ALUMINUM CHLORHYDROXIDE SUSPENSIONS Table III Effect of Particle Size Distribution on Viscosity Grade of Concen- Particle Size Distn. (%) Aluminum tration Viscosity Chlorhydroxide ( % ) 0-5• 5-10• ) 10• (cps) Impalpable 33 6.4 25.3 68.3 (control) 2900 Ultrafine 33 8.8 66.2 25.0 6700 159 the particles contained in the impalpable aluminum chlorhydroxide sus- pension are above 10 •. Seventy-five per cent of the particles contained in the ultrafine aluminum chlorhydroxide suspension are below 10/•. Additives to the suspension will influence the viscosity in different de- grees. Phenolic antibacterials, for instance, do not seem to have any effect on viscosity, as illustrated in Table I. Quaternary antibacterials, on the other hand, have a marked effect. In Table I, a comparison of suspensions with and without benzethonium chloride at 0.5% by weight is presented. Perfume as an additive will significantly influence viscosity. An ex- ample is given in Table I of the effect of a custom perfume at different levels in the concentrate suspension. SUMMARY As illustrated, the concentrate suspension system is very sensitive to water content. Water is an additive to the suspension and acts as a bridg- ing compound between the fumed silica particles that will increase the cell-like structure. Perfumes contain many polar ingredients that may also act as bridging compounds, thus increasing viscosity. In general, additives to the suspension should be screened carefully in order to avoid excessive viscosity. This will provide a broader base for the tolerance of water. The effect on viscosity of all the variables is cumulative. The vis- cosity threshold level ca• easily be exceeded when two or more variables contribute their VlSCOSlty.-lncreaslng influence. In screening possible suspending media for suspensions containing fumed silica, polarity is the most important factor. If the medium is completely nonpolar, the viscosity will increase at a given concentration of fumed silica. This is due to the fact that the influence of additives becomes greater. Mineral oil, for example, produces concentrate sus- pensions which have a thick paste-like consistency. More polar suspend-

160 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ing media produce lower viscosity. They tend to hydrogen bond with the fumed silica, depressing the formation of the cell-like structure. Ethyl alcohol is an example of a polar suspending medium which pro- duces a very low viscosity. Gonsiderations other than viscosity must be taken into account, however, in choosing a concentrate suspending me- dium. Mineral oil, for instance, is unacceptable at this concentration in an aerosol form for aesthetic properties and potential inhalation hazard. Ethyl alcohol may cause agglomeration in the finished aerosol product. Isopropyl esters are medium polar to nonpolar in character, provide ac- ceptable finished product characteristics, and usually produce low viscos- ity concentrate suspensions, but should be expected to vary from lot to lot. Particular attention should be paid to the concentration of aluminum chlorhydroxide used and the particle size distribution of the sample. The product should contain enough aluminum chlorhydroxide to be efficacious and have a small enough average particle size to provide aes- thetic value and to insure proper operation of the aerosol container. However, the viscosity of the concentrate system should be maintained below the threshold level to prevent manufacturing problems. These discussions have centered around the problems which exist in the formulation of suspension concentrates related to viscosity control. In the finished aerosol form there is no problem with viscosity. This is mainly due to the dilution of all ingredients approximately by a factor of 10. ACKNOWLEDGMENT The authors gratefully acknowledge the services provided by the Ana- lytical Chemistry Department of the Gillette Toiletries Company. (Received June 18, 1970) REFERENCES (1) Schumacher, G. C., Theoretical aspects of bulk compounding technology, II, Suspensions, Amer. J. Hosp. Pharm., 26 (9), 548-50 (1969). (2) Cartwright, A. C., The role and selection of gums and thickness for use in suspensions, presented at the 1969 Society of Cosmetic Chemists of Great Britain Symposium on "Gmns and Thickeners." (3) Loftman, K. A., Fumed silica in liquid specialtics, Soap Chem. Spec., 42 (9), 70-3 (1969). (4) Instruction Manual, Coulter Counter Industrial Model B, Coulter Electronics, Industrial Div., Hialeah, Fla. 33010.