Edge-to-face Research Articles

Tunable CH/π Interactions within a Tryptophan Zipper Motif to Stabilize the Fold of Long β-Hairpin Peptides.

The tryptophan zipper (Trpzip) is an iconic folding motif of β-hairpin peptides capitalizing on two pairs of cross-strand tryptophans, each stabilized by an aromatic-aromatic stacking in an edge-to-face (EtF) geometry. Yet, the origins and the contribution of this EtF packing to the unique Trpzip stability remain poorly understood. To address this question of structure-stability relationship, a library of Trpzip hairpins was developed by incorporating readily accessible nonproteinogenic tryptophans of varying electron densities. We found that each EtF geometry was, in fact, stabilized by an intricate combination of XH/π interactions. By tuning the π-electron density of Trpface rings, CH/π interactions are strengthened to gain additional stability. On the contrary, our DFT calculations support the notion that Trpedge modulations are challenging due to their simultaneous paradoxical engagement as H-bond donors in CH/π and acceptors in NH/π interactions.

Quantitative Evaluation of the Effects of Salt Concentration and pH on the Self-Assembly of Kaolin Nanoplatelets.

Diffusion of pollutants in the earth's strata threatens both the environment and human health. The clay soil microstructure that plays a crucial role in the diffusion of pollutants is significantly influenced by the pore water chemistry. However, there is still a lack of quantitative evaluation of pore water chemistry on clay fabric evolution. To bring new insights, we systematically examined the impact of water chemistry (mainly refers to salt ion concentration and pH) on the self-assembly form (fabric) of kaolin platelets and evaluated the fabric quantitatively. The results show that as the salt ion concentration increases, the "kaolin book" structure is formed, which can be captured by the (001) and (020) pole figures. Under acidic conditions, kaolin platelets turn randomly arranged; however, with the increase of pH, the edge-to-face (EF) microstructure of kaolin platelets gradually changes to a face-to-face (FF) structure. Under alkali-eq conditions, kaolin platelets form a dispersion assembly dominated by FF repulsion. However, the strong alkaline condition triggers the decomposition of kaolin, leading to a notable decrease in the maximum pole density. The conclusions were substantiated through insightful AFM tests. Moreover, we addressed the advantages and limitations of 1DXRD and 2DXRD by analyzing the trend between the OI and pole density, with 2DXRD being favored for its accuracy. Overall, this study provides insights into clay platelets and the self-assembly of kaolin under different water chemistry conditions, which have significant implications for predicting and modeling the physical properties of clay under special environmental conditions.

Quantitative examination of microstructural transformations of clay-rich sediments in river-dominated deltas under the influence of polluted pore water

Coastal water pollution has a significant impact on sedimentary environments, altering the microstructure of clay-rich sediments and further destabilizing river-dominated delta strata. However, the understanding of the microstructure of clay sediment, influenced by burial depth and pore water chemistry, remains limited due to challenges in quantitatively analyzing clay texture at varying depths. The perturbable of clay microstructures, and the cost of deep sampling have hindered such efforts. To address this issue, this study aims to quantitatively analyze the clay anisotropy at different depths and pore water chemistry through laboratory-simulated sediment samples by using centrifugal modeling and 2DXRD technology. The results suggest that 1DXRD (Orientation index) is prone to generating incorrect conclusions, whereas 2DXRD (pole density) yields more precise and reliable results. Specifically, the results indicated that the introduction of salt ions promoted clay precipitation and stabilized the oriented microstructure at shallower depths. In acidic solutions, clay sediment still contained a certain proportion of edge to face (EF) microstructure at depths less than 6 m, suggesting higher soil thixotropy and lower strength than that of clay sediments in other types of solutions. Overall, our findings provide valuable insights into the relationship between water pollution, delta disappearance, and ocean acidification, highlighting the urgent need for effective environmental management strategies to prevent further damage to fragile coastal ecosystems.

