The
phase behavior of nanoparticle (silica)–polymer (polyethylene
glycol) system without and with an electrolyte (NaCl) has been studied.
It is observed that nanoparticle–polymer system behaves very
differently in the presence of electrolyte. In the absence of electrolyte,
the nanoparticle–polymer system remains in one-phase even at
very high polymer concentrations. On the other hand, a re-entrant
phase behavior is found in the presence of electrolyte, where one-phase
(individual) system undergoes two-phase (nanoparticle aggregation)
and then back to one-phase with increasing polymer concentration.
The regime of two-phase system has been tuned by varying the electrolyte
concentration. The polymer concentration range over which the two-phase
system exists is significantly enhanced with the increase in the electrolyte
concentration. These systems have been characterized by small-angle
neutron scattering (SANS) experiments of contrast-marching the polymer
to the solvent. The data are modeled using a two-Yukawa potential
accounting for both attractive and repulsive parts of the interaction
between nanoparticles. The phase behavior of nanoparticle–polymer
system is explained by interplay of attractive (polymer-induced attractive
depletion between nanoparticles) and repulsive (nanoparticle–nanoparticle
electrostatic repulsion and polymer–polymer repulsion) interactions
present in the system. In the absence of electrolyte, the strong electrostatic
repulsion between nanoparticles dominates over the polymer-induced
depletion attraction and the nanoparticle system remains in one-phase.
With addition of electrolyte, depletion attraction overcomes electrostatic
repulsion at some polymer concentration, resulting into nanoparticle
aggregation and two-phase system. Further addition of polymer increases
the polymer–polymer repulsion which eventually reduces the
strength of depletion and hence re-entrant phase behavior. The effects
of varying electrolyte concentration on the phase behavior of nanoparticle–polymer
system are understood in terms of modifications in nanoparticle–nanoparticle
and polymer–polymer interactions. The nanoparticle aggregates
in two-phase systems are found to have surface fractal morphology
