Advanced Sponge phases and
studying the ouzo effect
Linear flow line Rheology on Ouzo
and Spongephase systems
Tibert van der Loop
UvA FNWI HIMS Complex fluids
Supervised by Dr Erika Eiser and Drs Anil Gaikwad
7-7-2006
This report is about my bachelor project at the Complex fluids group on the University of Amsterdam. Here I studied for three months on two different topics, sponge phases and Rheology on Ouzo. During both studies I was supervised by Dr. Erika Eiser and with the study on sponge phasesI was also supervised by Drs. Anil Gaikwad.
Tibert van der Loop 0252638
Abstract
I studied the rheological behavior of ouzo emulsion of 5% and 20% ethanol to determine the surface tension of the oil droplets in this system with a constructed dimensionless number (the Tibert number). The Rheometer appeared to be not enough accurate to measure the contribution of the oil droplets in the system.Therefore the surface tension could not be determined.
The second part of mystudywas on sponge phase systems (L3) of SDS, pentanol, water and cyclohexane at different cyclohexane/(SDS/water) ratios. I was interested in the effect of added polymers and goldclusters on the mechanic and structural properties of these sponge phases. The observed rheological properties of the pure sponge phase was different to those of the sponge phases doped with goldcluster and with polymers.
Based on ourrheology measurements their can be conclude that gold clusters stabilize the sponge phase for this system,while polymers had anopposite effect on the stability of the sponge phase.
The pictures on the front page are from the following websites.
Spongebob squarepants:
Bottle of Ouzo:
Populair wetenschappelijke samenvatting
Door het meten van viscociteit ensnelheid (shear rate) aan ouzo mengsels bij verschillende stress (shear stress) met behulp van een rheometerheb ik geprobeerd om de oppervlakte spanning van olie druppeltjes te bepalen die aanwezig zijn in een ouzo mengsel.De gebruikte rheometer bleek niet nauwkeurig genoeg te zijn om de invloed van de kleine hoeveelheid olie druppeltjes op de viscositeit in het ouzo mengsel te kunnen meten. Hierdoor kon geenoppervlakte spanning worden bepaald.
Als tweede onderdeel heb ik een fase bestudeerdvaneen zeep (SDS),water, alcohol (pentanol) en olie (cyclohexane) mengsel. De fase van het mengsel wordt bepaald door de structuur waarin de zeepmoleculen zich rangschikken. De bestudeerde fase wordt de spons fase genoemd.
Drie vershillende spons fases werden gemaakt namelijk;zonder toevoegingen, met toegevoegde goudclusters en met toegevoegde polymeren (polyacrylamide). Aan deze monsters heb ik rheologische metingen gedaan om bij verschillende stress de fase te kunnen bepalen. Uit dit onderzoek kon geconcludeerd worden dat pure spons fase, spons fase met goudclusters en spons fase met polymeren werden verkregen. Tevens kan er geconcludeerd wordendat de spons fase wordt gestabiliseerd door de goudclusters.
~Table of Content~
1Introduction
1.1Ouzo
1.2Spongephase
2EXPERIMENTAL SECTION
2.1Systems
2.1.1Sponge phase (L3)
2.1.2Goldclusters in the Spongephase
2.1.3Polymerization in the Spongephase
2.1.4Preparing Ouzo samples
2.2Techniques
2.2.1Rheology
2.2.1.1Fluid mechanics
2.2.1.2Rheology with Couette geometry
2.2.1.3Rheological measurements
3Results And Discussion
3.1Sponge phase
3.2Ouzo
4Conclusion
5Reference
1Introduction
1.1Ouzo
Ouzo is an alcoholic beverage that contains mainly water ethanol and trans-anethol. Trans- anethol, a aromatic compound, is showed in figure 1. It has been studied thoroughly because of its ability to form a stable oil-in-wateremulsion without the use of a surfactant. Ouzo, one can buy in a shop, comes in a three component mixture that is well in the one phase region. When sufficiently large amounts of water are added to ouzo the mixture is quenched into the “nucleation-and-growth” region of the two phase region, where very small oil droplets are formed spontaneously. This phenomenon occurs in liquid systems with two, three or more components. Generally, when there is no surfactant present in the system the oil droplets will usually grow either by coalescence or Ostwald ripening until the system is macroscopically phase separated into an oil-rich and a water rich phase. But for the water-rich Ouzo mixture there is a region in the phase diagram, where the droplets stop growing after a certain time so that the emulsion can stay stable for weeks.This is called the Ouzo effect.
