It has been generally recognized that the surface phenomena of the solid particles such as shrinking and swelling, water- holding capacity, water' movement, and cation exchange are important in understanding the physical properties of the soil. Clay is the most prevalent material in the colloidal fraction of many soils. Because of the complex nature of the surface of clays and the small size of the particles, the direct study of surface phenomena is difficult. Thermodynamic functions change in accordance with changes and organization within the system. Thus, an examination of the thermodynamics of surface phenomena provides some understanding of the reactions.