1 research outputs found
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 CaCl<sub>2</sub> 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 (<i>C</i><sub>s</sub>) collapse monotonically with increasing <i>C</i><sub>s</sub>. On the other hand, in divalent CaCl<sub>2</sub> solutions the brushes underwent four distinct regimes of (i) a low <i>C</i><sub>s</sub> collapse regime, (ii) a relatively broad plateau
regime (0.1 M ≤ <i>C</i><sub>s</sub> < 1 M), (iii)
a weak reswelling regime, and (iv) a high <i>C</i><sub>s</sub> collapse regime. The novel behavior in regimes ii–iv may
be attributed to weak interactions of the poly(AMPS) brushes with
Ca<sup>2+</sup>. 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 CaCl<sub>2</sub> solutions at extreme salinities