Crystalline Compounds Research Articles

Development of natural sodium montmorillonite as a cathode material

Montmorillonite (MMT) is naturally occurring clay mineral in Sri Lanka. It is an aluminosilicate crystalline compound with layered structure. In this structure an octahedral sheet of aluminum-oxygen (Al-O) is sandwiched between two tetrahedral sheets of silicon-oxygen (Si-O). The MMT structure can accommodate various monovalent and divalent cations such as H⁺, Li⁺, Na⁺, Mg²⁺ within the interlayer spacing. Natural Li-MMT and Na-MMT are found in various parts of the world. The MMT structure has an ability to exchange interlayer cations with various other divalent cations. MMT with interlayer alkali cations show appreciable ionic conductivity. Natural Na-MMT found in Sri Lanka was used for the present study. To improve the ionic conductivity of natural Na-MMT, the structure was further saturated with Na⁺ ions. The Na⁺ ion concentration of saturated Na-MMT was measured by Inductive Couple Plasma Mass Spectroscopy. The structural and electrical properties of Na saturated MMT were characterized by SEM and Complex Impedance Spectroscopy. As a result of Na⁺ ion saturation the bulk conductivity of Na-MMT was increased to 3.41×10^–4 S cm^–1 at 30⁰C . Saturated Na-MMT has negligible electronic conductivity which is less than 0.001%. For using Na-MMT as intercalation cathode materials in rechargeable batteries, an appreciable electronic conductivity must also be present. To enhance the electronic conductivity of saturated Na-MMT, carbon black was mixed. The electronic transference number of saturated Na-MMT pellets was measured by the DC polarization technique. The electronic transference number of saturated Na-MMT with 25% and 30% carbon black were 0.81 and 0.87 respectively.

Recent Trends in the Design, Synthesis and Biomedical Applications of Covalent Organic Frameworks.

The most recent and advanced class of crystalline and permeable compounds are covalent organic frameworks (COFs). Due to their exceptional qualities, such as their porous structure, high surface area, strong chemical and thermal stabilities, low density, good water stability, luminescent nature, and so on, COFs have seen remarkable growth over the past ten years. COFs have been successfully researched for a number of applications based on these characteristics. The current state of COFs has been reported in this study, with particular attention paid to their design, topology, synthesis, and a variety of biological applications, including drug delivery systems, photodynamic and photothermal therapy, biosensing, bioimaging, etc. Moreover, several miscellaneous applications, such as catalysis, gas storage and separation, photocatalysis, sensors, solar cells, supercapacitors, and 3D printers, have also been explored. It is significant that we have examined current research on COFs with a focus on the biological applications, which are infrequently covered in the literature. Descriptions of the difficulties and prospective outcomes have also been given.

The kihara potential function parameters of methane, ethane, propane, and i-butane: The effects on clathrate hydrate structure determination

Gas hydrates, or clathrate hydrates, are solid crystalline compounds, which are formed by combination of water and gas and/or some volatile liquid molecules. Prediction of hydrate stability/dissociation/equilibrium conditions of natural gases is important in separation processes, gas storage, and in preventing blockage of gas transmission pipelines. In this study, initially, the different sets of the Kihara Potential Function Parameters, KPFP, reported in the literature were used to predict the experimental hydrate dissociation conditions of methane, ethane, propane and i-butane and mixtures of these four compounds. In most cases, however, based on these sets of KPFP, the hydrate structure cannot be predicted correctly. Consequently due to incorrect estimation of the hydrate structure, especially for natural gas mixture, the predicted hydrate dissociation conditions are found inaccurate. For overcoming this fault and by using a genetic algorithm, a new set of KPFP were optimized based on the new definition of the objective function considering hydrate structure. The results show good agreement with experimental data, both in the prediction of hydrate dissociation conditions and hydrate structure.

