Selective solute adsorption and partitioning around single PNIPAM chains

Mechanical deformation affects the counterion condensation in highly-swollen polyelectrolyte hydrogels

Polyelectrolyte gels can generate electric potentials under mechanical deformation. While the underlying mechanism of such a response is often attributed to changes in counterion-condensation levels or alterations in the ionic conditions in the pervaded volume of the hydrogel, the exact molecular origins are largely unknown. By using all-atom molecular dynamics simulations of a polyacrylic acid hydrogel in explicit water as a model system, we simulate the uniaxial compression and uniaxial stretching of weakly to highly swollen (i.e., between 60-90% solvent content) hydrogel networks and calculate the microscopic condensation levels of counterions around the hydrogel chains. The counterion condensation under deformation is highly non-monotonic. Ionic condensation around the constituting chains of the deformed hydrogel tends to increase as the chains are stretched. This increase reaches a maximum and decreases as the chains are strongly stretched. The condensation around the collapsed chains of the hydrogel is weakly affected by the deformation. As a result, both compressing and stretching the model hydrogel lead to an overall increase in the counterion condensation. The effect vanishes for weakly swollen hydrogels, for which most ions are already condensed. The simulations with single, stretched polyelectrolyte chains show a qualitatively similar response, suggesting the effect of chain elongation on the ionic distribution throughout the hydrogel. Notably, this deformation-induced counterion condensation phenomenon does not occur in a polyelectrolyte solution at its critical concentration, indicating the role of hydrogel topology constraining the chain ends. Our results indicate that counterion condensation in a deforming polyelectrolyte hydrogel can be highly heterogeneous and exhibit a rich behaviour of electrostatic responses.

Product from sessile droplet evaporation of PNIPAM/water system above LCST: A block or micro/nano-particles?

PNIPAM as a stimuli-responsive polymer has generated extreme interests due to its versatile applications. However, there is no research report on whether PNIPAM micro/nano-particles can be extracted from its suspension after phase separation. In the present work, LCST-type phase separation in self-synthesized PNIPAM/water system was investigated in depth by dividing the DLS testing process into four stages. In addition to quenching duration, temperature rise process, quenching temperature and PNIPAM concentration all have a great influence on particle size of the suspension. Meanwhile, the steady-state rheology and dynamic viscoelasticity results show that PNIPAM micro/nano-particles in the suspension are "soft" that can deform. Finally, FE-SEM was used to observe the morphology of dehydrated PNIPAM extracted by sessile droplet evaporation under different conditions. The results indicate that these "soft" particles are easier to fuse together, do not have sufficient mechanical strength to maintain their spherical morphology after dehydration. But the above fusion can be suppressed by adjusting evaporation conditions to acquire smaller PNIPAM particles which have sufficient mechanical properties to keep their basic particle morphology. Further, by changing evaporation pressure to positive or negative pressure, dehydrated PNIPAM micro/nano-particles with excellent uniformity and separation can be obtained. This work will provide guidance for extracting micro/nano-particles from polymer/diluent systems with LCST.

Phenol release from pNIPAM hydrogels: scaling molecular dynamics simulations with dynamical density functional theory

We employed molecular dynamic simulations (MD) and the Bennett's acceptance ratio method to compute the free energy of transfer, ΔG_trans, of phenol, methane, and 5-fluorouracil (5-FU), between bulk water and water-pNIPAM mixtures of different polymer volume fractions, ϕ_p. For this purpose, we first calculate the solvation free energies in both media to obtain ΔG_trans. Phenol and 5-FU (a medication used to treat cancer) attach to the pNIPAM surface so that they show negative values of ΔG_trans irrespective of temperature (above or below the lower critical solution temperature of pNIPAM, T_c). Conversely, methane switches the ΔG_trans sign when considering temperatures below (positive) and above (negative) T_c. In all cases, and contrasting with some theoretical predictions, ΔG_trans maintains a linear behavior with the pNIPAM concentration up to large polymer densities. We have also employed MD to compute the diffusion coefficient, D, of phenol in water-pNIPAM mixtures as a function of ϕ_p in the diluted limit. Both ΔG_trans and D as a function of ϕ_p are required inputs to obtain the release halftime of hollow pNIPAM microgels through Dynamic Density Functional Theory (DDFT). Our scaling strategy captures the experimental value of 2200 s for 50 μm radius microgels with no cavity, for ϕ_p ≃ 0.83 at 315 K.

