Carbon dioxide, crystal constants

When the crystallinity of polyethylenes is increased, the gas permeability through the film decreases. The factors involved are the tortuosity of the gas path through the amorphous phase, and the effect of the crystals in restricting the mobility of the amorphous polymer chains (chain immobilisation factor). The logarithm of the permeability of nitrogen, argon and carbon dioxide decreased almost linearly with increased crystallinity of PE, with the ratio of the gas values remaining almost constant for a particular PE. 

The use of conductance as a measure of the solution composition is limited unless some other control of the composition is possible. Figure 6 illustrates a schematic of the apparatus used to follow the crystal growth of calcite from calcium bicarbonate solutions under constant composition conditions. In this case, the carbon dioxide concentration is kept constant by the flow of a water-saturated mixture of carbon dioxide and nitrogen gas. The conductance is monitored using an a.c. bridge with the analog signal... 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

Transfer the salt to a 150 ml beaker and add 5 g of barium carbonate and 5 ml of 95 per cent ethanol. Stir the mass into a paste so long as carbon dioxide is evolved. When the evolution of gas is complete add 10 ml of alcohol, and heat in a water bath to boiling, with constant stirring. Remove beaker from the water bath. Allow the solids to settle, and pour off the supernatant liquid through a folded filter paper into an evaporating dish. Repeat the extraction of the residue with 15 ml of alcohol. Evaporate the filtrates on a water bath until crystals of urea separate on the surface of the liquid. Remove the dish to a cool place, cover with a watch glass and allow to cool. Filter the crystals of urea with suction. If the product is colored, recrystallize from a small volume of alcohol. The yield is 2-3 g. 

Though salt dehydration was not accompanied [27] by particle disintegration, the anhydrous pseudomorph was shown by X-ray diffiaction measurements to be very poorly crystallized (a characteristic feature of many nickel carboxylates). Decomposition in air (554 to 631 K) proceeded at a constant rate (0.1 < nr < 0.8 and = 96 kJ mol" ), ascribed to the operation of an autocatalytic mechanism. The reaction in vacuum (562 to 610 K) gave a sigmoid ar-time curve which was fitted by the Prout-Tompkins equation. Because the activation energy was the same as that for reaction in air, it was concluded that the same mechanism operated. The reaction in air yielded residual nickel oxide, while reaction in vacuum gave the carbide with excess carbon and some oxide. In addition to carbon dioxide, the volatile products of decomposition included water and acetic acid. 

---