A reliable model to predict the methane-hydrate equilibrium: An updated database and machine learning approach

Gas hydrates are a type of crystalline compounds that consists of water and small gas molecules. A wide range of applications of gas hydrates in storing natural gas in the form of artificially created solid hydrates, known as solidified natural gas technology, gas separation processes, and seawater desalination technology, has attracted great interest in scientific and practical studies. Gas hydrate formation may also cause deleterious effects such as blockage of gas pipelines. Therefore, accurate prediction of equilibrium conditions for gas hydrates is of great interest. The purpose of this study was to propose machine learning based models to predict methane-hydrate formation temperature for a wide range of brines. In this regard, firstly, a comprehensive database including 987 data samples covering 15 different brines was gathered from the literature. After data cleaning and preparation, three different models of multilayer perceptron, decision tree, and extremely randomized trees were used and tested. The results showed that the extremely randomized trees is capable of predicting methane-hydrate formation temperature with good accuracy. The root mean squared error for this model for the testing dataset was acquired as 0.6248, which shows its great accuracy. The findings of this study can be used as a reliable tool to predict the methane-hydrate formation PT curve in the pure water, single-salt brines, and multi-salt brines.

Thermodynamic Morphological Study of Liquid Crystalline Compounds

Thermodynamic morphism study has been envisaged using search procedure of phase space of polymorphs at a prescribed temperature and pressure(NPT) of several azoxybenzenes based liquid crystalline compounds at room temperature using molecular structure, unit cell information and AMOEBA parameters employing Force Field computation (FFX) calculations. These calculations were carried out at various intermolecular strengths to understand the morphological walk of molecules with corresponding changes in free energy, change in a force field with a strength of the interactions and free energy difference between successive interaction strengths. This study will help us to understand the nature, range and type of mesophase in a series of azoxybenzene compounds.

Raman spectroscopy and DFT modelling of Tl2S-GeS2 crystals and glasses

Thallium thiogermanate glasses and crystals were studied by Raman spectroscopy accompanied by DFT modelling. The experimental Raman spectra of the known crystalline compounds in the Tl2S-GeS2 system are consistent with the crystal structure and DFT modelling of the respective Tl-Ge-S clusters, representing structural motifs in c-Tl4Ge4S10, c-Tl4Ge2S6, and c-Tl4GeS4. The Raman spectra of the (Tl2S)x(GeS2)1-x glasses can be divided into two groups: (1) semiconducting glasses at x ≤ 0.3, and (2) ion-conducting vitreous alloys at x > 0.3. At x ≤ 0.3, the continuous g-GeS2 network is transformed into a fragmented structure consisting of adamantane-like units Ge4S10 held together by thallium species. In the ion-conducting composition domain at x > 0.3 takes place further fragmentation of tetrahedral tetramers Ge4S10 into edge-sharing ES-Ge2S6 dimers and small corner-sharing CS-oligomers, i.e., CS-Ge2S7 dimers or CS-Ge3S10 trimers.

Compositionally Disordered Crystalline Compounds for Next Generation of Radiation Detectors.

The review is devoted to the analysis of the compositional disordering potential of the crystal matrix of a scintillator to improve its scintillation parameters. Technological capabilities to complicate crystal matrices both in anionic and cationic sublattices of a variety of compounds are examined. The effects of the disorder at nano-level on the landscape at the bottom of the conduction band, which is adjacent to the band gap, have been discussed. The ways to control the composition of polycationic compounds when creating precursors, the role of disorder in the anionic sublattice in alkali halide compounds, a positive role of Gd based matrices on scintillation properties, and the control of the heterovalent state of the activator by creation of disorder in silicates have been considered as well. The benefits of introducing a 3D printing method, which is prospective for the engineering and production of scintillators at the nanoscale level, have been manifested.

Classification of liquid crystal textures using convolutional neural networks

ABSTRACTWe investigate the application of convolutional neural networks (CNNs) to the classification of liquid crystal phases from images of their experimental textures. Three CNN classifier model types (Sequential, Inception and ResNet50) are tuned and trained on five individual phase group datasets. The complete dataset includes images of the cholesteric phase, chiral fluid smectic A and C phases and hexatic smectic I and F phases, all extracted from polarised microscopy videos of various liquid crystalline compounds. Three binary classification tasks, each including two liquid crystal phases, provide the foundational demonstration of CNN model viability. The average test set accuracies obtained are approximately (95 ± 2)%. More complex multi-phase datasets are also created and investigated, with a three-phase cholesteric, fluid smectic, and hexatic smectic set, in addition to a set containing all five individual phases. The average test set accuracies for these classification tasks are (85 ± 2)%.