Modeling Solution Behavior of Poly(N-isopropylacrylamide): A Comparison between Water Models

Water is known to play a fundamental role in determining the structure and functionality of macromolecules. The same crucial contribution is also found in the in silico description of polymer aqueous solutions. In this work, we exploit the widely investigated synthetic polymer poly(N-isopropylacrylamide) (PNIPAM) to understand the effect of the adopted water model on its solution behavior and to refine the computational setup. By means of atomistic molecular dynamics simulations, we perform a comparative study of PNIPAM aqueous solution using two advanced water models: TIP4P/2005 and TIP4P/Ice. The conformation and hydration features of an atactic 30-mer at infinite dilution are probed at a range of temperature and pressure suitable to detect the coil-to-globule transition and to map the P–T phase diagram. Although both water models can reproduce the temperature-induced coil-to-globule transition at atmospheric pressure and the polymer hydration enhancement that occurs with increasing pressure, the PNIPAM–TIP4P/Ice solution shows better agreement with experimental findings. This result can be attributed to a stronger interaction of TIP4P/Ice water with both hydrophilic and hydrophobic groups of PNIPAM, as well as to a less favorable contribution of the solvent entropy to the coil-to-globule transition.

Solvation shell thermodynamics of extended hydrophobic solutes in mixed solvents

The ability of various cosolutes and cosolvents to enhance or quench solvent density fluctuations at solute–water interfaces has crucial implications on the conformational equilibrium of macromolecules such as polymers and proteins. Herein, we use an extended hydrophobic solute as a model system to study the effect of urea and methanol on the density fluctuations in the solute’s solvation shell and the resulting thermodynamics. On strengthening the solute–water/cosolute repulsive interaction, we observe distinct trends in the mutual affinities between various species in, and the thermodynamic properties of, the solvation shell. These trends strongly follow the respective trends in the preferential adsorption of urea and methanol: solute–water/cosolute repulsion strengthens, urea accumulation decreases, and methanol accumulation increases. Preferential accumulation of urea is found to quench the density fluctuations around the extended solute, leading to a decrease in the compressibility of the solvation shell. In contrast, methanol accumulation enhances the density fluctuations, leading to an increase in the compressibility. The mode of action of urea and methanol seems to be strongly coupled to their hydration behavior. The observations from this simple model is discussed in relation to urea driven swelling and methanol induced collapse of some well-known thermo-responsive polymers.

Highly Heterogeneous Polarization and Solvation of Gold Nanoparticles in Aqueous Electrolytes

The performance of gold nanoparticles (NPs) in applications depends critically on the structure of the NP-solvent interface, at which the electrostatic surface polarization is one of the key characteristics that affects hydration, ionic adsorption, and electrochemical reactions. Here, we demonstrate significant effects of explicit metal polarizability on the solvation and electrostatic properties of bare gold NPs in aqueous electrolyte solutions of sodium salts of various anions (Cl^-, BF₄^-, PF₆^-, nitrophenolate, and 3- and 4-valent hexacyanoferrate), using classical molecular dynamics simulations with a polarizable core-shell model for the gold atoms. We find considerable spatial heterogeneity of the polarization and electrostatic potentials on the NP surface, mediated by a highly facet-dependent structuring of the interfacial water molecules. Moreover, ion-specific, facet-dependent ion adsorption leads to considerable alterations of the interfacial polarization. Compared to nonpolarizable NPs, surface polarization modifies water local dipole densities only slightly but has substantial effects on the electrostatic surface potentials and leads to significant lateral redistributions of ions on the NP surface. Besides, interfacial polarization effects cancel out in the far field for monovalent ions but not for polyvalent ions, as anticipated from continuum "image-charge" concepts. Far-field effective Debye-Hückel surface potentials change accordingly in a valence-specific fashion. Hence, the explicit charge response of metal NPs is crucial for the accurate description and interpretation of interfacial electrostatics (e.g., for charge transfer and interfacial polarization in catalysis and electrochemistry).

