RESEARCH
Agi Points to New Nucleating Agents Ratio of hydrophobic to hydrophilic sites determines effectiveness of possible cloud seeding agents A new series of artificial nucleating agents for possible use as cloud seeders in cloud modification work can now be produced. Now that the surface chemistry of the most effective nucleating agent (silver iodide) has been recognized, it's possible to seek out other materials which nucleate or promote crystallization in gaseous and liquid media such as water clouds, ac-
cording to Dr. A. C. Zettlemoyer of the surface chemistry laboratory of Lehigh University, Bethlehem, Pa. New and cheap cloud seeders hâve already been developed, some of which rival silver iodide in efficacy in laboratory tests, Dr. Zettlemoyer says. To produce these cloud seeders (or nucleating agents), inorganic materials are used as substrates. Silicas, usually
SERIES. Lehigh University's Dr. Albert C. Zettlemoyer, Dr. K. S. Narayan, and David R. Bassett have developed a series of nucleating agents that may be useful in cloud seeding studies. Some of the materials they are working with are silicas, carbon black, magnesite, bauxite, alumina, magnesia, and lime 58
C&EN
DEC.
9,
1963
of colloidal size, are very desirable inorganic substrates, the Lehigh chemist finds. Other substrates can be used, but it is difficult to find cheaper ones than silicas, he says. These include carbon black, magnesite, limestone, dolomite, clay, bauxite, alumina, magnesia, and lime. All of these materials have largely hydrophilic surfaces. To be effective nucleating agents, however, the surface of the substrate must consist mainly of hydrophobic sites. But hydrophilic sites covering a critical proportion of the area must also be present, Dr. Zettlemoyer says. The hydrophilic sites should cover from 5 to 40r/c of the surface, preferably 20 to 30*/r. (A site is an atomic or molecular area ranging from 2 to 20 square A.) In addition, particle size of the substrates should range from 0.01 to 10 microns, and preferably between 0.3 and 1 micron, he adds. Character. The special character of the surface of the nucleating agents is based on the nature of the silver iodide surface, determined by Dr. Zettlemoyer and his colleagues, N. Tcheurekdjian of Lehigh \ n d Charles L. Hosier of the College .of Mineral Industries of Pennsylvania State University. They found that the silver iodide surface is mostly hydrophobic but possesses from 7 to 37 r/c hydrophilic sites. Dr. P. G. Hall and Dr. F. C. Tompkins of Imperial College, London, later confirmed the hydrophobic nature of the silver iodide. Cloud physicists up to that time regarded silver iodide as largely hydrophilic and valuable as a cloud seeder because it is epitaxial with ice, Dr. Zettlemoyer says. But colloid chemists regarded silver iodide as hydrophobic because it flocculates and settles out of colloidal dispersion upon reduction of charged double layers. The conclusion reached by the colloid chemists was correct, but the argument was specious since the floccules simply settle out because of gravitational forces, he points out. Surface Conversion. Several techniques can be used to convert a surface which is largely hydrophilic to the desired proportion of hydrophobic and hydrophilic sites, Dr. Zettlemoyer says. For example, precipitated silica, which is largely hydrophilic, is converted to 26% hydrophilic sites when heat treated for two hours at 450° C , and from two to 10 hours at 650° C. (0.05 micron particle size). In a cloud chamber, the silica produces initial
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.
FREE!
MEW S&S Brochure on
I l LA1 CHROMAIM Every chemist interested in chromatography will find this new authoritative S&S brochure on Thin Layer Chromatography valuable in his work. Advantages include superior separation of enzymes, proteins, hormones, lipids, and nucleic acids. Even materials such as hemoglobin may be separated into fractions by this method. CONTENTS: Included is a background on the benefits of ion exchange celluloses, the advantages of thin layer chromatography; the types of ion exchangers available; their uses; prices of Selectacel® TLC Ion Exchange Celluloses; and a bibliography.
S&S TLC Advantages: Briefly, Selectacel TLC Ion Exchangers offer chemists two outstanding advantages over all known materials for thin layer chromatography work.
1 2
SAVINGS IN TIME. Quality control techniques result in extremely uniform particle size, outstandingly smooth TLC layers. BETTER ADHERENCE TO THE PLATE. Superior, extremely sharp separations even of closely related substances.
Selectacel Ion Exchange Celluloses are available in a complete range of types (DEAE, ECTEOLA, CM and P). Get full data. TEAR OFF COUPON AMD MAIL TODAY!
Φ
Carl Schleicher & Schuell Co. Keene, New Hampshire-Dept. CE312 Please send free the new Selectacel brochure on Thin Layer Chromatography.
Name Company Address City
Zone
State
Selectacel is produced by Brown Company and exclusively packaged and distributed for laboratory use by Carl Schleicher & Schuell Co.
DEC.
9, 196 3 C & E N
59
