The surface thermodynamic properties of some volcanic ash colloids

The surface tension components and parameters for nine naturally occurring colloidal-sized volcanic ash samples were determined by thin-layer wicking. These materials include rhyolites, trachytes, phonolites, and an andesite, and were collected from deposits in California, Arizona, Italy and Martinique. The samples are principally volcanic glass (with minor amounts of mixed materials of various compositions) with silica contents ranging (average values) from 74% (rhyolite) to 57% (andesite). The surface tension components were determined by means of thin-layer wicking to provide contact angles which were used to solve the Young-Dupré equation. The Lifshitz-van der Waals component of the surface tension, γ^LW, for these ash samples varies between 26.6 mJ/m²and 38.7 mj/m². The Lewis acid parameter, γ^⊕, values are small with values ranging from 0 mJ/m²to 1.8 mJ/m², while the Lewis base parameter, γ^⊕, values varied between 15.6 mJ/m²and 45.6 mJ/m². Of the nine samples, five were hydrophobic and the other four were hydrophilic. The hydrophobic ashes were produced by eruptions with a dominantly magmatic rupture mechanism: pyroclastic flow, nuèe ardente, ash cloud surge The hydrophilic ashes were generated by a strong hydromagmatic mechanism and most formed accretionary lapilli. The ζ-potential values were determined from the electrophoreuc mobility, and varied between -32.1 mV and -55.2 mV. Utilizing the surface tension values and the ζ-potential, an extended DLVO-rype (XDLVO) calculation of the free energy of interaction versus distance, including the Lewis acid-base contribution, between 1 μm spherical particles, showed that for the five hydrophobia ash samples there is a net attraction between particles in the presence of water, and for the four hydrophilic samples there is a strong net repulsion between particles. Assuming that the stability of an ash deposit (in the presence of water) on an inclined slope is related to the forces acting between neighboring ash particles, those deposits with repulsive interparticle forces would be more mechanically unstable and pose a risk of forming mud flows on remobilization.