A STRUCTURAL STUDY OF ALUMINUM-CHLORHYDRATE

DIRK LOWELL TEAGARDEN, Purdue University

Abstract

The structure of the antiperspirant aluminum chlorhydrate was determined to be the {Al(,13)O(,4)(OH)(,24)(H(,2)O)(,12)}('+7)Cl(,7). This structure is composed of a central aluminum in a tetrahedral configuration surrounded by twelve aluminum in a octahedral configuration. The structure is essentially spherical with a +7 charge equally distributed on the surface. Chloride ions act essentially as counterions.^ Chloride activity measurements indicated that the chloride ions were ionic in nature and not bound or trapped within the aluminum chlorhydrate structure.^ Coagulation studies indicated an approximate charge of from six to eight for the polycation.^ pH-stat reactivity and potentiometric titration studies suggested a structure with a high, symmetrical compact charge which was stable to proton attack and degraded with base addition.^ Reaction with Ferron showed the quantities of different aluminum species to be approximately 4% monomeric, 8% small polycation, and 88% larger polycation.^ X-ray diffraction work indicated a size on the order of 9 (ANGSTROM). This was approximately the size of the Al-13 complex.^ Comparison of the empirical formula Al(,2)(OH)(,5)Cl(.)2H(,2)O to the Al-13 complex showed excellent fit.^ Scanning electron microscopy indicated a very platety nature. This in conjunction with the single X-ray peak suggested that the Al-13 complex could only grow in one plane due to the uneven +7 charge.^ Infrared and ('27)Al NMR studies confirmed the existence of a tetrahedral aluminum characteristic for the Al-13 complex. The Al-O bond in the tetrahedral aluminum of the complex was longer than in aluminate, Al(OH)(,4)('-).^ Aluminum hydroxide was precipitated by the addition of base to aluminum chlorhydrate. Bayerite formed following the degradation of aluminum chlorhydrate upon addition of base.^ The existence of the Al-13 complex was also supported by the pH versus time profile following dilution. Degradation of the Al-13 complex by water in the pH range from 4.0-5.5 led to the formation of gibbsite, an aluminum hydroxide polymorph.^ A mechanism for the formation of the Al-13 complex was suggested whereby a condition of high pH formed at the surface of the metallic aluminum during the manufacture of aluminum chlorhydrate. The local high pH allowed clusters of tetrahedral aluminum to form. Some of the aluminums in the cluster were protected as the aluminums converted to an octahedral configuration as they diffused into the bulk solution. The cluster eventually formed the Al-13 complex.^ The Al-13 complex was related to the mechanism of antiperspirancy due to its high effective charge. ^

Degree

Ph.D.

Subject Area

Chemistry, Pharmaceutical

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