Direct Surface Force Measurement for Synthetic Smectites Using the Atomic Force Microscope

Abstract

An atomic force microscope with a colloid probe technique was used to measure forces interacting between smectites, that is, saponite and hectorite. The measured forces fit well to the forces calculated by the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory at separations larger than 4 nm over concentrations of 10-5−10-2 M NaCl and the pH range of 4−10. There is also a good agreement between zeta potential and the electrical double-layer (EDL) potential extracted from the best fit to the DLVO force curve. According to the analysis on the pH dependence of the EDL potential using the Gouy−Chapman−Stern−Graham model, the inner Helmholtz layer capacitance for the good fit of the outer Helmholtz plane potential to the EDL potential required a much lower value for hectorite (∼5 μF/cm2) than that for saponite (∼500 μF/cm2), reflecting the difference in the location of the lattice charge between saponite and hectorite. This provided evidence for the fact that the EDL force for smectites is dominated by the location of the charges as well as the density of the charges. Extra short-range repulsion was observed at separations below ∼3 nm with increasing NaCl concentration and pH and then disappeared with decreasing pH or the NaCl concentration. The short-range repulsion was extracted as a double-exponential function, that is, F/R = A1 exp(−D/D1) + A2 exp(−D/D2), by subtracting the DLVO forces from the measured forces in the same way as the study with a surface force apparatus (Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531). The values of two decay lengths (D1 and D2) for smectites showed a very similar result to those for muscovite mica rather than silica. It was also found that the sum of the force constants (A1 + A2) was closely related to the density and structure of the lattice charge.