The interactions following a reentrant phase transition of charged silica nanoparticles from one phase to two phases and back to one phase by varying the concentration of multivalent counterions have been examined. The observations are far beyond the framework of Debye-Hückel or even nonlinear Poisson-Boltzmann equations and demonstrate the universal behavior of multivalent counterion-driven charge inversion. We show that the interplay of multivalent counterion-induced short-range attraction and long-range electrostatic repulsion between nanoparticles results in reentrant phase behavior.
The
evolution of interactions in the bovine serum albumin (BSA)
protein solution on addition of mono and multivalent (di, tri and
tetra) counterions has been studied using small-angle neutron scattering
(SANS), dynamic light scattering (DLS) and ζ-potential measurements.
It is found that in the presence of mono and divalent counterions,
protein behavior can be well explained by DLVO theory, combining the
contributions of screened Coulomb repulsion with the van der Waals
attraction. The addition of mono or divalent salts in protein solution
reduces the repulsive barrier and hence the overall interaction becomes
attractive, but the system remains in one-phase for the entire concentration
range of the salts, added in the system. However, contrary to DLVO
theory, the protein solution undergoes a reentrant phase transition
from one-phase to a two-phase system and then back to the one-phase
system in the presence of tri and tetravalent counterions. The results
show that tri and tetravalent (unlike mono and divalent) counterions
induce short-range attraction between the protein molecules, leading
to the transformation from one-phase to two-phase system. The two-phase
is characterized by the fractal structure of protein aggregates. The
excess condensation of these higher-valent counterions in the double
layer around the BSA causes the reversal of charge of the protein
molecules resulting into reentrant of the one-phase, at higher salt
concentrations. The complete phase behavior with mono and multivalent
ions has been explained in terms of the interplay of electrostatic
repulsion and ion-induced short-range attraction between the protein
molecules.
The new V12 instrument at the Hahn-Meitner Institute in Berlin is a multiple setup combining several techniques to investigate the internal structure of bulk samples. It consists of two double-crystal diffractometers (DCDs) and an attenuation tomography device operating with monochromatic neutrons. The three instrument parts can be used independently at the same time. The DCDs are mainly used in the ultra-small-angle neutron scattering (USANS) regime, where they overlap the accessible range of small-angle neutron scattering instruments, while tomographic methods collect real-space information on a macroscopic scale. Together they enable structural investigations over six orders of magnitude (50 nm to 5 cm). Scattering and tomographic methods can even be combined by means of diffraction and scattering-enhanced imaging. The sample environment can be varied over a large range of temperatures and pressures for USANS measurements and a polarized USANS option is available.
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