Colloidal Interactions of Microplastic Particles with Anionic Clays in Electrolyte Solutions.

Homoaggregation of polystyrene microplastics (MPs) and heteroaggregation of MPs with anionic clay minerals, namely, layered double hydroxide (LDH), in different salt (NaCl, CaCl2, and Na2SO4) solutions were systematically investigated using light scattering techniques. The salt type and ionic strength had significant effects on the stability of both MPs and LDH particles individually and the results could be explained by DLVO theory and the Schulze-Hardy rule. However, once stable colloidal dispersions of the individual particles were mixed, heteroaggregation occurred between the oppositely charged MPs and LDH, which was also confirmed by transmission electron microscopy and X-ray scattering. Adsorption of the LDH particles resulted in neutralization and reversal of MPs surface charge at appropriate LDH doses. Once LDH adsorption neutralized the negative charges of the MP spheres, rapid aggregation was observed in the dispersions, whereas stable samples formed at high and low LDH concentrations. The governing interparticle interactions included repulsive electrical double-layer forces, as well as van der Waals and patch-charge attractions, the strength of which depended on the mass ratio of the interacting particles and the composition of the aqueous solvent. Our results shed light on the colloidal behavior of MPs in a complex aquatic environment and, in the long term, are also useful for developing LDH-based approaches for water remediation to remove contamination with MP particles.

Aggregation of Halloysite Nanotubes in the Presence of Multivalent Ions and Ionic Liquids.

Colloidal stability was investigated in two types of particle systems, namely, with bare (h-HNT) and polyimidazolium-functionalized (h-HNT-IP-2) alkali-treated halloysite nanotubes in solutions of metal salts and ionic liquids (ILs). The valence of the metal ions and the number of carbon atoms in the hydrocarbon chain of the IL cations (1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), 1-butyl-3-methylimidazolium (BMIM+), and 1-hexyl-3-methylimidazolium (HMIM+)) were altered in the measurements. For the bare h-HNT with a negative surface charge, multivalent counterions destabilized the dispersions at low values of critical coagulation concentration (CCC) in line with the Schulze-Hardy rule. In the presence of ILs, significant adsorption of HMIM+ took place on the h-HNT surface, leading to charge neutralization and overcharging at appropriate concentrations. A weaker affinity was observed for MIM+, EMIM+, and BMIM+, while they adsorbed on the particles to different extents. The order HMIM+ < BMIM+ < EMIM+ < MIM+ was obtained for the CCCs of h-HNT, indicating that HMIM+ was the most effective in the destabilization of the colloids. For h-HNT-IP-2 with a positive surface charge, no specific interaction was observed between the salt and the IL constituent cations and the particles, i.e., the determined charge and aggregation parameters were the same within experimental error, irrespective of the type of co-ions. These results clearly indicate the relevance of ion adsorption in the colloidal stability of the nanotubes and thus provide useful information for further design of processable h-HNT dispersions.

Melting upon cooling and freezing upon heating: fluid-solid phase diagram for Svejk-Hasek model of dimerizing hard spheres.

A simple model of dimerizing hard spheres with highly nontrivial fluid-solid phase behavior is proposed and studied using the recently proposed resummed thermodynamic perturbation theory for central force (RTPT-CF) associating potentials. The phase diagram has the fluid branch of the fluid-solid coexistence curve located at temperatures lower than those of the solid branch. This unusual behavior is related to the strong dependence of the system excluded volume on the temperature, which for the model at hand decreases with increasing temperature. This effect can be also seen for a wide family of fluid models with an effective interaction that combines short range attraction and repulsion at a larger distance. We expect that for sufficiently high repulsive barrier, such systems may show similar phase behavior.

Structure and Dynamics of a Model Polymer Mixture Mimicking a Levan-Based Bacterial Biofilm of Bacillus subtilis.

