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Subject: search.filters.subject.530Author: search.filters.author.Holm, ChristianPublication Type: search.filters.UBSTyp.Zeitschriftenartikel

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    Charge regulation and swelling of weak polyelectrolyte nanogels in divalent salt solutions
    (2026) Beyer, David; Holm, Christian
    We use computer simulations to investigate the behavior of a weak polyelectrolyte nanogel in a solution containing divalent salt. In our simulations, we systematically vary the bulk pH value and the bulk concentration of divalent salt, allowing us to study the influence of charge regulation and divalent ions on the ionization behavior, ion partitioning, and nanogel swelling. With regard to the ionization behavior, we observe that, with an increasing concentration of divalent salt, the suppression of ionization becomes weaker. Moreover, we find that the strongly non‐uniform ionization profile observed in the absence of divalent counterions becomes increasingly uniform as the concentration of divalent salt is increased. We also study the partitioning of monovalent and divalent counterions between the nanogel and the bulk solution; our analysis shows that the uptake of divalent ions may be enhanced by up to tenfold as compared to the mean‐field prediction. Finally, we consider the influence of divalent ions on the pH‐dependent swelling behavior of the nanogel. Here, we observe a two‐stage swelling driven by charge regulation and ion partitioning. Overall, our results highlight the complex interplay of ionization equilibria, valency effects, and ion partitioning in weak polyelectrolyte systems.
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    Overview: the Janus-nature of molecular CO2 in charge adjustment at wet surfaces
    (2026) Vogel, Peter; Qaisrani, Muhammad Nawaz; Rasenat, Mattis; Lützenkirchen, Johannes; Sulpizi, Marialore; Beyer, David; Holm, Christian; Palberg, Thomas
    Molecular CO2 readily dissolves in aqueous electrolyte solutions and partially dissociating to form carbonic acid. The decharging effects of the dissociation products mediated by the ensuing pH-shift and the additional salinity are well established. However, the effects of dissolved molecular CO2 have not been studied systematically. We summarize recent and novel investigations on the role of CO2 regarding charge control at surfaces submersed in aqueous electrolytes. In our electrokinetic and conductometric measurements on representative surfaces, we took special care to control and monitor the electrolyte composition in situ. We discriminate the effects of molecular and dissociated CO2via control experiments using HCl. Depending on the surface under investigation and the charging mechanisms involved, we find that molecular CO2 assists either charging, de-charging and/or recharging. This contrasting charge regulating behaviour reveals the Janus nature of dissolved molecular CO2 with respect to charge control at wet surfaces. In our complementary molecular dynamics simulations, Q4 silica and 9% ionized Q3 silica surfaces are studied as hydrophobic/hydrophilic, respectively charged/uncharged, analogues, as well as uncharged Q3 silica and molecularly rough Isoleucin-coated quartz surfaces. In all cases, we find that the charge-neutral CO2 molecule physisorbs in a thin diffusive layer close to the surface, which leads to pronounced re-structuring of the electric double layer. Based on this result, we suggest to interpret the experimentally observed Janus nature of molecular CO2 in terms of a local decrease of the dielectric permittivity. This in turn leads to a local strengthening of electrostatic interactions dominating the double layer structure next to charged surfaces. Specifically, we propose that CO2 induces a dielectric charge regulation for weakly acidic surface groups, assists the incorporation of OH− into the H-bond network at smooth inert surfaces, and induces significant ion-correlations promoting co-ion binding. Overall, we demonstrate that molecular CO2 allows for a controlled charge-adjustment in opposing directions. We anticipate that our findings on the one hand provide substantial challenges for analytical or numerical modelling as well as for controlled experimental work, but on the other hand bear important practical implications for applications ranging from desalination to bio-membranes.
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    MDSuite : comprehensive post-processing tool for particle simulations
    (2023) Tovey, Samuel; Zills, Fabian; Torres-Herrador, Francisco; Lohrmann, Christoph; Brückner, Marco; Holm, Christian
    Particle-Based (PB) simulations, including Molecular Dynamics (MD), provide access to system observables that are not easily available experimentally. However, in most cases, PB data needs to be processed after a simulation to extract these observables. One of the main challenges in post-processing PB simulations is managing the large amounts of data typically generated without incurring memory or computational capacity limitations. In this work, we introduce the post-processing tool: MDSuite. This software, developed in Python, combines state-of-the-art computing technologies such as TensorFlow, with modern data management tools such as HDF5 and SQL for a fast, scalable, and accurate PB data processing engine. This package, built around the principles of FAIR data, provides a memory safe, parallelized, and GPU accelerated environment for the analysis of particle simulations. The software currently offers 17 calculators for the computation of properties including diffusion coefficients, thermal conductivity, viscosity, radial distribution functions, coordination numbers, and more. Further, the object-oriented framework allows for the rapid implementation of new calculators or file-readers for different simulation software. The Python front-end provides a familiar interface for many users in the scientific community and a mild learning curve for the inexperienced. Future developments will include the introduction of more analysis associated with ab-initio methods, colloidal/macroscopic particle methods, and extension to experimental data.
