Long-range Repulsion Research Articles

Nucleation in confinement generates long-range repulsion between rough calcite surfaces

Fluid-induced alteration of rocks and mineral-based materials often starts at confined mineral interfaces where nm-thick water films can persist even at high overburden pressures and at low vapor pressures. These films enable transport of reactants and affect forces acting between mineral surfaces. However, the feedback between the surface forces and reactivity of confined solids is not fully understood. We used the surface forces apparatus (SFA) to follow surface reactivity in confinement and measure nm-range forces between two rough calcite surfaces in NaCl, CaCl2, or MgCl2 solutions with ionic strength of 0.01, 0.1 or 1 M. We observed long-range repulsion that could not be explained by changes in calcite surface roughness, surface damage, or by electrostatic or hydration repulsion, but was correlated with precipitation events which started at µm-thick separations. We observed a submicron-sized precipitate that formed in the confined solution. This liquid-like viscous precipitate did not undergo any spontaneous ripening into larger crystals, which suggested that confinement prevented its dehydration. Nucleation was significantly postponed in the presence of Mg2+. The long-range repulsion generated by nucleation between confined mineral surfaces can have a crucial influence on evolution of the microstructure and therefore the macroscopic strength of rocks and materials.

Waiting-time distributions of particle entrapments in clustered states generated by short-range attractive, long-range repulsive (SALR) interactions

Brownian dynamics simulations have been carried out for so-called SALR systems, comprising fluids of particles interacting via competing short-range attractions and long-range repulsions. As the attraction is strengthened, particles form long-lived clusters with an internal structure that becomes ordered. This process is correlated with a slowing-down of the dynamics, which shows subdiffusive behavior in this regime with multiple dynamical time scales. Here we demonstrate that the waiting-time distribution of particle entrapments in clusters reflects these variations in the dynamics. Two- and many-body effects are cleanly delineated by computing these waiting times using a first passage time theory for excursions in the pair potential. For internally ordered clusters the accompanying slower time scale is found to be on the order of the escape time for a particle buried in a crystalline cluster.

Assembly of particle strings via isotropic potentials.

Assembly of spherical colloidal particles into extended structures, including linear strings, in the absence of directional interparticle bonding interactions or external perturbation could facilitate the design of new functional materials. Here, we use methods of inverse design to discover isotropic pair potentials that promote the formation of single-stranded, polydisperse strings of colloids "colloidomers" as well as size-specific, compact colloidal clusters. Based on the designed potentials, a simple model pair interaction with a short-range attraction and a longer-range repulsion is proposed which stabilizes a variety of different particle morphologies including (i) dispersed fluid of monomers, (ii) ergodic short particle chains as well as porous networks of percolated strings, (iii) compact clusters, and (iv) thick cylindrical structures including trihelical Bernal spirals.

When hydrogen bonding overcomes Coulomb repulsion: from kinetic to thermodynamic stability of cationic dimers.

Quantum chemical calculations have been employed to study the kinetic and thermodynamic stability of hydroxy-functionalized 1-(3-hydroxyalkyl)pyridinium cationic dimers. For [Py-(CH2)n-OH+]2 structures with n = 2-17 we have calculated the robust local minima with clear dissociation barriers preventing their "Coulomb explosion" into separated cations. For n = 15 hydrogen bonding and dispersion forces fully compensate for the repulsive Coulomb forces between the cations allowing for the quantification of the pure hydrogen bond in the order of 20 kJ mol-1. The increasing kinetic stability even turns to thermodynamic stability with further elongated hydroxyalkyl chains. Now, quantum-type short-range attraction wins over classical long-range electrostatic repulsion resulting in negative binding energies and providing the first thermodynamically stable cationic dimers. The electronic, structural and spectroscopic signatures of the cationic dimers could be correlated to NBO parameters, supporting the existence of anti-electrostatic hydrogen bonds (AEHB) as recently suggested by Weinhold. In principle, these pure cationic dimers should be detectable in gas-phase experiments at low temperatures without the need of mediating molecules or counteranions.

Assembly of Helical Structures in Systems with Competing Interactions under Cylindrical Confinement.

