Abstract:The electrostatic charge density of particles is of paramount importance for the control of the dispersion stability. Conventional methods use potentiometric, conductometric or turbidity titration but require large amount of samples. Here we report a simple and cost--effective method called polyelectrolyte assisted charge titration spectrometry or PACTS. The technique takes advantage of the propensity of oppositely charged polymers and particles to assemble upon mixing, leading to aggregation or phase separation. The mixed dispersions exhibit a maximum in light scattering as a function of the volumetric ratio , and the peak position !"# is linked to the particle charge density according to ~ ! !"# where ! is the particle diameter. The PACTS is successfully applied to organic latex, aluminum and silicon oxide particles of positive or negative charge using poly(diallyldimethylammonium chloride) and poly(sodium 4--styrenesulfonate). The protocol is also optimized with respect to important parameters such as pH and concentration, and to the polyelectrolyte molecular weight. The advantages of the PACTS technique are that it requires minute amounts of sample and that it is suitable to a broad variety of charged nano--objects.

Interactions between charged surfaces in aqueous solutions, widespread in soft matter and biology, are very complex and, despite many efforts, their full explanation remains challenging. We support the idea that, in contrast to extremely small separations (d≤2  nm), where many effects, prominently those linked to the structure of liquid water, interfere, electrostatics alone rules over larger distances (d≥5  nm) at low ionic strength. We set up specially designed surface force apparatus (SFA) experiments to measure the elastic compressibility modulus of a stack of charged membranes with monovalent counterions, directly and with high precision. We demonstrate that electrostatics alone, if implemented beyond Poisson-Boltzmann theory, fully accounts for the data, nonelectrostatic contributions playing at best a minor role.

We investigate the formation/re-dissociation mechanisms of hybrid complexes made from negatively charged PAA2k coated γ-Fe2O3 nanoparticles (NP) and positively charged polycations (PDADMAC) in aqueous solution in the regime of very...