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    PDADMAC/PSS oligoelectrolyte multilayers : internal structure and hydration properties at early growth stages from atomistic simulations
    (2020) Sánchez, Pedro A.; Vögele, Martin; Smiatek, Jens; Qiao, Baofu; Sega, Marcello; Holm, Christian
    We analyze the internal structure and hydration properties of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) oligoelectrolyte multilayers at early stages of their layer-by-layer growth process. Our study is based on large-scale molecular dynamics simulations with atomistic resolution that we presented recently [Sánchez et al., Soft Matter 2019, 15, 9437], in which we produced the first four deposition cycles of a multilayer obtained by alternate exposure of a flat silica substrate to aqueous electrolyte solutions of such polymers at 0.1M of NaCl. In contrast to any previous work, here we perform a local structural analysis that allows us to determine the dependence of the multilayer properties on the distance to the substrate. We prove that the large accumulation of water and ions next to the substrate observed in previous overall measurements actually decreases the degree of intrinsic charge compensation, but this remains as the main mechanism within the interface region. We show that the range of influence of the substrate reaches approximately 3 nm, whereas the structure of the outer region is rather independent from the position. This detailed characterization is essential for the development of accurate mesoscale models able to reach length and time scales of technological interest.
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    Self- and Fick diffusion coefficients in implicit solvent simulations : influence of local aggregation effects and thermodynamic factors
    (2025) Tovey, Samuel; Holm, Christian; Smiatek, Jens
    In this article, we discuss the relationship and transition between self- and Fick diffusion coefficients in continuous implicit solvents across different particle densities. By applying the established expressions for self-diffusion and Fick diffusion coefficients in binary solutions, we analyze how the local environment influences diffusion through thermodynamic factors, which can be readily evaluated within the framework of Kirkwood-Buff (KB) theory. These thermodynamic factors, originally defined as derivatives of thermodynamic activity, vary with changes in local particle densities, particularly in the presence of aggregation effects. Consequently, the transition from self- to Fick diffusion coefficients can be understood as a reflection of variations in these thermodynamic factors. Langevin Dynamics simulations at low number densities show excellent agreement with the analytical expressions derived. Overall, our findings provide deeper insight into how local structural environments shape particle dynamics, clarifying the connection between KB theory and the transition from self- to Fick diffusion coefficients.
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    Efficient algorithms for electrostatic interactions including dielectric contrasts
    (2013) Arnold, Axel; Breitsprecher, Konrad; Fahrenberger, Florian; Kesselheim, Stefan; Lenz, Olaf; Holm, Christian
    Coarse grained models of soft matter are usually combined with implicit solvent models that take the electrostatic polarizability into account via a dielectric background. In biophysical or nanoscale simulations that include water, this constant can vary greatly within the system. Performing molecular dynamics or other simulations that need compute exact electrostatic interactions between charges in those systems is computationally demanding. We review here several algorithms developped by us that perform exactly this task. For planar dielectric surfaces in partial periodic boundary conditions, the arising image charges can be either treated with the MMM2D algorithm in a very efficient and accurate way, or with the ELC term that enables the user to use his favorite 3D periodic Coulomb solver . Arbitrarily shaped interfaces can be dealt with using induced surface charges with the ICC algorithm. Finally, the local electrostatics algorithm MEMD (Maxwell Equations Molecular Dynamics) allows even to employ a smoothly varying dielectric constant in the systems. We introduce the concepts of these three algorithms, and an extension for the inclusion of boundaries that are to be held fixed at constant potential (metal conditions). For each method, we present a showcase application to highlight the importance of dielectric interfaces.
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    An extensible lattice Boltzmann method for viscoelastic flows : complex and moving boundaries in Oldroyd-B fluids
    (2021) Kuron, Michael; Stewart, Cameron; de Graaf, Joost; Holm, Christian
    Most biological fluids are viscoelastic, meaning that they have elastic properties in addition to the dissipative properties found in Newtonian fluids. Computational models can help us understand viscoelastic flow, but are often limited in how they deal with complex flow geometries and suspended particles. Here, we present a lattice Boltzmann solver for Oldroyd-B fluids that can handle arbitrarily shaped fixed and moving boundary conditions, which makes it ideally suited for the simulation of confined colloidal suspensions. We validate our method using several standard rheological setups and additionally study a single sedimenting colloid, also finding good agreement with the literature. Our approach can readily be extended to constitutive equations other than Oldroyd-B. This flexibility and the handling of complex boundaries hold promise for the study of microswimmers in viscoelastic fluids.