In this paper, we report on the structure and dynamics of biologically important model polymer mixtures that mimic the extracellular polymeric matrix in native biofilm of Bacillus subtilis. This biofilm is rich in nonionic polysaccharide levan, but also contains other biopolymers such as DNA and proteins in small concentrations. Aiming to identify the contribution of each component to the formation of the biofilm, our investigations encompassed dynamic rheology, small-angle X-ray scattering, dynamic light scattering, microscopy, densitometry, and sound velocity measurements. As it turned out, this very powerful combination of techniques is able to provide solid results on the dynamical and structural aspects of the microbiologically and chemically complex biofilm formations. Macroscopic rheological measurements revealed that the addition of DNA to levan solution increased the viscosity, pseudoplasticity, and elasticity of the system. The addition of protein contributed similarly, but also increased the rigidity of the system. This confirms that the presence of minor biofilm components is essential for biofilm formation. DNA and proteins appear to confine levan molecules within their supramolecular structure and, in this way, restrict the role of levan to merely a filling agent. These findings were complemented by small-angle X-ray scattering data, which provided insight into the structure on a molecular scale. One of the essential goals of this work was to compare the structural properties of the native biofilm and synthetic biofilm mixture.

Testing Classical Approach to Polymer Solutions on SAXS Data of λ-Carrageenan, κ-Carrageenan and Methylcellulose Systems.

In this paper we present the performance of the classical approach to polymer solutions in evaluation and interpretation of the experimental SAXS data obtained for aqueous solutions of two gelling polysaccharides κ-carrageenan and methylcellulose and a non-gelling polysaccharide λ-carrageenan. In a systematic structural SAXS study of various types of polymer solutions we pointed out and discussed the issues encountered and connected to the fact that the studied gels are obviously not "homogeneous gels" in terms of the structural details that the SAXS technique can resolve and the issues connected to the limited experimental resolution of the SAXS technique for such systems. In parallel the necessary modifications of the classical approach equations for the evaluation of the SAXS data are discussed. Furthermore, the detailed structural results of the studied aqueous polymeric systems in liquid state, during the onset of the gelation, and even in the gel state are presented.

Evaluating SAXS Results on Aqueous Solutions of Various Bacterial Levan utilizing the String-of-Beads Model.

Polysaccharide levan is a homopolymer of fructose and is an important component of plants, yeast, fungi and some bacterial biofilms. In this paper we report on the structural properties of aqueous solutions of bacterial levan utilizing smallangle X-ray scattering and light microscopy. In addition to commercially available levan isolated from Zymomonas mobilis and Erwinia herbicola, we also studied levan isolated and purified from the biofilm of Bacillus subtilis. The smallangle X-ray scattering data were analyzed by the string-of-beads model that revealed qualitative differences in the structure of levan molecules. Levan can be represented as a semi-flexible chain that interacts intra- and inter-molecularly and therefore forms various suprastructures on larger size scales. Increasing the concentration of levan makes the levan structure more compact, which was observed on the nano as well as on the micro scale. The structures with most homogeneously distributed polymer local density were found in B. subtilis levan solutions.

Experimental and theoretical study of the silica particle interactions in the presence of multivalent rod-like ions.

The silica particle interactions in the presence of spermidine were systematically investigated both from experimental and theoretical points of view. The hydrodynamic radii and the corresponding polydispersity indices of the colloidal silica particles were determined by dynamic light scattering (DLS) as a function of spermidine concentration. Whereas the effective size of the silica particles increases with increasing spermidine concentration (pointing to the particle aggregation), the polydispersity index first increases reaches a maximum and then further decreases with the increasing spermidine concentration. From the mobility measurements it was concluded that the increase in spermidine concentration causes less negative values of zeta potential, meaning that the adsorption of spermidine leads to the less negative silica surface. Moreover, Monte Carlo (MC) simulations also confirmed that the addition of spermidine reduces the repulsion between silica particles. The MC concentration profiles of spermidine close to the charged silica particle are in a very good agreement with the results obtained by theory. An important motivation for our study is the effectiveness of multivalent ions to coagulate colloidal suspensions; e.g., the multivalent ions are exploited in the water purification process.