Structures, Characterization and DFT Studies of Four Novel Nickel Phenanthroline Complexes

Four Ni–phen complexes were prepared and characterized by FTIR and thermal analysis. The X-ray structure determination shows that the geometry around NiII ions in complexes 1, [Ni(phen)2(H2O)(ONO2)](NO3); 2, [Ni(phen)2(H2O)Cl]Cl; 3, [Ni(phen)(CH3CN)(ONO2)(O2NO)] and 4, [Ni(phen)3](NO3)2.H2O.C2H5OH is considerably distorted octahedral. The coordination core geometries in 1, 2, 3 and 4 are NiN4O2, NiN4OCl, NiN3O3 and NiN6, respectively. The crystal packing and crystal supramolecularity analysis of the complexes reveal the importance of aryl∙∙∙aryl interactions, and both offset face-to-face (OFF) and edge-to-face (EF) motifs are dominant. Hydrogen bonding interactions in these structures reinforce aryl∙∙∙aryl interactions. FTIR proved the coordination of M-N and M-Cl as well as the presence of nitro groups in the complexes, while thermal analysis revealed that the loss of water, chloro and nitro groups occurred before the degradation of phenanthroline ligand in the complexes. DFT study indicates that a strong correlation exists among theoretical and experimentally determined geometric parameters with distorted octahedral geometry. The charge density in HOMO is localized on the metal halogen bond and nitrate group (NO3−), whereas the LUMO cloud density is mainly distributed on the phen ring, which indicates the electroactive nature of the ring.

Effects of water chemistry on microfabric and micromechanical properties evolution of coastal sediment: A centrifugal model study

Water chemistry alteration induced strength weakening of natural sediment, which leads to the differential settlement of infrastructures in coastal areas, has caused numerous disasters and engineering failures. To thoroughly understand the underlying mechanisms of how water chemistry influences the microfabric and mechanical properties evolution of coastal sediments, herein, the authors adopted centrifuge test, X-ray diffraction (XRD), and atomic force microscope (AFM) to quantitatively study the structure anisotropy index (i.e., orientation index (OI)), micromorphological property (i.e., root mean square height (Sq)), and micromechanics (i.e., microscale apparent modulus) of clay sediments in different water chemistry conditions and gravity gradients. The results show that the change rule of OI is: OIsaline > OIalkaline > OIwater > OIacid, along the vertical sedimentary depth. Randomly distributed clay flocs and loose flocculated soil skeleton (mainly consisted by edge-to-face (EF) and edge-to-edge (EE) contact of the kaolinite platelets) are associated with the acidic water chemical conditions. The action of supergravity and face-to-face (FF) repulsive contact mode lead to high degree of anisotropy of kaolinite sediments in alkaline environment. Clay platelets are compacted closely under the synergetic effect of centrifugal pressure and prevailing van der Waals attraction (reduction of electric double layer repulsion) in saline environment. The change of 1/Sq is highly consistent with the change of OI at different depths in different water chemical environments. Along the sedimentary depth (i.e., transition from the normal gravity (1 g) to supergravity (8000 g)), the microscale apparent modulus of kaolinite sediment was found to be the highest in alkaline environment. As the water chemistry changes from alkaline to acidic, however, the microscale apparent modulus of kaolinite aggregate decreased, and it showed the smallest in the saline environment.

The impacts of high salinity and polymer properties on dewatering and structural characteristics of flocculated high-solids tailings

In seeking to better understand the inline flocculation technique for enhanced water recovery from concentrated fine-particle suspensions, polymers of different molecular weight (MW) and chemistry were used to treat a synthetic tailings slurry in a low-shear mixer for continuous flocculation under controlled conditions. The effects of dissolved salts in the slurry and polymer solution make-up water on dewatering performance were evaluated for conventional acrylamide/acrylate copolymers (BASF Magnafloc® products) and an alternative functional chemistry polymer (BASF Rheomax® DR 1050). The properties of the separated liquid and solid phases were examined to understand the impacts of salinity and polymer type on the formation of rapidly dewatering flocculated material. Higher concentrations of NaCl (0.006–0.6 M) and CaCl2 (0.006–0.06 M) salts in the unflocculated slurries formed larger coagulated structures, with focussed beam reflectance measurement (FBRM) showing greater pre-aggregation for the divalent salt. NaCl slurries show edge-to-edge (EE) and edge-to-face (EF) inter-particle associations that are open and easily disrupted while more compact face-to-face (FF) contacts are observed in CaCl2 slurries. For concentrated NaCl slurries, the dosing of medium-to-high MW polymers is generally preferred to maximise net water recovery, with the implication being that bridging mechanisms are still at play in high solids applications. In calcium-enriched slurries, the lower MW conventional copolymer gave comparatively better water returns than the other polymers when applied under low shear conditions, highlighting quite distinct aggregation processes compared to low solids flocculation. These results provide insight on the fundamental complexity of the ionic strength dependence of high solids-high dosage tailings flocculation, towards informing reagent polymer dosing and offering greater flexibility for end-of-pipe dewatering schemes.

