From the previous discussion, it is clear that an important factor in the dispersion efficiency of superplasticizers is their adsorbed conformation. While computational methods such as molecular dynamics may provide insight into such phenomena in the long term, it is currently more practical to measure the interaction between superplasticizer-coated surfaces directly in relevant solution conditions. The atomic force microscopy (AFM) method for measuring surface forces was first used by Ducker et al.8 They attached a spherical particle at the tip of the cantilever and obtained a force-displacement curve from the deflection of the cantilever as a function of intersurface separation. This versatile method was recently extended to spherical MgO particles,9, 10 which have a similar surface chemistry as cement but have the advantage of being much less reactive when subjected to water. 11
Figure 3 shows the results from direct-force measurements between a spherical MgO probe attached to the cantilever and flat MgO substrate immersed in an aqueous media at pH 10.12 It was found that the interaction is repulsive in a simple monovalent electrolyte (KCl), which probably can be related to the positive charge on the MgO surfaces (Figure 3a) and low attractive van der Waals forces. However, the addition of calcium resulted in an attraction between the surfaces that may originate from ion correlation forces, as described in the article by Pellenq and Van Damme in this issue. The addition of a comb-like copolymer having a negatively charged backbone with grafted poly(ethylene oxide) (PEO) chains of a relatively short length (PCP1) resulted in a stronger repulsion between the surfaces (Figure 3b). No attraction was observed, even in a calcium-rich electrolyte.
Addition of a comb-like copolymer having an identical backbone to PCP1 but with PEO side chains of a much longer length (PCP4) results in a interparticle repulsion that is much more long-range (Figure 3c). In addition, the effect of this polymer is influenced relatively little by the nature of the polyelectrolyte. This indicates that the steric repulsion is dominating and we can even get an indication of the thickness of the adsorbed layer on the surface. These results are currently being used in the design of superplasticizers with an optimal structure and can also be used for realistic estimates of the interparticle forces in larger-scale simulations of the rheology of particulate suspensions, as described in the section on "Interparticle Interactions/Yield Stress."
Figure 3. Atomic force microscopy colloidal probe results between MgO surfaces at pH 10: (a) without superplasticizer, (b) in the presence of PCP1 [short side chains of poly(ethylene oxide), and (c) in the presence of PCP4 (longer side chains of PEO). The errors on such measurements are about 5 µN/m.13
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