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    Triggered dissolution of electrostatically crosslinked hydrogels from star‐shaped polyampholytic block copolymers
    (2026) Grün, Jonas Julius; Beyer, David; Mons, Peter Johannes; Seitel, Sebastian; Fribiczer, Nora; Poudel, Purushottam; Könemann, Nicklas; Zank, Lynn Kendra Renate Jagna; Zylla, Paul Fabio; Košovan, Peter; Seiffert, Sebastian; Holm, Christian; Schacher, Felix Helmut
    We explore the possible reversible formation of hydrogels through electrostatic interactions between four‐arm star‐shaped block copolymers, consisting of a polyethylene glycol (PEG) inner block and either an anionic polystyrene sulfonate [PEG27‐b‐PSS108]4 or a zwitterionic polybetaine [PEG27‐b‐PCBMAAm110]4 as outer block. The combination of both can induce attractive or repulsive electrostatic interactions depending on the solution pH value and ionic strength. The polymers were synthesized using controlled atom transfer radical polymerization (ATRP). The charge of [PEG27‐b‐PCBMAAm110]4 was further investigated by potentiometric titration and zeta potential measurements. Using oscillatory shear rheology, we demonstrated the required conditions for hydrogel formation. Stable hydrogel formation is observed within a wide pH range (6.8 -9.5), corresponding to the protonation states of the carboxylic acid groups that facilitate electrostatic interactions. We also showed how the hydrogel stability is influenced by parameters like block copolymer concentration and ionic strength. Coarse‐grained simulations provided molecular‐scale insights, revealing charge regulation effects and the energetic favorability of electrostatic complexation up to high pH values. Overall, our results demonstrated the key design principles, as the polyelectrolyte length, ionic strength, and charge regulation effects, for the formation of partially reversible hydrogels, triggered by changes in the solution pH. Furthermore, we showed that understanding the desired conditions for hydrogel formation requires a combination of experimental characterization with modeling approaches.
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    A sequence-specific theory for charge-regulating IDPs
    (2026) Beyer, David; Holm, Christian; Wang, Zhen-Gang
    Intrinsically disordered proteins are a notable class of biological polymers whose physicochemical properties and biological functions are determined by an intricate interplay of chain connectivity, electrostatic interactions, the sequence of residues, and nonuniversal short-range interactions. An important phenomenon in these molecules is charge regulation, which arises from weakly acidic and basic residues, but is often neglected in theoretical descriptions. In this work, we use the Edwards-Singh variational method to derive an approximate theory that describes sequence effects in the charge regulation and chain conformation of intrinsically disordered proteins. The main result of our theory is a set of coupled algebraic equations yielding a renormalized Kuhn length and residue-specific mean-fields that determine the respective ionization states of the residues. We discuss limiting cases of these equations that underline the internal consistency of our theory and connect our results to earlier studies. To solve the full set of equations, we propose a simple numerical scheme. As test cases, we calculate the conformation and ionization state of a weak polyelectrolyte and of 30 sequence variants of the polypeptide (EK)25. For the weak polyelectrolyte, we show that the theory predicts phenomena such as the overall suppressed ionization due to electrostatic interactions and the enhanced ionization at the chain ends. In the case of the polypeptide (EK)25, we find strong effects of the sequence on ionization, with well-mixed sequences exhibiting a broad pH range where the polypeptide is net neutral, while blockier sequences exhibit a steeper ionization response. Moreover, we also observe pronounced sequence effects on the swelling behavior of the chains.
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    The presence of a wall enhances the probability for ring‐closing metathesis : insights from classical polymer theory and atomistic simulations
    (2020) Tischler, Ingo; Schlaich, Alexander; Holm, Christian
    The probability distribution of chain ends meeting when one end of the polymer is fixed to a certain distance to a reflecting wall is investigated. For an ideal polymer chain the probability distribution can be evaluated analytically via classic polymer theory. These analytical predictions are compared to atomistic MD simulations of one tethered alkane chain close to the wall. The results demonstrate that a confining wall can lead to a significant increase in the return probability for the chain ends, and thus, can increase the occurrence of ring‐closing reactions. It is further demonstrated that the excess return probability shows a maximum at a certain distance, thereby yielding an optimal catalyst position in the ring‐closing reaction.