109 research outputs found
Interaction between like-charged polyelectrolyte-colloid complexes in electrolyte solutions: a Monte Carlo simulation study in the Debye-H\"uckel approximation
We study the effective interaction between differently charged
polyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlo
simulations. These complexes are formed when short and flexible polyelectrolyte
chains adsorb onto oppositely charged colloidal spheres, dispersed in an
electrolyte solution. In our simulations the bending energy between adjacent
monomers is small compared to the electrostatic energy, and the chains, once
adsorbed, do not exchange with the solution, although they rearrange on the
particles surface to accomodate further adsorbing chains or due to the
electrostatic interaction with neighbor complexes. Rather unexpectedly, when
two interacting particles approach each others, the rearrangement of the
surface charge distribution invariably produces anti-parallel dipolar doublets,
that invert their orientation at the isoelectric point. These findings clearly
rule out a contribution of dipole-dipole interactions to the observed
attractive interaction between the complexes, pointing out that such
suspensions can not be considered dipolar fluids. On varying the ionic strength
of the electrolyte, we find that a screening length, short compared with the
size of the colloidal particles, is required in order to observe the attraction
between like charged complexes due to the non-uniform distribution of the
electric charge on their surface ('patch attraction'). On the other hand, by
changing the polyelectrolyte/particle charge ratio, the interaction between
like-charged polyelectrolyte-decorated (pd) particles, at short separations,
evolves from purely repulsive to strongly attractive. Hence, the effective
interaction between the complexes is characterized by a potential barrier,
whose height depends on the net charge and on the non-uniformity of their
surface charge distribution.Comment: 24 pages, 9 figure
On the effect of temperature on the reentrant condensation in polyelectrolyte-liposome complexation
In systems of highly charged linear polyelectrolytes and oppositely charged
colloidal particles, long-lived clusters of polyelectrolyte-decorated particles
form in an interval of concentrations around the isoelectric point, where
reentrant condensation connected to charge inversion of cluster is observed.
The mechanisms that drive the aggregation and stabilize, at the different
polymer/particle ratios, a well defined size of the aggregates are not
completely understood. Moreover, a central question still remains unanswered,
i.e., whether the clusters are true equilibrium or metastable aggregates. To
elucidate this point, in this work, we have investigated the effect of the
temperature on the formation of the clusters. We employed liposomes built up by
DOTAP lipid interacting with a simple anionic polyion, sodium polyacrylate,
over an extended concentration range below and over the isoelectric condition.
Our results show that the aggregation process can be described by a
thermally-activated mechanism.Comment: Submitted Langmui
Colorimetric detection of chromium(VI) ions in water using unfolded-fullerene carbon nanoparticles
Water pollution caused by hexavalent chromium (Cr(VI)) ions represents a serious hazard for human health due to the high systemic toxicity and carcinogenic nature of this metal species. The optical sensing of Cr(VI) through specifically engineered nanomaterials has recently emerged as a versatile strategy for the application to easy-to-use and cheap monitoring devices. In this study, a one-pot oxidative method was developed for the cage opening of C60 fullerene and the synthesis of stable suspensions of N-doped carbon dots in water–THF solutions (N-CDs-W-THF). The N-CDs-W-THF selectively showed variations of optical absorbance in the presence of Cr(VI) ions in water through the arising of a distinct absorption band peaking at 550 nm, i.e., in the transparency region of pristine material. Absorbance increased linearly, with the ion concentration in the range 1–100 µM, thus enabling visual and ratiometric determination with a limit of detection (LOD) of 300 nM. Selectivity and possible interference effects were tested over the 11 other most common heavy metal ions. The sensing process occurred without the need for any other reactant or treatment at neutral pH and within 1 min after the addition of chromium ions, both in deionized and in real water sam-ples
Kinetic Arrest in Polyion-Induced Inhomogeneously-Charged Colloidal Particle Aggregation
Polymer chains adsorbed onto oppositely charged spherical colloidal particles
can significantly modify the particle-particle interactions. For sufficient
amounts of added polymers, the original electrostatic repulsion can even turn
into an effective attraction and relatively large kinetically stable aggregates
can form which display several unexpected and interesting peculiarities and
some intriguing biotechnological implications. The attractive interaction
contribution between two oppositely particles arises from the correlated
adsorption of polyions at the oppositely charged particle surfaces, resulting
in a non-homogeneous surface charge distribution. Here, we investigate the
aggregation kinetics of polyion-induced colloidal complexes through Monte Carlo
simulation, in which the effect of charge anisotropy is taken into account by a
DLVO-like intra-particle potential, as recentely proposed by Velegol and Thwar
[D. Velegol and P.K. Thwar, Langmuir, 17, 2001]. The results reveal that in the
presence of a charge heterogeneity the aggregation process slows down due to
the progressive increase of the potential barrier height upon clustering.
Within this framework, the experimentally observed cluster phases in
polyelectrolyte-liposomes solutions should be considered as a kinetic arrested
state.Comment: 9 pages. 11 figure
Polyion-Induced Cluster Formation in Different Colloidal Polyparticle Aqueous Suspensions
The formation of aggregates in polyion-induced charged colloidal particles in aqueous suspension is characterized, under appropriate conditions, by two complementary effects, known as re-entrant condensation and charge inversion, which are considered as proof for the existence of a cluster phase. In this paper, we extend our previous investigation to a set of aqueous colloidal particle suspensions, such as polystyrene spheres, colloidal gold particles, and polylactic acid particles. These systems are characterized by the evolution of the average size of the aggregates and their surface electrical charge (charge inversion) by means of dynamic light-scattering measurements and laser Doppler electrophoretic techniques. The results, together with the previous ones concerning liposome particles, support the notion of a common behavior of this group of complex colloids characterized by short-ranged attractive interactions. The study provides some insights into these structures, which are potentially useful in biotechnological applications, such as multicompartmental carriers in nonviral drug delivery
- …