Emulsion is a colloid where the two immiscible liquid phases are present; it is a suspension where one of the liquid phases is dispersed in the other. Here the liquid which is dispersed as fine particles in called as dispersed phase and the liquid medium which holds the dispersed phase is known as continuous phase.
There are ways to disperse a liquid in another liquid phase which could also be achieved by mechanical agitation which breaks a liquid phase into fine droplets which gets dispersed into the continuous phase (e.g. oil in water). But the stability of such emulsion would be very low as the dispersed phase coalesces together and separate out into immiscible liquid phase with a distinct boundary.
The stability of an emulsion depends on various factors and the components which help in stabilizing are known as emulsifiers. The amphiphilic substances such as surfactants and polymers are generally used to stabilize the emulsion. Goodwin , Pashly-b
1.2 Emulsions in practice:-
(Foams and emulsions as soft matter in general especially in food systems, Aim of this study)
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Some of the emulsions in practice are food systems such as mayonnaise, milk, salad dressings and ice cream, etc. The other systems are creams in cosmetics and ointments in pharmaceuticals, etc. In all the cases stable emulsions which contain immiscible liquid phases are necessary which drive us to study more about the factors influencing their stability.
The food emulsions are often very complicated by their composition due to presence various substances which makes it hard to study and understand the influence of all those factors. One approach to such systems would be to study well defined simpler systems and apply this knowledge to address the problems in the complex systems.
So the aim of this study is to understand the underlying principles of interfacial science and to examine the behaviour of protein/surfactant mixtures adsorbed at the water/oil interface. E.dickinsen (Adv in food colloids)-b
1.3. Role of proteins and surfactants in emulsions: - (in general description of surfactant classes, few words of protein folding and different types of proteins, origin of amphiphilicity, interactions); also a few more words about the used surfactant.
The surfactant is a molecule which is amphiphilic in nature (i.e. which has both hydrophilic and hydrophobic part) and adsorbs at interface of two immiscible phases and thus reducing the interfacial tension.
The interfacial tension or surface tension is net unbalanced cohesive forces of layer of molecules at the surface of the bulk liquid phase. It is measured in dynes.cm-1 or N/m. The interfacial energy is identical parameter but it is measured in J/m2. For example the interfacial tension of water/oil is 30 to 40 mN/m and interfacial energy is 30 to 40 mJ/m2.
So at water/oil interface the hydrocarbon in oil phase orient in such a way to avoid the water molecules as they cannot interact strongly with each other. This is why they are not miscible but when disrupted mechanically the oil forms droplets which coalesce quickly and phase out. A stable emulsion could be formed by lowering the surface energy which is possible by addition of polymers or proteins and surfactants. There are also other factors which stabilize the emulsions such as electrostatic repulsion of ionic surfactants surfactants, increased interfacial viscosity due to proteins or polymers which give mechanical strength to the droplets and prevent liquid drainage between the films. (pashley)-b
Add here in more detail the role of the two compounds:
According to a well accepted hypothesis the small surfactant molecules in a mixed protein/surfactant system adsorb more quick, reduce the interfacial tension efficiently and contribute in this way to the optimum formation of an emulsion. The proteins are not very efficient in reducing the interfacial tension, but they provide the respective behavior to the emulsion films, and reduce drainage and stabilize the films against coalescence.
Following the main ideas of emulsion stabilization, the protein provides a steric stabilization of liquid films but gives also elastic and viscous properties to the film, which can be tuned via the composition of the protein/surfactant mixture.
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Therefore, investigations of the interfacial behavior, including the dilational and shear rheology, of such mixed layers is in the focus of interest presently.
There are many applications of protein/surfactant mixtures such as in food industries for stabilizing emulsions and foams (eric.dickinson) and many other biological systems.
