Oleg Rud | Charles University, Prague (original) (raw)
Papers by Oleg Rud
We developed a new method for coarse-grained simulations of acid-base equilibria in a system coup... more We developed a new method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt, that we term the Grand-reaction method. More generally, it can be used for simulations of any reactive system coupled to a reservoir of a known composition. Conceptually, it can be regarded as an extension of the reaction ensemble, combining explicit simulations of reactions within the system and Grand-canonical exchange of particles with the reservoir. To demonstrate its strength, we applied our method to a solution of weak polyelectrolytes in equilibrium with a reservoir. Our results show that the ionization and swelling of a weak polyelectrolyte are aected by the Donnan eect due to the partitioning of ions and by the polyelectrolyte eect due to electrostatic repulsion along the chain. Both eects lead to a similar shift in ionization and swelling as a function of pH; albeit for dierent physical reasons. By comparison with published results, 1 we showed that neglecting one or the other eect may lead to erroneous predictions or misinterpretations of results. In contrast, the Grand-reaction method accounts for both eects on the results and allows us to quantify them. Finally, we outline possible extensions and generalizations of the method and provide a set of guidelines for its safe application by a broad community of users.
Gels
One of the emerging water desalination techniques relies on the compression of a polyelectrolyte ... more One of the emerging water desalination techniques relies on the compression of a polyelectrolyte gel. The pressures needed reach tens of bars, which are too high for many applications, damage the gel and prevent its reuse. Here, we study the process by means of coarse-grained simulations of hydrophobic weak polyelectrolyte gels and show that the necessary pressures can be lowered to only a few bars. We show that the dependence of applied pressure on the gel density contains a plateau indicating a phase separation. The phase separation was also confirmed by an analytical mean-field theory. The results of our study show that changes in the pH or salinity can induce the phase transition in the gel. We also found that ionization of the gel enhances its ion capacity, whereas increasing the gel hydrophobicity lowers the pressure required for gel compression. Therefore, combining both strategies enables the optimization of polyelectrolyte gel compression for water desalination purposes.
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions fr... more We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions from a brine with low salinity to a brine with high salinity. As a driving force we use the fact that the volume of polyelectrolyte gel does depend on the surrounding salinity. While the salinity defines the gel volume in open system, i.e. in equilibrium with big aqueous solution bath, in closed system (in equilibrium with small bath) salinity is defined by the gel volume. The compression and swelling in open and closed system processes are combined into four stages thermodynamic cycle working between two bathes of different salinities. The cycle implies reversibility at any stage, so, in principle, the method can achieve the maximum thermodynamic efficiency. We have shown that for weak polyelectrolyte gels the dependence between the salinity and gel volume appears to be non-monotonic. Depending on the model parameters the surrounding salinity may increase or decrease during compression. In both cases we consider the possible use of this relation in desalination cycle.
Gels
We present results of MD and MC simulations of the equilibrium properties of swelling gels with c... more We present results of MD and MC simulations of the equilibrium properties of swelling gels with comb-like or bottlebrush subchains and compare them to scaling-theory predictions. In accordance with theory, the simulation results demonstrate that swelling coefficient of the gel increases as a function of the polymerization degree of the main chains and exhibits a very weak maximum (or is virtually constant) as a function of the polymerization degree and grafting density of side chains. The bulk osmotic modulus passes through a shallow minimum as the polymerization degree of the side chains increases. This minimum is attributed to the onset of overlap of side chains belonging to different bottlebrush strands in the swollen gel.
Gels
Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a resu... more Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a result of their swelling pressure. On the other hand, the insoluble cross-linked hydrogel network enables dewatering under the influence of external (thermal and/or mechanical) stimuli. Moreover, from a thermodynamic perspective, a polyelectrolyte hydrogel is already an osmotic membrane. These properties designate hydrogels as excellent candidates for use in desalination, at the same time avoiding the use of expensive membranes. In this article, we present our recent theoretical study of polyelectrolyte hydrogel usage for water desalination. Employing a coarse-grained model and the Gibbs ensemble, we modeled the thermodynamic equilibrium between the coexisting gel phase and the supernate aqueous salt solution phase. We performed a sequence of step-by-step hydrogel swellings and compressions in open and closed systems, i.e., in equilibrium with a large and with a comparably small reservoir of...
