Theoretical and experimental study of the surface charge density and the surface potential of coal—water suspensions in dissymmetrical electrolytes

Abstract

To estimate the physicochemical interactions between dispersed particles and the stability of a suspension it is necessary to know the surface charge and surface potential of the particles. In practice, the surface potential can reach very high values (±100 to ± 400 mV). In that case, the solution of the Debye—Hückel equation becomes inaccurate. The same holds if the particle radius a becomes very small (500-10 Å) and if κ a < 100 where κ is the Debye—Hückel reciprocal length. In the latter case, the Poisson—Boltzmann equation (implying parity between a spherical particle and a plane surface) becomes unsuitable. A more suitable and precise solution of the non-linear Poisson—Boltzmann equation for charged spherical particles in the presence of dissymmetrical electrolytes was worked out. The relationship between the electrostatic potential Ψ( x) and the surface charge σ 0 was established. It was found that electrolytes with trivalent anions (1–3 and 2–3 electrolytes) give higher surface densities. These theoretical results were confirmed by studying the surface charge and potential of coal-in-water suspensions in the presence of various dissymmetrical electrolytes such as Na 2SO 4, Na 5P 3O 10, CaCl 2, Na 3PO 4 and NACl. Using our theoretical model and experimental results we were also able to obtain the changes in electrostatic potential as a function of the distance x (from the particle surface) in various electrolytes and for different pH values. Our experimental results were in good agreement with the theoretical calculations. It was demonstrated (in order of decreasing surface charge density) that σ 0(NaTPP) > σ 0(Na 3PO 4) > σ 0(Na 2SO 4) > σ 0(NaCl) > σ 0(CaCl 2) or σ 0(1–5) > σ 0(1–3) > σ 0(1–2) > σ 0(1-1) > σ 0(2-1)