Fraction Of Charged Units Research Articles

REARRANGEMENTS IN THE CONFORMATIONAL STRUCTURE OF POLYELECTROLYTES ON THE SURFACE OF A FLATTENED METAL NANOSPHEROID IN AN ALTERNATING ELECTRIC FIELD

A mathematical model has been presented for the formation of the conformational structure of chain units in a polyelectrolyte adsorbed on a flattened conducting charged nanospheroid polarized in an external electric field, which harmonically varies at a frequency much lower than the plasma frequency of the nanospheroid metal. Molecular dynamics has been employed to study the rearrangements in the conformational structure of uniformly charged polypeptides adsorbed on the surface of the oppositely charged flattened gold nanospheroid in an external alternating electric field, the strength vector of which varies along the rotation axis of the nanospheroid. One-dimensional density distributions along the rotation axis, as well as radial distributions, have been plotted for atoms of the polypeptides adsorbed on the nanospheroid surface. At a low temperature, a narrow ring-shaped polyelectrolyte fringe is formed in the equatorial region of the flattened metal nanospheroid, and the fringe density increases with the total charge of the nanospheroid and the number of charged units in polyelectrolyte macrochains. At a high temperature, the formed narrow macromolecular ring periodically shifts along the rotation axis of the nanospheroid with redirections of the polarizing electric field vector. The amplitude of the shifts increases with a decrease in the total charge of the nanospheroid and an increase in the fraction of charged units in a polyelectrolyte.

Collapse–Decollapse of Thermosensitive Polyelectrolyte Gels in Aqueous Media

The effect of temperature on water sorption by gels based on the copolymers of N-isopropylacrylamide with sodium p-styrene sulfonate containing 1, 3, and 5 mol % of charged monomer units is studied in aqueous media in the presence of salt (NaCl, KCl) or urea. It is shown that the incubation of gels in salt solutions leads to compression of the gels. With increasing temperature, the collapse of gels is observed in all studied media. The difference in the gel collapse temperature in water and salt solution increases with an increase in the fraction of charged units in the polymer gel network and in the salt concentration in the solution. With a return temperature decrease and gel decollapse, the mass of all gels in salt solutions (0.5–5.0 wt %) is completely restored. The incubation of gels in aqueous solutions of urea (0.5–5.0 wt %) has practically no effect on water sorption by gels before the collapse and the collapse temperature. However, the degree of swelling of polyelectrolyte gels during decollapse caused by a temperature decrease is much lower than the initial value, in contrast to the completely reversible swelling of the poly(N-isopropylacrylamide) gel under similar conditions.

Self-assembly of an amphiphilic diblock copolymer in aqueous solutions: Effect of linear charge density of an ionogenic block

The effect of linear charge density of the ionogenic block on the selfassembly of the amphiphilic diblock copolymer based on polystyrene and poly(4�vin ylpyridine) partially alkylated by ethyl bromide in aqueous solutions at pH 9 is studied by UV spectrophotometry and dynamic and static light scattering. Dur� ing dispersion in water, the diblock copolymer forms micelles composed of the hydrophobic polystyrene core and the amphiphilic lyophilizing corona consisting of Nethyl�4�vinylpyridinium bromide ionic units and uncharged units of 4�vinylpyridine. It is shown that with a change in the fraction of charged units in the lyo� philizing block from 30 to 90 mol %, the thermodynamic quality of the solvent with respect to micelles is slightly monotonically improved and the hydrodynamic sizes of micelles in 0.05 M NaCl are increased. At the same time, such properties as the weightaverage deg ree of aggregation of macromolecules in a micelle, the dimensions of the corona in 0.05 M NaCl, and the dispersion stability of micelles in aqueous-saline solution abruptly change when the content of charged units in the lyophilizing block is 60-70 mol %. After addition of oppositely charged polyelectrolytes or lowmolecu larmass surfactants into the micellar solution, poly� electrolyte complexes form and the solubility of these complexes in an excess oppositely charged component likewise changes abruptly with variation in the fraction of charged units in the range from 60 to 70 mol %. A qualitative description is advanced to explain the effect of linear charged density of the lyophilizing block on the selfassembly and complexing behavior of diblock copolymers in aqueous solutions.

Interaction of two polyelectrolyte gels in solution of an oppositely charged surfactant

The behavior of two charged polymer networks in a solution of an oppositely charged surfactant was studied. It was shown that such a system (depending on preset parameters) can exist in different modes: without micelles in both networks, with micelles in one of the network, and with micelles in both networks. The dependences of network dimensions and ion concentrations inside the networks on the surfactant concentration in the solution, the fraction of charged units in one of the networks, and the relative size of the system were obtained. It is possible to affect the state of one network by varying the parameters (e.g., the proportion of charged units) of the other network. Different network swelling scenarios depending on the relative size of the system and the fraction of charged network units were revealed.

Spontaneous Charge Inversion of a Microgel Particle by Complexation with Oppositely Charged Polyelectrolytes

A combination of the mean-field theory and the random phase approximation is used to describe the effect of charge inversion (overcharging) of a spherical penetrable microgel particle by oppositely charged multiarm star polyelectrolytes in dilute solution. The overcharging is shown to occur due to the gain in the electrostatic self-energy and in the elastic free energy of the star macromolecules, while counterions can either promote or suppress the effect. It is shown that many parameters, such as the densities of the microgel and the stars, the fraction of charged units, etc., control the overcharging.

Effect of polyelectrolytes on aggregation of a cyanine dye

The effect of poly(acrylic acid) and poly(methacrylic acid) on the molecular state of a cationic cyanine dye (3,3′-diethyl-2,2′-thiacarbocyanine iodide) was studied. Gels based on the above polymers were shown to efficiently absorb an oppositely charged dye. The absorption of dye ions by a gel induced their aggregation. It was shown that, in the acrylic acid-based gels, H-aggregates, dimers, and single ions of the dye coexist. In gels containing methacrylic acid units, dye ions mainly formed H-aggregates. A comparison of aggregation of dye ions in the gels with that in aqueous solutions of corresponding polyelectrolytes was performed. When dye ions were immobilized in a gel network, the proportion of the ions forming the H-aggregates was considerably larger than that in solutions. The effect of the gel network charge density on the aggregation of dye ions was investigated. It was shown that an increase in the fraction of charged units in network chains facilitates aggregation.