Composites of bentonite mixed with natural fibers as acoustic absorption material

Noise pollution is a very common problem that causes a negative impact on society. Long exposure to high levels of noise might lead to dire health consequences such as irritability, headaches, stress, anxiety, and hearing loss. The use of acoustic sound-absorbing materials may decrease noise propagation when they are used as insulation and consequently reduces the negative impact. Different composite materials made with natural waste have been studied showing a wide range of thermal, physical, ecological, and economic properties that impact mainly the application of sound absorption. In this work we focused on a composite of natural bentonite clay mixed with coconut or nopal fibers, varying the fiber weight ratio. An impedance tube was used to quantify the reflection, transmission, and absorption acoustic coefficients in a frequency range of 50 Hz to 10,000 Hz. The type and percentage of crystalline compounds and the functional groups of the natural clay were analyzed by X-ray diffraction and FTIR spectroscopy, respectively. Morphological analysis of the fibers was carried out using a scanning electron microscope (SEM), and thermogravimetric analysis (TGA) technique was used to determine thermal stability. A hardness test was performed on the samples with 3 months of aging, this test was repeated 10 times for each of the samples and 2 different samples were used for each composition. The results show a difference in the three acoustic coefficients related to fiber weight ratio. The lower weight of clay the higher magnitude of the absorption coefficient. The clay-coconut composites present better absorbing properties than clay-nopal composites, although the magnitude of this coefficient increases as a function of frequency for both composites. The highest reflection coefficient is presented in the clay samples, and the lowest transmission coefficient is shown in the coconut sample. In addition, TGA suggested bentonite improves the thermal stability of the composite. The experimental acoustic characterization shows that both types of composites could be used as a green material for sound absorption application and thermally insulating.

The First Noncovalent-Bonded Supramolecular Frameworks of (Benzylthio)Acetic Acid with Proline Compounds, Isonicotinamide and Tryptamine

The co-crystallization of (benzylthio)acetic acid (HBTA) with L-proline (L-PRO), D-proline (D-PRO), DL-proline (DL-PRO), isonicotinamide (INA) and tryptamine (TPA) led to the formation of five novel crystalline compounds: L-PRO±·HBTA (1), D-PRO±·HBTA (2), DL-PRO±·HBTA (3), INA·HBTA (4) and TPA+·BTA- (5). The prepared supramolecular assemblies were characterized by single crystal X-ray diffraction, an elemental analysis, FT-IR spectroscopy and a thermal analysis based on thermogravimetry (TG) combined with differential scanning calorimetry (DSC). Additionally, their melting points through TG/DSC measurements were established. All fabricated adducts demonstrated the same stoichiometry, displayed as 1:1. The integration of HBTA with selected N-containing co-formers yielded different forms of multi-component crystalline phases: zwitterionic co-crystals (1-3), true co-crystal (4) or true salt (5). In the asymmetric units of 1-4, the acidic ingredient is protonated, whereas the corresponding N-containing entities take either the zwitterionic form (1-3) or remain in the original neutral figure (4). The molecular structure of complex 5 is occupied by the real ionic forms of both components, namely the (benzylthio)acetate anion (BTA-) and the tryptaminium cation (TPA+). In crystals 1-5, the respective molecular residues are permanently bound to each other via strong H-bonds provided by the following pairs of donor···acceptor: Ocarboxylic···Ocarboxylate and Npyrrolidinium···Ocarboxylate in 1-3, Ocarboxylic···Npyridine and Namine···Ocarboxylic in 4 as well as Nindole···Ocarboxylate and Naminium···Ocarboxylate in 5. The crystal structures of conglomerates 1-5 are also stabilized by numerous weaker intermolecular contacts, including C-H···O (1-3, 5), C-H···S (1, 2, 5), C-H···N (5), C-H···C (5), C-H···π (1-5) as well as π···π (4) interactions. The different courses of registered FT-IR spectral traces and thermal profiles for materials 1-5 in relation to their counterparts, gained for the pure molecular ingredients, also clearly confirm the formation of new crystalline phases.

