SYNTHETIC ESTER DISPERSING AGENTS gll agents such as talc, starch, bentonite, and silica derivatives and careful selec- tion of suitable valves (2). With regard to this, another important aspect is that the particle size of the solid to be dispersed must be about 50/.• or less (able to pass through a number 39,5 mesh screen). A combination of many of these factors is used in order to formulate suitable aerosols, which contain dispersed solids (3,4). In the past, various materials had been used as dispersing agents for these aerosols. They include isopropyl myristate, hexadecyl alcohol, mineral oil, and other similar substances. In addition, several surfactants, such as the sorbitan esters, have also been used as dispersing agents for these aerosols, although the use of these esters has been limited to those products containing very small quantities of powder such as is found in medicinal aerosols (5). Preliminary studies, which employed several other materials, which are po- tentially usehd as dispersing agents, revealed the possibility that the dispers- ing properties of the material may be related to the chemical structure of the compound. This observation has also been noted by others (6). Several materials were selected for this study. They included isopropyl my- fistate, hexadecyl alcohol, methyl myristate, isopropyl isostearate, g-ethyl- hexylpelargonate, propylene glycol dipelargonate, propylene glycol monoiso- stearate, and propylene glycol monoisostearate (90 per cent). The chemical structure of each of these materials is shown in Fig. 1. The effect of each of these materials upon the dispersion of aluminum chlorhydroxide complex, talc, and starch was determined. The solubility of each of the above materials was also determined in various propellants and propellant blends so that the solubility of the substance in the propellant could be correlated with its suspending properties. Since viscosity changes were noted when the dispersing agent was mixed with the solid (in particular, combinations of aluminum chlorhydroxide complex with silica products), the viscosity of these systems was determined. EXPERI1VIENTAL Development o[ a Simple Method for the Screening of Dispersi,g Agents This study was carried out to devise a simple technique for evaluating dis- persing agents, which are useful with solids in aerosol formulations. Further- more, it is desirable to effect this screening without the use of a pressurized aerosol system, thus allowing any laboratory without aerosol filling equipment to obtain a reasonable idea as to the potential performance of various prod- ucts in an actual aerosol formulation. It was felt that if a system in which a nonpolar solvent, such as petroleum ether, was substituted for the propellant in a typical aerosol formulation, measurement of suspension and redispersibili- ty properties of the solid material in this system would correlate to the same properties measured in an actual aerosol system. Several dispersing agents and

212 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aluminum chlorhydrate (as the model solid), were evaluated at atmospheric pressure relative to their suspension and redispersibility properties in a nonpolar solvent system, which simulated fl•e viscosity of a typical pressur- ized aerosol system. A. Simulated Aerosol Formulation Aluminum chlorhydrate* Fumed silica* Dispersing agent Petroleum ether 5.0 per cent 0.1 per cent 4.9 per cent 90.0 per cent B. Dispersing Agents Isopropyl myristate Propylene glycol monoisostearate Isostearyl isostearate Isopropyl isostearate Propylene glycol dipelargonate C. Preparation of Dispersion The aluminum chlorhydrate, fumed silica, and dispersing agent were mixed together. A 6.0-g sample of this concentrate was then added to a 100-ml grad- uated cylinder, followed by addition of 54.0 g of petroleum ether. The cylin- ders were shaken by hand to give complete dispersion. D. Measuremen,t o/Settling Rate and Ease of Redispersibility The cylinders containing a homogenous dispersion of aluminum chlorhy- drate and fumed silica were allowed to stand, and the volume of clear su- pernatant liquid was measured, starting with a 15-sec time lapse, until settling was completed. The final volume the solid occupied was also recorded. After 24 hours, the number of inversions required to redisperse the powder was measured. These results are shown in Table I. Solubility of Dispersing Agen,t in Propellant The approximate solubility of each of the dispersing agents studied was de- termined by adding a given weight of dispersing agent to an aerosol com- *Chlorhydrol, Impalpable Solid, Micro-dry Reheis Chemical Company, Berkeley Heights, N.J. tCab-O-Sil-M-5, Cabot Corporation, Boston, MA.