Microfabric characteristics of kaolinite flocculates and aggregates — Insights from large-scale molecular dynamics simulations

Clay particles are usually associated in flocculates or aggregates with different microfabric manners, which will govern the macroscale mechanical behaviors of clayey soils. In this study, the characteristics of different microfabric manners of defect-free kaolinite were explored applying large-scale full-atomistic molecular dynamics simulations. Due to the special crystal structure, kaolinite particles were more likely to flocculate in Edge-to-Face (EF) and Edge-to-Edge (EE) manners than aggregate in Face-to-Face (FF) manner when analyzed from the system energy point of view. For EF and EE manners, the 1st layer of adsorbed water of every single kaolinite particle was complete while the 2nd and the 3rd layers of adsorbed water arranged in T-, L-/inverted L- or cross-shape according to the wettability performance of surrounding kaolinite particles' surfaces. A series of new schematic diagrams of different microfabric manners with adsorbed water structures were developed according to the corresponding equilibrium topologies and physicochemical characteristics. The conceptual hypothesis of the kaolinite sedimentary process was verified and improved based on the simulation results analysis. The research results enhanced the comprehension of microfabric characteristics of clay colloids. The simulation method applied in this work would foster future investigations into clay colloids in various physicochemical environments.

Application of Tea Polyphenols as a Biodegradable Fluid Loss Additive and Study of the Filtration Mechanism.

Drilling fluids with poor filtration property are disadvantageous for well drilling, easily causing wellbore instability and formation collapse. This work reports the novel utilization of tea polyphenols (TPs) as a fluid loss additive in the bentonite–water-based drilling fluids (BT-WDFs). The influence of TP concentration and temperature on the filtration property of the fluids was described. The results showed that an increase in the TP concentration contributed to a decrease in fluid loss. Especially BT-WDFs added with 3.0 wt % TP exhibited a low fluid loss (less than or approximately 10 mL) at room temperature and high temperatures (∼150 °C), displaying better filtration property and temperature resistance than common fluid loss agents. Through the investigations on the viscosity, the particle size of TP/BT-WDFs, and micromorphology of filter cakes, the dispersion effect of TP was considered as the dominant factor for the filtration property of TP/BT-WDFs. TP molecules, containing many functional groups, could attach to the surface of bentonite platelets, improve the hydration of bentonite particles, and promote the dispersion of bentonite particles. At room temperature, TP facilitated the dispersion of hydrated bentonite. The existing “house-of-cards” structure was weakened, decreasing the particle size and viscosity of TP/BT-WDFs. At high temperature, bentonite dehydrated and aggregated, thereby increasing the particle size of bentonite particles, decreasing the viscosity of bentonite dispersion, and resulting in a high fluid loss. The addition of TP dispersed bentonite from face-to-face (FF) attraction to edge-to-face (EF) attraction, recovered the house-of-cards structure, and increased the viscosity of TP/BT-WDFs. Under the dispersion effect of TP, an appropriate grain composition of bentonite particles was formed and the pore throats were plugged to prevent the penetration of water. Finally, a compact and thin filter cake was built and the fluid loss was greatly reduced. The TP/BT-WDFs exhibited good filtration property. TP is a prospective candidate to be a high-performance and biodegradable fluid loss additive in well-drilling applications.

Macro- and microscale structure formation and aging in different arrested states of Laponite dispersions

Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most pronounced at pH = 8.5 occurs during gel or glass formation, but at longer times, all arrested states appear homogenous at the 0.2 μm length scale. Finally, all arrested states exhibit power law frequency dependence G″ ∼ ω0.75 at high frequencies attributed to internal bending modes of layers.Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most prono...

