COMPLEX FORMATION IN AEROSOLS 805 Stiffness values below about 30 indicate that the foams have low viscosity and are quite thin. Typical aerosol shaving lather foams have stiffness values ranging from about 50 to over 100. Foam Drainage Rates One of the most common indications of complex formation is the de- crease in the rate of foam drainage that occurs with the addition of a polar compound to a surfactant solution. This is considered to be due to an increase in the viscosity of the liquid foam films, or laminae, that occurs with complex formation. Foam drainage rates were determined in the present work by discharging a known quantity of foam into a funnel and determining the amount of liquid that drained from the foam at various time intervals. With most series of foams, there were certain time intervals after discharge when the differences in drainage rates between the various foams were the most distinct. The comparative foam drainages that showed the greatest differences in the foams were selected. The time required for this varied with the different series of foams and is given in the tables. Drainage from foams results from gravitational forces on the liquid films, which causes thinning of the foam laminae without rupture, or from rupture and collapse of the foam structure itself (26). Thus, the liquid which drains from a foam can have two different origins. Foam Stability Foam stability has two aspects, foam drainage and persistence of the foam. Thus, a foam may drain and become quite thin but may retain its structure. Foams also may collapse as a result of rupture of the foam structure. In the present study, comparative foam stabilities were judged visually by discharging a series of foams onto paper towels and noting the appearance of the foams after various time intervals. The rapidity with which the foam wet paper was an indication of the drainage, and the rate at which the foam collapsed was an indication of the stability of the foam structure. It was possible to line up a series of foams in order of their relative stabilities by this procedure, but it was not feasible to place any numerical value upon the stability. Foam Density Foam density was determined by weighing a known volume of the foam as soon after discharge as possible. The volume of the vessel used for density determinations was 360 cc.

8O6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Type of Discharge The type of discharge of the products is indicated in the tables. Some of the products had a quiet discharge, while others had a noisy dis- charge. Some samples also discharged as liquids which subsequently expanded into foams. These products are described as having a liquid discharge. Others discharged a mixture of liquid and foam which also expanded into a foam. These are described as having a semiliquid dis- charge. The remaining products gave an immediate foam during dis- charge. Microscopic Examination Observations of the foam structures were made using a Bausch and Lomb Sterozoom microscope, Model BVB-73, equipped with a 10X paired wide field eyepiece and a power pod magnification of 3 X. Ap- proximate bubble sizes in the foams were determined using a microm- eter disc (•31-16-08) which measured intervals of 0.001 in. The fact must be stressed that the bubble sizes reported are approxi- mate. Some of the cells appeared to be fairly round, while others were very irregular in shape. The dimensions given in the report indicate the largest dimension of a specific bubble. All of the foams had bubbles which varied considerably in size. The sizes of the smallest and the largest bubbles were determined in order to show the range of sizes. No attempt was made to determine the frequency of any particular size. Microphotographs were obtained with a Spencer trioeular single- stage microscope, manufactured by the American Optical Company. It was equipped with a 15X eyepiece and a 3.5 objective lens. The earnera was focused through 10X eyepieces with 3.5 objectives. The pictures were taken with a MP-3 Polaroid Multipurpose Industrial View earnera with a 4 X 5 film adapter, using surface illumination and a one second exposure. Preparation of the Aerosol Emulsions The artionic surfactants, sodium lauryl sulfate and the trierhanoi- amine salts of lauric, myristic, palmitie, and stearic acids were used in the present studies. Although sodium lauryl sulfate is not suitable for packaging in metal aerosol containers as a result of its corrosive effect, it has been widely used in complex formation studies in nonaerosol sys- tems and much information was already available about combinations of sodium lauryl sulfate and the long-chain alcohols. The trierhanoi- amine salts of the fatty acids are used extensively in aerosol products,