Impact of Monovalent and Divalent Cations on the Colloidal Stability of Negatively Charged Latex Particles Decorated with Poly(ethylene glycol)

The present work reports results from the interactions of monovalent (Na+, NH4+, and K+) and divalent (Ca2+ and Sr2+) cations, nitrate was the counterion, with negatively charged poly(methyl methacrylate) particles decorated with poly(ethylene glycol) (PMMA/PEG). Particle characterization was performed with dynamic light scattering, C-potential measurements, and scanning electron microscopy (SEM). Phase separation analyses were performed in the salt concentration range of 0.01 to 1.0 mol L-1. Among the monovalent cations at 1.0 mol L-1, the colloidal stability order was Na+ > K+ > NH4+ and only NH4+ bound specifically to PEG oligomers, causing the largest instability for concentrations higher than 0.75 mol L-1. In the case of divalent cations, three different situations were observed: (i) at 0.01 mol L-1 the dispersions were very stable, (ii) in the concentration range of 0.1 to 0.5 mol L-1 the particles aggregated due to charge screening, and (iii) at 1.0 mol L-1 the stability was larger than at the intermediate salt concentration because free divalent cations tend to chelate with PEG oligomers, building flocs. After phase separation all dispersions could be redispersed by manual shaking, recovering the original colloidal state. The colloidal behavior under experimental conditions of centrifugation, freeze/thaw cycle, and heating was investigated in the presence of 1.0 mol L-1 salt. The colloidal behavior of PMMA/PEG particles was discussed with basis on the high molecular density of PEG oligomers on the particle surface and specific ion binding.