DEM dynamic simulation of tetrahedral particle packing under 3D mechanical vibration

The transition from random loose packing (RLP) to random close packing (RCP) of mono-sized regular tetrahedral particles under 3D mechanical vibration was modeled by using discrete element method. The effects of vibration conditions and container size on the packing densification were systematically studied, and the macro and micro properties such as packing density, coordination number (CN), particle contact type, radial distribution function (RDF), particle orientation correlation, and forces between particles were characterized and analyzed. The randomness of the obtained dense packings and corresponding densification mechanisms were also investigated. The results show that RCP of the tetrahedral particles can be realized by properly controlling the vibration conditions. And the obtained maximum random packing density can reach about 0.7402 by extrapolating the packing densities in different sized containers. The average CN for RLP and RCP are 7 and 6.3, and the CN distribution for RCP is higher and narrower than that for RLP. From RLP to RCP, the frequency of face to face (F-F) contact between two particles increases, while that of vertex to face (V-F) contact and edge to face (E-F) contact decreases, and edge to edge (E-E) contact does not change much. RDF characterization shows four obvious peaks on the curves of RLP and RCP, where the height of the first peak (F-F contact) increases while that of other peaks decreases from RLP to RCP. The densification mechanism can be ascribed to the formation of wagon wheel local dense structures through rearrangement by ‘pushing filling’ and ‘jumping filling’.

Four supramolecular isomers of dichloridobis(1,10-phenanthroline)cobalt(II): synthesis, structure characterization and isomerization.

Crystal engineering can be described as the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding to design new solids with desired physical and chemical properties. Free-energy differences between supramolecular isomers are generally small and minor changes in the crystallization conditions may result in the occurrence of new isomers. The study of supramolecular isomerism will help us to understand the mechanism of crystallization, a very central concept of crystal engineering. Two supramolecular isomers of dichloridobis(1,10-phenanthroline-κ(2)N,N')cobalt(II), [CoCl2(C12H8N2)2], i.e. (IA) (orthorhombic) and (IB) (monoclinic), and two supramolecular isomers of dichloridobis(1,10-phenanthroline-κ(2)N,N')cobalt(II) N,N-dimethylformamide monosolvate, [CoCl2(C12H8N2)2]·C3H7NO, i.e. (IIA) (orthorhombic) and (IIB) (monoclinic), were synthesized in dimethylformamide (DMF) and structurally characterized. Of these, (IA) and (IIA) have been prepared and structurally characterized previously [Li et al. (2007). Acta Cryst. E63, m1880-m1880; Cai et al. (2008). Acta Cryst. E64, m1328-m1329]. We found that the heating rate is a key factor for the crystallization of (IA) or (IB), while the temperature difference is responsible for the crystallization of (IIA) or (IIB). Based on the crystallization conditions, isomerization behaviour, the KPI (Kitajgorodskij packing index) values and the density data, (IB) and (IIA) are assigned as the thermodynamic and stable kinetic isomers, respectively, while (IA) and (IIB) are assigned as the metastable kinetic products. The 1,10-phenanthroline (phen) ligands interact with each other through offset face-to-face (OFF) π-π stacking in (IB) and (IIB), but by edge-to-face (EF) C-H...π interactions in (IA) and (IIA). Meanwhile, the DMF molecules in (IIB) connect to neighbouring [CoCl2(phen)2] units through two C-H...Cl hydrogen bonds, whereas there are no obvious interactions between DMF molecules and [CoCl2(phen)2] units in (IIA). Since OFF π-π stacking is generally stronger than EF C-H...π interactions for transition-metal complexes with nitrogen-containing aromatic ligands, (IIA) is among the uncommon examples that are stable and densely packed but that do not following Etter's intermolecular interaction hierarchy.

Modeling marine mud: A four phase system

Fine-grained marine sediment consists of up to four phases: (1) clay Minerals often with some silt and sand, (2) saline water, (3) free gas, and (4) organic matter (OM). Shallow-water, shelf, and slope muds often have all four phases present. The clay minerals are the “building blocks” of mud deposits; these consist of multi-plate face-to-face domains. There are only three possible modes of domain contact (termed signatures): (1) edge-to-face (EF), (2) edge-to-edge (EE), and (3) face-to-face (FF), usually offset. The domain faces are negatively charged and have a large surface area and the edges are both negative and positive charged. OM is usually expressed as total organic carbon (TOC) in percent total dry mass and can range from <0.5% to several percent and the OM can occupy a significant portion of pore volume. OM is usually attached to clay domain signatures and faces with the location partially dictated by electrostatic potential energy fields. When several percent TOC is present, the OM drives up water contents and porosity considerably. When free gas is present, it replaces part of the seawater in the pore space. Several published organo-clay fabric models for mud deposits may improve prediction of acoustic properties.

