Explaining Giant Apparent pKA\mathrm{p}K_\mathrm{A} Shifts in Weak Polyelectrolyte Brushes

Recent experiments on weak polyelectrolyte brushes found marked shifts in the effective p$K_\mathrm{A}$ that are linear in the logarithm of the salt concentration. Comparing explicit-particle simulations with mean-field calculations we show that for high grafting densities the salt concentration effect can be explained using the ideal Donnan theory, but for low grafting densities the full shift is due to a combination of the Donnan effect and the polyelectrolyte effect. The latter originates from electrostatic correlations which are neglected in the Donnan picture and which are only approximately included in the mean-field theory. Moreover, we demonstrate that the magnitude of the polyelectrolyte effect is almost invariant with respect to salt concentration but depends on the grafting density of the brush. This invariance is due to a complex cancellation of multiple effects. Based on our results, we show how the experimentally determined p$K_\mathrm{A}$ shifts may be used to infer the grafting density of brushes, a parameter that is difficult to measure directly.Comment: First revised version of the manuscript; 15 pages, 16 figure

Desalination using polyelectrolyte hydrogels

When the gel is put into contact with aqueous salt solution, it absorbs a solution with the ion composition different from the original one. The absorbed solution can be easily squeezed out from the gel by means of sieve or microfiltration membrane. In our previous work we proposed a fully reversible desalination cycle made of compression and swelling steps, which can in principle work on ideal thermodynamic efficiency. In this work we simulate the desalination process using theoretical and coarse-grained models of gel and prove the concept by experiment. We used Monte Carlo and molecular dynamics molecular simulations in the reaction ensemble to predict the degree of ionization of the weak polyelectrolyte hydrogel when it is put in contact with salt solution, and calculate the salt partitioning between the gel and bulk salt solution. We constructed laboratory apparatus based on swelling and pressing cycles of the gel. First, we let the polyelectrolyte gel swell with salt solution of defined concentration. Then we press the gel and the liquid is released. Due to the ion exchange in polyelectrolyte hydrogel, this released liquid has lower salt concentration than the initial one. We measure the salinity of the solution before and after this procedure and we compare the results with theoretically obtained salt partitioning. We also measure the pressure applied on the gel and corresponding gel volume and compare these results with respective computational results

Intramolekulární samoorganizované struktury v amfifilních kopolymerech

59 ËÙÑÑ ÖÝ In this thesis we have presented results of our simulation studies concerning the intramolecular morphology of amphiphilic polymers. Amphiphilic polymers are those which contain two or more types of monomer units with different affinity for the solvent. Using the common terminology of polymer physics, we say that such polymers are in the selective solvent, i. e. such that it is poor for one type of units and good for the other type. In our definition of amphiphilic polymers we also include polyelectrolytes in which the good solvent conditions can be induced by the electrostatic repulsion among the charged monomer units. In our studies we have investigated both polyelectrolytes and neutral polymers of linear as well as branched topologies. While the poorly soluble monomer units have the tendency to collapse into compact globular structures, this tendency is counteracted by the excluded vol- ume repulsion among the well-soluble units. Equilibrium is attained when the two tendencies are balanced which often results in the so-called pearl-necklace confor- mation consisting of both collapsed domains (pearls) and stretched ones (strings). This behaviour has been first predicted for linear polyelectrolytes in poor solvents by Dobrynin and coworkers [8]. Later on similar structures have been predicted by...61 Ë ÖÒÙØ Tato práce obsahuje výsledky simulačních sutdií, které se zabývají intramolekul- ární morfologií amfifilních polymerů. Pod pojmem amfifilní se rozumí takový poly- mer, který obsahuje dva nebo více typů monomerních jednotek s různou afinitou vůči rozpouštědlu. V rámci polymerní fyziky pak mluvíme o tzv. selektivním rozpuštědle, t.j. takovém, které je dobré pro jeden druh monomerních jednotek a špatné pro druhý. V tomto textu budeme mezi amfifilní polymery zahrnovat i polyelektrolyty, u kterých jsou podmínky dobrého rozpouštědla dosaženy nepřímo skrze repulzi nabitých monomerních jendotek. V našich studiích jsme se zabývali jak polyelektrolyty, tak neutrálními polymery, které měly lineární nebo větvenou strukturu. Špatně rozpustné monomerní jednotky mají tendenci kolabovat a tvořit kom- paktní globulární struktury. Proti této tendenci působí repulze pocházející od vyloučeného objemu dobře rozpustných monomerních jednotek. V rovnováze jsou tyto dvě tendence navzájem vykompenzovány, přičemž často vznikají tzv. struk- tury perlového náhrdelníku (anglicky pearl-necklace structures). Tyto struktury obsahují jak kolabované domény (perličky) tak i natažené (řetízky). Tento typ chování poprvé popsali Dobrynin a kol. [8]. Později byl vznik podobných struktur předpovězen různými autory pro celou škálu rozmanitých...Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemieFaculty of SciencePřírodovědecká fakult