Weak Anion Binding to Poly(N-isopropylacrylamide) Detected by Electrophoretic NMR

Ion specific effects are ubiquitous in solutions and govern a large number of colloidal phenomena. To date, a substantial and sustained effort has been directed at understanding the underlying molecular interactions. As a new approach, we address this issue by sensitive ¹H NMR methods that measure the electrophoretic mobility and the self-diffusion coefficient of poly(N-isopropylacrylamide) (PNIPAM) chains in bulk aqueous solution in the presence of salts with the anion component varied from kosmotropes to chaotropes along the Hofmeister series. The accuracy of the applied electrophoretic NMR experiments is exceptionally high, on the order of 10^-10 m²/(V s), corresponding to roughly 10^-4 elementary charges per monomer effectively associated with the neutral polymer. We find that chaotropic anions associate to PNIPAM with an apparent Langmuir-type saturation behavior. The polymer chains remain extended upon ion association, and momentum transfer from anion to polymer is only partial which indicates weak attractive short-range forces between anion and polymer and, thereby and in contrast to some other ion-polymer systems, the lack of well-defined binding sites.

How the Shape and Chemistry of Molecular Penetrants Control Responsive Hydrogel Permeability

The permeability of hydrogels for the selective transport of molecular penetrants (drugs, toxins, reactants, etc.) is a central property in the design of soft functional materials, for instance in biomedical, pharmaceutical, and nanocatalysis applications. However, the permeation of dense and hydrated polymer membranes is a complex multifaceted molecular-level phenomenon, and our understanding of the underlying physicochemical principles is still very limited. Here, we uncover the molecular principles of permeability and selectivity in hydrogel permeation. We combine the solution-diffusion model for permeability with comprehensive atomistic simulations of molecules of various shapes and polarities in a responsive hydrogel in different hydration states. We find in particular that dense collapsed states are extremely selective, owing to a delicate balance between the partitioning and diffusivity of the penetrants. These properties are sensitively tuned by the penetrant size, shape, and chemistry, leading to vast cancellation effects, which nontrivially contribute to the permeability. The gained insights enable us to formulate semiempirical rules to quantify and extrapolate the permeability categorized by classes of molecules. They can be used as approximate guiding ("rule-of-thumb") principles to optimize penetrant or membrane physicochemical properties for a desired permeability and membrane functionality.

Contact Ion Pairs in the Bulk Affect Anion Interactions with Poly(N-isopropylacrylamide)

Salt effects on the solubility of uncharged polymers in aqueous solutions are usually dominated by anions, while the role of the cation with which they are paired is often ignored. In this study, we examine the influence of three aqueous metal iodide salt solutions (LiI, NaI, and CsI) on the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) by measuring the turbidity change of the solutions. Weakly hydrated anions, such as iodide, are known to interact with the polymer and thereby lead to salting-in behavior at low salt concentration followed by salting-out behavior at higher salt concentration. When varying the cation type, an unexpected salting-out trend is observed at higher salt concentrations, Cs⁺ > Na⁺ > Li⁺. Using molecular dynamics simulations, it is demonstrated that this originates from contact ion pair formation in the bulk solution, which introduces a competition for iodide ions between the polymer and cations. The weakly hydrated cation, Cs⁺, forms contact ion pairs with I^- in the bulk solution, leading to depletion of CsI from the polymer-water interface. Microscopically, this is correlated with the repulsion of iodide ions from the amide moiety.

Ion-Specific Adsorption on Bare Gold (Au) Nanoparticles in Aqueous Solutions: Double-Layer Structure and Surface Potentials