The behavior under confinement of nanoparticles interacting with the short-range attraction and long-range repulsion potential is studied by means of Monte Carlo simulations in the grand canonical ensemble. The study is performed at thermodynamic conditions at which a hexagonal cylindrical phase is the most stable phase in bulk. In these conditions, cylindrical confinement promotes the formation of helical structures whose morphology depends upon both the pore radius and boundary conditions. As the pore radius increases, the fluid undergoes a series of structural transitions going from single to multiple intertwined helices to concentric helical structures. When the pore ends are closed by planar walls, ring and toroidal clusters are formed next to these walls. Dependent upon the cylinder length, molecules away from the pore edges can either keep growing into ring and toroidal aggregates or arrange into helical structures. It is demonstrated that the system behaves in cylindrical confinement in the same way as the block copolymer systems. Such behavior has not been observed for the colloidal systems in cylindrical confinement with only repulsive interactions.

Non-monotonous Wetting of Graphene-Mica and MoS2-Mica Interfaces with a Molecular Layer of Water.

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale, the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS2 deposited on dry mica are used to investigate wetting of the two-dimensional (2D) material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17% for graphenes and 8-9% for MoS2 and concludes with a homogeneous layer at 25-30 and 15-20%, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25% RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter causing at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at submonolayer concentrations may also be a quite general phenomenon.

Dynamics of liquid-liquid phase separation of wheat gliadins

During wheat seeds development, storage proteins are synthetized and subsequently form dense protein phases, also called Protein Bodies (PBs). The mechanisms of PBs formation and the supramolecular assembly of storage proteins in PBs remain unclear. In particular, there is an apparent contradiction between the low solubility in water of storage proteins and their high local dynamics in dense PBs. Here, we probe the interplay between short-range attraction and long-range repulsion of a wheat gliadin isolate by investigating the dynamics of liquid-liquid phase separation after temperature quench. We do so using time-resolved small angle light scattering, phase contrast microscopy and rheology. We show that gliadins undergo liquid-liquid phase separation through Nucleation and Growth or Spinodal Decomposition depending on the quench depth. They assemble into dense phases but remain in a liquid-like state over an extended range of temperatures and concentrations. The analysis of phase separation kinetics reveals that the attraction strength of gliadins is in the same order of magnitude as other proteins. We discuss the respective role of competing interactions, protein intrinsic disorder, hydration and polydispersity in promoting local dynamics and providing this liquid-like behavior despite attractive forces.

Hydrogen evolution from water reactions with molybdenum sulfide cluster anions

A study of reactions between a distribution of 98MoxSy- clusters ranging from sub-sulfide (x ∼ y) to hypersulfide (y > 2x) is presented, compared with previous studies on homologous MoxOz- clusters, and analyzed with supporting computational studies of the reaction free energy pathways, giving insights into the molecular scale interactions involved in the electro- or photocatalytically driven hydrogen evolution reaction. First, reactions between MoxSy- clusters with water are approximately an order of magnitude slower than reactions between oxides with water, an effect attributed to longer-range repulsion between the cluster anions and water. However, sequential oxidation of the MoxSy- clusters, which generates oxosulfides and H2, is observed for several species. Evidence of oxidation of several hypersulfides with H2 evolution is also observed, indicating that oxidation of S2 groups is also possible. Based on the computational results on several of the bimetallic species, oxidation of the subsulfide Mo2S2− and Mo2S3− clusters is barrierless, while oxidation of the nascent Mo2S2O- cluster formed by oxidation of Mo2S2− is low-barrier. The stoichiometric Mo2S4- + H2O free energy path is predicted to have a modest barrier to the dissociative addition of water, which is consistent with the experimental observation of both oxidation (with H2 production) and water addition reactions. Calculations on the reaction free energy pathways of the oxosulfide species suggest that the formation of two dihydroxide groups, rather than a thiol and hydroxide group, is favored. Oxidation of the oxides is more thermodynamically favored than oxidation of sulfide analogs.

A new approach for characterization of hydrophobization mechanisms of surfactants on muscovite surface

A new approach ‘dynamic contact angle analysis’ has been used in the present investigation to characterize the hydrophobization mechanisms of single and mixed surfactants on muscovite surface. The hydrophobization of muscovite surface was achieved by anionic NaOL (sodium oleate), cationic DDA (dodecylamine) and combined DDA/NaOL surfactants. Besides dynamic contact angle analysis, AFM and flotation tests were carried out to gain detail information about the wettability of muscovite, adhesion ability of bubbles towards muscovite surface and adsorption of single and mixed DDA/NaOL surfactants on the muscovite surface. From flotation results, the highest recovery (>90%) was obtained by the mixed DDA/NaOL when they were used at a ratio of 1:3 and the concentration of 2 × 10−4 mol/L. The dynamic contact angle tests showed that the angles achieved by DDA/NaOL were bigger and the bubbles were adhered more easily, which are in agreement with the flotation results. AFM imaging indicated that, as compared to single collectors, the combined surfactants adsorbed uniformly on the muscovite surface as bulks and formed a tight adsorption multilayer. The attraction in mixed DDA/NaOL was stronger than that in single DDA with the same jump-in distance at around 15 nm and the long-range repulsion was observed in single NaOL.