We model a process of seawater desalination that employs compression of a weak polyelectrolyte hy... more We model a process of seawater desalination that employs compression of a weak polyelectrolyte hydrogel as the drawing force for ion transfer. We model seawater as an aqueous solution containing mono-and divalent ions, Na + , Cl − , and Ca 2+. We demonstrate that the compression of hydrogel in equilibrium with a bath containing such a solution creates two types of ion flow. The first type is the exchange of divalent Ca 2+ by monovalent Na +. The charge density during this process is preserved, so that per one absorbed Ca 2+ two Na + are released. The second type is the desalination itself which leads to the uptake of Na + and Cl − pairs from the surrounding solution. The first flow predominates when Ca 2+ are in excess; however, as their fraction in the solution decreases, desalination flow becomes more significant. We have shown that although the gel is a weak polyelectrolyte, its compression in the presence of divalent ions does not influence its ionization degree. This is caused by a significant screening of electrostatic interactions by divalent ions. When the density of Ca 2+ is small, the gel compression may lead to its discharge and to the release of counterions, which shows up in an increase of the solution ionic strength.
Hydrophobic blocks of amphiphilic block copolymers usually form glassy micellar cores with a rigi... more Hydrophobic blocks of amphiphilic block copolymers usually form glassy micellar cores with a rigid structure that limits their applications as nanocapsules for targeted delivery. We report here on the core/shell micelles with a soft core formedby self-assembly of block copolymer composed of hydrophobic and polycation blocks, poly(lauryl acrylate)-block-poly(trimethylammonioethyl acrylate) (PLA-QPDMAEA), in aqueous solution. Using scattering, microscopy and spectroscopy techniques, we showed that such copolymer forms spherical and cylindrical core/shell micelles with a fluid-like PLA core and a positively charged shell, and that these micelles can encapsulate and release hydrophobic solutes. Moreover, we discovered novel vesicle-like multicompartment structures containing both soft hydrophobic and interpolyelectrolyte (IPEC) layers formed by co-assembly of PLA-QPDMAEA core/shell micelles with another diblock copolymer composed of a hydrophilic block and polyanion block poly(ethylene ...
Soft Matter, 2019
This article recapitulates the state of the art regarding simulations of ionization equilibria of... more This article recapitulates the state of the art regarding simulations of ionization equilibria of weak polyelectrolyte solutions and gels.
Desalination, 2018
We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions fr... more We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions from a brine with low salinity to a brine with high salinity. As a driving force we use the fact that the volume of polyelectrolyte gel does depend on the surrounding salinity. While the salinity defines the gel volume in open system, i.e. in equilibrium with big aqueous solution bath, in closed system (in equilibrium with small bath) salinity is defined by the gel volume. The compression and swelling in open and closed system processes are combined into four stages thermodynamic cycle working between two bathes of different salinities. The cycle implies reversibility at any stage, so, in principle, the method can achieve the maximum thermodynamic efficiency. We have shown that for weak polyelectrolyte gels the dependence between the salinity and gel volume appears to be non-monotonic. Depending on the model parameters the surrounding salinity may increase or decrease during compression. In both cases we consider the possible use of this relation in desalination cycle.
Soft Matter, 2017
We present a novel approach to modeling polyelectrolyte gels, exploiting the analogy between star... more We present a novel approach to modeling polyelectrolyte gels, exploiting the analogy between star-branched polymers and polymer networks, as a computationally inexpensive, yet reliable alternative to full-scale simulations.
