Poster presented at the 4th World Congress on Emulsions Lyon 2006
T. Sobisch, D. Lerche,
L.U.M. GmbH, Rudower Chaussee 29 (OWZ), 12489 Berlin, Germany,
www.lum-gmbh.com, info@lum-gmbh.de
F. Aguilar, G.G. Badolato, H.P. Schuchmann,
Institute of Engineering in Life Sciences, Department of Food Process Engineering, University of Karlsruhe, Haid-und-Neu-Str. 9, 76131 Karlsruhe, Germany
Abstract
Destabilisation processes in emulsions are very complex. Aging and breaking of emulsions are the result of several processes – creaming or sedimentation, flocculation, coalescence, Ostwald ripening, phase inversion – which can occur separately or simultaneously. This is the main reason why the development of reasonable procedures for shelf life prediction is very difficult and always depends on the product which is subject to evaluation.
Multisample analytical centrifugation with photometric detection (STEP-technology) is a tool for an accelerated characterisation of any demixing processes (creaming, sedimentation, phase separation) as well as for the quantification of time dependent structural alterations (e.g. flocculation, coalescence, Ostwald ripening).
Model emulsions were designed exhibiting creaming, coalescence, flocculation or Ostwald ripening as the dominating mechanism for instability. To answer the question whether multisample analytical centrifugation can be used to predict the long term stability of emulsions results obtained are compared with rheological measurements, particle size analysis by light scattering and common accelerated stability tests like freeze/thaw cycling and storing at 45 °C.
In all cases the creaming (or sedimentation) velocity measured by analytical centrifugation markedly increased during storage induced by increasing salt concentration (flocculation), increasing droplet size (using emulsifier with lower HLB value) or increasing the fraction of clove oil (Ostwald ripening). During a relatively short period of observation only a small part of these alterations could be detected using conventional methods of emulsion characterization.
Method of multisample analytical centrifugation
The multisample analytical centrifuge (LUMiFuge, LUM GmbH, Berlin Germany) used in this study employs the STEP technology, which allows to measure the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (measurement scheme see Figure).
Measurement scheme of the multisample analytical centrifuge with photometric detection. Parallel NIR-light is passed through the sample cells and the distribution of local transmission is recorded at preset time intervals over the entire sample length.
The data are displayed as function of the position within the sample, representing the distance from the centre of rotation. The transmission profiles obtained are representative for the variation of droplet (particle) concentration inside the sample (low transmission means high, high transmission means low droplet concentration).
The progression of the transmission profiles provides information on the kinetics of the separation process and allows particle characterization.
At the same time up to 8 different samples can be analysed simultaneously at constant or variable centrifugal speed up to 2300 g. The separation behaviour of the individual samples can be compared and analysed in detail by tracing the variation in transmission at any part of the sample or by tracing the movement of any phase boundary.
As for a given type of sample cells the position corresponds to a defined sample volume the relationship between the position and the volume can be established by calibration. This allows to directly determine separated phase volumes and to calculate packing densities.
For further details of the application of the method for characterization of emulsions see [1,2,3].
Silicon oil-in-water emulsions with and without induced flocculation
Oil-in-water emulsions φ= 30 %
Sample preparation:
1. 40 % oil / 1% sodium dodecyl sulfate
Process:Microfluidizer® 500 bar
2. Diluted with deionized water or concentrated salt solution to result in 30 % oil and 0, 150 and 300 mM NaCl
Creaming without centrifugation (high salt concentration)
Centrifugation – creaming at 1100 g (low salt concentration)
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During the storage time no significant changes in droplet size and viscosity at 0 and 150 mM NaCl
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Creaming and water separation clearly visible after the addition of salt (c > 500 mM NaCl)
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For small concentration of salt the height of separated water (in a cylinder) not detected or remains constant
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The creaming velocity measured by the analytical centrifuge increases during storage, increasing the salt concentration and increasing the temperature
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All emulsions were very vulnerable to freeze/thaw cycling
Coalescencing vegetable oil-in-water emulsions –
stabilised with emulsifiers having different polarity
Oil-in-water emulsions j = 30 %
Process:Microfluidizer® 500 bar
Non ionic emulsifier Emulsions
η mPa/s x3,2 µm
Tween 20, HLB 16.7 3.73 0.66
Tween 80, HLB 15 3.11 0.62
Tween 65, HLB 10.5 3.12 1.35
Centrifugation – creaming at 1100 g
Movement of the position of the interface emulsion –water (distance to the centre of rotation) during centrifugation measured just after preparation of the emulsions.
Already at this stage distinct differences in stability traced due to bigger droplets (Tween 65) or higher viscosity (Tween 20)
- During storage no significant changes in droplet size and viscosity
- Creaming velocity increase due to bigger droplet sizes with decreasing emulsifier HLB value (more coalescence during production) or lower viscosity
- Increased destabilization at higher temperature
- Only unstable emulsion vulnerable to freeze-thaw cycling
Ostwald ripening in water-in-oil emulsions
Water-in-oil emulsions j = 30 %
Process:Microfluidizer® 500 bar
Continuous phase mixture clove/vegetable oil viscosity η
Sample 1 50/50 76 mPa/s
Sample 2 75/25 47 mPa/s
Sample 3 25/75 102 mPa/s
Sample 4 0/100 141 mPa/s
Emulsifier: polyglycerol polyricinoleat PGPR 90 – HLB 1
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Marked Ostwald Ripening only at mass fraction clove oil > 0.5
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Emulsions droplet size and sedimentation velocity increase during storage, however the viscosity remains unchanged
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Instability increases increasing the storage temperature
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Results of droplet size measurements and sedimentation velocity are in good agreement
Conclusion
The investigations revealed that analytical centrifugation is well suited for accelerated stability testing of emulsions of varying composition and for various types of slow destabilizing mechanisms. Whereas measurements of droplet size distributions and viscosity measurements were not able to always detect the beginning of destabilization, centrifugal analysis enabled to detect instabilities faster and more accurately. The multisample technique therefore is ideally suited for screening purposes in emulsion product development.
Acknowledgement
This study was supported by a grant from the Bundesministerium für Wirtschaft und Technologie (grant number KA 0047301AWZ3).
References
[1] T. Sobisch, D. Lerche, Rapid characterization of emulsions for emulsifier selection, quality control and evaluation of stability using multisample analytical centrifugation
SCI/RSC/SCS conference Cosmetics and Colloids, London 15 February 2005
http://www.soci.org/SCI/groups/col/2005/reports/pdf/gs3257_sob.pdf
[2] T. Sobisch, D. Lerche,
Rapid selection of dispersants and evaluation of emulsion stability by analytical centrifugation
La Rivista Italiana delle Sostanze Grasse 6/2005 308 – 316
[3] T. Sobisch, D. Lerche, 2007
Application of multisample analytical centrifugation for fast evaluation of long term stability and freeze/thaw stability of emulsions
http://docs.google.com/Doc?id=dcvcr5vw_1fqx3hp
