Stability of negatively charged latex particles in the presence of a strong cationic polyelectrolyte at elevated ionic strengths

Sulfate-terminated latex particles were investigated in the presence of poly(diallyldimethyl ammonium chloride) (PDADMAC) at pH 4.0 in aqueous KCl electrolyte solutions by dynamic light scattering and electrophoresis, in particular, at high ionic strengths. The polyelectrolyte adsorbs to the latex particles quantitatively until the adsorption plateau is reached. The adsorbed amount at this plateau and the corresponding layer thickness increase with increasing ionic strength. The resulting layers have a thickness of several nanometers. Colloidal stability is qualitatively consistent with electrostatic double layer forces, especially since the system can be fully destabilized at high ionic strengths even at high polyelectrolyte doses. Additional attractive forces due to lateral charge heterogeneities seem to contribute to the destabilization of the system, even for the adsorbed layers in the saturated state. However, this layer does not provide any additional stabilization mechanism due to steric repulsion forces, since the adsorbed polyelectrolyte layers are thin and laterally heterogeneous even in their saturated state

Influence of alkali metal counterions on the charging behavior of poly(acrylic acid)

Charging of poly(acrylic acid) (PAA) in the presence of different alkali metal counterions was studied by precision potentiometric titration. The charging behavior can be described with a novel cylinder Stern model quantitatively. This model is based on the Poisson–Boltzmann equation in the cylinder geometry and a constant Stern capacitance. One finds an increasing cylinder radius with increasing mass of the alkali metal ion and a correspondingly decreasing Stern capacitance. The intrinsic ionization constants for the uncharged polymer are found to decrease with ionic strength, similarly to weak acids of low molecular mass

Probing Nanometer-Thick Polyelectrolyte Layers Adsorbed on Oppositely Charged Particles by Dynamic Light Scattering

The thickness of adsorbed polyelectrolyte layers on oppositely charged particles can be measured by dynamic light scattering (DLS) with a precision of fractions of a nanometer. However, such data can be only reliably obtained when effects of particle aggregation are carefully eliminated by working at low particle number concentrations. In order to achieve a sufficient light scattering intensity at the same time, the size of colloidal particles must be chosen relatively large. We find that such measurements are best carried out with latex particles in the range of diameters of 150−300 nm. The precision of the measurement can be further enhanced with multiangle DLS. The thickness of adsorbed polyelectrolyte layers on oppositely charged particles is normally below 10 nm. At low ionic strengths, a typical thickness is merely 1−2 nm, while at higher ionic strengths one observes thicknesses between 6 and 9 nm. The transition between these two regimes occurs at ionic strengths 0.01−0.05 M. These observations were made with various highly charged cationic and anionic polyelectrolytes and can be considered as quite generic

Electrostatic Stabilization of Charged Colloidal Particles with Adsorbed Polyelectrolytes of Opposite Charge

Repulsive electrostatic double-layer forces are responsible for the stabilization of charged colloidal particles in the presence of adsorbed polyelectrolytes of opposite and high line charge densities. This mechanism is revealed by studies of electrophoretic mobility and colloidal stability performed with dynamic light scattering as a function of the polyelectrolyte dose and the ionic strength for two different types of latex particles and four different types of polyelectrolytes. The dependence of these quantities is very similar for bare charged latex particles and the same particles in the presence of the different oppositely charged polyelectrolytes. Positively charged particles in the presence of anionic polyelectrolytes behave analogously to negatively charged particles in the presence of cationic polyelectrolytes

Structure of an Adsorbed Polyelectrolyte Monolayer on Oppositely Charged Colloidal Particles

An adsorbed layer of a cationic polyelectrolyte, poly(diallyldimethyl-ammonium) chloride (PDADMAC) on negatively charged colloidal latex particles was investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). SANS gives a layer thickness of 8 ± 1 Å and a polymer volume fraction of 0.31 ± 0.05 within the film. DLS gives a somewhat larger thickness of 18 ± 2 Å, and the discrepancy is likely due to the inhomogeneous nature of the layer and the existence of polymer tails or loops protruding into solution. These results show that a highly charged polyelectrolyte adsorbs on an oppositely charged colloidal particle in a flat configuration due to the attractive forces acting between the polyelectrolyte and the substrate