Normal Melting Point Research Articles

Phase behavior of 1,3,5-tri- tert-butylbenzene–carbon dioxide binary system

1,3,5-tri- tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility in liquid and supercritical carbon dioxide. We present the phase behavior of TTBB–CO 2 binary system at temperatures between 298 and 328 K and at pressures up to 20 MPa. Phase diagrams showing the liquid–vapor, solid–liquid and solid–vapor equilibrium envelopes are constructed by pressure–volume–temperature measurements in a variable-volume sapphire cell. TTBB is highly soluble in CO 2 over a wide range of compositions. Single-phase states are achieved at moderate pressures, even with very high TTBB concentrations. For example, at 328 K, a binary system containing TTBB at a concentration of 95% by weight forms a single-phase above 2.04 MPa. TTBB exhibits a significant melting-point depression in the presence of CO 2, 45 K at 3.11 MPa, where the normal melting point of 343 K is reduced to 298 K. With its high solubility in carbon dioxide, TTBB has potential uses as a binder or template in materials forming processes using dense carbon dioxide.

Spontaneous liquifaction of isomerizable molecular crystals

A lattice vacancy raises the energy of the neighboring (flexible) molecule in a crystal, which may be enough to isomerize it to a tautomer that does not fit the lattice site, thus creating a liquidlike local region embedding the vacancy. Similar regions may appear elsewhere in the lattice and the regions may ultimately merge. Thus a crystal may spontaneously liquefy over a period of hours to years at a temperature below its normal melting point. Simultaneous heat capacity and heat absorption measurements of several such molecular crystals show that they spontaneously liquefy at a temperature far below their reputed melting point, according to a non-exponential rate kinetics and a temperature dependent rate constant, and do not crystallize on cooling.

Thermal stability of shear‐induced precursor structures in isotactic polypropylene by rheo‐X‐ray techniques with couette flow geometry

AbstractCombined in situ rheo‐SAXS (small‐angle X‐ray scattering) and ‐WAXD (wide‐angle X‐ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear‐induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long‐living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s−1 and duration ts = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo‐WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s−1 and duration, ts = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two‐phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006

Melting point depression by using supercritical CO 2 for a novel melt dispersion micronization process

The melting point of solid substances can be depressed considerably by using supercritical fluids that are highly dissolvable in the molten substance. This fact can be used in micronization processes where thermo-labile materials are liquefied at a temperature lower than their normal melting point in order to produce fine particles. In the present study, the melting points of four different materials under various pressures applied by CO 2 were determined visually with the first melting point method: phenanthrene, beta-sitosterol, an insecticide, and a pharmaceutical active. The aqueous suspensions of all materials except phenanthrene were also studied. Thus, it was possible to determine the solid–liquid–vapor equilibrium lines of these materials on a temperature–pressure plane. Finally, a thermodynamic model for describing the phase equilibria of such binary systems was developed implementing the Peng–Robinson equation of state and two different approaches concerning the fugacity of the solid phase.

Expanding the useful range of ionic liquids: melting point depression of organic salts with carbon dioxide for biphasic catalytic reactions

Large and previously unreported melting point depressions (even exceeding DeltaTm of 100 degrees C) were observed for simple ammonium and phosphonium salts in the presence of compressed CO2, bringing them well within the range of typical ionic liquids; possible applications include biphasic catalysis in IL/scCO2 systems as demonstrated for rhodium complex catalyzed hydrogenation, hydroformylation, and hydroboration of 2-vinyl-naphthalene using a CO2-induced molten sample of [NBu4][BF4] as a catalyst phase at temperatures in the range of 55-75 degrees C, i.e. 100 degrees C below the normal melting point of the organic salt.

Can simple models describe the phase diagram of water?

The melting point of ice Ih for the TIP3P, SPC, SPC/E, TIP4P, TIP4P/Ew and TIP5P models has beendetermined by computer simulation. It has been found that the melting points of iceIh for these models are 146, 190, 215, 232, 245 and 274 K respectively. Thus fromthe models of water available so far only TIP5P reproduces the experimentalmelting point of water. The relative stability of ice II with respect to iceIh at the normal melting point has also been considered. Ice II is more stable than iceIh for the TIP3P, SPC, SPC/E and TIP5P models. Only for the TIP4P and TIP4P/Ew models is iceIh more stable than ice II at low pressures. The complete phase diagram for the SPC/E,TIP4P and TIP5P models has been computed. It has been found that SPC/E and TIP5Pdo not correctly describe the phase diagram of water. However, TIP4P provides aqualitatively correct description of the phase diagram of water. A slight modification of theparameters of the TIP4P model yields a new model, denoted as TIP4P/ice, whichreproduces the experimental melting point of water and provides an excellent description ofthe densities of all ice phases.

