446 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

STATISTICAL APPROACH TO COMMON VARIABLES 447 The extent to which preparative variables are important for stability of the emulsion made from the emulsifiers and oils studied is indicated in Table 8 for o/w emulsions and in Table 9 for w/o emulsions. In these tables is listed, for each preparative variable, the level of the experimental design showing significance in addition, the number of emulsifier-oil pairs for each oil exhibiting significance at this level is shown. In the case of o/w emulsions, it is evident that higher emulsifier concentration (2.5 per cent), higher concentration of water (90 per cent) and higher temperature (70øC.) appear to be of importance in conferring stability on mineral oil, cottonseed oil and oleic acid o/w emulsions. Addition of oil to water, under the conditions used, was less frequently of importance to the forma- tion of stable o/w emulsions. Although placing the surfactant in water as opposed to oil is favored in the case of mineral oil, this factor is less important or nonexistent for the other nonaqueous liquids. Agitation by means of the Homo-Mixer instead of by propeller appears to be of value with mineral oil. The number of significant factors is appreciably less with methylphenyl silicone than with the other three oils. It may be con- cluded for the range of emulsifiers tested that the chemical or physical nature of this oil primarily determines the stability of its emulsions. Few preparative factors were found to be important for the stability of w/o emulsions. Of the 42 emulsifier-oil pairs which formed at least one w/o emulsion, there are 252 emulsifier-oil-variable comparisons possi- ble only 20 were found to be significant. A small proportion of water TABLE 8--FREO. UENCY OF VARIABLE LEVELS SIGNIFICANTLY FAVORING STABILITY OF O/W EMULSIONS (90-30) Emulsifier Water, Emulsifier Temp., Order of Oil Conc., % % Location øC. Addition Agitation 2.5 (6) 90 (6) w (4) 70 (7) o -+ w (4) H (5) o (1) P (1) 2.5 (6) 90 (3) o (1) 70 (5) o --• w (2) H (1) 30 (1) 2.5 (6) 90 (4) w (1) 70 (4) o-• w (3) H (2) o (2) 25 (1) 2. $ (2) 90 (1) w (1) 70 (2) o • w (1) H (1) 30 (1) o (1) Mineral Oil Cottonseed Oil Oleic Acid Methylphenyl Silicone Frequencies of significant levels are shown in parentheses. TABLE 9--FREO. UENCY OF VARIABLE LEVELS SIGNIFICANTLY FAVORING STABILrrY OF W/O EMULSIONS (90-30) Emulsifier Water, Emulsifier Temp., Order of Oil Conc. % Location øC. Addition Agitation Mineral Oil 30 (4) Cottonseed Oil 30 (1) Oleic Acid 30 (6) 25 (2) w -* o (2) Methylphenyl Silicone 30 (3) 25 (1) w --• o (1) Frequencies of significant levels are shown in parentheses.