THE MOLECULAR DYNAMICS, SPECIFIC SURFACE AREA, CRYSTAL COMPOSITION, AND SWELLING OF CLAY-WATER SYSTEMS

DAVID J MULLA, Purdue University

Abstract

Clay and water are intimately interrelated in nature, and their interaction, which governs many of the physical, chemical, and biological processes in soils and porous media, has long been the object of intense interdisciplinary study. The molecular basis for this interaction is examined in chapter I using a powerful molecular dynamics computer simulation of 256 water molecules between model pyrophyllite surfaces separated by 33 (ANGSTROM). Interaction potential energies between the water molecules and the surfaces are calculated and the equations of motion for all water molecules are integrated in order to generate date for their locations, velocities, and dipole moments as a function of time. Statistical mechanical calculations using this data indicate that the interfacial water differs substantially from bulk water by virtue of its slower rotational relaxation rate, smaller self-diffusion coefficient, disrupted hydrogen bond networks, and greater polarization near the surfaces. Hence, our results support the hypothesis that the mere presence of an uncharged surface significantly affects the structural and dynamical properties of interfacial water. In chapter II, we develop a new method for measuring the specific surface area (S) of a wide range of clay minerals, including a series of homoionic montmorillonites, soil clay minerals, and mixtures of montmorillonite and kaolinite. This method, which is both easier and more rapid than existing methods, relates the internal reflectance intensities of the infrared spectra for water in swollen clay gels to the values of S determined by the adsorption of 2-ethoxy ethanol. It is shown that the physical basis for changes in the spectral intensities as a function of S result from changes in the values of the real and imaginary components of the dielectric constant of the water. Furthermore, as shown in chapters IV and V, the relative proportions of water located on external surfaces, in large pores, between fully expanded layers, and between partially expanded layers differ for each of the swelling clay-water systems analyzed by reflectance spectroscopy. Hence, the reflectance intensities, which are highly correlated to the specific surface area of the mineral, are also affected by differences in the distribution of water.

Degree

Ph.D.

Subject Area

Agronomy|Cartography

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