Emulsions are used a lot in industry. Many food products and pharmaceutical products are emulsions. If a better understanding is gained over the Ouzo effect it might be possible to use the Ouzo effect in other emulsions.
Previous studies by Sitnikova et al.[1] show how the droplets of oil (trans-anethol) grow in time.The time-dependence of the growth showed what the mayor growth mechanism of the droplets is Ostwald Ripeningandthat no coalescence takes place.
If the surface tension of the oil droplets in its surroundings is known it might be possible to show why there is no coalescence taking place and why the emulsion can be stable.
In this study I tried to determine the surface tension of the oil droplets in solution by analyzing rheological data of this system.
1.2Spongephase
Surfactants and co-surfactants in mixtures of polar and apolar solvents can form all kinds of phase structures depending on the property and the ratios between the components.
Possible phasesthat can form are micellar, inverted micellar, hexagonal, lamellar and sponge phase. They are illustrated in Figure 2.
There are two methods to describe the phase behavior of these ternary systems. The first method is based on the curvature energy of the surfactant film. The second is based on the shapes of surfactants.
With the first method the energy of system can be written in terms of two different curvatures the mean curvatureH and the Gaussian curvatureK. These curvatures then depend on two perpendicular inverse radii of curvature, c1 and c2, that represent the two-dimensional curvatures along the x- and y-axis as can be seen in Figure 3. A membrane curvature is simply defined as the inverse radius of a sphere describing it.
Mean curvature:(1)
Gaussian:(2)
The curvature free energy is then approximated by equation 3. Here higher order terms and the effect of entropy are neglected. To get to equation 3 is rather difficult and you need a good understanding of tensor algebra to derive this equation from the inverse radii of curvature. So I will not go into details but refer to a reference in literature[2].
Curvature energy per unit area:(3)
, and c0 are material properties of the membrane. The spontaneous curvature c0 is dimensionless,while the elastic constants andare in units of energy. To get an idea of the Gaussian and Mean curvature I listed some phases in table 1.
Table 1: Mean and Gaussian curvature for different phases.
Mean curvature / Gaussiancurvature
Spherical micelles or vesicles / (c1 + c2)/2 / c1 c2
Cylindrical micelles / c1/2 / 0
Bicontinuous cubic phase
(L3 or sponge phase) / 0 to c1/2 / to 0
Lamellar phase / 0 / 0
Inverse bicontinuous cubic phase / -c1/2 to 0 / to 0
Inverse cylindrical micelles / - c1/2 / 0
Inverse spherical micelles or the inner layer of vesicles / -(c1 + c2)/2 /
The second method for describing the different surfactant phases is based on the packing of surfactant molecules. The surfactant molecule can be modeled as having a head groupwith aneffective area a, the total volumeof the molecule V, and the length of the fully extended molecule (figure 2). Then equation 4 can predict if the previously mentioned parameters are what the best packing for the surfactant film is. This is called the surfactant packing parameter, Ns.
(4)
Ns depends on the solvent and the surfactant concentration. Ns is also very dependent on the amount of co-surfactant, if present. Co-surfactants are molecules with slightly different geometry with respect to the main surfactant, and can therefore change Ns and the type of self-assembled structure. Already very small amounts of co-surfactant can shift the system to a different phase.
In figure 2 the structures for different values for Ns are shown.
In this study I studied the sponge phase made of sodium dodecylsulfate (SDS, surfactant), Pentanol (co surfactant), water and cyclohexane.A sketch of the phase diagram of a similar system is shown in figure 4. You can see that the pentanol ratio must be very accurate to make a sponge phase.
There were also sponge phases made where goldclustersand polymer were added to the sponge phase.
Predicted is that gold clusters would go into the surfactant film and the polymer would go into the organic phase.
Eventually the aim for this research is to evaporate all components except for the added polymers and gold clusters. This would be done in a way were the sponge phase structure is retained by the polymers.Metal nanoclusters made of platinum or ruthenium are expected to be a new class of catalystswith properties different to those of their atomic or bulk analogues. But such metal clusters also represent an environmental hazard, and they are very expensive. This is why we intend to trap these clusters in the sponge phase, which will retain the clusters after the catalytic reaction and at the same time offer a large reactive surface. In the present study gold clusters have been used, because they are inert and moreover, they have a plasmon frequency in the visible region. Thus the color of their aqueous solution gives us an idea about their size and their aggregation state.
2EXPERIMENTAL SECTION
2.1Systems
2.1.1Sponge phase (L3)
Preparing the sponge phase samples.