Structure and dynamics of a polysaccharide matrix: aqueous solutions of bacterial levan.

The polysaccharide levan is a homopolymer of fructose and appears in nature as an important structural component of some bacterial biofilms. This paper reports the structural and dynamic properties of aqueous solutions of levan of various origin obtained from dynamic rheological, small-angle X-ray scattering, static and dynamic light scattering, as well as density and sound velocity measurements, determination of polymer branching after per-O-methylation, and microscopy. Besides samples of commercially available levan from Zymomonas mobilis and Erwinia herbicola, we also isolated, purified, and studied a levan sample from the biofilm of Bacillus subtilis. The results of dynamic rheological and light scattering measurements revealed very interesting viscoelastic properties of levan solutions even at very low polymer concentrations. The findings were complemented by small-angle X-ray scattering data that revealed some important differences in the structure of the aqueous levan solutions at the molecular level. Besides presenting detailed dynamic and structural results on the polysaccharide systems of various levans, one of the essential goals of this work was to point out the level of structural information that may be obtained for such polymer systems by combining basic physicochemical, rheological, and various light scattering techniques.

Structural properties of archaeal lipid bilayers: small-angle X-ray scattering and molecular dynamics simulation study.

Aeropyrum pernix is an aerobic hyperthermophilic archaeon that grows in harsh environmental conditions and as such possesses unique structural and metabolic features. Its membrane interfaces with the extreme environment and is the first line of defense from external factors. Therefore, lipids composing this membrane have special moieties that increase its stability. The membrane of A. pernix is composed predominantly of two polar lipids 2,3-di-O-sesterterpanyl-sn-glicerol-1-phospho-1'(2'-O-α-D-glucosyl)-myo-inositol (AGI) and 2,3-di-O-sesterterpanyl-sn-glicerol-1-phospho-myo-inositol (AI). Both have methyl branches in their lipid tails and ether linkages and carbohydrates in their headgroup. These moieties significantly affect the structure and dynamics of the bilayer. To provide a molecular level insight into these characteristics, we used here Molecular Dynamics (MD) simulations of lipid bilayers of composition similar to those of the archaeal membranes. First, we show that the electron density profiles along the normal to the bilayers derived from the simulations are in good agreement with the profiles obtained by the small-angle X-ray scattering (SAXS) technique, which provides confidence in the force fields used. Analyses of the simulation data show that the archaeal lipid bilayers are less hydrated than conventional phosphatidylcholine (PC) lipids and that their structure is not affected by the salt present in the surrounding solution. Furthermore, the lateral pressure in their hydrophobic core, due to the presence of the branched tails, is much higher than that at PC-based lipid bilayers. Both the methyl branched tails and the special headgroup moieties contribute to slow drastically the lateral diffusion of the lipids. Furthermore, we found that the lipid head groups associate via hydrogen bonding, which affects their reorientational dynamics. All together, our data provide links between the microscopic properties of these membranes and their overall stability in harsh environments.

Structural study of simple organic acids by small-angle x-ray scattering and monte carlo simulations.

This study represents an extension of our previous research on the structural properties of simple organic liquids to the systems of organic acids. A set of simple acids from ethanoic to octanoic was modelled with the TraPPE-UA force field and configurational bias Monte Carlo (CBMC) simulations were used to obtain a number of configurations of each system. These data were subsequently used as a basis for the calculation of X-ray scattering intensities, partial radial distribution functions and statistical analysis of molecular aggregation in liquid organic acids. The comparison of simulated X-ray scattering curves with the results of SAXS measurements has shown the agreement to be overall somewhat poorer than in our previous studies on alcohols and aldehydes, although it did improve with increasing molecular length. Hydrogen bonds between the hydroxylhydrogen atom and the carbonyl oxygen have been found to have a profound effect on the structure of the liquid acids. However, the model-based results showed that the formation of intermolecular hydrogen bonds involving the hydroxyl oxygen was disproportionately scarce in these systems. Statistical evaluation of the model configurations has shown that only about 4% of acid molecules form such a type of hydrogen bonds, in contrast to 68% of molecules that form the hydrogen bond with the carbonyl oxygen. This suggests that the force field might be underestimating the hydrogen bonding via hydroxyl oxygen. The statistical analysis has also shown that the simulated molecules preferred to assemble into small molecular aggregates, particularly into double-bonded molecule pairs.

