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APPLICATION NOTE

AuthorAuthorAuthorAuthorAuthorKlaus OldörpThermo Fisher Scientific, Karlsruhe, Germany

Stability of cosmetic emulsions quantified withrheology and simultaneous DEA

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IntroductionIntroductionIntroductionIntroductionIntroductionCosmetic emulsions have to meet consumer demandsfrom many perspectives. They should have the desiredeffect on the skin, they should have a nice look and feeland they should not change their texture over severalmonths. All these properties are closely related to thelong-term stability of an emulsion. Only from a stableemulsion we can expect the active ingredients to stayhomogeneously distributed, an unchanged look andfeel and texture.

The Thermo Scientific™ HAAKE™ MARS™ rheometeris a versatile universal rheometer, which allows variouscombinations with other analytical techniques. For theverification of an emulsion’s stability, the integration ofa dielectric analysis DEA has been selected.

While the HAAKE MARS can measure the mechanicalproperties of an emulsion, the DEA can give informationabout the microscopic conditions inside the emulsionunder precisely defined stresses, deformations andtemperatures set by the rheometer.

Regarded in a simplified way, a DEA applies an oscillat-ing voltage to a sample between 2 electrodes and mea-sures the resulting current due to the mobility of chargedmolecules (ions) in the electrical field. This mobility isinfluenced by the resistivity of the sample matrix, whichis correlated to the viscosity or modulus of the sample. The results of a DEA measurement can be expressed with the so-called ion viscosity, which is the reciprocal ion mobility or reciprocal conductivity of the sample. In other words, the ion viscosity increases when the ion mobility is decreases.

SamplesSamplesSamplesSamplesSamplesTwo different batches of a skin cream have been select-ed for testing due to their difference in appearance.

Fig. 1:Fig. 1:Fig. 1:Fig. 1:Fig. 1: A lower measuring plate with Mini IDEX comb sensor.

Key wordsKey wordsKey wordsKey wordsKey wordsEmulsions, stability, rheology, simutaneous DEA

While one batch represented the usual quality of thecream with a matt white surface and a rather “solid“appearance the, other had a significantly glossiersurface, softer edges and also felt softer on the skin.

TTTTTest conditionsest conditionsest conditionsest conditionsest conditionsThe tests have been performed using a 20 mm plate/plate geometry with a gap of 1 mm. Sample loading hasbeen done avoiding unnecessary shearing and withreduced lift speed while closing the gap.

A lower measuring plate (TMP) with a DEA comb sen-sor [1] has been used to collect the ion viscosity atdifferent frequencies (Fig. 1). The sensor was connect-ed to a DEA 288 Epsilon (NETZSCH-Gerätebau, Selb,Germany). During the measurements, the NETZSCHProteus® software was triggered by the ThermoScientificTM HAAKETM RheoWinTM software at the start ofthe rheological measurement to guarantee simultaneousdata acquisition.

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Fig. Fig. Fig. Fig. Fig. 22222: : : : : Results of amplitude sweeps on 2 different cosmetic emulsions with the 2 perpendicular lines indicating the end of the respectivelinear viscoelastic range based on the storage modulus. For the less stable cream, the LVR ends at 1.4 Pa, for the stable cream it endsat 11.2 Pa.

HAAKE RheoWin 4.70.0006

Since the DEA sensor consists of a pair of interdigitat-ed electrodes, the DEA measurement is independent ofthe rheometer’s measuring gap, as long a s the gap sizeis bigger than the distance between the 2 electrodes.

With this setup, an amplitude sweep and a temperaturerun have been done on both samples. The amplitudesweep was performed at 20 °C and using the controlleddeformation (CD) mode, to have a better control of themeasurement conditions.

To characterise the temperature dependant behaviour,fresh samples have been heated up from 0 °C to 40 °Cwith a heating rate of 1 K/min.

Results and DiscussionResults and DiscussionResults and DiscussionResults and DiscussionResults and DiscussionThe appearance of a cream sold in a pot strongly dependson to what extent it flows under its own weight or not.Another part of the consumer perception is related to thecream’s handling properties. In other words, how doesthe cream feel, when taking a bit out of the pot andrubbing it onto the skin?