Smectite flocculation structure modified by Al 13 macro-molecules – As revealed by the transmission X-ray microscopy (TXM)

The aggregate structure which occurs in aqueous smectitic suspensions is responsible for poor water clarification, difficulties in sludge dewatering and the unusual rheological behaviour of smectite rich soils. These macroscopic properties are dictated by the 3D structural arrangement of smectite finest fraction within flocculated aggregates. Here, we report results from a relatively new technique, transmission X-ray microscopy (TXM), which makes it possible to investigate the internal structure and 3D tomographic reconstruction of the smectite clay aggregates modified by Al 13 Keggin macro-molecule [Al 13(O) 4(OH) 24(H 2O) 12] 7+. Three different treatment methods were shown resulted in three different micro-structural environments of the resulting flocculation. In case of smectite sample prepared in Methods 1 and 3 particles fall into the primary minimum where Van der Waals forces act between FF oriented smectite flakes and aggregates become approach irreversible flocculation. In case of sample prepared using Method 2, particles contacting by edges (EE) and edge to face (EF) orientation fell into secondary minimum and weak flocculation resulted in severe gelation and formation of the micelle-like texture in fringe superstructure, which was first time observed in smectite based gel.

Structural relationships between o-, m- and p-tolyl substituted R3EI2 (E = As, P) and [(R3E)AuX] (E = As, P; X = Cl, Br, I)

The compounds R3EI2 (R = o-tolyl, E = As, 1a; R = m-tolyl, E = P 1c; R = p-tolyl, E = As, 1d, P, 1e), which display the charge transfer spoke structure, and [(o-tolyl3As)AuCl] 2 have been synthesised and their solid state structures compared to the related complexes [(R3P)AuX] (R = o-tolyl, X = Cl, I, Ia; Br, II; I, III; R = m-tolyl, X = Cl, IV; R = p-tolyl, X = Cl, V, Va; Br, VI; I, VII) on the basis of a similarity of their molecular shape and volume. All of the new compounds 1a, 1c–1e and 2 have been fully spectroscopically characterised and by single crystal X-ray crystallography. The sterically demanding exo3o-tolyl ring conformation is observed for 1a, which is comparable to that reported for o-tolyl3PI21b, with a long As–I bond 2.7351(14) Å and short I⋯I distance 2.9528(11) Å. The exo3o-tolyl ring conformation is maintained on complexation to gold(I) in 2, but has no significant impact on the expected bond lengths, with As–Au 2.3443(15) Å and Au–Cl 2.284(4) Å. The exo3 conformation appears to be stabilised in both cases by the formation of a six-fold edge-to-face (EF)6 embrace. It is found that in some cases the structures of the dihalogen adducts and the gold(I) complexes are isomorphous indicating that ligand packing requirements are most significant i.e. for 1c and IV. Where the structures digress this is due either to the greater ability of the dihalogen adduct to engage in hydrogen bonding 1a, b and I–III; or subtle changes in the nature of the tolyl ring embraces 1d, e and V–VII. Subtle changes in the nature of the tolyl ring embraces also account for the different polymorphs I and Ia and V and Va. There is no credible evidence to suggest that the aurophilic contact, seen in only one polymorph Va, exerts any influence on the overall crystal packing. The structural comparisons presented here add further to the applicability of the recently recognised structural mimicking ability of the R3PX2 systems and [R3PAuX] complexes, and that the aurophilic contact is a poor supramolecular synthon.

