248 JOURNAL OF COSMETIC SCIENCE Table III Composition of Microcapsules and Oil Solutions for Stabilizing Reagents Components All trans-retinol All trans-retinol Ethyl Ethyl palmitate oil palmitate linoleate renoleate solution microcapsule oil solution microcapsule Internal oil phase All trans-retinol 0.2 0.2 palmitate Ethyl linoleate 0.2 0.2 Squarane 50 3.3 Liquid paraffin 90 9 CIO 49.79 3.3 9.75 1 Butyl hydroxy 0.01 0.01 0.05 0.05 toluene Water phase Butylene glycol 5.6 7 HCO-60 0.4 0.5 Water 35.65 30.75 Agar 1.5 1.5 Outer oil SC-9450N 1 1 phase Decamethyl 49 49 cyclopentasiloxane Remaining percentage of reagents at 50øC after 4 weeks 72% 87% 91% 95% the size of microcapsules because the viscosity of the agar-O/W emulsion mixture is high. However, thermal control is also effective for precise size control. A primary factor determining the strength of the microcapsules is the gel strength of the agar employed. Each agar has intrinsic rheological properties that depend upon the original seaweed, and the strength of agar gel is strongly related to its molecular weight and composition. In this study, an agar specimen that has one of the highest gel strengths was employed, since microcapsules for cosmetic use require thermostable and shear-resistant properties to withstand the usual emulsification process. Although the breaking intensities of the microcapsules were almost constant throughout the ratio of the internal oil, the Young's moduli were considerably affected by the ratio. In this case, because the breaking intensity indicating the strength of the solid portion (water phase) and the formulae of the portion were identical, it is reasonable that the breaking intensities were constant even though the ratio of the liquid portion (internal oil phase) was increased. On the other hand, Young's modulus indicates the hardness of the whole gel, and the modulus of the gel was affected by the space occupied by the oil droplets because the space could behave as vacant spaces for the gel. Thus, Young's modulus would decrease on increasing the vacant spaces. The strength of microcapsules is also affected by temperature because agar gel causes the thermoreversible sol-gel transition. Although no change in the breaking intensity was observed at temperatures between 25 o and 85øC, Young's modulus started to decrease above 70øC. This indicates that the flexibility of the agar network increases with increase in temperature, though the struc- ture of the gel is still maintained. Microencapsulation improved the stability of all trans-retinol palmitate and ethyl linole-

NEW SOFT CAPSULE 249 lOO 80 E o *" 60 E 40 0 5 10 15 20 25 30 Time I day Figure 8. Stabilizing effect of microencapsulation for all trans-retinol palmirate. Time course of the changes in the remaining percentage of all trans-retinol palmirate at 50øC in microcapsule (open circle, O) and in oil (closed circle, 0). The formulae employed are shown in Table Ill. ate. Due to its poly-ene structure, all trans-retinol is sensitive to oxygen, light, heat, acid, and metal ions. These factors cause oxidation, isomerization, and polymerization of retinol (36-39). Retinol palmitate is more stable than retinol however, similar decom- position processes are expected. Unsaturated fatty acid-like ethyl renoleate is also oxi- dized automatically and converted to peroxides (40). The conversion speed is accelerated with rise in temperature (41). Although the shielding effect of the microcapsule against thermal attack is poor, the shielding effect against oxygen permeation would be sig- nificant. The solubility of oxygen in hydrocarbons is approximately ten times as high as that in water (the Bunsen absorption coefficient that indicates the solubility of oxygen is 28.1 x 10 2 in octane and 2.83 x 10 2 in water at 25øC). Since the agar gel membrane prepared in this study contains 77.5% water, the membrane reduces the permeation of oxygen compared to a simple oil solution. Moreover, in the agar microcapsule, oxygen has to pass through both O/W and W/O interfaces to approach a reagent encapsulated in the innermost phase. This means that the agar microcapsule can protect an oxygen- sensitive reagent from oxygen invasion not only through a water layer, but also through an O/W or W/O interface. For the use of an antioxidant in the microcapsule system, both an oxygen-sensitive reagent and an antioxidant were formulated to be localized in the internal phase. In this system, because the antioxidant can be close to the oxygen-sensitive reagent in the small component (the microcapsule), the efficacy of the antioxidant was enhanced as compared to a system that is formulated with an antioxidant in the whole system. Therefore,