Optimizing the Impact Resistance of Matrix-free Spectra®Fiber-reinforced Composites

It has been discovered that Spectra® fiber can shift its peak melting temperature upward by almost 10 if it is maintained at constant length. Based on this, a novel processing method called high-temperature, high-pressure sintering has been developed in which Spectra® cloth is consolidated using heat and pressure to make a matrix-free fiber-reinforced composite. Because of the outstanding properties of the Spectra® fiber, the resulting product is a high modulus, high strength, and high-impact resistant ‘one-polymer composite.’ The three key processing parameters are temperature, time, and pressure. The high temperature is in the vicinity of the fiber’s normal melting point so as to induce adequate, but not excessive melting, which allows for the fusion of the cloths on recrystallization. Time is important in the sense of facilitating heat transfer and promoting interfiber and interlayer adhesion while conserving the fiber’s high crystallinity and orientation. The high pressure exerted laterally on the cloths is required to constrain them and prevent shrinkage, as well as eliminate the voids and consolidate the materials. The very long relaxation time and the rubbery nature of ‘melted ultrahigh molecular weight polyethylene’ retard the loss of orientation, and thus make this process possible. The influence of these parameters on the structure and properties of the consolidated structures are studied. Specifically, the crystallinity changes for specimens processed under different conditions are measured by DSC; the molecular orientation changes for specimens processed under different conditions are studied by WAXD; and the impact properties for specimens processed under different conditions are measured by instrumented puncture impact tests. The established process-structure-property relationship is used as a guideline to fine-tune and optimize the processing conditions in order to achieve the best possible impact properties.

The melting temperature of the most common models of water

The melting temperature of ice I(h) for several commonly used models of water (SPC, SPC/E,TIP3P,TIP4P, TIP4P/Ew, and TIP5P) is obtained from computer simulations at p = 1 bar. Since the melting temperature of ice I(h) for the TIP4P model is now known [E. Sanz, C. Vega, J. L. F. Abascal, and L. G. MacDowell, Phys. Rev. Lett. 92, 255701 (2004)], it is possible to use the Gibbs-Duhem methodology [D. Kofke, J. Chem. Phys. 98, 4149 (1993)] to evaluate the melting temperature of ice I(h) for other potential models of water. We have found that the melting temperatures of ice I(h) for SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P models are T = 190 K, 215 K, 146 K, 232 K, 245 K, and 274 K, respectively. The relative stability of ice I(h) with respect to ice II for these models has also been considered. It turns out that for SPC, SPC/E, TIP3P, and TIP5P the stable phase at the normal melting point is ice II (so that ice I(h) is not a thermodynamically stable phase for these models). For TIP4P and TIP4P/Ew, ice I(h) is the stable solid phase at the standard melting point. The location of the negative charge along the H-O-H bisector appears as a critical factor in the determination of the relative stability between the I(h) and II ice forms. The methodology proposed in this paper can be used to investigate the effect upon a coexistence line due to a change in the potential parameters.

Estimation of Melting Points of Organic Compounds

A combination of additive group contributions and nonadditive molecular parameters is employed to estimate the normal melting points of 1215 organic compounds. The melting points are calculated from the ratio of the total phase change enthalpy and entropy of melting. The total phase change enthalpy of melting is calculated from the enthalpic group contributions, whereas the total phase change entropy of melting is estimated using a semiempirical equation based on only two nonadditive molecular parameters. The average absolute error in estimating the melting points of these organic compounds is 33.2 K. This is a relatively low value considering the wide range of pharmaceutically and environmentally relevant organic compounds included in this data set.

