EMULSION QUALITY 751 EMUL. CONC. EXT. PHASE • + EMUL. CONC. EMULSION I NT. PHASE EMULSION Figure 5. Illustration of adding external phase and internal phase liquids to the emulsion concentrate fails to reinvert to form the intended type, an emulsion with poor stability or large droplets will form. This happens more frequently with emulsions having a fairly large amount of internal phase and is true with both W/O- and O/W-type emulsions. Figure 5 illustrates the difference in diluting an emulsion concentrate with an external phase liquid and an internal phase liquid. In general a dilution with an external phase liquid at the second stage is more desirable as dilution proceeds more smoothly and the droplet size of the final emulsion does not change significantly from that of the emulsion concentrate. On the other hand, when an emulsion concentrate is diluted with the internal phase liquid, as illustrated by the lower emulsion of Figure 5, a coarse emulsion or an emulsion with a wider droplet size distribution can result. This is easily understood since, during the addition of the cold phase, the temperature of the batch is lowered considerably and unless a high-shear mixer or a homogenizer is used in the second stage the resulting emulsion will be coarse. As a general rule in carrying out LEE, it is advisable to add the external phase liquid in the second stage after the completion of the first stage emulsification. If a homogenizing operation is desired, it is generally best to carry it out during the first stage since total batch volume at this stage is smaller and temperature is higher, making homogenization more effective. In executing LEE, the higher the value of ce, the greater is a conservation of energy expended to process the emulsion. Thus, it is of interest to determine the limit of ce within which emulsions can be prepared without significantly sacrificing the emulsion quality. In most LEE applications, the greater the ce value, the more concentrated and more viscous will be the emulsion concentrate. A practical limit of ce is thus dependent not only on the formulation but also on the process equipment, particularly the geometry of the kettles and the type and power of the mixers. For example, a marine-type propeller mixer can handle low-viscosity emulsions adequately, but not moderate- to high-viscosity emulsions.

752 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Nonionic O/W Emulsion Wt. % Light Mineral Oil 10.0 Stearyl Alcohol 3.0 P.O.E. (5.5) Cetyl Ether 1.2 P.O.E. (10) Cetyl Ether 2.0 Propylene Glycol 5.0 Water 78.8 100.0 A turbine mixer will handle a moderately high viscosity emulsion but probably not a heavy cream. A paddle-type mixer will handle a fairly viscous cream and allows LEE processing at a relatively high oz, although the rate of shear provided by such a low-speed mixer may not be sufficient for an adequate dispersion. Thus, in a practical operation, the limit of oz will be decided by a number of factors. It is, nevertheless, of interest to determine the upper limit of oz by carrying out experiments up to a very high oz region in the laboratory where a sufficient mixing can be provided. Table III shows an example of a low-solids, O/W emulsion stabilized with nonionic surfactants used in this series of experiments. A rather surprising result was obtained with this emulsion at high oz H range as shown in Figure 6. As ozH increased beyond 50%, the emulsion droplets became smaller and extremely fine emulsions having averaged droplet diameter in submicron range were obtained for oz• values greater than 70%. The sharp improvement in the emulsion quality at z o o z o o I I I I [ I I I I' NONIONIC O/W EMULSION - ß , • , , I , i •'0""--.-0-0 I0 20 :50 40 50 60 7'0 80 90 O0 % WATER WITHHELD, O• H Figure 6. Effect of o• H on droplet size of the nonionic O/W emulsion