Since these properties are related to the force requiredto make the cream yield, the results of the amplitudesweep have been plotted as a function of the stressapplied (Fig. 2), although the measurement has beendone with deformation control (CD mode).

The end of the linear viscoelastic range (LVR) has beencalculated based on the storage modulus (G’) and a 10% deviation from the plateau value. Between the softer(less stable, open symbols) and the harder (stable,filled symbols) cream lies a factor of about 8 in stress.

Regarding the results from the DEA (Fig. 3), we see anincrease of the ion viscosity at higher times, whichcorrespond to higher stresses. The less stable creamshows the earlier increase after 4.8 min, when thesample was exposed to a stress of 4.2 Pa, whereasthe stable sample shows this increase after 7.2 min orat 40 Pa (stress/time correlation not shown in Fig. 2).

It is interesting to see, that in both cases the onset ofthe ion viscosity increase occurs at stresses about 4times higher than the stresses for the end of the LVRcalculated from the rheological results. Similar to therheological results, the onset of the increase differs bya factor of almost 10.

2Fig. 4: Fig. 4: Fig. 4: Fig. 4: Fig. 4: The change in temperature dependent behaviour of both creams is almost the same with the less stable cream being less elastic.

Comparing the results from the temperature runs on therheometer, the 2 creams behave in a similar way, withthe less stable cream being less elastic throughout thewhole measurement (Fig. 4). After having almostconstant elasticity as indicated by the almost constantphase angle (δ), both samples start to lose elasticityabove 12 °C. The only significant difference lies in the

Fig. Fig. Fig. Fig. Fig. 33333: : : : : Results of amplitude sweeps on 2 different cosmetic emulsions. The data is plotted over time since the Proteus software doesnot read the current rheometer status during the measurement.

maximum of δ above 30 °C. At 33 °C the phase angleof the less stable cream reaches a maximum and startsdecreasing until the end of the measurement. For thestable cream, the maximum occurs at 35 °C and after-wards the decreasing part of the curve runs almostidentical with the curve of the less stable cream.

HAAKE RheoWin 4.70.0006

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ReferenceReferenceReferenceReferenceReference[1] Thermo Fisher Scientific product information P060 “Standard measuring geometries with integrated DEA-sensor for the HAAKE MARS rheometer plaform“ Cornelia Küchenmeister-Lehrheuer, Klaus Oldörp

SummarySummarySummarySummarySummaryWith the HAAKE MARS, the two different creams havesuccessfully been characterized. The visual differencebetween both products has been confirmed with signi-ficant differences in their mechanical response to differ-ent forces (stresses) and temperatures. The results fromthe DEA delivered additional information, which helpsto explain why the differences exist and can be usedas starting point for an analysis why these differencesappeared based on the formulation, temperature history,homogenisation technique used or other process para-meters.

Fig. 5: Fig. 5: Fig. 5: Fig. 5: Fig. 5: Results of amplitude sweeps on 2 different cosmetic emulsions. Due to the temperature range from 0 °C to 40 °C, the time axiscan accidentally be used as a temperature axis as well.

Comparing the DEA results collected during the tempe-rature increase, the stable cream shows a higher ionviscosity before the values start to drop at 26 °C (Fig. 5).The corresponding curves of the less stable cream al-ready go down from 24 °C on. These changes occur ata temperature, where the rheological curves do not showany similar change. Whether it is a coincidence to findthe same temperature difference here like between themaxima of the phase angle in Fig. 4 cannot be answeredbased on the data available here.The DEA results confirm the finding that the behaviourof both creams becomes the same above 34 °C. A pos-sible interpretation of the combined information couldbe that the stable cream has a better homogeneity withsmaller droplets of the aqueous phase, which wouldexplain the higher initial ion viscosity. The less stablecream seems to have a more coherent water phase fromthe beginning on and therefore the lower initial ion vis-cosity. Above 34 °C the temperature-induced coales-cence leads to the same conditions in both creams andhence the same properties.