Alternative metal grid structures formed by [M(terpy)2]2+ and [M(terpyOH)2]2+ complexes with small and large tetrahedral dianions, and by [Ru(terpy)2

Terpy embraces (TE) are intermolecular motifs involving metal complexes of the type [M(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine), based on concerted offset-face-to-face (OFF) and edge-to-face (EF) interactions between pyridyl rings. These embraces are well known to occur as extended two-dimensional grid motifs in crystals (called two-dimensional terpy embraces, 2D-TE). We have previously enunciated the layer-stacking principles for 2D-TE when the complexes are associated with monoanions which occupy cavities within grooves on the surfaces of the layers. We introduce here two new features of terpy embraces in crystals: (a) the use of tetrahedral dianions to control the stacking of layers and (b) an alternative layer motif, namely the two-dimensional inverted chain terpy embrace (2D-IC-TE). The standard 2D-TE is constructed with only translation of complexes, while the 2D-IC-TE also possesses inversion centres between complexes in the layer. The density of local OFF and EF interactions in the new 2D-IC-TE is the same as that in the standard 2D-TE, but the surfaces of the layers possess a halved number of enlarged troughs. In crystal [Ni(terpy)2](SO4)(nPrOH)4, 1, the orthogonal SO2 fins of dianionic sulfate engage the shallow grooves of the standard 2D-TE grids of [Ni(terpy)2]2+ complexes, causing them to stack in a fashion not previously observed. With a larger tetrahedral dianion, HgI42−, in crystal [Ni(terpy)2](HgI4), 2, the alternative 2D-IC-TE grids of [Ni(terpy)2]2+ complexes occur, with the anions occupying the troughs. Variation of the ligand to terpyOH (4-hydroxy-2,2′:6′,2″-terpyridine), with sulfate, yields crystals [Ni(terpyOH)2](SO4)(MeOH)(EtOH)(H2O)0.5, 3, which also contain 2D-IC-TE grids. Half of the sulfate together with methanol and ethanol occupy the troughs, and a hydrogen bonding network links terpyOH, MeOH, EtOH, SO42− and water. Two related crystal structures, [CoIII(terpyOH)(terpyO)](SO4)(EtOH)3(H2O)4, 4, and [CoIII(terpyOH)(terpyO)](BF4)2(H2O)4.5, 5, are also described. They are dominated by hydrogen bonding networks, and contain no terpy embraces. The one known related structure with no anions, [Ru(terpy)2]0 [CSD LOCRIE], is also analysed in detail for the first time. It contains modified 2D-IC-TE layers, interleaved with twinned pairs of one-dimensional TE chains. There are multiple OFF and EF interactions at the interfaces of these two motifs, but no new concerted embrace motifs. Analysis of this structure in the context of conventional [M(terpy)2]2+ complexes provides some guidance for the engineering of two-dimensional grids.

Micro-structure differences in kaolinite suspensions

SEM observations of the aqueous suspensions of kaolinite from Birdwood (South Australia) and Georgia (USA) show noticeable differences in number of physical behaviour which has been explained by different micro-structure constitution. Birdwood kaolinite dispersion gels are observed at very low solid loadings in comparison with Georgia KGa-1 kaolinite dispersions which remain fluid at higher solids loading. To explain this behaviour, the specific particle interactions of Birdwood kaolinite, different from interaction in Georgia kaolinite have been proposed. These interactions may be brought about by the presence of nano-bubbles on clay crystal edges and may force clay particles to aggregate by bubble coalescence. This explains the predominance of stair step edge-edge like (EE) contacts in suspension of Birdwood kaolinite. Such EE linked particles build long strings that form a spacious cell structure. Hydrocarbon contamination of colloidal kaolinite particles and low aspect ratio are discussed as possible explanations of this unusual behaviour of Birdwood kaolinite. In Georgia KGa-1 kaolinite dispersions instead of EE contact between platelets displayed in Birdwood kaolinite, most particles have edge-to-face (EF) contacts building a cardhouse structure. Such an arrangement is much less voluminous in comparison with the Birdwood kaolinite cellular honeycomb structure observed previously in smectite aqueous suspensions. Such structural characteristics of KGa-1 kaolinite particles enable higher solid volume fractions pulps to form before significantly networked gel consistency is attained.

