OCCLUSIVE PROPERTIES OF SLN 317 RESULTS AND DISCUSSION PRODUCTION PARAMETERS AND PARTICLE SIZE DETERMINATION The composition and production parameters of the formulations investigated in this study are shown in Table I. The SLN dispersions with cetyl palmirate as the matrix lipid were used for investigations regarding the dependency of the occlusion factor on particle size, applied sample volume, and lipid concentration. The formulations containing Dynasan 112-116 as the matrix lipid were used for investigations on the dependency of the occlusion factor on the crystallinity of the lipid matrix. Tables II and III show the PCS and LD data of the investigated formulations, respec- tively. Regarding the formulations with cetyl palmirate, the influence of the production parameters on particle size are evident. Three homogenization cycles at 500 bar lead to the smallest particles (247 nm) and to a very narrow particle size distribution (PI: 0.093). Using a homogenization pressure of 1500 bar leads to a low particle size as well (316 nm, PI: 0.182). However, the width of distribution is increased. This is due to the high amount of kinetic energy that is put into the particles at this pressure. The long-term stability of this SLN dispersion is decreased in comparison to the 500-bar formulation (unpublished data). A pressure of 150 bar for one cycle is less efficient for obtaining SLN. However, 76% of the particles are still in the nanometer range. The width of distribution is further increased and stability is consequently decreased. Simply cooling down the pre-emulsion leads to the formation of lipid microparticles. At this lipid concentration (40%), high-speed stirring for one minute is not sufficient to produce nanoparticles. For this batch, PCS is not an adequate sizing method because of the size range covered by this technique (3 nm-3 pm). Larger particles cannot be detected with this method. Due to the differences in size, the PCS data and LD data differ--apart from the differences in the measuring principle between the two methods. Regarding the ultrasonic production method, it can be stated that this method is in principle suitable for the production of SLN by definition. However, only 92% of the particles are within the nanometer range when processing a high lipid concentration of 40%. In addition, stability is decreased due to the broader particle size distribution. As for the Dynasan formulations, it can be stated that identical production conditions lead to almost identical mean particle size and particle size distribution. All investigated SLN formulations are physically stable at 32øC, well over the duration of the occlusion tests. OCCLUSION--DEPENDENCY UPON PARTICLE SIZE Figure 1 shows the dependency of the occlusion factor F upon particle size. For this in vitro test, 200-mg samples of SLN formulations consisting of 40% cetyl palmirate were used (CPa - CPe). There is an evident correlation between occlusion and particle size. Particles in the micrometer range do not show pronounced occlusive characteristics. For these particles, the occlusion factor remains under 15 throughout the duration of the in vitro test. This is due to the fact that these particles form a film on the filter paper with larger pores. Water evaporation takes place through the pores, and if the pore diameter is increased, water evaporation takes place more readily.

318 JOURNAL OF COSMETIC SCIENCE 80 70 60 50 40 30 20 10 0 ....... i:: '::: .:: :!: 6h 24h 48h :::. :::5 -:.r :-:- :.:..:.: ß 200nm [] 300nm ß 400nm ß 600nm :D 4000nm Figure 1. Effect of particle size on occlusion factor F (40% lipid content, formulations CPa, CPb, CPc, CPd, CPe). Particles in the nanometer range revealsdepending on their mean diameter--an oc- clusion factor which is two to three times as high as the factor obtained by micrometer particles. This is due to the fact that the particles are smaller and film formation can thus take place more easily. Complete film formation has been observed for 200-nm particles in scanning electron microscopy studies (19). Water evaporation is decreased to the greatest extent here. Consequently, it has been found that smaller particles show higher occlusion factors. For particles with a size from 200 nm to 400 nm, there is only a slight decrease in occlusivity. However, for particles that are 600 nm and larger, the occlusion factor is decreased strongly. It can be stated that for higher occlusivity, mean particle sizes of 400 nm are sufficient. Regarding the time dependency of the occlusivity, it can be seen that for particles between 200 nm and 400 nm, the occlusion factor remains fairly constant throughout the experiment. This is supported by findings that film formation takes place within the first hour. After film formation is completed, water evaporation should remain at a constant rate, as found in this study. OCCLUSION--DEPENDENCY UPON SAMPLE VOLUME Figure 2 shows the dependency of the occlusion factor F upon sample volume applied to the membrane. The occlusive effect is more pronounced if the thickness of the layer is greater. For this invitro test, sample sizes of 100 mg, 150 mg, and 200 mg of formu- lation CPa (40% cetyl palmitate, 227+2 nm) were used, i.e., 5.3 mg/cm 2, 8 mg/cm 2, and 10.6 mg/cm 2. The difference of the occlusion factor F between sample sizes of 5.3 mg/cm 2 and 8 mg/cm 2 is greater than the difference between sample sizes of 8 mg/cm 2 and 10.6 mg/cm 2. This is due to the fact that the denseness of the particles on the filter is already