1.4. Interfacial studies of adsorbed layers
(in general dynamic interfacial studies - interfacial tension, interfacial rheology - different kind of deformation, adsorption phenomena, structure formation, folding -unfolding)
Note in particular: The rheological properties are very important for many applications. There are two types of deformation, shear and dilational. While shear changes the shape of the interface by keeping the size, in dilation the shape is kept constant and the size is changed. Therefore, shear rheology is essentially sensitive to structure formation in the interfacial layers.
1.5. Importance of shear rheology:-
The interfacial shear rheology is study of deformation of adsorbed layer structure with stresses applied on it. It gives the measure of rigidity and mechanical stability (better to write flow behaviour and mechanical properties of interfacial layers) of the adsorbed layers. The rate of formation of structure can also be studied with this technique.
Note here, that the number of studies of the shear rheology of layers between two immiscible liquids is very small and feasible only since recently due to the availability of the required experimental tools.
2. MATERIALS AND METHODS
2.1. Model systems under investigation:-
Here ß-Lactoglobulin with anionic surfactant SDS was chosen to study with hexane and Medium Chain Triglycerides (MCT) as oil phase. ß-Lactoglobulin is a protein present in the bovine milk. It is a globular protein with molecular weight 18.4 kDa and isoelectric point of 5.3. Sodium dodecyl-sulphate (SDS) is an anionic surfactant of molecular weight 288.38 g/mol.
2.2. Materials used.-
ß-Lactoglobulin purified with 90% agarose gel electrophoresis was purchased from Sigma-Aldrich. Sodium dodecyl sulphate (SDS) was purchased from Sigma-Aldrich. The solutions were prepared with 10mM sodium phosphate buffer, pH 7, prepared by mixing appropriate stock solutions of Na2HPO4 and NaH2PO4 with Milli-Q water. Hexane was purchased from Aldrich and purified with aluminium oxide. The interfacial tension of NaH2PO4/Na2HPO4 buffer was 72.5 and 49 mN/m at the water/air and water/hexane interface, respectively. The medium chain triglycerides (MCT) oil was purchased from Danisco and interfacial tension measured against NaH2PO4/Na2HPO4 buffer was 26 mN/m.
All the glasswares used were flushed with water and then immersed in concentrated sulphuric acid for 2 hours. Later, they were flushed well with water and rinsed with Milli Q water and then dried and stored in clean place.
2.3. Instruments used and their working principle:-
2.3.1. Profile Analysis Tensiometer (PAT1):-
The kinetics of adsorption for ß-Lactoglobulin and SDS mixtures were measured using profile analysis tensiometer PAT1 (SINTECH/Germany). The principle of this method is to determine the surface tension of liquid from the shape of a pendent drop. This shape is determined by Gauss-Laplace equation, which gives a relationship between the curvature of meniscus and the surface tension ï§.
Y (1/R1+1R2) = delta.P0 + delta.ro.gz
R1 and R2 are the radii of curvature, ï„P0 is the pressure difference in reference plane, ï„ï² is the density difference, g is the acceleration due to gravity, and z is the vertical height of the drop measured from the reference plane.
The surface tension can be obtained from fitted the Gauss-Laplace equation to the co-ordinates of a drop, using ï§ as the fitting parameter. The shape of the drop is captured by CCD camera and the volume of the drop is kept constant by active control loop of the instrument.
The drop profile tensiometry can also be applied for studies of the dilational rheology of interfacial layers. For this purpose, the drop is generated to harmonic volume oscillations and the resulting area changes lead to compressions and expansions of the adsorbed layer. The relation behavior of the interfacial layer as response to these perturbations can be obtained by measuring the interfacial tension during the oscillations. The dilational elasticity and viscosity are obtained finally via a Fourier analysis of the harmonic interfacial tension response.
2.3.2. Forced oscillation interfacial shear rheometer (MCR301):-
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The MCR 301 rheometer equipped with the interfacial cell IRS from Anton Paar (Germany) was used to measure the shear elastic and loss modulus of ß-lactoglobulin and SDS mixtures. This instrument performs the oscillations at controlled stress or controlled strain conditions and the parameters which determine the structure of adsorbed layers such as shear elasticity, shear viscosity were calculated. The experimental data presented below are based on controlled strain conditions where the amount deformation of the interfacial layer is kept constant at same frequency of oscillation. Thus the kinetics of structure formation at the interface can be recorded over defined length of time. This instrument can measure very rigid layers but less sensitive to weak layers.