Soft Matter, 2014
Dendron brushes are molecular structures built up of treelike macromolecules, with multiple gener... more Dendron brushes are molecular structures built up of treelike macromolecules, with multiple generations of branches, grafted with a root segment to a surface (particle) or to a backbone chain (dendronized polymer) with a sufficiently high grafting density so that the dendrons interact laterally. Recent advances in the theory of dendron brushes are highlighted and complemented by insights from numerical self-consistent field modelling. Our focus is on controversial issues, which are still under debate, such as, the strain distribution in individual dendrons and the appearance of distinct populations with a different extent of stretching for dendrons in planar brushes. We anticipate that dendritic brushes (i) show a strong resistance against bending, which may manifest in a high apparent persistence length of dendronized polymers, and (ii) have an unusually large beneficial effect on the colloidal stability due to the sharp steric repulsive interaction observed when these surface layers are pushed towards the overlap.
Macromolecules, 2013
Equilibrium structural properties of polymer brushes formed by dendrons grafted via the root segm... more Equilibrium structural properties of polymer brushes formed by dendrons grafted via the root segment onto spherical surfaces (dendritic spherical polymer brushes, DSPB) are studied by means of the Scheutjens−Fleer self-consistent field (SF-SCF) numerical approach and scaling analysis. In particular, we focus on the effects of the variable curvature of the surface on the polymer volume fraction distribution and extension of the individual dendrons in DSPB. A systematic comparison with spherical polymer brushes formed by linear polymer chains (LSPB) end-grafted to the surfaces of the same curvature radii is performed. We demonstrate that the difference in internal structural organization of DSPB and LSPB is most pronounced at small curvature radius of the grafting surface. In particular, the radial distribution of polymer volume fraction in DSPB is close to uniform, whereas in LSPB it decays in the radial direction following a power law. The quasi-plateau polymer volume fraction distribution in DSPB is ensured by wide radial distribution of the end segments. In contrast, in LSPB the end segments of the chains are localized preferentially close to the periphery of the brush. An increase in the curvature radius of the surface is accompanied by emerging segregation into two (or more, for larger number of generations) populations of dendrons: the less extended and the more extended ones. The former ones fill the space in the central region of the DSPB, and the latter bring the majority of the monomer units closer to the periphery of the DSPB. The theoretical results are in line with experimental findings on hydrodynamic radii of linear and dendritic poly(ethylene glycol) brushes end-grafted onto Fe 3 O 4 nanoparticles.
Polymer Science Series A, 2013
ABSTRACT Changes in the conformations of two polyelectrolyte stars with amphiphilic segments duri... more ABSTRACT Changes in the conformations of two polyelectrolyte stars with amphiphilic segments during their convergence are investigated by the Scheutjens-Fleer method (a numerical method of solving self-consistent field equations). The stars are placed in an aqueous-salt solvent at a pH close to the pK of the segments of polyelectrolyte arms. Individual stars have a two-phase quasi-micellar conformation at a sufficiently strong hydrophobicity of segments in the uncharged state and at a sufficiently low salt concentration. This conformation is formed via separation of star arms into two groups: One consists of uncharged arms and forms a dense core in the star center, while the other consists of arms coming through and forming a corona with charged arms. When stars in this conformation converge, the transition of arms from the corona on the side of the oncoming star to the core occurs inside both stars. When stars touch each other with their cores, the stars merge into a united quasi-micelle. The free energy of interaction of stars is a nonmonotonic function of distance D between their centers. This value grows with a decrease in D until the cores come into contact. Upon the contact and merging of the cores, the free-energy values are characterized by a local minimum corresponding to an ellipsoid-shaped united quasi-micelle. With an increase in the concentration of salt in the solution, individual stars adopt extended conformations. In this case, the interaction of two stars is repulsive at all D values. However, if the salt concentration very slightly exceeds the threshold value of formation of a core in an isolated star, then the initially extended stars during their approach toward each other adopt (at first, each of them separately) a quasi-micellar conformation, while their cores grow as they converge and merge upon contact.