Kinetically Governed Formation of d(G4T2G4) Assemblies.

The most frequently appearing form of DNA is a double helical structure in which two single strands are held together by Watson-Crick base pairs. In addition, guanine rich DNA sequences are known to adopt several unusual structures with G-quartet as a basic repeating motif. Recently large self-assembling nanostructures, called G-wires have become of great interest because of their potential use in molecular electronics. To better understand the forces driving structural transitions of G-wires formed from d(G4T2G4) oligonucleotide in Na+ solutions, we employed a number of techniques such as UV and CD spectroscopy, differential scanning calorimetry (DSC), gel electrophoresis, small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). All the experimental techniques showed that thermally induced folding transitions depend on the cooling rate. In addition, DSC shows that thermally induced unfolding transition depends on heating rate, thus leading to the conclusion that structural transitions of d(G4T2G4) are kinetically governed processes.

Delamination of Layered Double Hydroxide in Ionic Liquids under Ambient Conditions.

Liquid phase delamination of layered materials into single- or few-layer nanosheets leads to stable nanoscale dispersions of 2D materials. The delamination of layered double hydroxide (LDH) to double hydroxide nanosheets was studied in two ionic liquids (ILs): ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium thiocyanate (BMIMSCN). The as-prepared lamellar structure of LDH disappeared upon dispersing it in ILs due to delamination into 2D nanosheets confirmed by X-ray scattering and diffraction techniques and further evaluated by height profile assessment of the nanoparticles by atomic force microscopy. The results showed that both the thickness and lateral size of the dispersed particles decreased in the IL-based samples, indicating that cleavage of the LDH materials can be observed in addition to delamination. The findings prove the concept of delamination of layered materials by ILs under ambient conditions─an excellent way to prepare 2D double hydroxide nanosheet dispersions in one step using nonvolatile green solvents.

Correlating Structural Properties with Electrochemical Behavior of Non-graphitizable Carbons in Na-Ion Batteries.

We report on a detailed structural versus electrochemical property investigation of the corncob-derived non-graphitizable carbons prepared at different carbonization temperatures using a combination of structural characterization methodology unique to this field. Non-graphitizable carbons are currently the most viable option for the negative electrode in sodium-ion batteries. However, many challenges arise from the strong dependence of the precursor's choice and carbonization parameters on the evolution of the carbon matrix and its resulting electrochemistry. We followed structure development upon the increase in carbonization temperature with thorough structural characterization and electrochemical testing. With the increase of carbonization temperature from 900 to 1600 °C, our prepared materials exhibited a trend toward increasing structural order, an increase in the specific surface area of micropores, the development of ultramicroporosity, and an increase in conductivity. This was clearly demonstrated by a synergy of small- and wide-angle X-ray scattering, scanning transmission electron microscopy, and electron-energy loss spectroscopy techniques. Three-electrode full cell measurements confirmed incomplete desodiation of Na+ ions from the non-graphitizable carbons in the first cycle due to the formation of a solid-electrolyte interface and Na trapping in the pores, followed by a stable second cycle. The study of cycling stability over 100 cycles in a half-cell configuration confirmed the observed high irreversible capacity in the first cycle, which stabilized to a slow decrease afterward, with the Coulombic efficiency reaching 99% after 30 cycles and then stabilizing between 99.3 and 99.5%. Subsequently, a strong correlation between the determined structural properties and the electrochemical behavior was established.

Solvation of nonionic poly(ethylene oxide) surfactant Brij 35 in organic and aqueous-organic solvents.

