DISPERSE SYSTEMS AS TOPICAL VEHICLES 221 18 16 14 12 10 8 6 4 2 0 0 I I I 20 40 60 80 100 shear rate (s -•) ^ 24hu -•-24 h I v ß •. 15'ev.u X 15'ev.I 1• 30'ev.u 0 30'ev.I Figure 3. Rheograms at 25 ø + 0.1øC of micellar solution D in the presence of 2.0% w/w laureth-2 and 1.0% w/w PCA just prepared and after evaporation. u = upper curve. 1 = lower curve. Lastly, the addition of 0.50% w/w PEG-120 methyl glucose dioleate to micellar solu- tions containing 1.0% or 2.0% w/w PCA determined a pseudoplastic behavior, also maintained after 30-minute evaporation (Figure 4). An increase in the structuring of micellar solutions could, of course, have been obtained by employing the common thickening agents of aqueous phase. Rather, we preferred to consider substances that, in some cases, depending on their amphyphilic structure, probably locate at the core/bulk interface, partially modifying it. Thus it was possible to evaluate whether the presence of a different composition of the interface could influence the rheology of the system. Moreover, the addition of the above-mentioned substances allowed us to eliminate ethanol from the formulations, thus increasing their mildness. As the micellar solutions containing 0.50% w/w PEG-120-methyl glucose dioleate and 2.0% w/w PCA were found to possess the best rheological properties, they were further examined to evaluate microbial growth by means of a challenge test. The results ob- tained with the plate count method are reported in Table VI, expressed in colony- forming units (c.f.u.) of viable microorganisms per gram. It may be seen that, under storage at room temperature, the c.f.u. at all times are below 10 per gram of product. Results of the challenge test indicate the following: at 24 hours the rapidity of the action

222 JOURNAL OF COSMETIC SCIENCE 8 6 4 2 0 8 6 4 2 0 0 I I 40 60 shear rate (s -1) I lOO -•-24 h u [] 24hl --•-- 15' ev. u X 15' ev. I 30' ev. u -•-30' ev. I Figure 4. Rheograms at 25 ø _+ 0.1øC of micellar solution D in the presence of 1.0% w/w PCA and 0.50% w/w PEG-120 methyl glucose dioleate just prepared and after evaporation. u = upper curve. 1 = lower curve. Table VI Challenge Test on Miceliar Solution: Survival of Microorganisms (c.f.u.) Time Gram(+)ve Gram(-)ve Yeasts Molds 24 hours 10 10 10 10 7 days 10 10 10 10 30 days 10 10 10 10 of a preserving system, at seven days the validity of such a system, and at 30 days any resistant strain become evident. It is clear that no microbial growth took place in the system under study, whose behavior was in complete agreement with that of a well- preserved system. This noteworthy resistance to microbical attack, probably due to the presence of the surfactants, hexylene glycol, and also, to some extent, linalool, is par- ticularly interesting, considering that the system contains lecithin, which is well known as a substrate for microbial growth. In view of the eventual practical use of these miceliar