Lucie Nová, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles Universi... more Lucie Nová, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic lucie.nova@natur.cuni.cz Oleg V. Rud, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic Peter Košovan, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic
Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for... more Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for water desalination. This partitioning also affects the H+ ions, leading to a decrease of the pH in the gel as compared to supernatant solution. When using weak polyelectrolyte gels, this lower pH may decrease their ionization degree, and hence diminish their desalination capacity. Using the recently developed grand-reaction ensemble, we performed particle-based computer simulations of a gel in contact with a reservoir of salt solution at pH ≃ 7. We showed that the effect of ion partitioning on the ionization degree is negligible, as long as the salinity of the reservoir is high. However, when the reservoir salinity approaches that of fresh water, the ionization degree of weak polyelectrolyte gel is significantly decreased, thus making it a less efficient desalination agent. Furthermore, we demonstrated that the ionization degree of the gel is decreased upon compression. However, up to p...
By this article, we continue studying weak polyelectrolyte hydrogels and their application as a d... more By this article, we continue studying weak polyelectrolyte hydrogels and their application as a desalination agent. We modeled the desalination process as a four-step reversible thermodynamic cycle transferring ions from a low salinity solution to a high salinity one. The cycle implies reversibility at any stage, therefore the method can achieve the maximum thermodynamic efficiency, comparable to the reverse osmosis. As a driving force for ions movement, we use the fact that compression of the gel leads to a decrease in the gel ionization degree, and therefore to a release of ions entrapped in the gel. We considered the gel composed of ionogenic units modified by hydrophobic pendants and showed that this modification significantly increases the number of transferred ions. This increase is caused by a first-order phase transition originated from an interplay between repulsive electrostatic and attractive steric interactions. The transition happens during hydrogel compression. At a ce...
On 16th September 2019 we found a major flaw in the original manuscript, that has been uploaded t... more On 16th September 2019 we found a major flaw in the original manuscript, that has been uploaded to ChemRXiv. To prevent readers from implementing and using a flawed algorithm, we decided to replace the manuscript with this notice. We anticipate to upload a corrected version within few weeks.
Soft Matter
Conformational transitions and nanoscale self-organization triggered in double pH- and thermo-res... more Conformational transitions and nanoscale self-organization triggered in double pH- and thermo-responsive molecular brushes by varying environmental conditions are studied by means of analytical mean-field theory and numerical Scheutjens-Fleer self-consistent field...
We developed a new method for coarse-grained simulations of acid-base equilibria in a system coup... more We developed a new method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt, that we term the Grand-reaction method. More generally, it can be used for simulations of any reactive system coupled to a reservoir of a known composition. Conceptually, it can be regarded as an extension of the reaction ensemble, combining explicit simulations of reactions within the system and Grand-canonical exchange of particles with the reservoir. To demonstrate its strength, we applied our method to a solution of weak polyelectrolytes in equilibrium with a reservoir. Our results show that the ionization and swelling of a weak polyelectrolyte are aected by the Donnan eect due to the partitioning of ions and by the polyelectrolyte eect due to electrostatic repulsion along the chain. Both eects lead to a similar shift in ionization and swelling as a function of pH; albeit for dierent physical reasons. By comparison with published results, 1 we showed that neglecting one or the other eect may lead to erroneous predictions or misinterpretations of results. In contrast, the Grand-reaction method accounts for both eects on the results and allows us to quantify them. Finally, we outline possible extensions and generalizations of the method and provide a set of guidelines for its safe application by a broad community of users.
Gels
One of the emerging water desalination techniques relies on the compression of a polyelectrolyte ... more One of the emerging water desalination techniques relies on the compression of a polyelectrolyte gel. The pressures needed reach tens of bars, which are too high for many applications, damage the gel and prevent its reuse. Here, we study the process by means of coarse-grained simulations of hydrophobic weak polyelectrolyte gels and show that the necessary pressures can be lowered to only a few bars. We show that the dependence of applied pressure on the gel density contains a plateau indicating a phase separation. The phase separation was also confirmed by an analytical mean-field theory. The results of our study show that changes in the pH or salinity can induce the phase transition in the gel. We also found that ionization of the gel enhances its ion capacity, whereas increasing the gel hydrophobicity lowers the pressure required for gel compression. Therefore, combining both strategies enables the optimization of polyelectrolyte gel compression for water desalination purposes.
