019 characterization of a water/oil emulsion by...
TRANSCRIPT
Characterization of a Water/Oil Emulsion
by Laser Diffraction
Introduction
019
Emulsions are mixtures of non-miscible liquids that form a dispersion of fine droplets in a continuousphase. Such diphasic systems are very extensively used in a wide range of industrial sectors, such as thepharmaceutical and cosmetics industries, agro-foods or petrochemicals, for example.
Two types of emulsions, direct and indirect may be produced; they correspond to the dispersion of oil inwater (O/W) or water in oil (W/O) respectively.Such emulsions may be stabilised by an emulsifier, the action of which maintains the dispersion ofdroplets.
The particle size analysis of the droplets enables us to:
- Fix the conditions for the formation of the emulsion
- Assess its stability
Description of an ideal Water/Oil system
Water/Oil mixtures were made to assess theinfluence of the measurement parameters on theappearance of the particle size distribution.
The influence of the power and duration of theultrasonic waves were assessed for various binarysystems at increasing water concentrations.
The aim of these operations was to define:
- The influence of the dispersion parametersused on changes in size and obscuration
- The maximum concentration of thedispersed phase for an accurate andreproducible measurement.
Figure 1: Photograph of an Oil/Water emulsion
examined under an optical microscope
Influence of the mode of dispersion
The use of agitation and pumps producesdroplet sizes of between 75 and 97µmafter 600s of treatment.
Ultrasound on the other hand, generatesmuch more energy in the system andproduces sizes of 2.7µm in less than200s.
Figure 2: Changes to D50
according to the mode of
emulsification ([C]=0.19%)
- Significant reduction in size- Asymptotic- Slight increase
It is possible to identify 3 conditions:
Finally, we observed an increase in the diameter of the bubbles after 325sof exposure to ultrasound at 50W. Coalescence starts and opposes theincreases in surface area imposed by the emulsification processes.100 µm
100 µm
Various diphasic systems of the Water/Oil type were produced and analysed by laser particle sizeanalysis. The agitation of the mixture and operation of the peristaltic pump, which are essential to make aparticle size measurement, are enough to separate the droplets of water in the continuous phase.For a constant volume of water, the number of small droplets increases. Concentration in terms ofnumber and, therefore obscuration of the medium also increase.
For this study, the influences of three modes of dispersion were compared: agitation, the peristalticpumps and ultrasound. The results set out in the figure 2 indicate that the mode of emulsification thekinetics of the reduction of the size of the droplets is different.
Observations of particle size distribution enable us to add that just after thestart of the emulsification process, particle size distributions arepolydisperse. The reduction in D50 is not due to the shift of a monodispersepopulation but rather the disappearance of entire populations between10µm and 100µm.
This phenomenon was revealed by examination of the samples by opticalmicroscope. The initial population is highly polydispersed and the addition ofultrasound (50W) results in a fine particle size distribution centred between 1and 10µm.
Therefore, the bubbles fuse to formlarger bubbles.
AgitationPump 1 (60 rpm)Pump 2 (120 rpm)Ultrasounds 20WUltrasounds 35WUltrasounds 50W
Assessment of the maximum water concentration
The determination of particle size distribution by laser diffraction is only possible in a small concentrationrange. Concentration should be sufficient so that the quantity of matter analysed is representative of thesample but should not exceed a limit value at which multiple-scattering starts.Figure 4 shows that for all concentrations, the size of the droplets is reduced by 200µm to 100µm after600s of agitation.
At the same time, the obscuration of the solution, which is due directly to the concentration in thedispersed phase, increases continuously in time (figure 5).
Figure 4 : Changes to diameter D50 in time
for W/O systems
Figure 5 : Changes to obscuration in time
for W/O systems
For dispersion by simple agitation, the obscuration of a system with 1% water rises to 30%. This valuecorresponds to the limits after which measurement by laser diffraction is made impossible due to multiplediffusion.
For this type of system and mode of dispersion, it is possible to use a maximum concentration of 1% interms of volume of water.
0
50
100
150
200
250
0 0.05 0.1 0.15 0.2
D5
0 (
µm
)
1/t (s-1)
References
Conclusion
[1] Techniques de l’ingénieur : Procédés d’émulsification - Mécanismes de formation des émulsions
This study shows that the determination of the size of droplets of diphasic emulsions ispossible by laser particle size analysis. In particular, such measurements may be madefor concentrations up to 1% in volume for the system concerned.The emulsification process required energy may be supplied to system by the dispersionand transfer functions built-into the particle size analyser.The modelling of the kinetics of the reduction in size of the droplets shows that thisinstrument may be used both for characterisation and for the generation and study ofemulsions, in its own right..
Modelling of the emulsification mechanism
Figure 5 : Modelling of changes to D50 for a
W/O system of 0.19% and ultrasound of 50W
Experimental results
Modelling
The value of D50 is due directly to the duration and power of the ultrasonic waves interfacial surfaceenergy (γ) of water [1]. A mathematical model is used to describe the mechanisms governing thereduction of size of particles in time.
[2] Techniques de l’ingénieur : Emulsification - Elaboration et étude des émulsions
It may be written as follows.
with and
The correlation between theexperimental results and the modelling isgiven in figure 5.
The coefficient R2=0.95 indicates that themathematical model used is appropriatefor describing this type of system. It isalso used to calculate the value of P, thepower absorbed by the system andtherefore the efficiency of theemulsification process.
This efficiency should be associated withthe attenuation of the acoustic wave andviscous dissipation [2].