We study the solvation and electrostatic properties of bare gold (Au) nanoparticles (NPs) of 1-2 nm in size in aqueous electrolyte solutions of sodium salts of various anions with large physicochemical diversity (Cl^-, BF₄^-, PF₆^-, Nip^- (nitrophenolate), 3- and 4-valent hexacyanoferrate (HCF)) using nonpolarizable, classical molecular dynamics computer simulations. We find a substantial facet selectivity in the adsorption structure and spatial distribution of the ions at the AuNPs: while sodium and some of the anions (e.g., Cl^-, HCF^3-) adsorb more at the "edgy" (100) and (110) facets of the NPs, where the water hydration structure is more disordered, other ions (e.g., BF₄^-, PF₆^-, Nip^-) prefer to adsorb strongly on the extended and rather flat (111) facets. In particular, Nip^-, which features an aromatic ring in its chemical structure, adsorbs strongly and perturbs the first water monolayer structure on the NP (111) facets substantially. Moreover, we calculate adsorptions, radially resolved electrostatic potentials as well as the far-field effective electrostatic surface charges and potentials by mapping the long-range decay of the calculated electrostatic potential distribution onto the standard Debye-Hückel form. We show how the extrapolation of these values to other ionic strengths can be performed by an analytical Adsorption-Grahame relation between the effective surface charge and potential. We find for all salts negative effective surface potentials in the range from -10 mV for NaCl down to about -80 mV for NaNip, consistent with typical experimental ranges for the zeta potential. We discuss how these values depend on the surface definition and compare them to the explicitly calculated electrostatic potentials near the NP surface, which are highly oscillatory in the ±0.5 V range.

Designing Solute-Tailored Selectivity in Membranes: Perspectives for Water Reuse and Resource Recovery

Treatment of nontraditional source waters (e.g., produced water, municipal and industrial wastewaters, agricultural runoff) offers exciting opportunities to expand water and energy resources via water reuse and resource recovery. While conventional polymer membranes perform water/ion separations well, they do not provide solute-specific separation, a key component for these treatment opportunities. Herein, we discuss the selectivity limitations plaguing all conventional membranes, which include poor removal of small, neutral solutes and insufficient discrimination between ions of the same valence. Moreover, we present synthetic approaches for solute-tailored selectivity including the incorporation of single-digit nanopores and solute-selective ligands into membranes. Recent progress in these areas highlights the need for fundamental studies to rationally design membranes with selective moieties achieving desired separations.

SJ, van der Vegt NFA. Characterizing Polymer Hydration Shell Compressibilities with the Small-System Method

The small-system method (SSM) exploits the unique feature of finite-sized open systems, whose thermodynamic quantities scale with the inverse system size. This scaling enables the calculation of properties in the thermodynamic limit of macroscopic systems based on computer simulations of finite-sized systems. We herein extend the SSM to characterize the hydration shell compressibility of a generic hydrophobic polymer in water. By systematically increasing the strength of polymer-water repulsion, we find that the excess inverse thermodynamic correction factor (Δ1/Γs∞) and compressibility (Δχs) of the first hydration shell change sign from negative to positive. This occurs with a concurrent decrease in water hydrogen bonding and local tetrahedral order of the hydration shell water. The crossover lengthscale corresponds to an effective polymer bead diameter of 0.7 nm and is consistent with previous works on hydration of small and large hydrophobic solutes. The crossover lengthscale in polymer hydration shell compressibility, herein identified with the SSM approach, relates to hydrophobic interactions and macromolecular conformational equilibria in aqueous solution. The SSM approach may further be applied to study thermodynamic properties of polymer solvation shells in mixed solvents.

Aqueous Nanoclusters Govern Ion Partitioning in Dense Polymer Membranes

The uptake and sorption of charged molecules by responsive polymer membranes and hydrogels in aqueous solutions is of key importance for the development of soft functional materials. Here, we investigate the partitioning of simple monatomic (Na⁺, K⁺, Cs⁺, Cl^-, I^-) and one molecular ion (4-nitrophenolate; NP^-) within a dense, electroneutral poly(N-isopropylacrylamide) membrane using explicit-water molecular dynamics simulations. Inside the predominantly hydrophobic environment, water distributes in a network of polydisperse water nanoclusters. The average cluster size determines the mean electrostatic self-energy of the simple ions, which preferably reside deeply inside them; we therefore find substantially larger partition ratios K ≃10^-1 than expected from a simple Born picture using a uniform dielectric constant. Despite their irregular shapes, we observe that the water clusters possess a universal negative electrostatic potential with respect to their surroundings, as is known for aqueous liquid-vapor interfaces. This potential, which we find concealed in cases of symmetric monatomic salts, can dramatically impact the transfer free energies of larger charged molecules because of their weak hydration and increased affinity to interfaces. Consequently, and in stark contrast to the simple ions, the molecular ion NP^- can have a partition ratio much larger than unity, K ≃10-30 (depending on the cation type) or even 10³ in excess of monovalent salt, which explains recent observations of enhanced reaction kinetics of NP^- reduction catalyzed within dense polymer networks. These results also suggest that ionizing a molecule can even enhance the partitioning in a collapsed, rather hydrophobic gel, which strongly challenges the traditional simplistic reasoning.