Tailoring of the Gd–Y–Lu ratio in quintuple (Gd, Lu, Y)3Al2Ga3O12:Ce ceramics for better scintillation properties

Ceramics from cerium-doped quintuple garnet-type compounds (Gd,Y,Lu)3Al2Ga3O12:Ce with different contents of Y and Lu have been fabricated and studied as a prospective scintillating material enabling improvement of scintillation properties by tuning of the composition of the matrix-forming crystalline compound. The coprecipitation method was applied to produce powders of complex garnet compounds, which were subsequently compacted and sintered. The influence of the matrix composition on photoluminescence and scintillation of the ceramics is studied using luminescence spectroscopy at steady state and pulsed excitation. Partial substitution of Lu by Y ions was found to be an effective tool for tailoring of scintillation properties of the material. A light yield of the Gd2Y0.5Lu0.5Al2Ga3O12 composition was measured to be 41 000 ph/MeV, whereas a scintillation kinetics was measured to have dominant fast scintillation components: 14 ns (84%) and 78 ns (16%).

Crystallization Kinetics on Melt Spun and HPT-Processed Zr62Cu22Al10Fe5Dy1 Metallic Glass

The influence of conventional and accumulative HPT (Acc. HPT) process on structural properties of Zr62Cu22Al10Fe5Dy1 metallic glass (MG) phase is examined by X-ray diffraction (XRD) technique. The XRD results showed that HPT process leads to free volume generation in MG. Free volume increases with the number of anvil turns in the conventional HPT process, while the highest free volume increment was observed in Acc. HPT process. To understand the role of latent crystalline phases on free volume, thermal stability and crystallization kinetics of Zr62Cu22Al10Fe5Dy1 MG are investigated by non-isothermal heating using differential scanning calorimetry (DSC). Thermal investigation indicates Dy addition results in increased activation energy along with lower nucleation rate during crystallization. As the initial amorphous phase significantly influences crystallization behavior, d-spacing analysis of amorphous hump is carried out to predict latent crystalline phases. Strong correlation was observed between latent crystalline phases and intermetallic compounds in devitrified Zr62Cu22Al10Fe5Dy1 alloy.

Phase Sequence, Kinetics of Crystallization and Molecular Dynamics of the Chiral Liquid Crystalline Compound Forming a Hexatic Smectic Glass

The vitrification of the antiferroelectric hexatic smectic XA* phase and cold crystallization are reported for (S)-4′-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]benzoate. The kinetics of isothermal cold crystallization and melt crystallization are investigated, revealing that both are controlled mainly by diffusion, as indicated by decrease in the characteristic crystallization time with increasing temperature of crystallization, with an activation energy of 114 kJ/mol. A weak relaxation process is detected in a crystal phase, with an activation energy of 38 kJ/mol, implying the conformationally disordered crystal phase. The estimated fragility parameter of the investigated glass former is equal to 94.5, which indicates rather high fragility.

Influence of sample measuring cell on isothermal crystallization of liquid crystalline compound forming the smectic CA* phase

ABSTRACT Crystallization of liquid crystalline (S)-4′-(1-methylheptyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2,3-difluorobenzoate in isothermal conditions is investigated by polarizing optical microscopy and differential scanning calorimetry. It is shown that the sample container influences the crystallization kinetics, as a faster development of the crystal phase is observed on microscopic textures, for a sample in an electro-optic cell with a planarly aligning polymer layer, than for a sample in an aluminum pan used in a differential scanning calorimetry experiment. However, in both cases the total time of melt crystallization in the 291–303 K range is shorter than 10 min. Only at the highest investigated temperature, 305 K, the crystallization process is completed after 1.4 h because of a long initialization time. The difference with the glassforming (S)-4′-(1-methylheptyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2-fluorobenzoate is explained based on the thermodynamic driving force of crystallization.

Structure of an amorphous calcium carbonate phase involved in the formation of Pinctada margaritifera shells.

