Untersuchungen mit NMR und Ultraschall an kosmetischen perlglanzhaltigen Cremes

13C-CP measurements on the stearic acid pearlescent system show that it is very suitable for detection of molecules in less mobile fractions of emulsions ready for the market such as crystallites. Comparison of characteristic signals of raw materials with signals of finished emulsions allows conclusions on the development of pearlescence in emulsions. Stearic acid and possibly also cetyl alcohol not only serve as the emulsifier in pearlescent emulsions but also are present in excess amounts and crystallize out, producing the pearlescent effect. When the temperature is increased, the crystals melt, a phenomenon that correlates with disappearance of pearlescence. Upon subsequent storage at room temperature, the crystals again form: the emulsions become pearlescent again. Concerning monoglyceride pearlescent systems the aim of this work was to study the relationship between the phase transitions of the emulsifier and the pearlescent properties of creams using NMR spectroscopy and ultrasound. Since the pearlescent effect is attributable to the formation of a certain phase of the emulsifier system, the coagel, first the different phases of the emulsifier system in the cream were studied in order to better understand formation of the coagel and to control it as much as possible. Then the aim was to determine the influence of changes in the composition of the emulsifier. All four phases of the monoglyceride emulsifier system – coagel, gel phase, liquid-crystalline lamellar phase and cubic phase – can be characterized in creams by means of NMR spectroscopy. The phase transition temperatures can be determined by ultrasonic velocity measurements, which were further developed in this work. When the temperature is increased the pearlescent effect disappears: the coagel is transformed into the liquid-crystalline lamellar phase. Visual observations showed that there is a temperature range for all pearlescent creams in which the disappearance of pearlescence is reversible on a narrow time scale. The reversibility of pearlescence correlates with the reversible ultrasonic velocity curves for transition between the coagel and liquid-crystalline lamellar phase. Once the temperature range of short-term reversibility of the disappearance of pearlescence is exceeded, the coagel will not form again until after several hours or weeks. Back-formation of the coagel can be followed by means of isothermal ultrasound measurements. It is a random nucleation process. These insights into the emulsifier system were useful for studying the pearlescence properties of the coagel: the amount of emulsifier in the coagel can be quantified indirectly by 13C-HPDEC measurements. On the one hand, the amount of coagel needed to generate pearlescence could be determined. For instance it could be shown that more that 27% of the emulsifier system of a cream containing 15% glyceryl laurate and 5% glyceryl myristate must be in the coagel for the pearlescent effect to become visible. On the other hand, ripening of the cream could be followed quantitatively and it was found that the intensity of pearlescence increased with the time elapsing after preparation. This correlated with the increase in the amount of emulsifier system in the coagel. The ripening process can take up to approx. 15 months. Changes in the composition of the emulsifier have an influence on phase transitions and on the intensity of the pearlescent effect of creams. It could be shown that the ratio of glyceryl laurate to glyceryl myristate plays a more important role for phase transitions than the total amount of emulsifier. For creams containing only glyceryl laurate or more glyceryl laurate than glyceryl myristate, the phase transition from the coagel to the liquid-crystalline lamellar phase began at room temperature and was accompanied by a disappearance of pearlescence at temperatures lower than 40°C. However, it could also be shown that the pearlescence properties of two different creams do not only depend on the absolute amount of coagel present but rather on their consistency. If creams were capable of flowing, the pearlescence is more intense at room temperature, even if the emulsifier system contained less coagel. The total amount of emulsifier therefore was not important for the intensity of pearlescence: creams containing only 3% emulsifier system may contain a smaller amount of coagel than creams with 20% but because they are much thin fluid, the pearlescent effect is just as strong. The preferred formulation for a pearlescent cream would therefore be one in which only glyceryl myristate is used: the pearlescence would then be retained even at temperatures above 50°C, with the result being fewer problems during storage and transport