23 research outputs found
Sorption and spatial distribution of protein globules in charged hydrogel particles
We have theoretically studied the uptake of a non-uniformly charged
biomolecule, suitable to represent a globular protein or a drug, by a charged
hydrogel carrier in the presence of a 1:1 electrolyte. Based on the analysis of
a physical interaction Hamiltonian including monopolar, dipolar and Born
(self-energy) contributions derived from linear electrostatic theory of the
unperturbed homogeneous hydrogel, we have identified five different sorption
states of the system, from complete repulsion of the molecule to its full
sorption deep inside the hydrogel, passing through meta- and stable surface
adsorption states. The results are summarized in state diagrams that also
explore the effects of varying the electrolyte concentration, the sign of the
net electric charge of the biomolecule, and the role of including
excluded-volume (steric) or hydrophobic biomolecule-hydrogel interactions. We
show that the dipole moment of the biomolecule is a key parameter controlling
the spatial distribution of the globules. In particular, biomolecules with a
large dipole moment tend to be adsorbed at the external surface of the
hydrogel, even if like-charged, whereas uniformly charged biomolecules tend to
partition towards the internal core of an oppositely-charged hydrogel.
Hydrophobic attraction shifts the states towards internal sorption of the
biomolecule, whereas steric repulsion promotes surface adsorption for
oppositely-charged biomolecules, or the total exclusion for likely-charged
ones. Our results establish a guidance for the spatial partitioning of proteins
and drugs in hydrogel carriers, tuneable by hydrogel charge, pH and salt
concentration.Comment: 16 pages, 5 figure
Effect of polymer-polymer interactions on the surface tension of colloid-polymer mixtures
The density profile and surface tension for the interface of phase-separated
colloid-polymer mixtures have been studied in the framework of the square
gradient approximation for both ideal and interacting polymers in good solvent.
The calculations show that in the presence of polymer-polymer excluded volume
interactions the interfaces have lower widths and surface tensions compared to
the case of ideal polymers. These results are a direct consequence of the
shorter range and smaller depth of the depletion potential between colloidal
particles induced by interacting polymers.Comment: 12 pages, 5 figures, accepted for J. Chem. Phy
A density--functional study of interfacial properties of colloid--polymer mixtures
Interfacial properties of colloid--polymer mixtures are examined within an
effective one--component representation, where the polymer degrees of freedom
are traced out, leaving a fluid of colloidal particles interacting via
polymer--induced depletion forces. Restriction is made to zero, one and
two--body effective potentials, and a free energy functional is used which
treats colloid excluded volume correlations within Rosenfeld's Fundamental
Measure Theory, and depletion--induced attraction within first--order
perturbation theory. This functional allows a consistent treatment of both
ideal and interacting polymers. The theory is applied to surface properties
near a hard wall, to the depletion interaction between two walls, and to the
fluid--fluid interface of demixed colloid--polymer mixtures. The results of the
present theory compare well with predictions of a fully two--component
representation of mixtures of colloids and ideal polymers (the Asakura--Oosawa
model), and allow a systematic investigation of the effects of polymer--polymer
interactions on interfacial properties. In particular, the wall surface tension
is found to be significantly larger for interacting than for ideal polymers,
while the opposite trend is predicted for the fluid--fluid interfacial tension.Comment: submitted to J. Phys. Chem. B, special issue in honour of David
Chandle
Cosolute partitioning in polymer networks: Effects of flexibility and volume transitions
We study the partitioning of cosolute particles in a thin film of a
semi-flexible polymer network by a combination of coarse-grained
(implicit-solvent) stochastic dynamics simulations and mean-field theory. We
focus on a wide range of solvent qualities and cosolute-network interactions
for selected polymer flexibilities. Our investigated ensemble
(isothermal-isobaric) allows the network to undergo a volume transition from
extended to collapsed state while the cosolutes can distribute in bulk and
network, correspondingly. We find a rich topology of equilibrium states of the
network and transitions between them, qualitatively depending on solvent
quality, polymer flexibility, and cosolute-network interactions. In particular,
we find a novel `cosolute-induced' collapsed state, where strongly attractive
cosolutes bridge network monomers albeit the latter interact mutually
repulsive. Finally, the cosolutes' global partitioning `landscape', computed as
a function of solvent quality and cosolute-network interactions, exhibits very
different topologies depending on polymer flexibility. The simulation results
are supported by theoretical predictions obtained with a two-component
mean-field approximation for the Helmholtz free energy that considers the chain
elasticity and the particle interactions in terms of a virial expansion. Our
findings have implications on the interpretation of transport processes and
permeability in hydrogel films, as realized in filtration or macromolecular
carrier systems.Comment: Macromolecules (2017
Short- and long-range topological correlations in two-dimensional aggregation of dense colloidal suspensions
We have studied the average properties and the topological correlations of computer-simulated two-dimensional (2D) aggregating systems at different initial surface packing fractions. For this purpose, the centers of mass of the growing clusters have been used to build the Voronoi diagram, where each cell represents a single cluster. The number of sides (n) and the area (A) of the cells are related to the size of the clusters and the number of nearest neighbors, respectively. We have focused our paper in the study of the topological quantities derived from number of sides, n, and we leave for a future work the study of the dependence of these magnitudes on the area of the cells, A. In this work, we go beyond the adjacent cluster correlations and explore the organization of the whole system of clusters by dividing the space in concentric layers around each cluster: the shell structure. This method allows us to analyze the time behavior of the long-range intercluster correlations induced by the aggregation process. We observed that kinetic and topological properties are intimately connected. Particularly, we found a continuous ordering of the shell structure from the earlier stages of the aggregation process, where clusters positions approach a hexagonal distribution in the plane. For long aggregation times, when the dynamic scaling regime is achieved, the short- and long-range topological properties reached a final stationary state. This ordering is stronger for high particle densities. Comparison between simulation and theoretical data points out the fact that 2D colloidal aggregation in the absence of interactions (diffusion-limited cluster aggregation regimen) is only able to produce short-range cluster-cluster correlations. Moreover, we showed that the correlation between adjacent clusters verifies the Aboav-Weaire law, while all the topological properties for nonadjacent clusters are mainly determined by only two parameters: the second central moment of number-of-sides distribution mu(2)=Sigma P(n)(n-6)(2) and the screening factor a (defined through the Aboav-Weaire equation). We also found that the values of mu(2) and a calculated for two-dimensional aggregating system are related through a single universal common form a proportional to mu(2)(-0.89), which is independent of the particle concentration