Surface charge heterogeneity and aggregation of clay lamellae in aqueous suspensions

Abstract

Montmorillonite (2 1 layer silicate) is the most often studied swelling clay mineral. The faces of the lamellae have permanent negative charges owing to isomorphic substitutions, and pH-dependent charges develop on the surface hydroxyls at the edges. The acid–base titration of sodium montmorillonite suspensions at 0.01, 0.1 and 1 M NaCl was evaluated by fitting of potentiometric data measured between pH 4 and 9 using FITEQL. A surface complexation model assuming ion exchange on faces, and protonation–deprotonation processes at edges in the negative electrostatic field emanating from the particle face were introduced. This model provides excellent fitting of the measured points. The calculated number of edge sites was 3–4 mmol/100 g besides a permanent charge of 100±2 mmol/100 g on the faces. The intrinsic equilibrium constants of the protonation and deprotonation processes were logK a 1 i nt>= 5.1±0.1 and logK a 2 i nt>=–7.9±0.1, respectively. The point of zero charge for edge sites was at pH∼6.5, i.e., positive charges can develop on the Al–OH sites of edges only at pHs below around 6.5. Study of the pH-dependent particle aggregation showed the formation of stable suspensions at low (below 0.01 M NaCl) ionic strength even at low pH, where the edges are positively charged. However, an increase in salt concentration induces edge-to-face aggregation below pH∼6.5. X-ray diffraction patterns of montmorillonite films prepared from slightly acidic suspensions proved that the formation of well-ordered lamella packages is hindered by the attraction between edges and faces. Characteristic changes in gel formation and in the plastic, elastic and viscous properties induced by decreasing pH in dense suspensions containing 0.01 M NaCl provided experimental evidence for the structure of the particle network. The significant increase in thixotropy and yield values and also the formation of viscoelastic gels only below pH∼6.5 verify that attractive interaction exists between oppositely charged parts of lamellar particles.