension). Because surface free energies measurements for soil colloids are scarce it was the purpose of this paper to report values of the surface free energy for soil clay fraction (<2 μm) of Torrifluvents. X-ray diffraction patterns indicated that the clay fractions of the P5 and P9 soil samples (Nile delta, Egypt) were mainly formed of smectite, followed by kaolinite and illites. Chlorite, vermiculite, and mixed layers appeared only in traces. The spreading surface pressures of soil clays were determined using water vapor adsorption isotherms at 28 o C and a nonlinear least squares fitting the BET equation to the experimental data. The values: 36.93 and 38.37 mJ. m -2 were computed for the spreading pressures of the two soil-clay samples P5 and P9, respectively. Then, the surface free energies for the soil clays were computed following a recently developed procedure that utilized a formula obtained by combining the Young equation with the general equation of pair interaction. This procedure gave the values 183.76 and 191.45 mJ m -2 for the surface free energies of the soil clays P5 and P9, respectively. These values seemed comparable but little less than values of the surface free energy of clay minerals; and soil clays seemed less hydrophilic than pure clay minerals. The Helmy’s method used in this study for determining the surface free energy is independent of the extra-thermodynamics assumed by the other methods and seems useful for other soils. Keywords: Adsorption isotherms, Young equation, BET equation, Spreading pressure, _________________________________________________________________ INTRODUCTION The interaction of water with soil colloids plays a critical role in all areas of soil science. Water repellency (Bauters et al., 2000), floatability (Chibowski and Holysz, 1986), hydrophobicity ( Zettlemoyer, 1969; van Oss and Giese, 1995), swelling (Low, 1987), wetting (Dekany et al., 1986), water adsorption (Dontsova et al., 2004), and structure of adsorbed water (Jurinak, 1961; Low, 1982; Sposito and Prost, 1982; Elprince, 1986) are examples of interactions at interfaces. A property of solids involved in these interactions is the surface free energy (tension). Despite the fact that surface free energy has been measured for many of the clay minerals (Schultz et al., 1977ab; Janczuk and Bialopiotrowicz, 1988; Christenson, 1993; Helmy et. al., 2003, 2004, 2006), measurements for soil colloids are scarce (Goebel et al., 2004). Several approaches can be used (Zettlemoyer, 1969; Wu, 1980) but no direct method for the determination of the surface free energy exists. One method relies on the determination of the adsorption isotherm of a liquid vapor. From this isotherm the spreading pressure, , can be calculated. Based on the  value, the surface free energy of the solid can be estimated (Zettlemoyer, 1969; Staszczuk, 1984; Helmy et. al., 2003, 2004, 2006). This method employs the Fockes type formula (Fockes, 1964 and 1968), the Lifshitz-VDW-acid/base approach (Giese and van Oss, 2002), or the Helmy’s procedure (Helmy et al., 2006). The advantage of the Helmy’s procedure is that it is independent of the extra-thermodynamics assumed by the other approaches.