ALUMINUM CHLORHYDROXIDE SUSPENSIONS 155 cell-like structure similar to a honeycomb. Each cell traps a portion of the suspension and restricts free movement of fluid and particles (3). In addition, there is probably a build-up of positive charge by the aluminum chlorhydroxide particles, which is partially neutralized by the negative charge of fumed silica. This overcomes the electrokinetic force of repulsion between particles and allows the riocs to form. Fumed silica in the concentrate retards sedimentation by increasing viscosity. It produces a system which is pseudoplastic and thixotropic in nature. EXPERIMENTAL Materials Antiperspirant Concentrate The following typical antiperspirant concentrate was used as a con- trol. When the concentration of any of the ingredients was varied, the difference was made up with isopropyl ester. Aluminum chlorhydroxide Fumed silica Additives Isopropyl ester 33.0% 4.0% 3.9% q.so About 10% of this concentrate was then diluted with a low pressure fiuorocarbon propellant to produce the finished product. ,4 lureinure Chlorhydroxide Two commercially available grades of aluminum chlorhydroxide were selected. (a) Impalpable powder with the following particle size distribution: Micron size 0-2 2-5 5-10 10-17 17-27 27-34 34 % composition 1.9 4.5 25.3 28.9 31.1 7.5 0.8 This was evaluated at two concentrations (33% and 41% by weight in the suspension). (b) Ultrafine powder with the following particle size distribution: Micron size 0-2 2-5 5-10 10-17 17-27 27-34 34 % composition 0.7 8.1 66.2 23.3 1.6 0.1 ___ Both particle size distributions are typical for these grades of alumi- num chlorhydroxide.

156 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Fumed Silica A commercially available grade of fumed silica was selected for use. It has a surface area of 200 m2/g. Evaluations were made at 3.0, 3.5, and 4.0% by weight in the concentrate suspension. Additives The following additives to the concentrate suspension were evaluated. Perfume--A custom perfume was evaluated at 3.5% and 5.0% by weight. Phenolic Antibacterial--A commercially available grade of hexachlo- rophene was evaluated at 0.4% and 1.0% by weight. Quaternary Antibacterial--A commercially available grade of ben- zethonium chloride was evaluated at 0.5% by weight. Isopropyl Ester A commercially available product containing approximately 95% isopropyl myristate and 5% various other isopropyl esters was selected. Test Procedures Water Determination A modified Karl Fischer water determination was performed by add- ing a 50% excess of Karl Fischer reagent, allowing 10 min for complete reaction, then back-titrating with standard water-in-methanol reagent. Particle Size Distribution (PSD) PSD was determined with a Coulter Counter, Model B, * containing a 70-• aperture (4) and equipped with a volume converter, Model M.* Viscosity Measurement After the suspensions were allowed to sit for 2 rain after preparation. measurements of apparent viscosity were made using a Brookfield RVT Viscometer,* equipped with a disc spindle No. 3 at 10 rpm. Readings were taken after the fourth revolution. Preparation of Suspensions Quantities (300 g) of each suspension were prepared using the follow- ing order of addition: isopropyl ester, additives, almninum chlorhy- droxide, fumed silica. * Coulter Electronics, Industrial Div., 590 W. 20th St., Hialeah, Fla. 33010. ? Brookfield Engineering Laboralories, Inc., 240 Cushing St., Stoughton, Mass. 02072.