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions fr... more We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions from a brine with low salinity to a brine with high salinity. As a driving force we use the fact that the volume of polyelectrolyte gel does depend on the surrounding salinity. While the salinity defines the gel volume in open system, i.e. in equilibrium with big aqueous solution bath, in closed system (in equilibrium with small bath) salinity is defined by the gel volume. The compression and swelling in open and closed system processes are combined into four stages thermodynamic cycle working between two bathes of different salinities. The cycle implies reversibility at any stage, so, in principle, the method can achieve the maximum thermodynamic efficiency. We have shown that for weak polyelectrolyte gels the dependence between the salinity and gel volume appears to be non-monotonic. Depending on the model parameters the surrounding salinity may increase or decrease during compression. In both cases we consider the possible use of this relation in desalination cycle.
Gels
We present results of MD and MC simulations of the equilibrium properties of swelling gels with c... more We present results of MD and MC simulations of the equilibrium properties of swelling gels with comb-like or bottlebrush subchains and compare them to scaling-theory predictions. In accordance with theory, the simulation results demonstrate that swelling coefficient of the gel increases as a function of the polymerization degree of the main chains and exhibits a very weak maximum (or is virtually constant) as a function of the polymerization degree and grafting density of side chains. The bulk osmotic modulus passes through a shallow minimum as the polymerization degree of the side chains increases. This minimum is attributed to the onset of overlap of side chains belonging to different bottlebrush strands in the swollen gel.
Gels
Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a resu... more Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a result of their swelling pressure. On the other hand, the insoluble cross-linked hydrogel network enables dewatering under the influence of external (thermal and/or mechanical) stimuli. Moreover, from a thermodynamic perspective, a polyelectrolyte hydrogel is already an osmotic membrane. These properties designate hydrogels as excellent candidates for use in desalination, at the same time avoiding the use of expensive membranes. In this article, we present our recent theoretical study of polyelectrolyte hydrogel usage for water desalination. Employing a coarse-grained model and the Gibbs ensemble, we modeled the thermodynamic equilibrium between the coexisting gel phase and the supernate aqueous salt solution phase. We performed a sequence of step-by-step hydrogel swellings and compressions in open and closed systems, i.e., in equilibrium with a large and with a comparably small reservoir of...
We model a process of seawater desalination that employs compression of a weak polyelectrolyte hy... more We model a process of seawater desalination that employs compression of a weak polyelectrolyte hydrogel as the drawing force for ion transfer. We model seawater as an aqueous solution containing mono-and divalent ions, Na + , Cl − , and Ca 2+. We demonstrate that the compression of hydrogel in equilibrium with a bath containing such a solution creates two types of ion flow. The first type is the exchange of divalent Ca 2+ by monovalent Na +. The charge density during this process is preserved, so that per one absorbed Ca 2+ two Na + are released. The second type is the desalination itself which leads to the uptake of Na + and Cl − pairs from the surrounding solution. The first flow predominates when Ca 2+ are in excess; however, as their fraction in the solution decreases, desalination flow becomes more significant. We have shown that although the gel is a weak polyelectrolyte, its compression in the presence of divalent ions does not influence its ionization degree. This is caused by a significant screening of electrostatic interactions by divalent ions. When the density of Ca 2+ is small, the gel compression may lead to its discharge and to the release of counterions, which shows up in an increase of the solution ionic strength.
Hydrophobic blocks of amphiphilic block copolymers usually form glassy micellar cores with a rigi... more Hydrophobic blocks of amphiphilic block copolymers usually form glassy micellar cores with a rigid structure that limits their applications as nanocapsules for targeted delivery. We report here on the core/shell micelles with a soft core formedby self-assembly of block copolymer composed of hydrophobic and polycation blocks, poly(lauryl acrylate)-block-poly(trimethylammonioethyl acrylate) (PLA-QPDMAEA), in aqueous solution. Using scattering, microscopy and spectroscopy techniques, we showed that such copolymer forms spherical and cylindrical core/shell micelles with a fluid-like PLA core and a positively charged shell, and that these micelles can encapsulate and release hydrophobic solutes. Moreover, we discovered novel vesicle-like multicompartment structures containing both soft hydrophobic and interpolyelectrolyte (IPEC) layers formed by co-assembly of PLA-QPDMAEA core/shell micelles with another diblock copolymer composed of a hydrophilic block and polyanion block poly(ethylene ...
