Sublimation theory

Sublimation and distUlation are closely related. Crystals of soUd substances that sublime, when placed in an evacuated container, wUl graduaUy generate molecules in the vapor phase by the process of evaporation (i.e., the soUd has a vapor pressure). Occasionally, one of the vapor molecules wtU strike the crystal surface or the waUs of the container and be held by attractive forces. This process, condensation, is the reverse of evaporation. 

Sublimation is the complete process of evaporation from the soUd phase to condensation from the gas phase to directly form crystals without passing through the Uquid phase. 

Many solid substances have a sufficiently high vapor pressure near their melting point that allows them to be sublimed easily under reduced pressure in the laboratory. Sublimation occurs when the vapor pressure of the solid equals the pressure of the sample s environment. 

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The theory of sublimation, t.e. the direct conversion from the vapour to the sohd state without the intermediate formation of the liquid state, has been discussed in Section 1,19. The number of compounds which can be purified by sublimation under normal pressure is comparatively small (these include naphthalene, anthracene, benzoic acid, hexachloroethane, camphor, and the quinones). The process does, in general, yield products of high purity, but considerable loss of product may occur. 

Thermal Theory. The thermal approach to flame retardancy can function in two ways. Eirst, the heat input from a source may be dissipated by an endothermic change in the retardant such as by fusion or sublimation. Alternatively, the heat suppUed from the source maybe conducted away from the fibers so rapidly that the fabric never reaches combustion temperature. 

After cooling, the solution was diluted with 1.5 liters of water and subjected to three extractions with ether. The amine was extracted from the ethereal solution with 2 N HCI and liberated therefrom by the addition of solid NaOH (while cooling). The alkaline solution was extracted with ether and the ethereal solution was dried with soiid NaOH. Distii-lation resulted in 10.6 grams (70% of the theory) of 1-aminoadamantane which, after sublimation, melted at 180° to 192°C (seal capillary). It is converted to the hydrochloride. 

Figure 15 A schematic of the model for evaluating sublimation rate and evaporation rate from kinetic theory.

CrCl2 molecules have been isolated in solid inert-gas matrices and their i.r. spectrum has been recorded 33—1000 cm The isotopic shifts and i.r. selection rules indicate a linear 10°) structure. The d-d spectrum of gaseous CrClj has been discussed in terms of ligand field theory.The heat capacity of anhydrous CrCl3 in the temperature interval 2—20 K has been determined and the sublimation and decomposition pressures of the compound have been recorded. ... 

A related study of 3,6-diphenyl-l,2,4,5-tetroxane resulted in enthalpies of formation of the sohd and gaseous species of 134.0 1.3 and 99.7 1.3 kJmor. Again, while computational theory and experiment are in good agreement, the sublimation enthalpy of 34.3 kJmoU seems too low. We would have suggested a lower bound of ca 71 kJmoU based on our enthalpy of vaporization estimation approach given above. 

The quasi-steady-state theory has been applied particularly where a condensed phase exists whose volume changes slowly with time. This is true, for example, in the sublimation of ice or the condensation of water vapor from air on liquid droplets (M3, M4). In the condensation of water vapor onto a spherical drop of radius R(t), the concentration of water vapor in the surrounding atmosphere may be approximated by the well-known spherically symmetric solution of the Laplace equation ... 

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. 

The theory of the sublimation process has been discussed in detail elsewhere41 the following describes the practical aspects of this technique which is applicable down to a few milligrams of material. 

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