ice at —3° C ; a significant mass of ice at —10° C ; and massive amounts of ice at —14° C. Silver iodide, however, usually requires —4° C. to initiate ice formation. Another approach is ion impregnation of a largely hydrophilic substrate to produce hydrophobic sites. For example, when 50 parts of precipitated silica (0.05 micron particle size) is ground together with one part of silver nitrate and heated to about 850° C. for four hours, it is converted into about 29% hydrophilic sites. In a cloud chamber, trace ice formation occurs at — 2° C ; a significant amount of ice at —12° C ; and massive amounts of ice at —14° C. The nucleating agent is at least as good as silver iodide at most temperatures, Dr. Zettlemoyer says. At temperatures below - 8 ° C. to about - 2 0 ° C , it seems better, he adds. Other impregnating agents that can be used include metallic sulfides, metallic halides, zinc nitrate, and silver chromate. Still other approaches to making hydrophobic silica and other hydrophilic materials include esterifying the surface of the substrate with an aliphatic or aromatic alcohol, reacting the surface with phenol and formaldehyde to produce a resin coating, and adsorption on the surface of proteins or amino acids. Different Method. A different method to produce a nucleating agent is to create a surface that initially has the proper proportion of hydrophobic and hydrophilic sites, Dr. Zettlemoyer says. This can be done effectively by making silica of colloidal sizes by flame hydrolysis. For example, when silicon tetrachloride (0.01 to 0.1 micron size range) is injected into a flame of hydrogen burning in oxygen, effective nucleating agents are produced. These have about 25% hydrophilic sites. Also, carbon black produced by the decomposition of carbonaceous fuel has initially an oxidized surface which is partly hydrophobic and partly hydrophilic. This surface can be converted to bring the hydrophilic surface area within the optimum range of ordinary heat treatment in an inert atmosphere. Dr. Zettlemoyer has applied for patents for both the method of nucleating crystallization of a hydrogen-bonding crystal from liquid and gaseous media, and the nucleating agents for use in such media and their production.
Pore Diffusion Is Important Adsorption Step Diffusion within the pores of solid adsorbents is a relatively important ratedetermining step in many adsorption processes. This diffusion may take place mostly along the adsorbent's surface rather than mostly in the gas phase, as previously supposed. These two facts have emerged from analyses of adsorption data and their comparison with rate equations, Dr. J. M. Smith of the University of California, Davis, told the 30th Annual Chemical Engineering symposium. The symposium was sponsored by the ACS Division of Industrial and Engineering Chemistry and held at the University of Maryland, College Park. Adsorption of gases on porous solids is a significant process in drying, gas separations, and heterogeneous catalysis. It is now fairly generally agreed that in such separation processes, over-all adsorption rate depends on rates of diffusion from the gas to the solid surface, diffusion inside the pores of the solid, and adsorption of the gas on the solid surface. Thus the rate of movement through the body of the gas to the particle's surface is a first step, Dr. Smith explains. Porous solids achieve their adsorptive or catalytic properties with surface areas in the hundreds of square meters per gram. Virtually all of this surface lies inside the millions of pores of the adsorbent. So diffusion within the pores becomes important. Activation energies for physical adsorption are small. Therefore, diffusion rates may very well control total absorption rates, Dr. Smith points out. In their approach to the problem, Dr. Smith and Shinobu Masamuna measured adsorption rates at liquid nitrogen temperatures for nitrogen on Vycor glass (a case of physical adsorption) and ethyl alcohol on silica gel above 90° C. (a case of chemisorption ). They then compared the "breakthrough" or relative concentration curves obtained with theoretical breakthrough curves, in which they assumed that one or more of the three rate-determining processes controls the over-all rate. For the adsorption of nitrogen on Vycor glass, they find that the rates of diffusion to the particle and surface adsorption on the particle are too rapid to constitute any effective resistance to the over-all adsorption process.
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This, by elimination, makes it appear that diffusion within the pore is the important rate-controlling step. In fact, an equation with this variable in control produces a curve that very nearly fits the data, indicating that diffusion within the pore is controlling. Evaluate. Once this was established, the Davis workers could use it to evaluate the nature of the diffusion process within the pore, called intraparticle diffusion. Diffusivity—a measurement related to the rate of diffusion—could then be used to shed light on the two possible mechanisms of intraparticle diffusion: two-dimensional diffusion on the pore wall, or gas-phase diffusion in the pore volume. In the past, studies of the effectiveness of porous catalysts had considered pore diffusion to be in the gas phase. Application of the theory established by Dr. R. M. Barrer in England allows devising an equation for the relative effects of the surface and pore diffusion in the nitrogen-Vycor case. These indicate that surface diffusion is more important. But more detailed proof comes from the adsorption of ethyl alcohol on silica je\ between 90° and 155° C. This is a case not of physical adsorption but of chemisorption, including significant activation energies. So pore diffusion alone is not rate-controlling in this case, but is joined by a low rate of adsorption. With small particle sizes at only slightly above 90° C , surface adsorption controls the rate. Its effect can thus be determined and plugged into situations where adsorbent particles are larger and the temperature higher to determine the pore diffusion effect. This way, the Davis workers found the rate of pore diffusion to be strongly temperature-dependent. The rate of diffusivity would be nearly independent of temperature if pore diffusion were mostly in the gas phase, Dr. Smith says. But this is not the case. An Arrhenius plot of the surface adsorption rate with temperature is linear. Measured over-all rates fit this plot at lower temperatures, but experimental points pass through a maximum and drop off sharply at higher temperatures. Departures from the curve are not unusual. But a decrease of the rate constant is, Dr. Smith says. This indicates diffusivity in the pores must have dropped off, pointing to diffusion via migration of adsorbed particles on the pore walls rather than gas phase diffusion in the pore volume.