Soft Matter, 2019
This article recapitulates the state of the art regarding simulations of ionization equilibria of... more This article recapitulates the state of the art regarding simulations of ionization equilibria of weak polyelectrolyte solutions and gels.
Desalination, 2018
We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions fr... more We propose a novel desalination scheme which employs the hydrogels as a tool to draw salt ions from a brine with low salinity to a brine with high salinity. As a driving force we use the fact that the volume of polyelectrolyte gel does depend on the surrounding salinity. While the salinity defines the gel volume in open system, i.e. in equilibrium with big aqueous solution bath, in closed system (in equilibrium with small bath) salinity is defined by the gel volume. The compression and swelling in open and closed system processes are combined into four stages thermodynamic cycle working between two bathes of different salinities. The cycle implies reversibility at any stage, so, in principle, the method can achieve the maximum thermodynamic efficiency. We have shown that for weak polyelectrolyte gels the dependence between the salinity and gel volume appears to be non-monotonic. Depending on the model parameters the surrounding salinity may increase or decrease during compression. In both cases we consider the possible use of this relation in desalination cycle.
Soft Matter, 2017
We present a novel approach to modeling polyelectrolyte gels, exploiting the analogy between star... more We present a novel approach to modeling polyelectrolyte gels, exploiting the analogy between star-branched polymers and polymer networks, as a computationally inexpensive, yet reliable alternative to full-scale simulations.
Soft Matter, 2014
Dendron brushes are molecular structures built up of treelike macromolecules, with multiple gener... more Dendron brushes are molecular structures built up of treelike macromolecules, with multiple generations of branches, grafted with a root segment to a surface (particle) or to a backbone chain (dendronized polymer) with a sufficiently high grafting density so that the dendrons interact laterally. Recent advances in the theory of dendron brushes are highlighted and complemented by insights from numerical self-consistent field modelling. Our focus is on controversial issues, which are still under debate, such as, the strain distribution in individual dendrons and the appearance of distinct populations with a different extent of stretching for dendrons in planar brushes. We anticipate that dendritic brushes (i) show a strong resistance against bending, which may manifest in a high apparent persistence length of dendronized polymers, and (ii) have an unusually large beneficial effect on the colloidal stability due to the sharp steric repulsive interaction observed when these surface layers are pushed towards the overlap.
Macromolecules, 2013
Equilibrium structural properties of polymer brushes formed by dendrons grafted via the root segm... more Equilibrium structural properties of polymer brushes formed by dendrons grafted via the root segment onto spherical surfaces (dendritic spherical polymer brushes, DSPB) are studied by means of the Scheutjens−Fleer self-consistent field (SF-SCF) numerical approach and scaling analysis. In particular, we focus on the effects of the variable curvature of the surface on the polymer volume fraction distribution and extension of the individual dendrons in DSPB. A systematic comparison with spherical polymer brushes formed by linear polymer chains (LSPB) end-grafted to the surfaces of the same curvature radii is performed. We demonstrate that the difference in internal structural organization of DSPB and LSPB is most pronounced at small curvature radius of the grafting surface. In particular, the radial distribution of polymer volume fraction in DSPB is close to uniform, whereas in LSPB it decays in the radial direction following a power law. The quasi-plateau polymer volume fraction distribution in DSPB is ensured by wide radial distribution of the end segments. In contrast, in LSPB the end segments of the chains are localized preferentially close to the periphery of the brush. An increase in the curvature radius of the surface is accompanied by emerging segregation into two (or more, for larger number of generations) populations of dendrons: the less extended and the more extended ones. The former ones fill the space in the central region of the DSPB, and the latter bring the majority of the monomer units closer to the periphery of the DSPB. The theoretical results are in line with experimental findings on hydrodynamic radii of linear and dendritic poly(ethylene glycol) brushes end-grafted onto Fe 3 O 4 nanoparticles.