Transfer Free Energies and Partitioning of Small Molecules in Collapsed PNIPAM Polymers

A central quantity in the design of functional hydrogels used as nanocarrier systems, for instance, for drug delivery or adaptive nanocatalysis, is the partition ratio, which quantifies the uptake of a molecular substance by the polymer matrix. By employing all-atom molecular dynamics simulations, we study the solvation and partitioning (with respect to bulk water) of small subnanometer-sized solutes in a dense matrix of collapsed poly( N-isopropylacrylamide) (PNIPAM) polymers above the lower critical solution temperature in aqueous solution. We examine the roles of the solute's polarity and its size on the solubility properties in the thermoresponsive polymer. We show that the transfer free energies of nonpolar solutes from bulk water into the polymer are favorable and scale in a good approximation with the solute's surface area. Even for small solute size variation, partitioning can vary over orders of magnitude. A polar nature of the solute, on the other hand, generally opposes the transfer, at least for alkyl solutes. Finally, we find a strong correlation between the transfer free energies in the gel and the adsorption free energies on a single extended polymer chain, which enables us to relate the partition ratios in the swollen and collapsed state of a PNIPAM gel.

Cross-linker effect on solute adsorption in swollen thermoresponsive polymer networks

Molecular dynamics study on the solute adsorption to thermoresponsive polymers estimating the cross-link impact on particle partitioning in swollen hydrogels.

P-NIPAM in water–acetone mixtures: experiments and simulations

The lower critical solution temperature (LCST) of poly-N-isopropylacrylamide (p-NIPAM) diminishes when a small volume of acetone is added to the aqueous polymer solution, and then increases for further additions, producing a minimum at a certain acetone concentration.

Aqueous ionic liquids in comparison with standard co-solutes : Differences and common principles in their interaction with protein and DNA structures

Ionic liquids (ILs) are versatile solvents for a broad range of biotechnological applications. Recent experimental and simulation results highlight the potential benefits of dilute ILs in aqueous solution (aqueous ILs) in order to modify protein and DNA structures systematically. In contrast to a limited number of standard co-solutes like urea, ectoine, trimethylamine-N-oxide (TMAO), or guanidinium chloride, the large amount of possible cation and anion combinations in aqueous ILs can be used to develop tailor-made stabilizers or destabilizers for specific purposes. In this review article, we highlight common principles and differences between aqueous ILs and standard co-solutes with a specific focus on their underlying macromolecular stabilization or destabilization behavior. In combination with statistical thermodynamics theories, we present an efficient framework, which is used to classify structure modification effects consistently. The crucial importance of enthalpic and entropic contributions to the free energy change upon IL-assisted macromolecular unfolding in combination with a complex destabilization mechanism is described in detail. A special focus is also set on aqueous IL-DNA interactions, for which experimental and simulation outcomes are summarized and discussed in the context of previous findings.

Catalysis by Metallic Nanoparticles in Solution: Thermosensitive Microgels as Nanoreactors