Reverse temperature injection molding of Biopol? and effect on its properties

A novel reverse temperature profile for the injection molding of Biopol™ was studied. It was found that both the mechanical properties and the part quality of Biopol™ were improved with this new reverse temperature process. When injection molded, most conventional thermoplastic polymers are processed at 30 to 70°C above the melting temperature; under these conditions, Biopol™ degraded rapidly and the resulting material showed poor mechanical properties. In contrast, when using a reverse temperature molding process, where Biopol™ was melted in the first zone and then was conveyed through the barrel with a decreasing temperature pathway and was injection-molded at a temperature below its melting point, the resulting material showed higher mechanical properties. The processing of Biopol™ was also greatly improved. The reverse temperature process uses the characteristically slow crystallization rate of Biopol™, which can be easily injected as hot melt even below its normal melting point. DSC analysis suggested that the reverse temperature process resulted in a more homogeneous crystalline phase than the conventional process. GPC analysis also indicated that thermal degradation of Biopol™ was largely reduced in the reverse temperature injection-molding process compared with conventional methods. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 483–491, 2004

Prospects and limitations of the rational engineering of fibrillar collagens.

Recombinant collagens are attractive proteins for a number of biomedical applications. To date, significant progress was made in the large-scale production of nonmodified recombinant collagens; however, engineering of novel collagen-like proteins according to customized specifications has not been addressed. Herein we investigated the possibility of rational engineering of collagen-like proteins with specifically assigned characteristics. We have genetically engineered two DNA constructs encoding multi-D4 collagens defined as collagen-like proteins, consisting primarily of a tandem of the collagen II D4 periods that correspond to the biologically active region. We have also attempted to decrease enzymatic degradation of novel collagen by mutating a matrix metalloproteinase 1 cleavage site present in the D4 period. We demonstrated that the recombinant collagen alpha-chains consisting predominantly of the D4 period but lacking most of the other D periods found in native collagen fold into a typical collagen triple helix, and the novel procollagens are correctly processed by procollagen N-proteinase and procollagen C-proteinase. The nonmutated multi-D4 collagen had a normal melting point of 41 degrees C and a similar carbohydrate content as that of control. In contrast, the mutant multi-D4 collagen had a markedly lower thermostability of 36 degrees C and a significantly higher carbohydrate content. Both collagens were cleaved at multiple sites by matrix metalloproteinase 1, but the rate of hydrolysis of the mutant multi-D4 collagen was lower. These results provide a basis for the rational engineering of collagenous proteins and identifying any undesirable consequences of altering the collagenous amino acid sequences.

Developing an ionic medium for ligandless-palladium-catalysed Suzuki and Heck couplings

Abstract High-melting-point alkylammonium tetrafluoroborates melt with water or toluene to form biphasic mixtures at much lower temperatures than their normal melting points, thus could serve as green reaction media for Suzuki and Heck couplings. Advantages of using the aqueous-ammonium biphasic catalytic system include: (1) working well with a ligandless palladium catalyst, (2) solving the solubility problem of organic substrates and (3) simplifying work-up procedures for both separation of products and recycling of reaction media.

Electroacoustic Study of the Crystallization of n-Eicosane Oil-in-Water Emulsions

When sodium dodecyl sulfate-stabilized emulsions of n-eicosane are supercooled from about 40 °C, solidification of the droplets occurs below 24 °C; reheating the suspension causes the particles to melt at the normal melting point of 36-37 °C, and the emulsion returns to its initial state without any aggregation or coalescence observed. The crystallization changes the density of the particles and the volume fraction of the suspension. When this is properly considered, the dynamic mobility spectra obtained from electroacoustic measurements can be readily interpreted in terms of the particle size and ζ potential of the droplets with very low fit errors. The droplet size appears, however, to be significantly different at different salt concentrations. This anomalous result is an indication of stagnant layer conduction on the emulsion drops. When this effect is also taken into account, the droplet size becomes independent of the salt concentration. The results then become entirely self-consistent and indicate that the theory, which is used to describe the dynamic mobility, appropriately describes the effect for both solid particles and surfactant-stabilized oil droplets.

Are metastable, precrystallisation, density-fluctuations a universal phenomena?

