Interactions of silver halide sols with hydrolyzed hafnium species

Summary Coagulation and reversal of charge effects of freshly prepared and heated solutions of hafnium tetrachloride have been studied as a function of the p_H using aged silver halide sols and solsin statu nascendi. It was shown that the critical coagulation concentration and the critical stabilization concentration (due to charge reversal) increased with increasing p_H. These observations have been related to the hydrolysis of the hafnium ion. At p_H values above 4 essentially the entire amount of hafnium is present in the form of the neutral soluble, species Hf(OH)₄. This accounts for the inability of the hafnium solutions to reverse the charge of the sols at higher p_H values. The adsorption measurements carried out with the aid of the radioactive isotope¹⁸¹Hf showed that the neutral hydrolyzed species are strongly adsorbed on negatively charged silver iodide particles. The adsorbed amounts of hafnium on a AgI sol are considerably larger than in the case of charged hydrolyzed ions (such as hydrolyzed thorium ions) on a similar sol. This is explained by the ability of the neutral hafnium species, Hf(OH)₄. to form a close-packed adsorbed layer. The results confirm previous findings that the enhanced adsorption of hydrolyzed ions is caused by the presence of the hydroxyl group, whereas the ionic charge plays a negligible role in this case.

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Zusammenfassung Die Koagulations- und Umladungserscheinungen von frisch dargestellten und durch Erwärmung gealterten Lösungen von Hafniumchlorid wurden an Silberhalogenid-Solen in der Abhängigkeit vom p_H untersucht. Es wurde festgestellt, daß die kritische Koagulationskonzentration und die kritische Stabilisationskonzentration (die Umladungsgrenze) mit steigendem p_H höher werden. Diese Beobachtungen wurden durch die Hydrolyse des Hafnium-Ions erklärt. Wenn p_H>4 ist, bestcht nahezu die ganze Menge von Hafnium als neutrale, gelöste Hf(OH)₄ Moleküle. Damit wird es erklärt, daß die Silberhalogenid-Teilchen bei höheren p_H-Werten durch Hafniumsalze nicht umgeladen werden können. Die mittels des radioaktiven Isotopen¹⁸¹Hf durchgeführten Messungen zeigten, daß die neutralen Hf(OH)₄ — Moleküle stark an den negativ geladenen Silberjodid-Teilchen adsorbiert sind. Die adsorbierte Menge per Mol von AgI ist beträchtlich größer als die Menge der geladenen, hydrolysierten Ionen (wie z. B. ThOH³⁺), die an dem gleichen Sol bestimmt wurde. Dieser Effekt ist leicht erklärt, da man mit den neutralen adsorbierten Molekülen eine fest gepackte Adsorptionsschicht bilden kann. Diese Resultate bestätigen die früheren Befindungen, daß die höhere Adsorptionsfähigkeit der hydrolysierten Ionen durch die Hydroxylgruppe verursacht wird, während die Ionenladung eine zu vernachlässigende Rolle spielt.

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Supported by the U.S. Army Research Office (Durham), Grant No. DA-ARO (D)-31-124-G 656.

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Part of a PhD Thesis byL. J. Stryker, supported by a NASA Traineeship.     

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