Interfacial characterization and stability of emulsions

Contant Persons: C. Koukiotis [koukiotis@gmail.com], M. Doutsi [m_doutsi@yahoo.gr]
 

Emulsions are complex soft – materials where fluid is dispersed into a liquid matrix. This kind of systems is widespread in nature and in everyday life. Some interesting applications of emulsions can be found in foods, cosmetics and many industrial products. Food and cosmetics emulsions are multi-component dispersed systems consisting of two immiscible phases a non polar and a polar one containing soluble or dispersed macromolecules such as different kinds of emulsifiers. These systems tend to separate into distinct phases especially in liquid –type products. So, for this reason stabilization and destabilization is a problem that concerns modern technology. Their stability against phase separation during storage is critically dependent on the type of emulsifying and stabilizing agents present at the interfaces and the distribution of the dispersed phases.

Several methods maybe employed in order to investigate the emulsion stability, some intrusive and non-intrusive, which determine the droplet size distribution from either direct visual observations or from measurements performed on withdrawn samples with various techniques. Measurement of the electrical conductance appears to be a tempting non-invasive method to measure the spatial fraction and distribution of two phases over a large area inside an opaque dispersion. In the case of the emulsification process, as the initial large oil droplets disintegrate into smaller ones due to the shearing action of mixing, water disperses more and more in the intra-droplet spaces and the electrically accessible water becomes progressively less, reflecting the structural changes during emulsification. The inverse process happens when the mixing stops, the drops progressively gather in agglomerates and the emulsion starts to destabilize.

This project examines the stability of emulsions and foams using a novel electrical conductance tomography technique which allows to identify the longitudinal volume fraction and phase distribution along a test vessel. Together with the study of emulsion stability this work investigates some interfacial properties (static and dynamic interfacial tension, interfacial rheology, dilational viscosity and elasticity) with in- house and commercial equipment such as PAT- 1S (Fig. 1), ODBA-1 (Fig. 2) and BPA-1S (Fig. 3) from Sinterface Technology, the TE2 (Fig. 4) and TVT 2 (Fig. 5) from Lauda Gmbh, custom made electrical technique (Fig. 6) etc. The aim is to understand the correlation between emulsion stability and the interfacial properties of the systems under test.

Further to the above a low-energy emulsification method by emulsion phase inversion is also studied in several systems. This can be obtained by changing the water volume fraction (emulsion inversion point method) by successively adding water into the mixture of oil and surfactant. Initially water droplets are formed in a continuous oil phase. Increasing the water volume fraction changes the spontaneous curvature of the surfactant from initially stabilizing a W/O emulsion to an O/W emulsion at the inversion locus (Fig. 9). This technic has been successfully applied in a study to produce stable nanoemulsions of an aminopolysiloxane (Fig. 10) and is further applied in a study for emulsification of paraffin oils with various non ionic surfactants surfactants (Fig. 11, Fig. 12, Fig. 13).

In this study, the non-ionic surfactants in use are Ethylan 1005 and Ethylan 1008, which have different chemical structure. Emulsification takes place using 20% of paraffin oil, 75% of millipore water and 5% of pure surfactant or mixture of the two surfactants mentioned previously in different proportions. Destabilization of the emulsions over time is observed with non-intrusive methods (e.g. cameras) and with the use of in-house Axiostar Plus (Carl Zeiss Inc.) microscope, the correlation between emulsion stability and size of droplets is under research. Special attention is given to the emulsions prepared with pure surfactants, as data analysis has shown that the emulsion with Ethylan 1005 consists of small sized droplets (1-50μm) but shows low stability, while the emulsion with Ethylan 1008 consists of larger droplets (1-300μm) but shows better stability. In order to understand this phenomenon, interfacial properties were tested with in-house equipment such as PAT-1S (Sinterface Technology) and TE1/2 (Lauda Gmbh) and also SITE 100 (Krüss) and a research on steric phenomena and theory is in progress.

Particle size distributions are measured by DLS (Fig. 7) for the nanoemulsions and a microscope equipped with a camera and image analysis software (Fig. 8) for emulsions with particles in the micro range.

Figure 1: Profile analysis tensiometer (PAT-1, Sinterface)

Figure 2: Oscillating Drop and Bubble Analyser (ODBA-1, Sinterface)

Figure 3: Maximum bubble pressure tensiometer (BPA-1S, Sinterface)

Figure 4: Ring and plate tensiometer (TE 2, LAUDA)

Figure 5: Drop volume tensiometer (TVT 2, LAUDA)

Figure 6: Time stability of sunflower oil/water (50/50) and 3% Soybean Protein Isolate emulsion at different positions inside the test vessel as measured by the custom made electrical technique.

Figure 6: Zetasizer Nano ZS ZEN3600 for DLS measurements.

Figure 7: Zetasizer Nano ZS ZEN3600 for DLS measurements.

Figure 7: Microscope equipped with a camera and image analysis software (Axiostar Plus, Carl Zeiss Inc.)

Figure 8: Microscope equipped with a camera and image analysis software (Axiostar Plus, Carl Zeiss Inc.)

Figure 8: Representation of emulsion phase inversion method [Image taken from McClements DJ. Soft Matter 2011;7:2297].

Figure 9: Representation of emulsion phase inversion method [Image taken from McClements DJ. Soft Matter 2011;7:2297].

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Figure 10: Z-average particle size of polysiloxane emulsion by DLS vs the HLB of the used C13 ethoxylated surfactant.

 

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Figure 11: Phase Separation of a paraffin oil emulsion in water with Ethylan 1005 surfactant.

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Figure 12: Photos under microscope of paraffin oil emulsions in water with surfactants (a) Ethylan 1005, (b) Ethylan 1008.

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Figure 13: Comparison of Droplet Volume Distribution for parafin oil emulsion in water with Ethylan 1005 & Ethylan 1008 surfactants.