The instrument has high resolution optical sensor for position control of shear rate and strain. The EC motor for deflection is magnetic non contact drive motor which is sensitive to small torques. A biconical disk is fixed to the EC motor drive and its sharp edge is placed at the interface of fluids. The measuring cell is fitted to the thermostat for maintaining the constant temperature. (this is wrong! The temperature is controlled by a Peltier element - which needs a own cooling)
[few more words about the measuring geometry and measuring principles (amplitude, frequency and time sweep modes)]
2.3.3. Damped oscillation interfacial shear rheometer (ISR1):-
ISR 1 interfacial shear rheometer from Sinterface works on the principle of damping of oscillating pendulum suspended by a torsion wire. The apparatus consists of a drive comprising of a stepper motor, transmission and motor controller for the deflection. The torsion wire is fixed to the drive and a biconical disk is suspended to it which is a measuring body. The vessel holding the system under investigation is surrounded by water jacket for temperature control. The biconical disk is immersed in the liquid by placing knife edges exactly at the interface. The angular position of the biconical disk is recorded by a mini laser and position-sensitive photosensor. Due to the sensitivity of the photodiode and , the circular motion of the edge can be measured with an accuracy of f 0.01 Hz and deflection angle of 2'.This should be improved! The frequency is given by the torsion wire parameter; the maximal defection angle due to the positioning of PSD - an important parameter is the resolution of sensor which limits the smallest defection angle. More details measuring principle, geometry etc.
2.4. Preparation of stock solutions:-
10 ml of 1e-4 M ß-Lactoglobulin stock solution was prepared by dissolving 0.018g of ß-lactoglobulin crystals in 10 ml of NaH2PO4/Na2HPO4 buffer at pH 7 with 1-2 ppm sodium azide to prevent microbial degradation of stock solution and stored in refrigerator.
A new stock solution is prepared once in every 10-12 days.
100 ml of 1e-3 M SDS stock solution is prepared by dissolving 0.028g pure SDS in 100ml of NaH2PO4/Na2HPO4 buffer at pH 7 and stored in refrigerator.
The desired concentration of ß-lactoglobulin and SDS mixtures were attained by further dilution with NaH2PO4/Na2HPO4 buffer solution.
2.5. Modeling of adsorption layers
2.5.1. Surfactant adsorption layers
2.5.2. Protein adsorption layers
2.5.3. Mixed protein/surfactant adsorption layers
3.1. ß-lactoglobulin and SDS mixtures adsorbed at water/hexane interface:-
3.1.1. Pendant drop measurement of dynamic interfacial tension.
Fig 3.1.1. (a) The isotherm comprising of interfacial tension values at the water/hexane interface corresponding to concentration of ß-lactoglobulin after the equilibrium has been reached.
Fig 3.1.1. (b) The isotherm comprising of interfacial tension values corresponding
to concentration of SDS mixed with 1e-6 mol/lß-lactoglobulin after the equilibrium has been reached.
Do we have also such data at the water/MCT interface?
3.1.2. Shear rheology results:-
Fig 3.1.2. (a) Increase in shear viscosity of 3e-7M ß-lactoglobulin mixed with different concentrations of SDS adsorbed layer at water/hexane interface and measured under controlled strain forced oscillations performed by MCR 301 Anton Paar shear rheometer at 0.2% deformation and 0.7 Hz frequency at pH 7 and temperature 20o C.
Do we have any studies with both rheometers and can compare the results?
Fig 3.1.3. (b) Trend of total increase in the shear viscosity observed in the fig.3.1.2. (a)
Show also the problem of repeatability and discuss possible reasons.
