Adhesion and the Surface Energy Components of Natural Minerals and Aggregates

A range of geochemical reactions are controlled by the interfacial characteristics of rocks and minerals. Many engineered and natural systems are affected by geochemical reactions that occur at interfaces. Asphalt-aggregate adhesion in road construction is influenced by the interfacial characteristics of the aggregate. Likewise, the remediation of nonaqueous-phase liquid contaminants, such as trichloroethylene or methyl tert-butyl ether, is controlled by the interactions between mineral surfaces and the organic liquid. Many natural systems are also influenced by reactions at interfaces. The migration of petroleum in sedimentary basins is influenced by the wettability of the surfaces of the basin pore space. Adhesion of organisms, such as bacteria or lichens, to rock surfaces is controlled by the interactions of proteins and mineral surfaces. Rock and mineral surfaces are described by surface energy. Surface energy is a thermodynamic construct defined as the amount of work required to form more of a surface. Surface energy can be divided into van der Waals, Lewis acid, and Lewis base components. The ability to predict the magnitude of surface energy components is valuable in understanding species behavior. Surface energy is controlled by three master variables: surface chemistry, surface morphology, and surface coatings. While the surface energy of a number of minerals and aggregates has been characterized, there has not yet been a comprehensive study of the surface energies of a variety of the most common minerals and aggregates using consistent methodology. In addition there has not yet been a study of the effect of these three master variables on surface energies of natural minerals and rocks. This study measured the surface energy of 22 common minerals and 7 aggregates. The samples? bulk and surface chemistries were characterized with wavelength and energy dispersive spectra analyses on an electron microprobe and x-ray photoelectron spectroscopy. The XPS was also used to quantify the organic and inorganic coatings on the surfaces. Results showed that van der Waals surface energy is typically between 40 and 60 ergs/cm2. Polar surface energy varies by 1 to 3 orders of magnitude, and thus is likely the most important component in accounting for changes between natural minerals.