An SDS/H20 (1:2.5 (v/v)) solution was made under constant stirring at about 25ºC. The solution was kept at 30˚C. Respectively, SDS/H20 pentanol and cyclohexane where added in amounts according to table 2. For sample 1,2 and 5, 20μL of the total amount of the pentanol was added afterwards. The samples became clear after vigorously stirring. All samples where anisotropic.
Table 2:Composition of sponge phase samples.
SDS/H20 / Pentanol / Cyclohexane / Total amountVolume (mL) / Volume fraction (%) / Volume (mL) / Volume fraction (%) / Volume (mL) / Volume fraction (%) / Volume
(mL)
1) / 0.69 / 23 / 0.21 / 7 / 2.1 / 70 / 3
2) / 0.68 / 18.4 / 0.23 / 6.3 / 2.74 / 75 / 3.65
3) / 0.54 / 14.8 / 0.22 / 6 / 2.88 / 79 / 3.65
4) / 0.4 / 9.9 / 0.23 / 5.7 / 3.4 / 84.5 / 4.03
5) / 0.2 / 4.98 / 0.22 / 5.5 / 3.6 / 89.6 / 4.02
6) / 3.57 / 23.8 / 1.03 / 6.9 / 10.40 / 69.3 / 15
2.1.2Goldclusters in the Spongephase
Solutions of sodium citrateand HAuCl4where made by adding a sodiumcitrate solution (34mM, 10gram/L) to HAuCl4-solution in amounts as listed in table 2. The solutions were vigorously stirred and heated at 100˚C for 5min. A clear red solution was obtained.
The red solution was centrifuged for four minutes. The supernatant, a light red solution, was removed by a pipette. The total volume of the remaining viscous pallet (dark red solution) was determined roughly with a pipette.
A part of the dark red solution was added in amounts listed in table 3to the sponge phase samples (70% SDS/H20):
* For goldclustersolution 7 to 9 the concentrations of spongephase 1 (see table 1)were used.
* For goldcluster solution 10 the concentration of spongephase sample 6 was used.
Sample 7 became clear after approximately 1,5 hours. Sample 8 and 9 became clear after adding 2 μL pentanol and vigorously stirring. Sample 10 became clear purple after vigorously shaking and adding 25µL pentanol. See Figure 5.By assuming that all metal gold clusters where on the bottom after centrifuging the amount of gold clusters in the sponge phase could be estimated (See Table 2).
Table 3: Amounts of gold cluster that where added to spongephase samples
HAuCl4-solution (1mM, 340gram/L)(mL) / Sodiumcitrate solution (34mM, 10gram/L)
(mL) / Volume of conc. goldclusters added to sponge phase (μL) / Number of Au atoms (μmol) / Spongephase sample
7) / 1.2 / 0.150 / 15 / 1.2 / 1)
8) / 2.00 / 0.250 / 30 / 2 / 1)
9) / 2.5 / 0.3125 / 37.5 / 2.5 / 1)
10) / 4.44 / 0.555 / 150 / 4.44 / 6)
2.1.3Polymerization in the Spongephase
A solution of 0.29mL Acryl amide, 1.46mL Demin. Nano Pure H2O, 12,5µL Aminopersulfate and 1,5µL TEMED was made. 100µL was directly added to a sponge phase sample of concentration 1) in Table 1. The sample became hazy and lumpy.After waiting forhalf an hourand adding 30µL pentanol and 120µL cyclohexane and after shaking vigorously the sample became clear.
2.1.4Preparing Ouzo samples
Four different ethanol water trans-ethanol mixtures where made. Respectively, ethanol trans- anethol and water where added in a glass bottle in concentration of table 4. Samples 1 and 2 formed a white emulsion. The third and fourth sample phase separated macroscopically and where therefore not used for further studies. The exact amounts are listed in table 4.
Table 4: Ouzo Samples
Ethanol / Trans-anethol / H2OVolume (mL) / Volume fraction (%) / Volume (µL) / Volume fraction (%) / Volume (mL) / Volume fraction (%)
1) / 6.32 / 5% / 50.8 / 0.05 / 94.95 / 94.95
2) / 25.28 / 20% / 121.6 / 0.1 / 79.90 / 79.90
3) / 63.30 / 50% / 356.0 / 3.5 / 46.50 / 46.50
4) / 63.30 / 50% / 203.4 / 2 / 48 / 48
Sample 1 and 2 where also prepared without trans- anethol to see the effect of the oil droplets. These samples are referred to as, respectively sample 5 and 6. All the samples where kept in the refrigerator when not used. The samples could only be used for a few days. After a longer time white fungus flakes appeared in the bottles.
