Shear response of concentrated calcium carbonate suspensions.

The rheology of concentrated calcium carbonate suspensions is investigated with respect to addition of solution and dispersion polymers. System materials and composition are chosen to be similar, generically, to those in use in the coating of paper. Specifically, we investigate the particle volume fraction dependence of the relative viscosity, using both capillary and steady-shear concentric cylinder measurement methods to cover a broad range of concentrations. The results are interpreted in terms of semi-empirical models, such as the Krieger-Dougherty model. Oscillatory shear measurements are also employed to investigate the viscoelastic behavior of the concentrated suspensions. The measurements indicate that a common solution polymer thickener, carboxymethyl cellulose (CMC), causes depletion flocculation of calcium carbonate suspensions.

Interpretation of Competitive Adsorption Isotherms in Terms of Affinity Distributions

In the present study we evaluate affinity distributions for competitive adsorption isotherms which involve several components. In such a multicomponent situation, the affinity distribution becomes a function of several affinity constants, and already in the case of two components, little is known about their features. In the two-component situation, we have calculated the affinity distributions from the adsorption isotherms with a numerical inversion technique. This technique is based on a constrained least-squares algorithm and uses a regularization function which biases the resulting affinity distribution toward a smooth function. The applicability of the procedure was tested with a newly derived isotherm, which is based on a fully uncorrelated affinity distribution, and with the generalized Langmuir-Freundlich (LF) isotherm, which is known to have a perfectly correlated distribution. The present study demonstrates that the extended Henderson-Hasselbalch (HH) isotherm has an underlying affinity distribution, which displays a partial correlation, while the non-ideal competitive adsorption (NICA) isotherm has an affinity distribution with a varying degree of correlation. In the competitive situation, the affinity distribution thus provides an interesting means to characterize the corresponding isotherms. As an illustration of the present techniques, experimental data of metal ion binding for a humic acid are analyzed in the same context.

On the difference in ionization properties between planar interfaces and linear polyelectrolytes.

Ionizable planar interfaces and linear polyelectrolytes show markedly different proton-binding behavior. Planar interfaces protonate in a single broad step, whereas polyelectrolytes mostly undergo a two-step protonation. Such contrasting behavior is explained using a discrete-charge Ising model. This model is based on an approximation of the ionizable groups by point charges that are treated within a linearized Poisson-Boltzmann approximation. The underlying reason as to why planar interfaces exhibit mean-field-like behavior, whereas linear polyelectrolytes usually do not, is related to the range of the site-site interaction potential. For a planar interface, this interaction potential is much more long ranged if compared with that of the cylindrical geometry as appropriate to a linear polyelectrolyte. The model results are in semi-quantitative agreement with experimental data for fatty-acid monolayers, water-oxide interfaces, and various linear polyelectrolytes.