Gelation of plasmonic metal oxide nanocrystals by polymer-induced depletion attractions

Gelation of colloidal nanocrystals emerged as a strategy to preserve inherent nanoscale properties in multiscale architectures. However, available gelation methods to directly form self-supported nanocrystal networks struggle to reliably control nanoscale optical phenomena such as photoluminescence and localized surface plasmon resonance (LSPR) across nanocrystal systems due to processing variabilities. Here, we report on an alternative gelation method based on physical internanocrystal interactions: short-range depletion attractions balanced by long-range electrostatic repulsions. The latter are established by removing the native organic ligands that passivate tin-doped indium oxide (ITO) nanocrystals while the former are introduced by mixing with small PEG chains. As we incorporate increasing concentrations of PEG, we observe a reentrant phase behavior featuring two favorable gelation windows; the first arises from bridging effects while the second is attributed to depletion attractions according to phase behavior predicted by our unified theoretical model. Our assembled nanocrystals remain discrete within the gel network, based on X-ray scattering and high-resolution transmission electron microscopy. The infrared optical response of the gels is reflective of both the nanocrystal building blocks and the network architecture, being characteristic of ITO nanocrystals' LSPR with coupling interactions between neighboring nanocrystals.

Ordering of sedimenting paramagnetic colloids in a monolayer.

Sedimentation enables self-assembly of colloidal particles into crystalline structures, as needed for catalysis or photonics applications. Here we combine experiments, theory, and simulations to investigate the equilibrium structure of a colloidal monolayer with tunable interparticle repulsion via an applied external magnetic field. Experimental observations of the equilibrium structure are in excellent agreement with density functional theory. Within a (zero-temperature) local density approximation, we derive a simple analytical expression that quantitatively captures the inhomogeneous ordering ranging from solid to liquidlike states. Monte Carlo simulations corroborate these findings and explore an even wider range of sedimentation conditions, thus providing a global view of the sedimentation-mediated ordering in colloidal monolayers with tunable long-ranged interparticle repulsions. Our findings shed further light on the classical sedimentation problem in colloidal science and related areas.

Entanglement Induced Interactions in Binary Mixtures.

We establish a conceptual framework for the identification and the characterization of induced interactions in binary mixtures and reveal their intricate relation to entanglement between the components or species of the mixture. Exploiting an expansion in terms of the strength of the entanglement among the two species enables us to deduce an effective single-species description. In this way, we naturally incorporate the mutual feedback of the species and obtain induced interactions for both species which are effectively present among the particles of same type. Importantly, our approach incorporates few-body and inhomogeneous systems extending the scope of induced interactions where two particles interact via a bosonic bath-type environment. Employing the example of a one-dimensional ultracold Bose-Fermi mixture, we obtain induced Bose-Bose and Fermi-Fermi interactions with short-range attraction and long-range repulsion. With this, we show how beyond species mean-field physics visible in the two-body correlation functions can be understood via the induced interactions.

Hydrogen bubble nucleation by self-clustering: density functional theory and statistical model studies using tungsten as a model system

Low-energy, high-flux hydrogen irradiation is known to induce bubble formation in tungsten, but its atomistic mechanisms remain little understood. Using first-principles calculations and statistical models, we studied the self-clustering behaviour of hydrogen in tungsten. Unlike previous speculations that the hydrogen self-clusters are energetically unstable owing to the general repulsion between two hydrogen atoms, we found that 2D platelet-like hydrogen self-clusters could form at high hydrogen concentrations. The attractive binding energy of the hydrogen self-cluster becomes larger as the cluster size increases and plateaus at 0.38 eV/H around size of 40. We found that hydrogen atoms would form 2D platelet-like structures along planes. These hydrogen self-clustering behaviours can be quantitatively understood by the competition between long-ranged elastic attraction and local electronic repulsion among hydrogens. Further analysis showed hydrogen self-clusters to be kinetically feasible and thermodynamically stable above a critical hydrogen concentration. Based on this critical hydrogen concentration, we predicted the hydrogen irradiation condition required for the formation of hydrogen self-clusters. Our predictions showed excellent agreement with the experimental results of hydrogen bubble formation in tungsten exposed to low-energy hydrogen irradiation. Finally, we proposed a possible mechanism for the hydrogen bubble nucleation via hydrogen self-clustering. This work provides mechanistic insights and quantitative models towards understanding of plasma-induced hydrogen bubble formation in plasma-facing tungsten.