Polymer Science Series A, 2013
ABSTRACT Changes in the conformations of two polyelectrolyte stars with amphiphilic segments duri... more ABSTRACT Changes in the conformations of two polyelectrolyte stars with amphiphilic segments during their convergence are investigated by the Scheutjens-Fleer method (a numerical method of solving self-consistent field equations). The stars are placed in an aqueous-salt solvent at a pH close to the pK of the segments of polyelectrolyte arms. Individual stars have a two-phase quasi-micellar conformation at a sufficiently strong hydrophobicity of segments in the uncharged state and at a sufficiently low salt concentration. This conformation is formed via separation of star arms into two groups: One consists of uncharged arms and forms a dense core in the star center, while the other consists of arms coming through and forming a corona with charged arms. When stars in this conformation converge, the transition of arms from the corona on the side of the oncoming star to the core occurs inside both stars. When stars touch each other with their cores, the stars merge into a united quasi-micelle. The free energy of interaction of stars is a nonmonotonic function of distance D between their centers. This value grows with a decrease in D until the cores come into contact. Upon the contact and merging of the cores, the free-energy values are characterized by a local minimum corresponding to an ellipsoid-shaped united quasi-micelle. With an increase in the concentration of salt in the solution, individual stars adopt extended conformations. In this case, the interaction of two stars is repulsive at all D values. However, if the salt concentration very slightly exceeds the threshold value of formation of a core in an isolated star, then the initially extended stars during their approach toward each other adopt (at first, each of them separately) a quasi-micellar conformation, while their cores grow as they converge and merge upon contact.
Lucie Nová, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles Universi... more Lucie Nová, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic lucie.nova@natur.cuni.cz Oleg V. Rud, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic Peter Košovan, Dept. of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Prague, Czech Republic
Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for... more Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for water desalination. This partitioning also affects the H+ ions, leading to a decrease of the pH in the gel as compared to supernatant solution. When using weak polyelectrolyte gels, this lower pH may decrease their ionization degree, and hence diminish their desalination capacity. Using the recently developed grand-reaction ensemble, we performed particle-based computer simulations of a gel in contact with a reservoir of salt solution at pH ≃ 7. We showed that the effect of ion partitioning on the ionization degree is negligible, as long as the salinity of the reservoir is high. However, when the reservoir salinity approaches that of fresh water, the ionization degree of weak polyelectrolyte gel is significantly decreased, thus making it a less efficient desalination agent. Furthermore, we demonstrated that the ionization degree of the gel is decreased upon compression. However, up to p...
By this article, we continue studying weak polyelectrolyte hydrogels and their application as a d... more By this article, we continue studying weak polyelectrolyte hydrogels and their application as a desalination agent. We modeled the desalination process as a four-step reversible thermodynamic cycle transferring ions from a low salinity solution to a high salinity one. The cycle implies reversibility at any stage, therefore the method can achieve the maximum thermodynamic efficiency, comparable to the reverse osmosis. As a driving force for ions movement, we use the fact that compression of the gel leads to a decrease in the gel ionization degree, and therefore to a release of ions entrapped in the gel. We considered the gel composed of ionogenic units modified by hydrophobic pendants and showed that this modification significantly increases the number of transferred ions. This increase is caused by a first-order phase transition originated from an interplay between repulsive electrostatic and attractive steric interactions. The transition happens during hydrogel compression. At a ce...
On 16th September 2019 we found a major flaw in the original manuscript, that has been uploaded t... more On 16th September 2019 we found a major flaw in the original manuscript, that has been uploaded to ChemRXiv. To prevent readers from implementing and using a flawed algorithm, we decided to replace the manuscript with this notice. We anticipate to upload a corrected version within few weeks.
Soft Matter
Conformational transitions and nanoscale self-organization triggered in double pH- and thermo-res... more Conformational transitions and nanoscale self-organization triggered in double pH- and thermo-responsive molecular brushes by varying environmental conditions are studied by means of analytical mean-field theory and numerical Scheutjens-Fleer self-consistent field...