26 research outputs found
Monte Carlo simulations of polyion−macroion complexes. 2. Polyion length and charge density dependence
The complexation between a polyion and an oppositely charged spherical macroion in the framework of the
primitive model has been studied by the use of Monte Carlo simulations. The polyion length, linear charge
density, and bare persistence length are varied systematically, while the properties of the macroion are kept
constant. The polyion charge to macroion charge ratio is varied between 1/4 and 4. The structure of the
complex is investigated by direct visualization; polyion bead complexation probability; loop, tail, and train
characteristics; degree of overcharging; and tail joint probability functions. The strongest complexes are
observed for flexible chains, where the polyion is folded around the macroion. In the case of fully flexible
chains, a transition from a collapsed state to a fluctuating two-tail state and eventually to a one-tail state are
observed as the chain length is increased. As the stiffness is increased, several complex structures, such as
multiloop, single-loop, and solenoid arrangements, and finally a structure involving only a single contact
between the polyion and the macroion occur. In particular, for long and highly charged polyions, a transition
from the one-tail state to a two-tail state appears as the chain stiffness is increased. A discussion with recent
theories and other simulation studies is also provided
Diblock polyampholytes grafted onto spherical particles : effect of stiffness, charge density, and grafting density
The structure of spherical brushes formed by symmetric diblock polyampholytes end-grafted onto small spherical
particles in aqueous solution is examined within the framework of the so-called primitive model using Monte Carlo
simulations. The properties of the two blocks are identical except for the sign of their charges. Three different chain
flexibilities corresponding to flexible, semiflexible, and stiff blocks are considered at various polyampholyte linear
charge densities and grafting densities. The link between the two blocks is flexible at all conditions, and the grafted
segments are laterally mobile. Radial and lateral spatial distribution functions of different types and single-chain
properties are analyzed. The brush structure strongly depends on the chain flexibility. With flexible chains, a disordered
polyelectrolyte complex is formed at the surface of the particle, the complex becoming more compact at increasing
linear charge density. With stiff blocks, the inner blocks are radially oriented. At low linear charged density, the outer
blocks are orientationally disordered, whereas at increasing electrostatic interaction the two blocks of a polyampholyte
are parallel and close to each other, leading to an ordered structure referred to as a polyampholyte star. As the grafting
density is increased, the brush thickness responds differently for flexible and nonflexible chains, depending on a
different balance between electrostatic interactions and excluded volume effects
Diblock polyampholytes grafted onto spherical particles : Monte Carlo simulation and lattice mean-field theory
Spherical brushes composed of diblock polyampholytes (diblock copolymers with oppositely charged
blocks) grafted onto solid spherical particles in aqueous solution are investigated by using the primitive
model solved with Monte Carlo simulations and by lattice mean-field theory. Polyampholyte chains of two
compositions are considered: a copolymer with a long and a short block, A100B10, and a copolymer with
two blocks of equal length, A50B50. The B block is end-grafted onto the surface, and its charge is varied,
whereas the charge of the A block is fixed. Single-chain properties, radial and lateral spatial distributions
of different types, and structure factors are analyzed. The brush structure strongly depends on the charge
of the B block. In the limit of an uncharged B block, the chains are stretched and form an extended
polyelectrolyte brush. In the other limit with the charges of the blocks compensating each other, the chains
arecollapsedandformapolyelectrolytecomplexsurroundingtheparticles.Atintermediatechargeconditions,
a polyelectrolyte brush and a polyelectrolyte complex coexist and constitute two substructures of the
spherical brush. The differences of the brush structures formed by the A100B10 and A50B50 polyampholytes
are also analyzed. Finally, a comparison of the predictions of the two theoretical approaches is made