Cluster formation in binary fluids with competing short-range and long-range interactions

The equilibrium phase behaviour of a model binary fluid is investigated through Monte Carlo simulations and by developing a molecular thermodynamic model. Both fluid components interact through a hard core with short-range attractions (SA), but one of the components exhibits an additional long-range repulsion (SA+LR). We find that phase behaviour for this system is controlled by the cross-interaction between the two types of particles as well as their chemical potentials. For a weak cross-interaction, the system displays behaviour that is a composite of the behaviour of the individual components, i.e. the SA component can display bulk vapour/liquid phase separation, while the SALR component can display giant micelle-like clusters for a suitable combination of SA and LR interactions. For a strong cross-interaction, qualitatively different behaviour is observed, with the resulting clusters typically composed of a more equal mixture of SA and SALR particles. Moreover, these mixed clusters can exist even when the SA component by itself would be undersaturated or supercritical, and/or when the SALR component by itself would not form giant clusters. These insights should help to identify the mechanisms for clustering in experimental systems where giant equilibrium clusters are observed.

Communication: Is directed percolation in colloid-polymer mixtures linked to dynamic arrest?

Using computer simulations, we study the dynamic arrest in a schematic model of colloid-polymer mixtures combining short-ranged attractions with long-ranged repulsions. The arrested gel is a dilute rigid network of colloidal particles bonded due to the strong attractions. Without repulsions, the gel forms at the spinodal through arrested phase separation. In the ergodic suspension at sufficiently high densities, colloidal clusters form temporary networks that percolate space. Recently [M. Kohl et al., Nat. Commun. 7, 11817 (2016)], it has been proposed that the transition of these networks to directed percolation (DP) coincides with the onset of the dynamic arrest, thus linking structure to dynamics. Here, we evaluate for various screening lengths the underlying gas-liquid binodal and the percolation transitions. We find that DP shifts the continuous percolation line to larger densities, but even beyond this line the suspension remains ergodic. Only when approaching the spinodal does dynamic arrest occur. Competing repulsions thus do not modify the qualitative scenario for non-equilibrium gelation, although the structure of the emerging percolating network shows some differences.

Adsorption Free Energies of Imidazolinium-Type Surfactants in Infinite Dilution and in Micellar State on Gold Surface.

We report adsorption behavior of imidazolinium-type surfactant molecules in different aggregation states on metal-water interfaces studied using all-atom molecular dynamics simulations. Surfactant molecules with two different alkyl tail lengths, a 10-carbon and a 17-carbon tail (henceforth referred to as imid-10 and imid-17, respectively), have been considered. Six layers of face-centered cubic lattice of gold atoms submerged in water represent the metal-water interface. Our simulations reveal that, in infinite dilution, both types of surfactant molecules strongly adsorb onto the metal-water interface in a configuration with their alkyl tail lying parallel to the surface. This adsorption occurs through a barrierless transition with an adsorption free energy of ∼30 kB T and is found to be enthalpically driven and entropically unfavorable. Surfactant micelles, on the other hand, experience a long-range repulsion from the metal surface at distances as large as 50-60 Å due to the presence of a large "corona" around the micelles that comprises counterions and their solvation layer. Surfactant micelles have an adsorption free energy barrier of ∼13-16 kB T, which is associated with the removal of adsorbed water from the metal surface. Micelles are thermodynamically stable in the bulk aqueous phase, and the adsorbed micellar state is only metastable.

Orientational ordering of lamellar structures on closed surfaces.

Self-assembly of particles with short-range attraction and long-range repulsion interactions on a flat and on a spherical surface is compared. Molecular dynamics simulations are performed for the two systems having the same area and the density optimal for formation of stripes of particles. Structural characteristics, e.g., a cluster size distribution, a number of defects, and an orientational order parameter (OP), as well as the specific heat, are obtained for a range of temperatures. In both cases, the cluster size distribution becomes bimodal and elongated clusters appear at the temperature corresponding to the maximum of the specific heat. When the temperature decreases, orientational ordering of the stripes takes place and the number of particles per cluster or stripe increases in both cases. However, only on the flat surface, the specific heat has another maximum at the temperature corresponding to a rapid change of the OP. On the sphere, the crossover between the isotropic and anisotropic structures occur in a much broader temperature interval; the orientational order is weaker and occurs at significantly lower temperature. At low temperature, the stripes on the sphere form spirals and the defects resemble defects in the nematic phase of rods adsorbed at a sphere.