Intramolecular self-organised structures in amphiphilic (co)polymers

59 ËÙÑÑ ÖÝ In this thesis we have presented results of our simulation studies concerning the intramolecular morphology of amphiphilic polymers. Amphiphilic polymers are those which contain two or more types of monomer units with different affinity for the solvent. Using the common terminology of polymer physics, we say that such polymers are in the selective solvent, i. e. such that it is poor for one type of units and good for the other type. In our definition of amphiphilic polymers we also include polyelectrolytes in which the good solvent conditions can be induced by the electrostatic repulsion among the charged monomer units. In our studies we have investigated both polyelectrolytes and neutral polymers of linear as well as branched topologies. While the poorly soluble monomer units have the tendency to collapse into compact globular structures, this tendency is counteracted by the excluded vol- ume repulsion among the well-soluble units. Equilibrium is attained when the two tendencies are balanced which often results in the so-called pearl-necklace confor- mation consisting of both collapsed domains (pearls) and stretched ones (strings). This behaviour has been first predicted for linear polyelectrolytes in poor solvents by Dobrynin and coworkers [8]. Later on similar structures have been predicted by..

Intramolekulární samoorganizované struktury (ko) polymerů

This dissertation presents an overview of our computer simulation studies of the conformational behaviour of amphiphilic polymers. In the studies we showed that the polymers form a variety of self-organised structures at a single molecule level. These include the pearl-necklace structure, spherical and cylindrical intramolecular micelles and bundles. From the simulations we were able to obtain a deeper insight into the behaviour of amphiphilic polymers and conditions under which the above-mentioned structures are formed. Some of the results were compared to experimental or theoretical studies of similar polymers. In some cases the simulations confirmed earlier interpretations which were based on analogy or intuition. In other cases they revealed new phenomena which had not been considered before.Tato dizertační práce představuje shrnutí studií konformačního chování amfifilních polymerů, provedených s použitím počítačových simulací. V těchto studiích jsme ukázali, že amfifilní polymery tvoří řadu samoorganizovaných struktur na úrovni jediné molekuly. Jedná se zejémna o strukturu perlového náhrdelníku, sférické a cylindrické intramolekulární micely a svazky řetězců. Ná základě provedených simulací jsme získali hlubší pochopení chování studovaných polymerů a podmínek, za jakých vznikají jmenované struktury. Některé z výsledků byly srovnávány s experimentálními či teoretickými předpověďmi pro podobné polymery. V některých případech výsledky simulací potvrdily dřívější interpretace, které byly založené na analogii či intuici. V jiných případech ukázaly simulace nové vlastnosti, které předtím vůbec nebyly předpovězeny.Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemieFaculty of SciencePřírodovědecká fakult

Intramolecular self-organised structures in amphiphilic (co)polymers

This dissertation presents an overview of our computer simulation studies of the conformational behaviour of amphiphilic polymers. In the studies we showed that the polymers form a variety of self-organised structures at a single molecule level. These include the pearl-necklace structure, spherical and cylindrical intramolecular micelles and bundles. From the simulations we were able to obtain a deeper insight into the behaviour of amphiphilic polymers and conditions under which the above-mentioned structures are formed. Some of the results were compared to experimental or theoretical studies of similar polymers. In some cases the simulations confirmed earlier interpretations which were based on analogy or intuition. In other cases they revealed new phenomena which had not been considered before

Intramolecular self-organised structures in amphiphilic (co)polymers

This dissertation presents an overview of our computer simulation studies of the conformational behaviour of amphiphilic polymers. In the studies we showed that the polymers form a variety of self-organised structures at a single molecule level. These include the pearl-necklace structure, spherical and cylindrical intramolecular micelles and bundles. From the simulations we were able to obtain a deeper insight into the behaviour of amphiphilic polymers and conditions under which the above-mentioned structures are formed. Some of the results were compared to experimental or theoretical studies of similar polymers. In some cases the simulations confirmed earlier interpretations which were based on analogy or intuition. In other cases they revealed new phenomena which had not been considered before

Thermodynamic model for a reversible desalination cycle using weak polyelectrolyte hydrogels

International audienceThe recently proposed use of hydrogels for water desalination is based on the decrease of salt concentration in the gel upon compression. In the first experiments, desalination cycles using hydrogels involved an irreversible mixing step, which inevitably reduced the thermodynamic efficiency. This approach could become competitive with membrane-based desalination methods if it could work close to maximum thermodynamic efficiency. In this work, we develop a thermodynamic model for compression of weak polyelectrolyte hydrogels in open and closed systems. We use this model to design a fully reversible desalination cycle which can, in principle, achieve maximum thermodynamic efficiency. We also show that compressing weak polyelectrolyte hydrogels at low salinity decreases their ionization, thereby leading to a non-monotonic dependence of salt concentration on the gel compression. Therefore, our model shows how to redesign the desalination cycle when using weak polyelectrolytes at low salinities