The extent to which various liquid drops may be supercooled has been observed by examining the appearance of clouds of such drops in a beam of light a t different temperatures. For most of the liquids studied the great majority of the drops did not freeze until a comparatively narrow range of temperature had been reached far below the melting point. In this range where the drops freeze spontaneous homogeneous nucleation is assumed to occur. This is discussed in the light of current theories of nucleation phenomena in which a key role is played by the interfacial tension between solid and liquid. An attempt is made to determine values of this quantity, and correlate them with the corresponding heats of fusion.IN recent years measurements have been made, particularly by Turnbull and his colleagues, on the extent to which liquid metals will supercool in the absence of catalysts (Turnbull and Cech, J . AppZ. Physics, 1950, 21, 804). An essential feature of these experiments was that observations were made on numerous, isolated drops, in the expectation that while some of these might have occluded impurities the majority should not. Experiments on the same lines with molecular liquids with the single exception of water have not been described. In this paper we present some observations on the supercooling of some common organic liquids and of some liquefied gases. An attempt will be made to interpret these on current theories of homogeneous nucleation. EXPERIMENTALThe problem was to find the temperature a t which a large number of isolated drops of liquid froze. One may in principle either examine a cloud of drops simultaneously or else a large number of individual drops. An unsuccessful attempt was made to follow the latter course, by adapting Rumpf and Seigl's method (2. Physik, 1938, I l l , 301), in which individual drops were held stationary and observed in a cold stream of air flowing upwards. It was found that it was not easy to hold a drop stationary, that the drops steadily evaporated, and that freezing could not readily be detected.The method had been used for water, though few experimental details were given (Schaefer, Bull. Auner. Mat. SOC., 1948, 29, 175), and it was reported that the water clouds froze completely over the range -39°f0-10.Consequently, a cloud method was adopted.
Equilibrium isotherms for water have been measured on three samples of Saran charcoal between 10" C and 60" C. The heats of adsorption at very low surface coverage are as high as 15,000 callmole, but for most of the water adsorbed are only slightly greater than the heat of liquefaction. On desorption, there is no hysteresis loop.Water adsorbs more slowly than a hydrocarbon of comparable size. At low relative
A hard, highly porous charcoal has been prepared by the pyrolysis of polyvinylidene chloride. Adsorption isotherms were determined for water, helium, n-pentane and nitrogen. Isosteric heats of adsorption were obtained for nitrogen, and anomalous results were observed below a pressure of 20 microns. Rates of adsorption have been studied for a number of simple hydrocarbons. Large bulky molecules were adsorbed slowly, while linear and planar molecules were adsorbed quickly. Fick's diffusion law was obeyed by those molecules which were adsorbed at measurable rates. The temperature dependence of the rate showed that activated diffusion occurred, and activation energies were calculated. Variations of these energies do not wholly account for the variable rates of adsorption for different molecules, so that steric limitations must play a part.Saran is the generic term for a series of thermoplastic resins derived from vinylidene chloride, which thus have the formula (CHCI),. Pyrolysis drives off hydrogen chloride and yields a form of charcoal. This material was shown by Pierce, Wiley and Smith 1 to be highly porous and to give Langmuir type isotherms with most organic adsorbates. There were indications that the pores in this charcoal were very small ; thus Emmett 2 reported that one specimen adsorbed one-twelfth as much iso-octane as nitrogen. The work described in this paper was chiefly concerned with an investigation of this selective adsorption. EXPERIMENTAL MATERIALSCharcoal.-Pure polyvinylidene chloride was obtained from the Dow Chemical Co. as a powder. It could be readily pressed at 100" C into hard cylinders, using metal moulds and a hydraulic press. This moulded material was heated in a quartz tube in DCICUO, at first at 180" C for several hours which resulted in about 35 % loss of weight, and then the temperature was raised in steps to 700" C, when decomposition was nearly complete as indicated by approximately 74 % loss of weight (calc. for CHCl --f C + HCl, 75-3 % weight loss). The product was a remarkably hard piece of charcoal which, if the heating had not been too rapid, largely retained the shape of the original moulded piece of Saran. Heating the unpressed Saran powder gave a friable mass with different adsorptive properties (see later). Contrary to the experience of Pierce et al., with this Saran it was not necessary to treat the charcoal at 600" C with hydrogen in order to obtain a water isotherm that showed a sharp rise.Helium was obtained by collecting the gas from the boiling liquid. Nitrogen was from a Matheson " prepurified" sample, and was passed over hot copper turnings and through a liquid-air trap. A mass spectrograph analysis showed the sample to be better than 99.9 % pure. n-Pentane was Phillips research grade ; it was washed with H2SO4, NaOH and water, and dried with CaC12 and P2O5.neo-Pentane was Phillips research grade, and was dried with P205. iso-Octane and cyclohexane were washed with concentrated H2SO4 and distilled from cis-and trans-Butenes were Phillips research grade.sodium.
The pyrolysis a t 300°C. of vinylidene chloride monomer adsorbed on Saran charcoal alters the pore structure of the charcoal so that low boiling gases such as nitrogen are adsorbed slowly. The rates of adsorption of nitrogen, argon, and methane have been measured. They were found t o vary with pressure and temperature, and from the temperature variation a n activation energy may be calculated. A new method of deter~nining this energy is described which involves changing the temperature during only one adsorption experiment. INTRODUCTIONPrevious work had shown that pressed Saran (polymerized vinylidene chloride) gave, when pyrolyzed i n vacuo, a porous charcoal in which the pores were comparable in size with the size of simple hydrocarbon molecules (3). T h e pores were not cylindrical for although large bullcy molecules such as neopentane were only adsorbed slowly, flat molecules such as benzene were adsorbed very rapidly as were thin molecules such as n-pentane. I t seemed therefore as though there were slot-like constrictions in the charcoal, perhaps arising from the presence of graphitic platelets. T h e molecules which were adsorbed slowly diffused into the charcoal by an activated process, and the energy of activation, E, could be measured. I t was suggested that E corresponded to the energy required to move a molecule from one position t o another over the carbon surface, though for larger molecules, the energy required to squeeze through a small constriction may also play a part. Unfortunately the kinetic experiments, which were performed a t temperatures ranging from about -50°C. upwards, were not always precisely reproducible even with consecutive runs on the same piece of charcoal. After each run it was necessary to heat the charcoal in order that the desorption should occur in a reasonable time, and it was thought that changes in the charcoal might take place during this heating, accounting for the irreproducibility. T o offset these difficulties an attempt was made to block the interstices of the charcoal to a controlled extent, so t h a t smaller and lighter molecules would be adsorbed slowly a t low temperatures, desorption occurring a t room temperature. The charcoal was successfully blocked, but the behavior was still not reproducible.A t the same time a modified method of determining E during one adsorption run was devised. EXPERIMENTAL Materials CharcoalSaran charcoal was prepared in the manner previously described (3).Oxygen was used initially in an attempt to block the charcoal, which was For personal use only.
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