Influence of disorder on the superconducting critical temperature in indium-opal nanocomposites

Transport properties of bulk indium-opal and indium-porous glass superconducting nanocomposites possessing moderate and strong disorder are investigated. A strongly nonmonotonous dependence of the global critical temperature Tc versus normal state conductivity of samples is found. The maximum, which is observed at moderate disorder, has Tc higher than that of clean bulk indium. The increasing part can be explained by the Eliashberg equations with disorder and an additional mechanism of interaction between superconducting and dielectric granules. The descending part of the maximum at higher disorder can be explained by the increasing of long-range Coulomb repulsion due to diffusion of charges. Negative slope in magnetic field dependence of resistivity and a peak in the temperature dependence of resistivity, observed in the sample near the proximity to the disorder-induced superconductor–insulator transition (SIT). A large difference between the onset temperature of superconducting fluctuations, Tcon, and global critical temperature Tc is found and considered in the framework of the weak multifractal theory. Slow time-logarithmic relaxation of the resistivity between Tc and Tcon is observed, which assumes existence of the precursor state near the SIT. This unusual state is discussed in the scope of the many-body localization theory.

Self-assembly of rarely polymer-grafted nanoparticles in dilute solutions and on a surface: From non-spherical vesicles to graphene-like sheets

Rarely polymer-grafted nanoparticles are shown to be able to self-assemble into stable nanostructures of different morphologies. The driving force for the assembly is a competition between a short-range attraction of the nanoparticles and long-range repulsion of the grafted chains. Effects of the solvent quality for the nanoparticles, grafting density and the length of the chains are studied. We have shown that non-spherical vesicles resembling a monocrystal can be stable in dilute solution. In them, the wall represents a sheet of hexagonally packed nanoparticles which sharply bends to become closed forming facets and edges. Improving solvent quality leads to a cascade of transitions: vesicles - tubes - perforated sheets - threads. Adsorption of the nanoparticles on a solvophobic surface can result in their 2D self-assembly. We have predicted ordering of the nanoparticles into a hexagonal network similar to the structure of graphene sheets. Schematic diagrams of states in 2D and 3D are constructed.

Short-time dynamics of lysozyme solutions with competing short-range attraction and long-range repulsion: Experiment and theory.

Recently, atypical static features of microstructural ordering in low-salinity lysozyme protein solutions have been extensively explored experimentally and explained theoretically based on a short-range attractive plus long-range repulsive (SALR) interaction potential. However, the protein dynamics and the relationship to the atypical SALR structure remain to be demonstrated. Here, the applicability of semi-analytic theoretical methods predicting diffusion properties and viscosity in isotropic particle suspensions to low-salinity lysozyme protein solutions is tested. Using the interaction potential parameters previously obtained from static structure factor measurements, our results of Monte Carlo simulations representing seven experimental lysoyzme samples indicate that they exist either in dispersed fluid or random percolated states. The self-consistent Zerah-Hansen scheme is used to describe the static structure factor, S(q), which is the input to our calculation schemes for the short-time hydrodynamic function, H(q), and the zero-frequency viscosity η. The schemes account for hydrodynamic interactions included on an approximate level. Theoretical predictions for H(q) as a function of the wavenumber q quantitatively agree with experimental results at small protein concentrations obtained using neutron spin echo measurements. At higher concentrations, qualitative agreement is preserved although the calculated hydrodynamic functions are overestimated. We attribute the differences for higher concentrations and lower temperatures to translational-rotational diffusion coupling induced by the shape and interaction anisotropy of particles and clusters, patchiness of the lysozyme particle surfaces, and the intra-cluster dynamics, features not included in our simple globular particle model. The theoretical results for the solution viscosity, η, are in qualitative agreement with our experimental data even at higher concentrations. We demonstrate that semi-quantitative predictions of diffusion properties and viscosity of solutions of globular proteins are possible given only the equilibrium structure factor of proteins. Furthermore, we explore the effects of changing the attraction strength on H(q) and η.