3.2. ß-lactoglobulin and SDS mixtures adsorbed at water/MCT interface:-
3.2.1. Pendant drop measurement of dynamic interfacial tension.
Fig 3.2.1. (a) Adsorption isotherm for 3e-7M ß-lactoglobulin
Consisting of dynamic interfacial tension at Water/MCT interface
at pH 7 and temperature 20 C.
3.2.2. Shear rheology results:-
Fig 3.2.2 (a) Increase in shear viscosity of 3e-7M ß-lactoglobulin mixed with different concentrations of SDS adsorbed layer which is aged for 18 hours at water/MCT interface and measured under controlled strain forced oscillations performed by MCR 301 Anton Paar shear rheometer at 0.2% deformation and 0.7 Hz frequency at pH 7 and temperature 20o C.
Do we have any studies with both rheometers and can compare the results?
Fig 3.2.2 (b) Increase in shear viscosity of 3e-7M ß-lactoglobulin mixed with different concentrations of SDS adsorbed layer at water/MCT interface and measured by damped oscillations performed by ISR 1 shear rheometer at an angle 0.75 at pH 7 and temperature 20o C.
Fig 3.2.2 (c) Increase in shear viscosity of 3e-7M ß-lactoglobulin with different concentrations of Monolein in oil phase, adsorbed layer at water/MCT interface and measured by damped oscillations performed by ISR 1 shear rheometer at an angle 0.75 at pH 7 and temperature 20o C.
Do we have any studies with both rheometers and can compare the results?
The adsorption kinetics studied by drop profile analysis gives good information about the adsorbed layer at the interface. The fig 3.1.1. (a) Shows that adsorbed layer is saturated with ß-lactoglobulin molecules at a concentration 2e-7 M or more. There is formation of second layer of ß-lactoglobulin at a concentration of more than 1e-6 M which is obtained from surface pressure data (Vincent). So to study the rheology of adsorbed monolayer the concentration of 3e-7M is chosen.
The fig 3.1.1. (b) tells us that surfactants dominate the interface by replacing the protein, so higher the concentration of surfactant greater the dominance over the protein which is shown in surface tension isotherm. It is also important to note that at low concentrations of ionic surfactant, complexes are formed with protein which has almost similar surface tension value compared to ß-lactoglobulin without any surfactant.
The effect of low concentrations of SDS mixed with ß-lactoglobulin is visible in the fig 3.1.2. (b). The trend in increase of the shear viscosity initially and then decrease with increasing surfactant concentration was seen, however the reproducibility of interfacial shear rheology data is bad so no literature is published on this topic at water/oil interface. Therefore rheological data obtained cannot be analyzed quantitatively but significant difference with various ratios of protein/surfactant mixtures can be analyzed qualitatively. The higher shear viscosity at lower ratios of ß-lactoglobulin/SDS mixtures could be due to ionic interaction and unfolding of the protein molecule and then binding of more SDS molecules via hydrophobic interaction with more exposed protein. The interaction between the protein-surfactant complexes becomes more due to increase in more bonding sites, thus increasing the rigidity of the 2-dimensional structure formed at the interface.(R.Miller)
The kinetics of structure formation at water/MCT interface was very slow compared to that of water/hexane. So the adsorbed layer was aged for about 18 hours before running the experiment with MCR 301 as the structure has to reach visco-elastic regime at that measuring conditions. In both cases (hexane and MCT) the protein-surfactant complex formed more rigid layers than the protein. The water/MCT system showed better reproducibility than the water/hexane system.
The strategy should be: Show that
- BLG has its particular shear rheology at the water/oil interface
- there are differences at water/hexane and water/MCT interfaces
- all this gives in general the same picture like at the water/air interface, but on the other hand also quantitative differences
- successive addition of surfactant changes the shear rheology such that we can assume the protein is step by step removed from the interface
- this is in equivalence to the water/air interface
- the adsorption data (isotherm and kinetics, and maybe also dilational rheology) support this picture
- use finally a cartoon that demonstrates the main idea (from either of the recent papers by Alahverdjieva, or Kotsmar, or Pradines)