The Intramolecular Edge‐to‐Face Interactions of an Aryl CH Bond and of a Pyridine Nitrogen Lone‐Pair with Aromatic and Fluoroaromatic Systems in Some [3,3]Metaparacyclophanes: A Combined Computational and NMR Study

Simple model systems based on the [3,3]metaparacyclophane skeleton have been designed to study the effect of fluorination of the "face" ring on the edge-to-face (EtF) interactions with the C(Ar)-H bond of a phenyl ring or the nitrogen lone-pair of pyridine. Calculations established that in their more stable conformation the model systems adopt a tilted EtF disposition with the rim of the meta-substituted ring pointing towards the face of the para-substituted ring. Topomerization occurs by flipping of the meta-substituted ring, a process that involves the formation of an intermediate featuring an orthogonal EtF disposition of the arenes, which is less stable than the tilted arrangement. The energy barriers to the isomerization were determined by variable-temperature NMR spectroscopy and were well reproduced by DFT calculations. The variation in the energy barrier as a function of the substitution of the para-substituted ring could be rationalized well by a polar interpretation of the EtF interaction in the cases of model systems presenting the PyN...pi interaction but not in the cases of models featuring the C(Ar)-H...pi interaction.

Crystal packing in metal complexes of 4′-phenylterpyridine and related ligands: occurrence of the 2D and 1D terpy embrace arrays

The crystallisation and resulting crystal structures of complexes [M(4′-Ph-terpy)2](PF6)2 (4′-Ph-terpy = 4′-phenyl-2,2′ : 6′,2″-terpyridine) are described. Three crystal structure types retain the 2D terpy embrace (2D-TE) layers common in crystals of unsubstituted [Mterpy)2]2+ complexes, while one structure type has the phenyl substituents intercalated into the 2D-TE, generating 1D terpy embrace chains. Where the 2D-TE is retained there are edge-to-face (EF) or offset-face-to-face (OFF) interactions between the phenyl substituents in the interlayer domain which also contains solvent, while in the intercalated structure the phenyl substituents engage the outer pyridyl rings of terpy in EF and OFF interactions. Anions occupy the major grooves, as usual. Solvent identity is influential, but enigmatic.

Complementary Wave-Based Characterizations of Sedimentation Processes

In this paper, the sedimentation behavior of two kaolinite samples with distinct fabric associations is characterized using mechanical and electromagnetic wave-based techniques. The two different fabric formations, the edge-to-face (EF) flocculated structure (i.e., Sample A) and the dispersed and deflocculated structure (i.e., Sample B), were produced by changing the pH of the pore fluid. The anisotropy of the shear-wave velocity and DC conductivity was not observed in the sediment of Sample A because of EF isotropic fabric associations, but it was detected in Sample B as a result of face-to-face aggregation. An open card-house structure of the Sample A sediment results in a higher relaxation strength of the bulk water, Δ κw owing to a higher water content; the smaller Δ κw measured in the Sample B sediment indicates denser packing. In both samples, sediment consolidation gives rise to a decrease in the bulk-water relaxation strength, but an increase in the bound-water relaxation strength owing to increas...

Structure characteristics and mechanical properties of kaolinite soils. I. Surface charges and structural characterizations

In this study on the surface charges and structural characterization of kaolinite clay, it is found that the highly pH dependent edge charges are important to the interparticle forces and associated fabric formations. When the pH is below the isoelectrical point of the edge surfaces, IEPedge, the Coulombian attraction between faces and edges dominates, which favors the formation of an opened edge-to-face (EF) association and a higher final sediment volume. At pH > IEPedge, the double-layer repulsion prevails at all surfaces to result in a deflocculated and dispersed fabric, which tends to form denser soil packing in the sediment. Increasing the ionic strength (with NaCl as the electrolyte) leads to the opposite behavior in clay suspensions below and above the IEPedge: the degree of fabric flocculation and the final sediment volume decrease with an increased ionic strength at pH < IEPedge but increase at pH > IEPedge. The measured pore-size distribution reveals a distinct dual-porosity characteristic in the pH 4 with salts (0.15 mol/L NaCl) specimen: it consists of inter-aggregate spaces (~1.10 µm) and intra-aggregate pores (~0.30 µm). Soils with a higher degree of EF flocculation due to the prevailing Coulombian or van der Waals attraction can render a higher liquid limit.Key words: pH, fabric associations, isoelectrical point, dual porosity, interparticle forces, liquid limit.