Some mollusc shells are formed from an amorphous calcium carbonate (ACC) compound, which further transforms into a crystalline material. The transformation mechanism is not fully understood but is however crucial to develop bioinspired synthetic biomineralization strategies or accurate marine biomineral proxies for geoscience. The difficulty arises from the simultaneous presence of crystalline and amorphous compounds in the shell, which complicates the selective experimental characterization of the amorphous fraction. Here, we use nanobeam X-ray total scattering together with an approach to separate crystalline and amorphous scattering contributions to obtain the spatially resolved atomic pair distribution function (PDF). We resolve three distinct amorphous calcium carbonate compounds, present in the shell of Pinctada margaritifera and attributed to: interprismatic periostracum, young mineralizing units, and mature mineralizing units. From this, we extract accurate bond parameters by reverse Monte Carlo (RMC) modeling of the PDF. This shows that the three amorphous compounds differ mostly in their Ca-O nearest-neighbor atom pair distance. Further characterization with conventional spectroscopic techniques unveils the presence of Mg in the shell and shows Mg-calcite in the final, crystallized shell. In line with recent literature, we propose that the amorphous-to-crystal transition is mediated by the presence of Mg. The transition occurs through the decomposition of the initial Mg-rich precursor into a second Mg-poor ACC compound before forming a crystal.

Inhibitor-Resistant Calcites Causing Solids Fouling Identified, Studied

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 209475, “Unraveling the Mystery of Inhibitor-Resistant Calcite Affecting Onshore and Offshore Operations,” by Hugh M. Bourne, SPE, Adil A. Abbasov, and Ben E. Smith, BP. The paper has not been peer reviewed. _ Magnesian calcite has been identified as the main constituent of solids fouling in several of an operator’s facilities in the North Sea and Azerbaijan and in partner operations in other locations. These solids were, for many years, thought to be calcite scale. The complete paper describes how this “pseudoscale” was detected and its prevalence, using field case examples. The Discovery: Case 1 Soon after the start of operations in May 2017, fouling of the rundown coolers was observed on a North Sea floating production, storage, and offloading vessel. The increase in differential pressure across the coolers was believed to be the result of wax deposition. The cooler performance, however, progressively deteriorated with every dewaxing cycle to a point at which running the cooler without cooling medium had no effect on the differential pressure. One of the rundown coolers was removed and returned to the manufacturer for inspection and cleaning. This revealed that hard, scale-like solids were filling the space between the plates completely (Fig. 1). The retrieved solids dissolved in acid, were calcium-rich, and appeared visually and mechanically to be a scale; thus, they were assumed initially to be calcium carbonate. The first analysis of solids from the rundown cooler and other parts of the plant suggested that up to 33% of the composition of these solids was calcium, which would be equivalent to greater than 80% of these solids being calcium carbonate. Scaling in the rundown cooler was not expected and raised concerns about uncontrolled scaling in other parts of the plant. A wider review of plant performance was initiated, and more-detailed analysis of the solids collected in the rundown coolers was undertaken using X-ray diffraction spectroscopy (XRD). The rundown cooler solids analysis revealed an inorganic crystalline compound consisting of magnesian calcite (calcium/magnesium carbonate) that had the appearance of a compacted transported solid. The dewaxing frequency was increased to prevent this accumulation, which ultimately proved an effective mitigation strategy. The wider review of plant performance identified frequent fouling of the produced water-pump suction strainers and a gradual decrease in the performance of the crude-oil heaters. Analysis of the solids fouling the produced-water-pump suction strainers also was dominated by magnesian calcite, suggesting that the reservoir fines were transported in both the oil and the produced-water streams. Reservoir solids fouling is not uncommon, but, by the beginning of 2018, the decrease in the differential pressure trend across the crude-oil heaters became more prominent. By the end of the first quarter of 2018, the degradation was limiting the ability of the plant to meet temperature set points in the separators, potentially limiting production rates.

Quantifying Micromolar Crystallinity in Pharmaceutical Materials Utilizing 19F Solid-State NMR.

