Molecular Distillation and Sublimation

One type of laboratory-scale molecular still is called a batch or pot still. Batch stills have too low a capacity and are too slow for most industrial needs. A continuous operation is required. Industrial models are usually of the eentrifligal or the cylindrical flowing film type depicted later. 

Equation 7-1 is Langmuir s method to determine the maximum amount of material that can distill Phys. Rev., 8, 149, 1916)  

However, the pressure is given in dynes/cm which is inconvenient. Equation 7-2 revises Langmuir s equation to have the pressure in torr and the amount distilled in grams  

The above equation expresses what should happen theoretically. The problem is that few molecules go directly from the evaporating surface to the condenser in a straight line and at a 90° angle. A correction factor, f, called the evaporative coefficient, developed by Burrows Molecular Distillation, Oxford Clarendon Press, 1960) considers three factors (1) the fraction of molecules reaching the condenser without collision, (2) the fraction that collides first and how many reach the condenser, and (3) those that have collided and reach the condenser by random motion. 

By combining the constants in equation 7-2 and adding f, the following equation is obtained  

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The basic differences between a molecular distillation and a sublimation at low pressures are that (1) the molecular distillation goes from solid to liquid to gas, which usually requires more energy than a sublimation that goes from the solid directly to the gas, and (2) the surface of the subliming compound is continuously renewed by evaporation, convection, or diffusion. As a result, a sublimation in many cases is faster and more economical. 

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

Batch Distillation Evaporation and Condensation Continuous Distillation Fractionation Rectification Reflux Distillation Vacuum Distillation Steam Distillation Azeotropic Extractive Distillation Destructive Distillation Molecular Distillation Distillation by Compression and Sublimation)... 

Uranium hexafluoride is probably the most interesting of the uranium fluorides. Under ordinary conditions, it is a dense, white solid with a vapor pressure of about 120 hull ai room temperature. It can readily be sublimed or distilled, and it is by far the most volatile uranium compound known. Despite its high molecular weight, gaseous UFg is almost a perfect gas, and many of the properties of the vapor can be predicted from kinetic theory. 

An analysis of the extensive literature data and the data of the authors of this book [1640, 1642, 1655] permits a conclusion that the variations in the physicochemical characteristics of Al(OR)3 are caused by the existence of different types of aggregates and the possibility oftheir mutual transformations. The molecular composition depends on the conditions in the course of sample preparation — temperature, nature ofthe solvent applied, and the age ofthe sample. An important role is also played by the presence in the samples of microamounts of oxocomplexes, which are always formed on sublimation, distillation, and any other thermal treatments. 

The ionic nitrates generally melt to liquids which are stable to various degrees above their melting points. These liquids can be distilled under reduced pressure.11 The covalent nitrates are generally not stable as liquids. When heated, they first sublime, frequently giving molecular vapors, and then decompose. The cation influences the stability of the anion through its ability to distort its structure in the same manner as for carbonates and sulfates, as discussed in Chapters 2 and 4. 

Materials. Athabasca asphaltene was prepared from an oil sand sample from the GCOS (now Suncor Inc.) quarry according to a standard procedure used in this laboratory (9). Solvents were refluxed over CaH2 and redistilled. Nitromethane was distilled and stored over molecular sieves. AlCl3 was sublimed and ground under nitrogen ZnCl2 was dried at 110° C in a vacuum oven. 

As demonstrated in the first experiment, sublimation can occur readily at atmospheric pressure. For substances with lower vapor pressures vacuum sublimation is used. At very low pressure the sublimation becomes very similar to molecular distillation, where the molecule leaves the warm solid and passes unobstructed to a cold condensing surface and condenses in the form of a solid. 

Molecular weight determinations were consistent with a dimeric structure, [U(OCH2CH3)5]2, and the compound can be distilled at 123 °C (0.001 Torr). Species that are thermally stable, distillable or sublimable are desirable for use in the separation of metal isotopes. The number of ensuing reports describing various synthetic routes to [U(OCH2CH3)5]2 are evidence of that motive.Some of these methods are described by Equations (38) to (43) ... 

Acceptor compounds (TCPA, chloranil, i,3,5-trinitrobenzene, benzoquinone, phthallic anhydride, and 1,3,5-tricyanobenzene) were recrystallized and sublimed. Acetonitrile (MaI Iinckrodt reagent) was distilled from P2O5 and redistilled, retaining middle cuts. Dichloromethane (MaI Iinckrodt reagent) was dried over calcium hydride. XVIII was prepared by Shaw Chu (27) and has a molecular weight of 110,000 by membrane osmometry. XIX was prepared according to the procedure of J.W. Schwietert (30). 

The solution of dihalocyclopropane is often added to the solution of base at lower than j-t. -The crude reaction mixture can also be distilled and the distillate worked up further. For higher molecular weight products the mixture can be evaporated to dryness and then extracted with benzene. The use of freshly sublimed f-BuOK reportedly gives higher and more consistent yields. ... 

What is sublimation and how does it differ from a molecular distillation ... 

These compounds (Table IV) can be distilled or sublimed in vacuo and are soluble in common organic solvents, readily hydrolyzed, and converted by alcoholysis into metal alkoxides. Molecular weight determinations in boiling benzene show that the dimethylamino derivatives are significantly polymerized, whereas the diethylamino derivatives are monomeric. 

A typical definition that is often made when describing OLEDs is to categorize them as either small molecule or polymer-based devices. However, this distinction is somewhat misleading since small molecule- and polymer-based OLEDs do not markedly differ in their operating principles. The mechanistic discussions given Section 12.04.2.1 apply equally well to both types of devices. A more logical way to differentiate OLEDs is by the fabrication method. Molecular materials (typically referred to as small molecules) can be distilled or sublimed at reduced pressure, and are... 

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with H, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the het-erometallic species in the vapor phase. The information gained by infrared spectroscopy is limited the technique is mostly useful for the identification of solvates M(OR) (ROH)x (vOH absorption 3400-3100 cm-l or of chemically modified (heteroleptic) alkoxides (probe for the vCO stretching of P-diketonate or carboxylate ligands, for instance). 

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