HYPOTHESIS: By combining the experimental small- and wide-angle x-ray scattering (SWAXS) method with molecular dynamics simulations and the theoretical 'complemented-system approach' it is possible to obtain detailed information about the intra- and inter-molecular structure and dynamics of the solvation and hydration of the surfactant in organic and mixed solvents, e.g., of the nonionic surfactant Brij 35 (C12E23) in alcohols and aqueous alcohol-rich ternary systems. This first application of the complemented-system approach to the surfactant system will promote the use of this powerful methodology that is based on experimental and calculated SWAXS data in studies of colloidal systems. By applying high-performance computing systems, such an approach is readily available for studies in the colloidal domain. EXPERIMENTS: SWAXS experiments and MD simulations were performed for binary Brij 35/alcohol and ternary Brij 35/water/alcohol systems with ethanol, n-butanol and n-hexanol as the organic solvent component at 25 °C. FINDINGS: We confirmed the presence of solvated Brij 35 monomers in the studied organic media, revealed their preferential hydration and discussed their structural and dynamic features at the intra- and inter-molecular levels. Anisotropic effective surfactant molecular conformations were found. The influence of the hydrophobicity of the organic solvent on the hydration phenomena of surfactant molecules was explained.

The complemented system approach: a novel method for calculating the x-ray scattering from computer simulations.

In this paper, we review the main problem concerning the calculation of x-ray scattering of simulated model systems, namely, their finite size. A novel method based on the Rayleigh-Debye-Gans approximation was derived, which allows sidestepping this issue by complementing the missing surroundings of each particle with an average image of the system. The method was designed to operate directly on particle configurations without an intermediate step (e.g., calculation of pair distribution functions): in this way, all information contained in the configurations was preserved. A comparison of the results against those of other known methods showed that the new method combined several favorable properties: an arbitrary q-scale, scattering curves free of truncation artifacts, and good behavior down to the theoretical lower limit of the q-scale. A test of computational efficiency was also performed to establish a relative scale between the speeds of all known methods: the reciprocal lattice approach, the brute force method, the Fourier transform approach, and the newly presented complemented system approach.

Ion Specific Effects on the Stability of Halloysite Nanotube Colloids-Inorganic Salts versus Ionic Liquids.

Charging and aggregation processes were studied in aqueous dispersions of halloysite nanotubes (HNTs) in the presence of monovalent inorganic electrolytes and ionic liquid (IL) constituents. The same type of co-ion (same sign of charge as HNT) was used in all systems, while the type of counterions (opposite sign of charge as HNT) was systematically varied. The affinity of the inorganic cations to the HNT surface influenced their destabilizing power leading to an increase in the critical coagulation concentration (CCC) of HNT dispersions in the Cs+ < K+ < Na+ order. This trend agrees with the classical Hofmeister series for negatively charged hydrophobic surfaces. For the IL cations, the CCCs increased in the order BMPY+ < BMPIP+ < BMPYR+ < BMIM+. An unexpectedly strong adsorption of BMPY+ cations on the HNT surface was observed giving rise to charge neutralization and reversal of the oppositely charged outer surface of HNT. The direct Hofmeister series was extended with these IL cations. The main aggregation mechanism was rationalized within the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek, while ion specific effects resulted in remarkable variation in the CCC values. The results unambiguously proved that the hydration level of the surface and the counterions plays a crucial role in the formation of the ionic composition at the solid-liquid interface and consequently, in the colloidal stability of the HNT particles in both inorganic salt and IL solutions.

Structural, rheological and dynamic aspects of hydrogen-bonding molecular liquids: Aqueous solutions of hydrotropic tert-butyl alcohol.

