NEW SOFT CAPSULE 247 x 10 4 35000 2500 30000 25000 o o o 2000 60 80 1500 20000 ' 1000 20 40 100 Temperature / øC Figure 7. Effect of temperature on the breaking intensity and Young's modulus of agar aqueous gels. The open (O) and closed (O) circles represent the breaking intensity and Young's modulus, respectively. encapsulation improved the stability of all trans-retinol palmitate compared to that of the oil solution. The remaining percentages of all trans-retinol palmitate at 50øC for four weeks in oil solution and in the microcapsule were 72% and 87%, respectively. The effect of the microcapsulation on ethyl linoleate was also studied. The remaining per- centages of ethyl linoleate in oil solution and in microcapsules were 91% and 95%, respectively (Figure 9). DISCUSSION The microcapsule in this study has a polynuclear structure. The average diameter of internal oil droplets (nucleus) depends on the conditions for preparing the primary O/W emulsion (35). The average diameter of internal oil droplets was small enough (0.5-2 pm) for preparing an O/W/O emulsion without a significant loss of internal oil. The size of the microcapsule itself is governed by conditions during the addition of the agar-O/W emulsion mixture to the outer oil phase. The average diameter of microcap- sules was controlled by changing the stirring speed and the process temperature. The average diameter decreased as the stirring speed increased because of the increase of mechanical shear. The process temperature also contributed to the control of average diameter because the interfacial tension is reduced with the rise in temperature. The magnitude of the contribution to the average diameter of the microcapsule by changing the stirring speed was greater than that caused by changing the temperature. This result shows that controlling mechanical power is the most effective way to control

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-