Novel Phenomena of Crystallization and Emulsification of Hydrophobic Solute in Aqueous Solution

Emulsification of lauric acid in an aqueous ethanol solution including lauric acid solute has been observed during cooling before crystallization of lauric acid occurs. The nature of two different solubility curves was explained for the system of lauric acid and aqueous ethanol solution. The mutual solubility of the two liquid phases controls emulsification; the solid solubility of lauric acid controls crystallization. The mutual solubility curve appears at relatively high temperature, and the solid solubility curve at relatively low temperature. Crystallization essentially generates a solid metastable zone under the solid solubility curve. A supersaturated solution can be obtained in the metastable zone. However, no nucleation occurs in the metastable zone. The metastable zone, therefore, still caused emulsification at low temperature before crystallization of lauric acid occurred. The hypothetical mutual solubility curve for the aqueous solution including hydrophobic solutes appeared invariably even at low temperature in the metastable zone under the solid solubility. Copyright 2001 Academic Press.     

Characterization of capillary inner surface conditions with streaming potential

Streaming potential is created when an electrolyte solution is forced to flow pass a charged surface. For an uncoated fused silica capillary, the streaming potential is measured between the inlet and outlet vials while applying a pressure across the capillary. The changes in streaming potential can be used to characterize the properties of the capillary inner surface. In this work, HCl, NaCl, and NaOH solutions ranging from 0.4 to 6 mM were used as the background electrolyte (BGE) at temperatures of 15 to 35 °C for the mesurements. The streaming potential decreases with the increase in BGE concentration, and the trend is amplified at higher temperatures. When buffer solutions in the pH range of 1.5 to 12.7 were used as the BGE, streaming potential was shown to be sensitive to changes in pH but reaches a maximum at around 9.5. At pH < 3.3, no streaming potentials were observed. The pH of zero surface charge (streaming potential equals 0) changes with temperature, and is measured to be 3.3 to 3.1 when the temperature is changed from 15 to 35°C. Zeta potentials can be calculated from the measured streaming potential, conductivity, and the solution viscosity. Surface charge densities were calculated in this work using the zeta potentials obtained. We demonstrated that capillary surface conditions can significantly change the streaming potential, and with three different solutions, we showed that analyte-dependent adsorption can be monitored and mitigated to improve the peak symmetry and migration times reproducibility.     

Calculation of the minimum energy conformation of acetylcholine using a global optimization technique

The conformational energy of acetylcholine is minimized with respect to the distances between nonbonded atoms with the help of the Bremermann method of unconstrained global optimization. The set of distances for which the energy is the absolute minimum is then used to calculate the coordinates of all the atoms and hence the conformation of the molecule. The simplest type of potential function, namely the classical potential function is chosen for the calculation. The major advantages of this method are (i) that the starting point need not be close to the actual solution, (ii) that it gives the global minimum, irrespective of the starting point, (iii) that it is very general and can be used for any type of potential function, and (iv) that it does not require the computation of gradients. The results obtained are in very good agreement with those of other workers.     

Diffusion of Noble Metals,Zn and Ge in GaSb Single Crystals

Diffusion of Cu, Ag, Au, Ge and Zn in single crystal gallium antimonide has been carried out by measuring Hall effect according to van der Pauw, conductivity, energy dispersive X-ray (EDX) and surface electron microscopy. The best results have been obtained in excess of antimony. The resulting diffusion data in GaSb are diffusivity D_o, activation enthalpy Q, carrier density p and mobility μ at 300 K: Ag: D_o=1.8·10⁻⁴ cm² s⁻¹, Q=1.2 eV, p=6·10¹⁸ cm⁻³, μ=550 cm² (Vs)⁻¹ Au: D_o=6.6·10⁺³ cm² s⁻¹, Q=2.7 eV, p=5·10¹⁸ cm⁻³, μ=500 cm² (Vs)⁻¹ Cu: D_o=3.2·10⁺⁸ cm² s⁻¹, Q=2.7 eV, p=2·10¹⁸ cm⁻³, μ=150 cm² (Vs)⁻¹ Zn: D_o=9.2·10⁻² cm² s⁻¹, Q=1.8 eV, p=2·10²⁰ cm⁻³, μ=80 cm² (Vs)⁻¹ Ge: D_o=1.0·10⁻¹ cm² s⁻¹, Q=1.7 eV, p=1·10¹⁹ cm⁻³, μ=320 cm² (Vs)⁻¹. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characteristics of Cyclodextrin Adsorption onto Activated Carbons

Whereas the amount of cyclodextrin (CD) adsorbed onto the large-pore activated carbon A (AC-A) increased with the number of glucose units, the amount adsorbed onto the small-pore activated carbon B (AC-B) showed the opposite tendency. This behavior can be accounted for in terms of a molecular exclusion. It is known that a good linear relationship is obtained between the Freundlich constants log K and 1/N for hydrophobic adsorption. The adsorption of CDs onto AC-A obeyed this relation, but, because of the molecular exclusion, the plots of AC-B deviated greatly. The adsorption of CDs onto AC-A was not explainable in terms of solubility. This could be because, in the case of a solid compound, adsorbability depends on the chemical potential of the molecule in aqueous solution whereas solubility depends also on the heat of fusion of the solid. In order to estimate the relative chemical potential of CDs in water, a method based on the numbers of carbon atoms and oxygen atoms in the molecule was devised which allowed a more accurate estimation of CD adsorbability than did solubility. The mean pore diameter of AC-A increased after CD adsorption, while that of AC-B showed little change. Copyright 2000 Academic Press.     

Thermodynamic study of fatty acids adsorption on different adsorbents

This work has as objective the study about the adsorption behavior of fatty acids (acetic, propionic, and butyric) on activated carbon and on modified and unmodified montmorillonite clays as a function of temperature and initial concentration of the adsorbate, through adsorption isotherms and their thermodynamic parameters (ΔG, ΔH, and ΔS). The activated carbon presented a higher adsorption capacity due to its relatively large surface area, compared to others adsorbents. The polar characteristic of fatty acids decreased with the increase in the length of non-polar hydrocarbon chain, improving the affinity between the activated carbon (non-polar adsorbent) and the acids. The adsorption capacity of modified montmorillonite (polar adsorbent) was favored due to the presence of the organic cation among its layers, which make the surface more hydrophobic and organophilic when compared to the unmodified montmorillonite surface. The amount of fatty acids adsorbed in the adsorbents surface increased with the concentration, at constant temperature, and decreased with the increase of temperature, at constant concentration. The amount of fatty acids adsorbed in the three adsorbents was related to the surface area and polarity of the adsorbent, concentration and solubility of the adsorbate and temperature of the solution. The negative values of ΔG and ΔH showed that the adsorption on activated carbon and on modified and unmodified montmorillonite clays was a spontaneous and an exothermic process. The decrease in the values of ΔG, with the increase of temperature, demonstrated that the adsorption was benefited by the high temperature and the positive values of ΔS showed that the fatty acids molecules were in a more randomic condition in the adsorbed state than in solution. The experimental results obtained at the temperatures of (298, 303, 313, and 323) K showed that experimental data were well represented by the Langmuir and Freundlich isotherms models.     

Solubility and thermodynamics of carbon dioxide in aqueous ethanol solutions

Solubilities of carbon dioxide in aqueous ethanol solutions, determined at 15, 20, 25, 30 and 35°C at atmospheric pressure, show a minimum at each temperature. Thermodynamics functions for the solution process were calculated and compared with the predictions of the Pierotti gas solubility theory. The abnormal behavior of these solutions is demonstrated.     

Aqueous solubility calculation of aromatic acids within a wide temperature range using a modified regular solution model

Abstract  

The modified RSFH model, based on the regular solution theory coupled with the Flory-Huggins entropy term, was extended to calculate the solubility of aromatic acids in water within wide temperature ranges. The aqueous solubility data on aromatic acids from the published literature were assembled and validated. A total of 1,009 aqueous solubility data points for 25 aromatic acids within the temperature range of 273–463 K were selected for modeling. The calculation results showed that the solubility of aromatic acids in water could be well represented by the proposed four-parameter solution model within a wide temperature range. The overall absolute average deviation (δ _AAD) is 6.76%. The estimated cohesive energies of the aromatic acids were found to be about 20–30 kJ mol⁻¹. For the majority of the aromatic acids investigated, the cohesive energy could be considered as a constant. Strong temperature dependency, however, was also observed for a few aromatic acids, and misleading results may be obtained if this dependency is neglected. The model also has a certain prediction ability and could be extrapolated to a high temperature range where no experimental solubility data are available.     

Crystalline carbon nitrides: thin films and bulk samples

A new method was developed for the preparation of bulk samples of crystalline carbon nitride on exposure of an amorphous nitrogen- and carbon-containing material to high temperature and ultrahigh pressure in the presence of crystallization seeds. Amorphous carbon nitride whose composition was close to C₃N₄was used as a starting material. Thin films of crystalline carbon nitrides prepared by the laser-electric discharge method were used as crystallization seeds.     

Binuclear and polynuclear transition metal complexes with macrocyclic ligands. 2. New macrocyclic Schiff"s base in the reaction of 4-tert-butyl-2,6-diformylphenol with 1,2-diaminobenzene. Synthesis and structural,spectroscopic, and theoretical study

Condensation of 4-tert-butyl-2,6-diformylphenol with 1,2-diaminobenzene in ethanol is accompanied by partial reduction of the azomethine double bonds to form symmetrical macrocyclic Schiff"s base containing the alternating >C=N and >CH—NH fragments. In solution, this compound exists as the only isomer in which two endocyclic hydrogen atoms are bound to the oxygen atoms of the phenol groups and two other endocyclic H atoms are attached to the nitrogen atoms of the CH₂—NH fragments. All endocyclic protons are involved in hydrogen bonding and undergo rapid exchange with each other at room temperature. In the crystal, the planar macrocyclic molecules are arranged in closely packed stacks. The steric hindrances resulting from overlapping of the bulky tert-butyl groups are eliminated through rotation of the molecules with respect to each other in the adjacent layers. Study of the potential energy surface for the Schiff"s base under consideration by the DFT method demonstrated that the structure corresponding to the global minimum is similar to that found in solution. However, the isolated molecule is nonplanar, its macrocycle adopting a ladder conformation. The local minimum on the potential energy surface whose energy is 2.6 kcal mol^–1 higher than that of the global minimum corresponds to the zwitterionic structure in which all four endocyclic hydrogen atoms are attached to the nitrogen atoms and the macrocycle adopts a tub conformation. Flattening of the ring is considered as a consequence of stacking interactions between the molecules in the crystal.     

Transport behavior of carbon disulfide into poly aryl ether ether ketone

The sorption/desorption of carbon disulfide into/from PEEK as a function of crystallinity and temperature was investigated. The sorption curves of carbon disulfide into PEEK show only two major regions: (a) an increase of penetrant weight with time, and (b) a limiting equilibrium value (solubility). This is in contrast to the sorption of toluene into PEEK which shows three regions. The solubility of carbon disulfide decreases with increasing crystallinity, but temperature has little effect on the solubility in the temperature range of 25–40°C. An acceleration in penetration rate at the later stage of diffusion is observed for PEEK films whose crystallinity is greater than 13.4%, suggesting Supercase II diffusion. Carbon disulfide can be desorbed completely from PEEK in contrast to other fluids, such as toluene or methylene chloride, which are difficult to desorb. The normalized weight loss of carbon disulfide during desorption is an exponential function of square-root time. Solvent-induced crystallization was observed. Crystallinity was estimated from both the measured density and microhardness of the desorbed polymer and polymer which had undergone a sorption/desorption/resorption/desorption cycle. © 1996 John Wiley & Sons, Inc.     

Protein crystal nucleation: is the pair interaction potential the primary determinant of kinetics?

Fundamental understanding of protein crystal nucleation facilitates crystallization of biological macromolecules for structure determination and control of crystal size distribution. In the studies presented here, nucleation kinetics of hen egg-white lysozyme crystals were measured at solution conditions that exhibited equal solubility by adjusting pH, temperature, or sodium chloride concentration. It was observed that solution conditions that lead to equal solubility resulted in equal nucleation rates and hence kinetic parameters. Since the solubility of globular proteins correlates with the osmotic second virial coefficient, B(22), an integral measure of the protein pair interaction potential, this observation indicates that the protein pair interaction plays a key role in determining nucleation kinetic parameters.     

A study of the crystallization of the n-alkane C246H494 from solution: Further manifestations of the inversion of crystallization rates with temperature

The newly available, strictly uniform n-alkane, C₂₄₆H₄₉₄, has been crystallized from dilute solution. The rates of crystallization were followed by differential scanning calorimetry (DSC) as a function of temperature. Two pronounced rate inversions were registered. The dissolution temperatures of the crystals formed show a sharp discontinuity at the temperature of the rate minimum. From this it is inferred (reinforced by the precedent of previous work on C₁₉₈H₃₉₈) that a transition from extended to once folded crystallization is taking place at the temperature of the minimum. The methods by which the rate curves were constructed are laid out in step by step detail, leaving no possible doubt about the reality of the rate inversion. The rate inversion is attributed to “self-poisoning,” and this concept is extended to embrace the wider issue of mutually interacting competition of possible phase variants (“polymorphs”) of which the extended and folded chain crystals represent one special example. In addition, some further effects are noted and discussed regarding solubility behavior. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1775–1791, 1997     

Effect of Temperature on the Solubility of Short-Chain Carboxylic Acids in Water

The aqueous solubilities of pentanoic and hexanoic acids were measured between 298.15 and 333.15 K at intervals of 5.00 K. Nonlinear dependences were found for the variation of solubility with temperature. The temperature dependences of the solubility of the acids in water are described by nonlinear van’t Hoff plots. Experimental data were fitted by the method of least-squares to a second-order polynomial equation, which was then used to determine the differential solution enthalpies at selected temperatures. The values for the apparent Gibbs energy and the apparent entropy of solution are calculated.     

Improved solubilization of pyromellitic dianhydride and 4,4′-oxydianiline in ionic liquid by the addition of zwitterion and their polycondensation

Three different ionic liquids were prepared and examined as solvents for polyimide synthesis. The solubility of 4,4′-oxydianiline and pyromellitic dianhydride as starting materials in ionic liquids was first evaluated, and then their polycondensation was carried out. Although these starting materials were hardly soluble in 1-benzyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide (3), addition of imidazolium type zwitterion, 1-(1-butyl-3-imidazolio)butane-4-sulfonate (ZI), certainly improved their solubility. When 3 containing 40 mol % ZI was used, nothing was phase separated from this mixed solution containing both starting materials after cooling down to room temperature. After preparing prepolymer in 3 containing 40 mol % of ZI at room temperature, polycondensation was carried out in the same solution at 100, 200, and then 300 °C for every 1 h to obtain polyimide. An inherent viscosity of the obtained polyimide (0.05 g in 10 ml concentrated sulfuric acid) was 1.3 dL g⁻¹, higher than that prepared in only 3 (0.9 dL g⁻¹). The higher average molecular weight of the polyimide was attributed to the improved solubility of the starting materials by the addition of ZI that enabled the preparation of the prepolymer, poly(amide acid), without heating before imidation.     

On the prediction of equilibrium phase behavior of amino acids in aqueous and aqueous-electrolyte solutions using SAFT equation of state

We present an approach based on the statistical associating fluids theory (SAFT) to predict the solubility of amino acids in aqueous and aqueous-electrolyte solutions. This approach can describe the association interactions and their effects on the solubility of amino acids. Using the experimental data of activity coefficients of amino acids in water, the parameters of SAFT model for amino acids are obtained. The solubility of several amino acids in the temperature range of 273.15–373.15 K is predicted. Results obtained from the model are in a good accordance with the experimental data. Also, we examine the effect of pH on the solubility of dl-methionine. Addition of an extra amino acid to the binary solution of amino acid + water makes the system more complex. To check the accuracy of model, we study the ternary solution of dl-serine + dl-alanine + water and dl-valine + dl-alanine + water. Predicted results depict that the proposed model has the ability to describe the ternary solution of amino acids, accurately. Finally, the solubility of amino acids in aqueous-electrolyte solutions is investigated. The long-range interactions caused by the presence of ions affects the solubility of amino acids, leading them to be salted in or out. To treat this kind of interaction, the restrictive primitive mean spherical approximation (RP-MSA) is coupled with the SAFT equation of state. The proposed model can accurately predict the solubility of amino acids in aqueous-electrolyte solutions.     

A Molecular Dynamics Study of Nickel Crystallization at Strong Supercoolings

The homogeneous crystallization of liquid nickel models containing 2048 particles in the basic cube was studied by molecular dynamics. The potential of the embedded atom method was used. The models were constructed under zero pressure or constant volume conditions. The state of the structure was evaluated from the number of atoms with the coordination number 12. The concentration of such atoms in the stable and metastable liquids increased as the temperature decreased. At the selected potential of the embedded atom model, the equilibrium crystallization temperature at zero pressure was 1415 K. The existence of the lower boundary of liquid nickel supercooling was established. The liquid crystallized under isothermal conditions by the cluster mechanism with the formation of a predominantly closely packed structure below 850 K at zero pressure and below 1075 K at a constant volume (6.588 cm³/mol). The mechanism of nucleation was different from that accepted in classic nucleation theory. Nucleation was accompanied by an increase in the number of atoms with the coordination number 12, the formation of bound groups (12-clusters) from these atoms, and the growth of these groups, as with the crystallization of rubidium under strong supercooling conditions and coagulation of impurities from supersaturated solutions. At the initial stage, bound groups had a very loose structure and contained a large number of atoms with coordination numbers other than 12; the linear size of the largest group rapidly approximated the basic cube size. These atoms played a leading role in crystallization and activated the transfer of atoms in bound groups having different coordination numbers into the coordination state corresponding to a closely packed lattice. An important role in the formation of 12-clusters of the threshold (critical) size played cluster size fluctuations, which were especially strong close to the lower boundary of liquid supercooling.     

A study of the stability of aqueous suspensions of functionalized carbon nanotubes

The properties of aqueous suspensions of carbon nanotubes have been studied as depending on the conditions of their functionalization in a mixture of sulfuric and nitric acids. The elemental composition and contents of carboxyl, lactone, and hydroxyl groups in carbon nanotubes have been determined at different durations and temperatures of functionalization. The influence of functionalization conditions on the value of the electrokinetic potential of carbon nanotubes in aqueous suspensions and the nanotube solubility in water has been investigated. It has been found that the absolute value of the electrokinetic potential of nanotubes and their solubility in water increase with both the duration and temperature of functionalization due to a rise in the number of functional groups on their surface. The optimal regimes of functionalization of carbon nanotubes have been determined from the point of view of preserving their structure and stability in aqueous dispersions.     

具有SiC缓冲层的Si衬底上直接沉积碳原子生长石墨烯

石墨烯是近年发现的一种新型多功能材料.在合适的衬底上制备石墨烯成为目前材料制备的一大挑战.本文利用分子束外延(MBE)设备,在Si 衬底上生长高质量的SiC 缓冲层,然后利用直接沉积C原子的方法生长石墨烯,并通过反射式高能电子衍射(RHEED)、拉曼(Raman)光谱和近边X 射线吸收精细结构谱(NEXAFS)等实验技术对不同衬底温度(800、900、1000、1100 °C)生长的薄膜进行结构表征.实验结果表明,在以上衬底温度下都能生长出具有乱层堆垛结构的石墨烯薄膜.当衬底温度升高时,碳原子的活性增强,其成键的能力也增大,从而使形成的石墨烯结晶质量提高.衬底温度为1000 °C时结晶质量最好.其原因可能是当衬底温度较低时,碳原子活性太低不足以形成有序的六方C-sp2环.但过高的衬底温度会使SiC 缓冲层的孔洞缺陷增加,衬底的Si 原子有可能获得足够的能量穿过SiC薄膜的孔洞扩散到衬底表面,与沉积的碳原子反应生成无序的SiC,这一方面会减弱石墨烯的生长,另一方面也会使石墨烯的结晶质量变差.