HYPOTHESIS: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small- and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the 'complemented-system approach' method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular and supramolecular structures at the intra-, inter-, and supra-molecular scales. EXPERIMENTS: The SWAXS and x-ray diffraction experiments and MD simulations were performed for aqueous t-butanol solutions at 25 °C. Literature viscosity and self-diffusion data were also used. FINDINGS: The interpretive power of the proposed scheme was demonstrated by the extensive and diverse results obtained for aqueous t-butanol solutions across the whole concentration range. Four composition ranges with qualitatively different structures and viscosity trends were revealed. The experimental and calculated zero-shear viscosities and molecular self-diffusion coefficients were successfully related to the corresponding structural details. The hydrogen bonds that were, along with hydrophobic effects, recognized as the most important driving force for the formation of t-butanol aggregates, show intriguing lifetime trends and thermodynamic properties of their formation.

Exploring the structural properties of simple aldehydes: a Monte Carlo and small-angle X-ray scattering study.

The structure of simple linear alkanals from propanal to nonanal was studied utilizing configurational bias Monte Carlo (MC) simulations of the aldehydes modeled according to the transferable potential for phase equilibria-united atom force field (TraPPE-UA) and was compared to experimental small-angle X-ray scattering (SAXS) results. This was done by exploiting a recently developed approach for calculating the scattering intensities from theoretically obtained MC data by utilizing the Debye equation (Tomsic et al. J. Phys. Chem. B 2007, 111, 1738). Similar calculations were also performed utilizing a well-established approach based on the reciprocal lattice. Comparison of the calculated scattering data with the experimental SAXS results in the first instance revealed information on the molecular organization in simple aldehydes and in addition also served as a good structural test of the TraPPE-UA force field used to model the aldehydes studied. However, it turned out that such a structural test is a rather strict test for the model which otherwise showed good agreement with the experimental data from the thermodynamic point of view.

Supramolecular structure vs. rheological properties: 1,4-Butanediol at room and elevated temperatures.

HYPOTHESIS: The molecular hydrogen-bonding process in liquids with relatively small molecules, like butan-1,4-diol (BDO), results in interesting molecular-aggregation phenomena. These are reflected in a supramolecular self-assembly type of structure that is also expected to importantly influence the rheological properties of the system. The combination of experimental and theoretical methods should be able to reveal such influences in the important feedstock chemical BDO at low and moderate temperatures and further provide an application of the verified BDO model for a theoretical study of the structure at higher temperatures, for which serious health hazards make extensive experimental studies difficult. EXPERIMENTS: BDO was studied in the temperature range between 25 and 225 °C utilizing the experimental small- and wide-angle x-ray scattering method, molecular dynamics simulations, and the 'complemented-system approach'. FINDINGS: The theoretical results were experimentally verified and revealed important novel temperature-dependent structure and viscosity-related information, e.g., on aggregate sizes and self-diffusion coefficients, that would remain hidden in purely experimental studies. BDO has diverse applicability in many fields of research and industry, where it is often exposed to high temperatures. Using an experimentally verified model we were able to calculate reliable viscosity and self-diffusion coefficient values for the BDO molecules under such conditions.

Structural properties of a model system with effective interparticle interaction potential applicable in modeling of complex fluids.

We report grand canonical ensemble Monte Carlo (MC) simulation and theoretical studies of the structural properties of a model system described by an effective interparticle interaction potential, which incorporates basic interaction terms used in modeling of various complex fluids composed of mesoscopic particles dispersed in a solvent bath. The MC results for the bulk radial distribution function are employed to test the validity of the hard-sphere bridge function in combination with a modified hypernetted chain approximation (MHNC) in closing the Ornstein-Zernike (OZ) integral equation, while the MC data for the density profiles in different inhomogeneous environments are used to assess the validity of the third-order+second-order perturbation density functional theory (DFT). We found satisfactory agreement between the results predicted by the pure theories and simulation data, which classifies the proposed theoretical approaches as convenient tools for the investigation of complex fluids. The present investigation indicates that the bridge function approximation and density functional approximation, which are traditionally used for the study of neutral atomic fluids, also perform well for complex fluids only on condition that the underlying effective potentials include a highly repulsive core as an ingredient.