Density gradients, crystallization

Low and Richards used a density gradient column to study the density of crystals of "mixed gramicidin fractions"56. 

DEAE-cellulose column was further purified on Sephadex G-200 but resisted all efforts at crystallization. Figure 14 (4) shows the polyacrylamide gel electrophoretic pattern of crude rat liver extract with two bands, the resolution of these bands by DEAE-cellulose, and the crystalline peak I preparation. The crystalline enzyme had an approximate molecular weight of 100,000 estimated by sucrose density gradient... 

Crystal density is measured in a graduated cylinder by suspending the crystal in a density gradient made by mixing water-saturated organic solvents such as xylene and carbon tetrachloride. The crystal will settle through the liquid until its density matches that of the liquid mixture and then remain suspended there. Drops of salt solutions of known density are used to calibrate the gradient. 

This result is useful in understanding the variation of the field dependence of the TOF measured mobility from sample to sample, following the carrier density gradients (Fc dn/cbc). For example, the role of the diffusion carrier stream would explain the field dependence of jx in single crystals whenever their near-surface layer is strongly populated... 

Density gradients are established at several stages in the crystallization process (Fig. 5). As molecules attach to the growing crystal surface, the solution near the crystal is depleted of solute and becomes less dense than the bulk solution. Under the influence of gravity, such density differences result in convection currents. However, in microgravity, solutions with different densities are not subject to convection, so that solutions mix with less turbulence (Littke and John, 1984) and equilibration between solutions is much slower (DeLucas et al., 1986). 

The density of a polymer sample can be readily determined by allowing it to float in a density-gradient column, which is a vertical column containing a mixture of liquids with different (known) densities. The density of a small piece of polymer is determined from the position it adopts when it is dropped into the column. The density of the crystalline regions pc can be calculated from a knowledge of the crystal structure [19]. The amorphous density pa can sometimes be measured directly if the polymer can be obtained in a completely amorphous form, for example by rapid cooling of a polymer melt. Otherwise it can be determined by extrapolating either the density of the melt to the temperature of interest or that of a series of semicrystalline samples to zero crystallinity. 

The weight of all the atoms or ions in the unit cell can be calculated if the unit cell dimensions and the density of the crystal are known. This means that crystal density measurements are important in the early stages of a crystal structure analysis. A vertical column with a density gradient, calibrated with crystals of known density, is used to measure protein densities. These are determined by where along the column the crystal setdes and so that, by interpolation, its density can be estimated (39). This method must be done with care to ensure that the measurement is good. The density of a crystal is given by... 

The various densities along the column can be determined by adding objects of known density and noting their positions. The least expensive method is to add a few crystals of several different compounds that are insoluble in the density gradient liquid. Table 38-2 lists several crystals than can be used in organic liquids. 

Caesium sulphate Cesium sulfate Dicesium sulfate EINECS 233-662-6 Sulfuric acid, dicesium sail. Used In brewing, mineral waters, for density gradient in ultracentrifuge separation. Crystals mp = 1019° d = 4,24 soluble in H2O, insoluble in organic solvents. Atomergic Chemetals Cabot Carbon Ltd. Cerac Sigma-Atdrich Fine Chem. 

Crystallization. We measured the density of initially amorphous PET films following exposures to methylene chloride at unit activity, thereby obtaining crystallization kinetics. The density gradient technique was used (18) with carbon tetrachloride/n-heptane mixtures as the immersion media. To avoid spurious readings, efforts were made to remove residual solvent from the crystallized samples and to prevent the entrapment of air in surface cavities during the density... 

The rate of crystallization of polyethylene terephthalate was measured using the density-gradient method (Keller et al., 1954) by following the changes in density as a function of time for a given crystallization temperature. Data for T = 383 K are tabulated as follows ... 

The density of the crystalline material can be obtained from the chemical formula of the polymer and the lattice parameters, which can be found in the literature. A difficulty is that the lattice parameters of polymer crystals often depend on the precise conditions under which crystallisation took place, so a value obtained for a sample prepared in as similar a way as possible to the one whose crystallinity is to be determined should be chosen. The density of the amorphous material is quite difficult to determine because it too depends on the method of preparation. When fully amorphous material cannot be produced the densities of a series of samples whose crystallinities have been determined by another method ean be extrapolated to zero crystallinity. The density of the sample is usually obtained by means of a density-gradient column, the principles of which are explained in section 2.3. 

On the basis of the example of pipe extrusion. Fig. 74 shows the influence of cooling on the density gradient across the pipe wall thickness. It is clear that this pipe production involved external water cooling and internal pressurized air. The result is low density on the outside (low degree of crystallization), maximum density in the middle of the wall, shifted somewhat toward the inside diameter... 

The determination of the proportion of the solid that is crystalline (its ciystallinity) is of considerable fundamental and often practical significance. The easiest method for measuring the crystallinity is the determination of specific volume (the specific volume is the inverse of the density). The simplest methods are Archimedes method and the density gradient colunm. The measurement, Ity either method, is most convenientty performed at room temperature, say 2(f C. At this temperature, let the specific volumes of the crystal and amorphous fractions be and v. Then if X is the fraction of the mass that is crystalline and (1 -x) the fraction that is amorphous, the specific volume v of the specimen is... 

FIGURE 7.9 (a) Structure 01-126. (b) Structure 01-127. Noncovalent interaction (NCI) plot of predicted crystal structure. Reduced density gradient isosurface (IS) cutoff set to 0.25. Isosurfaces represent either unfavorable, weak, or strong favorable interactions, (see insert for color representation of the figure.)... 

Sample density can be measured using a variety of methods density-gradient column, dilatometry, pycnometry, and flotation or buoyancy. The density-gradient column approach is probably used most frequently to determine sample densities for crystallinity measurements. When thermostated and appropriately calibrated with floats, this approach permits measurements to accuracies of 0.2 mg/cm or better. Dilatometers are well suited for measuring specific volumes and following crystallization, as a function of temperature. 

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