After a preliminary survey with the electron microscope of various preparations of colloidal gold, a study was made of the process of nucleation and growth in gold colloids. It was shown that nucleating agents may be identified with reducing agents which form a mixed polymer with chlorauric ion before the reduction to the nucleus takes place. It was also shown that the law of growth is exponential. The average size, the deviation from the average size and the character of the particle size distribution curve are determined by the amount of gold, the nucleation process and the law of growth.Colloidal gold may be considered as a typical hydrophobic colloid with particle size falling below the resolution limit of the optical microscope. With the development of the electron microscope which has s resolution permitting the examination of the individual colloidal particles 1 it was natural to make an extensive study of the shape, mean size and size distribution of the various preparations of colloidal gold and determine the factors that govern these properties. Previous work on the subject was limited to one or two preparations.4 l1 Hauser and Lynn, Ex9eriments in Colloid Chemistry (McGraw Hill, 1g40), p. 18.
Kinetics of coagulation with NaCIOl of 200 A. colloidal gold which was stabilized by citrate ions was investigated by electron microscopy. The results were compared with the Derjaguin-Verwey-Overbeek theory and an agreement was obtained only in the very early stage of coagulation a t low electrolyte concentration. The discrepancies observed otherwise were explained on the basis of variation of the stabilizing potential due to adsorption of cation and anion of the electrolyte introduced. The Hamaker constant A was calculated as 2.3 X The morphology of the aggregates formed under various conditions was also explained on the basis of the behavior of the electrical double layer. erg.
An electron microscope study has been made of the process of reduction of gold chloride to colloidal gold. A method was found of producing gold of uniform size in the diameter range of 200-1500 A. It was found that the particle size distribution curve of colloidal gold preparations is not an error curve but is determined by the kinetics of the process: the nucleation stage and the growth stage. A theory of nucleation is proposed based on the formation of auric ion-organic reducing agent complex which on attaining proper size decomposes to form a gold nucleus. This theory is shown to be in agreement with experiment. The exponential law of growth of the colloidal gold was also established. 4
Colloidal gold has a long history (1). Gold's lustre, chemical stability and high price fascinatedmany ancients. Its preparation to give beautifulruby red `solution' intriguedtbe alchemists. Unusualmedicalproperties wereascribedto this potablegold'andhope was expressedthat this may be the 'elixir of life.' This hope found a modest realization in thè DanzigerGold, analcoholicconcoction containingfinely-dividedgold Stillavailable, its exhilaratingeffects are due not to thegoldbutto its alcoholiccontent. Gold ruby glass was man ufactured atthe end of the 16th centuryandthe `Purple of Cassius; prepared by the reduction of soluble gold salts by tin chloride, has been kn own since the 1 7th century.
A method has been developed for the preparation of uniform palladium particles of diameter from 55 to 450 angstroms. Uniform particles of gold layered on palladium were also synthesized. Hydrothermal treatment of aluminum hydroxide sol was used to prepare rods of alumina with uniform cross section from 100 to 500 angstroms and of varying lengths. The palladium was adsorbed as individual particles on alumina rods, both present in aqueous suspension. Then the suspension was dried to give a catalyst containing metal particles of uniform size dispersed in open pores produced by the intermeshing of the alumina rods. This procedure guaranteed the absence of diffusion control in the rate of reactions observed experimentally. All stages of the preparation were monitored with the electron microscope. The kinetics of the ethylene-hydrogen reaction were examined by means of a pulse technique. The number of active sites determined by carbon monoxide titration of the surface was equal to the number of surface atoms as determined by the calculation of the quantities of compounds involved in the synthesis and electron microscope examination. Furthermore, the activity per site depended on the method of preparation, being four times smaller when sodium formate was used as a reducing agent instead of sodium citrate. This may be due to the fact that the shape of particles makes certain crystallographic planes more favorable. Decrease in the size of particles to 56 angstroms produced no effect on catalytic activity beyond that due to the increase in the number of surface atoms. The activity of commercial 5 percent palladium on alumina diluted 100-fold with alumina gave 80.4 percent conversion with propylene and 82.7 percent conversion with ethylene. Thus there was little difference in the behavior of the two olefins.
An analysis of the vibrations of the pyridine molecule was made on the basis of infra-red, Raman, and ultraviolet absorption spectra. The fundamental frequencies were assigned and the thermodynamic properties of pyridine were calculated on the basis of such an assignment.
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