Behavior of Spherical Poly(2-acrylamido-2-methylpropanesulfonate) Polyelectrolyte Brushes on Silica Nanoparticles up to Extreme Salinity with Weak Divalent Cation Binding at Ambient and High Temperature

The colloidal stability of nanoparticles (NPs) stabilized by grafted polyelectrolyte (PE) brushes in concentrated divalent ion solutions, at either ambient or high temperature, is of interest in a wide variety of applications including medicine, personal care products, oil and gas recovery, reservoir imaging, and environmental remediation. Previous attempts to determine the length of PE brushes at these conditions have been limited by lack of colloidal stability particularly when divalent ions form complexes with the charges on the brushes. We find that brushes of highly acidic strong PE poly(2-acrylamido-2-methylpropanesulfonate, AMPS) end-grafted to silica NPs provide colloidal stability at salinities up to 4.5 M CaCl₂ or NaCl. Thus, the brush behavior could be studied with dynamic light scattering (DLS) and the electrophoretic mobility by phase analysis light scattering (PALS) from the salt-free condition to the extreme salinities of 4.5 M. In monovalent NaCl solutions, the highly extended poly(AMPS) brushes at low salt concentration (C_s) collapse monotonically with increasing C_s. On the other hand, in divalent CaCl₂ solutions the brushes underwent four distinct regimes of (i) a low C_s collapse regime, (ii) a relatively broad plateau regime (0.1 M ≤ C_s < 1 M), (iii) a weak reswelling regime, and (iv) a high C_s collapse regime. The novel behavior in regimes ii-iv may be attributed to weak interactions of the poly(AMPS) brushes with Ca²⁺. We also find that the brushes are more extended at 90 °C as thermal energy weakens interchain bridging, which is consistent with the behavior of free polymer chains dissolved in CaCl₂ solutions at extreme salinities.