Metallic nanoparticles have been used as catalysts for various reactions, and the huge literature on the subject is hard to overlook. In many applications, the nanoparticles must be affixed to a colloidal carrier for easy handling during catalysis. These “passive carriers” (e.g. dendrimers) serve for a controlled synthesis of the nanoparticles and prevent coagulation during catalysis. Recently, hybrids from nanoparticles and polymers have been developed that allow us to change the catalytic activity of the nanoparticles by external triggers. In particular, single nanoparticles embedded in a thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have become the most-studied examples of such hybrids: immersed in cold water, the PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic substrates can diffuse easily through the network, and react at the surface of the nanoparticles. Above the volume transition located at 32°C, the network becomes hydrophobic and shrinks. Now hydrophobic substrates will preferably diffuse through the network and react with other substrates in the reaction catalyzed by the enclosed nanoparticle. Such “active carriers”, may thus be viewed as true nanoreactors that open new ways for the use of nanoparticles in catalysis. In this review, we give a survey on recent work done on these hybrids and their application in catalysis. The aim of this review is threefold: we first review hybrid systems composed of nanoparticles and thermosensitive networks and compare these “active carriers” to other colloidal and polymeric carriers (e.g. dendrimers). In a second step we discuss the model reactions used to obtain precise kinetic data on the catalytic activity of nanoparticles in various carriers and environments. These kinetic data allow us to present a fully quantitative comparison of different nanoreactors. In a final section we shall present the salient points of recent efforts in the theoretical modeling of these nanoreactors. By accounting for the presence of a free-energy landscape for the reactants’ diffusive approach towards the catalytic nanoparticle, arising from solvent-reactant and polymeric shell-reactant interactions, these models are capable of explaining the emergence of all the important features observed so far in studies of nanoreactors. The present survey also suggests that such models may be used for the design of future carrier systems adapted to a given reaction and solvent.

Maximizing the absorption of small cosolutes inside neutral hydrogels: steric exclusion versus hydrophobic adhesion

In this work the equilibrium absorption of nanometric cosolutes (which could represent drugs, reactants, small globular proteins and other kind of biomacromolecules) inside neutral hydrogels is studied. We specially focus on exploring, for different swelling states, the competition between the steric exclusion induced by the cross-linked polymer network constituting the hydrogel, and the solvent-induced short-range hydrophobic attraction between the polymer chains and the cosolute particle. For this purpose, the cosolute partition coefficient is calculated by means of coarse-grained grand canonical Monte Carlo simulations, and the results are compared to theoretical predictions based on the calculation of the excluded and binding volume around the polymer chains. For small hydrophobic attractions or large cosolute sizes, the steric repulsion dominates, and the partition coefficient decreases monotonically with the polymer volume fraction, ϕ_m. However, for large enough hydrophobic attraction strength, the interplay between hydrophobic adhesion and the steric exclusion leads to a maximum in the partition coefficient at certain intermediate polymer density. Good qualitative and quantitative agreement is achieved between simulation results and theoretical predictions in the limit of small ϕ_m, pointing out the importance of geometrical aspects of the cross-linked polymer network, even for hydrogels in the swollen state. In addition, the theory is able to predict analytically the onset of the maximum formation in terms of the details of the cosolute-monomer pair interaction, in good agreement with simulations too. Finally, the effect of the many-body attractions between the cosolute and multiple polymer chains is quantified. The results clearly show that these many-body attractions play a very relevant role determining the cosolute binding, enhancing its absorption in more than one order of magnitude.

Catalyzed Bimolecular Reactions in Responsive Nanoreactors

We describe a general theory for surface-catalyzed bimolecular reactions in responsive nanoreactors, catalytically active nanoparticles coated by a stimuli-responsive "gating" shell, whose permeability controls the activity of the process. We address two archetypal scenarios encountered in this system: the first, where two species diffusing from a bulk solution react at the catalyst's surface, and the second, where only one of the reactants diffuses from the bulk while the other is produced at the nanoparticle surface, e.g., by light conversion. We find that in both scenarios the total catalytic rate has the same mathematical structure, once diffusion rates are properly redefined. Moreover, the diffusional fluxes of the different reactants are strongly coupled, providing a behavior richer than that arising in unimolecular reactions. We also show that, in stark contrast to bulk reactions, the identification of a limiting reactant is not simply determined by the relative bulk concentrations but is controlled by the nanoreactor shell permeability. Finally, we describe an application of our theory by analyzing experimental data on the reaction between hexacyanoferrate(III) and borohydride ions in responsive hydrogel-based core-shell nanoreactors.

Preferential adsorption of the additive is not a prerequisite for cononsolvency in water-rich mixtures

NMR studies reveal the distinct molecular interactions accounting for cononsolvency.