Quantifying low-level components in solid-state analysis presents a significant challenge for most thermal, diffractometric, vibrational, and spectroscopic techniques. In pharmaceutical analysis, identifying and quantifying the physical form of the drug substance in solid dosages is a critical task to ensure the quality of drug products. For example, recrystallization of active pharmaceutical ingredients in amorphous solid dispersions can compromise the stability and bioavailability of drug products. Herein, we have developed and demonstrated fluorine-19 solid-state nuclear magnetic resonance (19F ssNMR) methods and pushed the boundary to quantify minor crystalline contents in amorphous pharmaceuticals. Calibration curves suggest that 19F direct polarization and 1H-19F cross-polarization ssNMR can readily quantify 0.1% w/w crystalline compound I, a commercial fluorinated drug molecule developed by Merck & Co., Inc., Rahway, NJ, U.S.A., in its amorphous formulation. 1H-19F multiple cross-polarization (MultiCP) has been implemented, for the first time, and compared with conventional cross-polarization methods. Most importantly, a relaxation-filtered 19F ssNMR method was utilized to unambiguously identify and quantify as low as 0.04% w/w crystalline components, that is, 6 μmol in a 100 mg tablet at 25% drug loading, by suppressing the signal from the amorphous counterpart. Such a low level of detection offers high confidence and sensitivity to quantify trace amounts of phase change in pharmaceutical amorphous materials in the solid state, which can facilitate formulation development as well as quality control.

Near-Infrared Electrochromic Behavior of Dibenzothiepin Derivatives Attached with Two Michler's Hydrol Blue Units.

10,11-Bis[bis(4-dimethylaminophenyl)methylene]dibenzo[bf]thiepin (1) and -oxepin (2) were prepared as stable yellow crystalline compounds, which are the cyclic analogues of electron-donating hexaarylbutadienes. Upon two-electron oxidation, they are reversibly transformed into the title dications (12+ and 22+ ) exhibiting near-infrared (NIR) absorptions, which were also isolated as stable salts. These redox pairs can serve as new entries into less well-explored organic NIR-electrochromic systems, and the separation of redox peaks (electrochemical bistability) was attained for 1/12+ and 2/22+ , thanks to drastic geometrical changes between neutral and dicationic states, as revealed by a series of X-ray analyses. Thiepin-S,S-dioxide analogue (3/32+ ) exhibits quite similar dynamic redox behavior due to nonaromatic nature of the dibenzothiepin and -oxepin unit in 12+ and 22+ , whereas the thiepin-S-oxide derivative (4/42+ ) does not exhibit bistability due to the smaller change in geometry upon electron transfer, showing that a subtle change of a bridging atom in the central seven-membered ring can modify the redox properties.

Growth, optical, mechanical, dielectric properties of UV-nonlinear optical single-crystal of [(NH4) (Cd(NCS)3)]·C12H24O6

Single-crystal of ammonium (18-crown-6-ether) cadmium (II) tri-thiocyanate [(NH4)[Cd(NCS)3]; (ACCTC) has been grown with dimension 8 × 4 × 4 mm3 was grown from aqueous solution via slow evaporation solvent technique. The S-XRD was utilized to determine the grown single-crystal structure, where the crystalline compound of ACCTC adopts to be orthorhombic crystal system with the space group of Cmc21, and the calculated cell parameters are a = 14.7568 Å, b = 15.4378 Å and c = 10.6383 Å, volume V = 2423.54 Å3. The optical lower cut-off wavelength of the title compound of ACCTC is UV–vis-NIR spectra examined 239 nm. Moreover, the laser-induced surface damage threshold (LISDT), Vickers micro-hardness, and dielectric properties have been examined by the grown title compound of ACCTC. Overall, the experimental results suggest that a single-crystal of ACCTC can be the best nonlinear optical material and possess superior power density (Pd) compared to the KDP single-crystal.

CH/CH2 Group Clusters Doping Methane Hydrate Cages.

Methane hydrate is a crystalline compound with methane molecules as guest species trapped in host water cages. In this study, we detected methane hydrate with water cages doped by (Caromatic-H)5 clusters, (Caromatic-H)6 clusters, and (3Caliphatic-H2 + 2H2O) clusters using current spectroscopic techniques and differential scanning calorimetry (DSC). Methane molecules are trapped in the doped cages with type sI forming in nanoscale silica gel pores. The relative quantity ratio of host carbon to guest carbon in the doped hydrate sample reaches approximately 3.58. Methane hydrate doped by CH/CH2 group clusters greatly improves the ability of the hydrate unit cell to store methane and increases the stability of methane hydrate. Fast proton diffusion in the doped methane hydrate was confirmed. The results of this study will provide efficient and energy saving technical support for disruptive changes in hydrate storage and transportation of methane gas technology with a doped and dense solid phase.