In-situ observations of crystallisation in minerals and organic polymers have been made by simultaneous, time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) techniques. In isotactic polypropylene slow quiescent crystallisation shows the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with long range order before the development of crystalline peaks in the WAXS region. Block copolymers self-assemble into mesophases in polymer melts above a critical chain length (or above a critical temperature) and this self-assembly process is shown to be susceptible to an incipient crystallisation. Mesophase formation is observed at anomalously high temperatures in ethylene-oxide containing block copolymers below the normal melting point of the polyoxy ethylene chains. Formation of calcium carbonate from aqueous solutions of sodium carbonate and calcium nitrate is observed to be a two-stage process and precipitation proceeds by the production of an amorphous metastable phase. This phase grows until it is volume filling and leads to the formation of the two polymorphs Calcite and Vaterite. These three sets of results suggest pre-nucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation. In due course, this phase undergoes transformation to "normal" crystals.

Orientation-induced crystallization in isotactic polypropylene melt by shear deformation

Development of orientation-induced precursor structures (nuclei) prior to crystallization in isotactic polypropylene melt under shear flow was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and rheo-optical techniques. SAXS patterns at 165°C immediately after shear (rate = 60 s−1, ts = 5 s) showed emergence of equatorial streaks due to oriented structures (microfibrils or shish) parallel to the flow direction and of meridional maxima due to growth of the oriented layer-like structures (kebabs) perpendicular to the flow. SAXS patterns at later times (t = 60 min after shear) indicated that the induced oriented structures were stable above the nominal melting point of iPP. DSC thermograms of sheared iPP samples confirmed the presence of two populations of crystalline fractions; one at 164°C (corresponding to the normal melting point) and the other at 179°C (corresponding to melting of oriented crystalline structures). Time-resolved optical micrography of sheared iPP melt (rate = 10 s−1, ts = 60 s, T = 148°C) provided further information on orientation-induced morphology at the microscopic scale. The optical micrographs showed growth of highly elongated micron size fibril structures (threads) immediately after shear and additional spherulities nucleated on the fibrils at the later stages. Results from SAXS and rheo-optical studies suggest that a stable scaffold (network) of nuclei, consisting of shear-induced microfibrillar structures along the flow direction superimposed by layered structures perpendicular to the flow direction, form in polymer melt prior to the occurance of primary crystallization. The scaffold dictates the final morphological features in polymer.

Monotonic Increase of Nitrite Yields in the Photolysis of Nitrate in Ice and Water between 238 and 294 K

The quantum yield, φ, of nitrite formation in the 302 nm band photolysis of fluid or frozen aqueous nitrate solutions increases monotonically with temperature over the range 238−294 K. The presence of formate increases φ 5-fold but does not modify its temperature dependence. Considering that the detection of nitrite as a product is only possible after the initial photofragments (NO_2^- + O) escape the solvent cage and that the diffusivity of ice, D_(ice), is about 6 orders of magnitude smaller than that of supercooled water, D_(aq), at the same temperature, we infer that nitrate photodecomposition takes place in similar liquidlike media at all temperatures. We found that the nitrite dispersed into the bulk is subsequently degraded by OH radicals, another primary photoproduct that can be scavenged by formate. The fact that experimental φ values in ice are actually larger than those derived from linear φ vs D_(aq)T^(1/2) extrapolation of aqueous phase data, as expected for cage processes in homogeneous media, suggests that the photochemically relevant properties of the quasi-liquid layer covering ice below the normal melting point resemble those of bulk supercooled water, but other effects, such as the dissipation of excess photon energy into the medium, may also play a role.

Group-contribution based estimation of pure component properties

A new method for the estimation of properties of pure organic compounds is presented. Estimation is performed at three levels. The primary level uses contributions from simple groups that allow describing a wide variety of organic compounds, while the higher levels involve polyfunctional and structural groups that provide more information about molecular fragments whose description through first-order groups is not possible. The presented method allows estimations of the following properties: normal boiling point, critical temperature, critical pressure, critical volume, standard enthalpy of formation, standard enthalpy of vaporization, standard Gibbs energy, normal melting point and standard enthalpy of fusion. The group-contribution tables have been developed from regression using a data set of more than 2000 compounds ranging from C=3–60, including large and complex polycyclic compounds. Compared to the currently used group-contribution methods, the new method makes significant improvements both in accuracy and applicability.