2.2Techniques
2.2.1Rheology
Rheology is the study of the viscoelastic flow of fluids. How fluids behave depends on their intrinsic properties and their molecular interaction. Fluids can be Newtonian or non-Newtonian. A characteristic property of a Newtonian fluids is that the viscosity(1) is constant under different shear rate or shear stress. The change in share rate is then equal the change in shear stress. Here, is the viscosity, is the shear stress, and is the shear rate.
(1)
There are also different properties by witch a fluid can be classified as Newtonian or non Newton. This is for instance fluid memory, the Weisberg effect and Die Swell. Newtonian fluids do not have these properties but non-Newtonianfluids can have these properties. However, latter are not important in this report.
With a Rheometer (Haake, RS150) the viscosityand the shear rate of fluids can be determined. This is done by applying a constant torque or a constant angular velocity on a fluid by a rotor of a known geometry in a container of known geometry (figure 6). By solving the Navier-Stokes equation (see next paragraph) for the right geometries, the shear stress, the shear rate and viscosity can be determined. Our Rheometer is a stress controlled rheometer, but it can also be used in an applied constant angular velocity mode, as long as the variation in the shear rate is not too large. Constant shear stress CSS Rheometers in the Couette-geometry used here actually means a constant torque is applied to the sample.When we use the rheometer in the constant shear rate CSR mode, the shear rate is related to the angular velocity.
2.2.1.1Fluid mechanics
To understand something about Rheology, you have to know something about fluid mechanics. Here I will explain some basic tools for fluid mechanics. These are conservation of momentum and conservation of mass. The conservation of mass leads to the continuity equation that must hold in every point in a liquid. Here ρ is the density, t is time and is the velocity vector. The density change over time is equal to minus the gradient of the density velocity.
Continuity equation: (2)
The conservation momentum leads to the equation of motion. Here the same variables account as for the previous formula and is the stress tensor andis a gravitational force vector. The equation of motion tells us that momentum change over time is equal to three separate contributionsthat cause a flux. Fluid flow can take place due to convection, which is caused by external force like gravity or an electric and magnetic field, and due to diffusion, which comes from the thermal motion within the fluid. In equation (3) only the gravitational force has been taken in account.
Equation of motion:(3)
The stress tensoris the sum of the extra stress tensor and the pressure p times the identity tensor
Stress tensor: (4)
The equation that specifies the extra stress tensor is called the constitutive equation. The constitutive equation becomes equal to zero when the fluid is at rest.
For a Newtonian liquid the constitutive equation is:
(5)
Here is the bulk viscosity that is not important in this report.
For incompressible Newtonian fluids the constitutive equation can be more simplified.
Because the density is then constant over time the continuity equation becomes zero.
So the tensor product of the nabla operator and the velocity vector becomes zero and also becomes zero. Since the Rate of strain tensor is defined as:
Rate of strain tensor (6)
The Newtonian constitutive equation for incompressible fluid becomes:
(7)
Now the equation of motion can simplified for Newtonian incompressible fluid, this is what is called the Navier-Stokes equation. Here the Navier-Stokes written in terms of velocity:
(9)
[3]
2.2.1.2Rheology with Couette geometry
If the shear stress and shear rate has to be calculated for forinstance the Couette geometry as Z41/Z43 in next paragraph, the Navier-Stokes in equation for cylindrical coordinates has to be used.
Navier Stokes equation in cylindrical coordinates[4]:(10)
(11)
(12)
The first term of all three equations becomes zero because the system assumed to be in equilibrium so the equation becomes independent of time. Then we say there is only a velocity in de θ-direction, the gravitational force is pointed in the z-direction and p depends only on r and z. With these assumptions the equations 10,11 and 12 simplify to:
(13)
(14)
(15)
Differential equation 14 gives the velocity distribution.
(16)
(17)
(18)
(19)
By filling in the boundary conditions at equation 19 the integration constants C1 and C2 can be solved.
The boundary conditions are v= RΩ at r=κR and v=0 at r=R. Where Ω is the angular velocity of the rotor, R is the radius of the cylindrical container and r is the ratio between radius of the rotor and the radius of the cylindrical container.
With the second boundary conditions is assumed that the slip length of the velocity distribution is zero.
This gives and .Now the velocity distribution is: