Cosmetic emulsions have to meet consumer demands from various perspectives. They need to feel and look nice, they should not change their texture over several months and they should have the desired effect on the skin.
These properties are all closely related to the long-term stability of an emulsion. The active ingredients can only be expected to remain homogeneously distributed and maintain an unchanged look, texture and feel, if they are from a stable emulsion.
The Thermo ScientificTM HAAKETM MARSTM Rheometer is a universal, versatile rheometer that can be combined with other analytical techniques. The integration of a dielectric analysis (DEA) has been chosen for the verification of the stability of an emulsion.
Although the HAAKE MARS Rheometer can measure the mechanical properties of an emulsion, the DEA can give additional information about the microscopic conditions inside the emulsion when it is under precisely defined stresses, temperatures and deformations set by the rheometer.
A DEA applies an oscillating volume to a sample between two electrodes. It then measures the current that is created due to the mobility of charged molecules (ions) in the electrical field. The resistivity of the sample matrix, which is correlated to the modulus or viscosity of the sample, influences the mobility. The DEA measurement results can be expressed with the ion viscosity, which is the reciprocal conductivity or reciprocal ion mobility of the sample. Simply put, when the ion mobility decreases, the ion viscosity increases.
Samples
Two different batches of a skin cream were chosen for testing because they differed in appearance. One batch represented the usual cream quality – with a rather “solid” appearance and a matt white surface. The other had softer edges, had a significantly glossier surface and felt softer on the skin.
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Figure 1. A lower measuring plate with Mini IDEX comb sensor.
Test Conditions
Tests were carried our using a 20 mm plate/plate geometry and a gap of 1 mm. Sample loading avoided unnecessary shearing using a reduced lift speed when closing the gap.
Ion viscosity was collected at different frequencies using a lower measuring plate (TMP) with a comb sensor1 (Fig. 1). Connected to the sensor was a DEA 288 Epsilon (NETZSCH-Gerätebau, Selb, Germany). To guarantee simultaneous data acquisition, the NETZSCH Proteus® software was triggered by the Thermo ScientificTM HAAKETM RheoWinTM Measuring and Evaluation Software at the start of the rheological measurement.
As long as the gap size is bigger than the distance between the two electrodes, the DEA measurement is independent of the rheometer’s measuring gap. This is because the DEA sensor consists of a pair of interdigital electrodes.
With this setup, a temperature run and an amplitude sweep were done on both samples. To have better control of the measurement conditions, the amplitude sweep was performed at 20 oC using the controlled deformation (CD) mode.
Fresh samples were heated up from 0 oC to 40 oC with a heating rate of 1 K/min to characterize temperature dependant behavior.
Results and Discussion
The appearance of a cream that is solid in a container depends greatly on the extent that it flows under its own weight. The handling properties of the cream are another part of consumer perception. For example: how does the cream feel when rubbed onto the skin?
These properties are related to the force needed to make the cream yield and so the results of the amplitude sweep have been plotted as a function of the stress applied (Fig. 2). The measurement has, however, been done with CD mode.
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Figure 2. Results of amplitude sweeps on 2 different cosmetic emulsions with the 2 perpendicular lines indicating the end of the respective linear viscoelastic range based on the storage modulus. For the less stable cream, the LVR ends at 1.4 Pa; for the stable cream it ends at 11.2 Pa.
The storage modulus (G’) and a 10% deviation from the plateau value have been used to calculate the end of the linear viscoelastic range (LVR). A factor of about 8 in stress separates the softer (less stable, open symbols) and the harder (stable, filled symbols) cream.
The results from the DEA (Fig. 3) show an increase of the ion viscosity at higher times, corresponding to higher stresses. The less stable sample shows an increase after 4.8 min when it was exposed to a stress of 4.2 Pa. The stable sample only increases after 7.2 min or at 40 Pa (stress/time correlation not shown in Fig. 2).
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Figuire 3. Results of amplitude sweeps on 2 different cosmetic emulsions. The data is plotted over time since the Proteus software does not read the current rheometer status during the measurement.
Interestingly, in both cases the onset of the ion viscosity increase happens at stresses approximately four times higher than the stresses for the end of the LVR calculated from the rheological results. The onset of the increase differs by a factor of nearly ten, which is similar to the rheological results.
The two creams behave similarly when comparing the results from the temperature runs on the rheometer. The less stable cream is more elastic through the entire measurement (Fig. 4). Both samples begin to lose elasticity above 12 oC. Up until that point, they both had almost constant elasticity as indicated by the almost constant phase angle (δ).
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Figure 4. The change in temperature dependent behavior of both creams is almost the same with the less stable cream being less elastic.
The only significant difference is the maximum phase angle above 30 oC. The phase angle of the less stable cream reaches a maximum at 33 oC and then decreases until the end of the measurement. The stable cream reaches its maximum at 35 oC and then decreases in a curve almost identical to that of the less stable cream.
In regard to DEA results collected during the temperature increases, the stable cream shows a higher ion viscosity than the less stable cream, before the values start to drop at 26 oC (Fig. 5). The corresponding curve of the less stable cream starts to decrease at 24 oC. These changes happen at a temperature where the rheological curves do not change. From the data available here it is uncertain whether finding the same temperature difference, as between the maxima of the phase angles in Fig. 4, is a coincidence or not.
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Figure 5. Results of amplitude sweeps on 2 different cosmetic emulsions. Due to the temperature range from 0 °C to 40 °C, the time axis can accidentally be used as a temperature axis as well.
The DEA results confirm that the behavior of both creams above 34 oC is the same. It could be that the stable cream has a better homogeneity with smaller droplets of the aqueous phase and this would explain the higher initial viscosity. The less stable cream appears to have a lower initial ion viscosity because it has a more coherent water phase from the beginning. The coalescence induced by temperatures above 34 oC leads to the same conditions in both creams and therefore the same properties.
Summary
Two different creams were successfully characterized with the HAAKE MARS Rheometer. Visual differences between the two creams were confirmed by significant differences in their mechanical response to different temperatures and forces (stresses).
Additional information was derived from the DEA and this helped to explain why the differences are present. This understanding can be used as a starting point for analysis on why the differences appeared with regard to the temperature history, homogenization technique used or other process parameters.
Acknowledgments
Produced from materials originally authored by Klaus Oldörp from Thermo Fisher Scientific, Karlsruhe, Germany.
References and Further Reading
- Thermo Fisher Scientific product information P060 “Standard measuring geometries with integrated DEA-sensor for the HAAKE MARS rheometer platform" Cornelia Küchenmeister-Lehrheuer, Klaus Oldörp
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.