Analysis of the enthalpy of mixing data of binary and ternary [rare earth (Nd, La, Y, Yb), Al–alkali metal]–fluoride systems

We analyzed the enthalpy of mixing data of several liquid [rare earth and aluminum–alkali metal] fluoride systems using the Hoch–Arpshofen solution model. We investigated the binary systems AlF 3–MF (M is Li, Na, K), (LaF 3, YF 3, YbF 3)–MF (M is Li, Na, K, Rb, Cs) and NdF 3–MF (M is Li, Na, K) and the ternary LaF 3–NaF–LiF system. We also analyzed the LaBr 3–MBr (M is Li, Na, K, Rb, Cs) system, to show the difference to fluoride systems. In the fluoride systems the experimental data is carried out with solid MeF 3 and thus the analysis depends on the enthalpy of fusion of the MeF 3 much below the normal melting point. In the fluoride systems the enthalpy of mixing can be represented by a regular solution; in other systems the enthalpy of mixing leans toward the alkali metal. There is a linear relationship between the maximum enthalpy of mixing and the radius of the alkali metal cation, and also the halide anion radius. In the ternary system LaF 3–NaF–LiF the enthalpy of mixing could be computed from the binary systems. The relationship between the maximum of the enthalpy of mixing and the radii of the three valent cations, the alkali cations and the halide anions is: (a) only fluorides: H m,max (kK)= −(7.714±0.876)+(10.293±0.709)∗r M 3+ −(4.285±0.537)∗r M + R=0.9803 (b) all [Al, rare earth–alkali metal]–halides analyzed: H m,max (kK)= −(10.814±0.605)+(10.017±0.622)∗r M 3+ −(3.001±0.267)∗r M + +(1.592±0.337)∗r L − R=0.9643

Melting and freezing of water in ordered mesoporous silica materials

The melting and freezing of water in a series of mesoporous silica materials with a hexagonal arrangement of unidimensional cylindrical pores and narrow pore-size distribution (MCM-41 with pore diameters from 2.9 to 3.7 nm, and SBA-15 with pore diameters from 4.4 to 11.7 nm) was studied by differential scanning calorimetry (DSC). A lowering of the melting temperature ΔTm = Tmb − Tm(R) up to 50 K was found for water in pores of decreasing radius R. The melting point data can be represented by a modified Gibbs–Thomson equation, ΔTm(R) = K/(R-t), with K = 52 K nm and t = 0.4 nm, in agreement with an earlier study of water in MCM-41 materials of pore width 2 to 4 nm. The value of K agrees with an estimate of the Gibbs–Thomson constant based on thermodynamic data for the normal melting point of ice, and the parameter t can be taken as the thickness of a surface layer of non-frozen water at the pore wall. DSC scans of the freezing of H2O and D2O in partially filled pores of SBA-15 reveal a peak pattern depending on the degree of pore filling ϕ. The different peaks are attributed to different states of the liquid in the pore space, iz. pore water in completely filled regions, and water as an adsorbed film at the pore wall. These two states coexist in a pore filling range ϕmin<ϕ<1, but only the film state exists at ϕ<ϕmin. The freezing peak of pore water appears to be nucleated by external bulk ice (at ϕ>1) but exhibits substantial supercooling at ϕ<1. On the other hand, the peak attributed to the freezing of film water exhibits no significant supercooling and is found at temperatures near 237 K, almost independent of ϕ and the pore width of the SBA-15 sample. The nature of a further (small) peak near 233 K is not yet understood. Contrary to cooling scans, only one DSC peak is observed in heating scans, independent of the pore filling. This finding indicates that the adsorbed liquid film is metastable relative to the frozen pore liquid.

A Combined Group Contribution and Molecular Geometry Approach for Predicting Melting Points of Aliphatic Compounds

A combined approach that utilizes both group contribution and simple molecular geometric parameters is employed to predict normal melting points for a variety of aliphatic compounds. The melting points are estimated from the ratio of the enthalpy and the entropy of melting. The former is calculated from the sum of enthalpic group contributions and correction factors, whereas the latter is calculated using a modification of Walden's rule. Approximately 1040 melting point data were compiled and analyzed by multiple regression. The root-mean-square error of the estimation is 34.4 K. This is relatively low given the complexity of melting and the diversity of the database used. A comparison of the proposed method with the method of Joback and Reid8 was performed on 50 aliphatic compounds that were not used in the training set. The average absolute error for this method is approximately 20%, whereas that for the Joback and Reid data is 34%. The higher prediction accuracy of the proposed method suggests that the...