The anomalous viscometric behavior of AOT water-in-oil microemulsions

Diploma Thesis, 2007

54 Pages, Grade: 1,0


Table of Contents

1 Introduction

2 Theory
2.1 Microemulsions
2.1.1 Droplet Interactions
2.2 Phase Diagram
2.2.1 Droplet Structures
2.3 Light Scattering
2.3.1 Dynamic Light Scattering (DLS)
2.3.2 Static Light Scattering (SLS)
2.4 Rheology

3 Experimental
3.1 Materials and Samples
3.2 Light Scattering
3.2.1 Dynamic Light Scattering (DLS)
3.2.2 Static Light Scattering (SLS)
3.3 Phase Diagram
3.4 Viscosity Measurements
3.5 Microscopy

4 Results and Discussion
4.1 The Viscosity Anomaly
4.2 Light Scattering
4.3 Phase Behavior
4.4 Diffusion Coefficient
4.5 Microscopy
4.6 Size Estimates
4.7 Quantitative Analysis of the Viscosity Maximum

5 Conclusions and Future Outlook

6 Acknowledgements

7 References


1 Introduction

Microemulsions are solutions consisting of water, oil, surfactant, electrolyte and cosurfactant. They are thermodynamically stable, isotropic and optically transparent solutions[1]. They show low interfacial tension and a large interfacial area between the water and oil microphases. Because of these unique properties their microstructures have been of huge interest over the past few decades [2-4] and a wide field of applications, ranging from nanoparticle synthesis[5], use as corrosion inhibitors and lubricants[6], to applications in cosmetics[7] and as coating materials[8], has developed. Nowadays, an increasing application of microemulsions is in pharmacy[9] and biotechnology[10] as they promote a high stability and homogeneity of the system[11].

Using the anionic surfactant AOT (sodium bis-(2-ethylhexyl)-sulphosuccinate) together with oil and water, ternary microemulsions are formed without any need for electrolyte or cosurfactant. Because of the single water-in-oil phase persisting over a wide range of concentration and temperature and the need for only three components, this system is frequently thought of as a simple, model water-in-oil microemulsion [12, 13] and it is often used for particle synthesis [14, 15] and fundamental studies of microemulsions [16, 17].

In reality it is far from an ideal model system as there are still many remaining questions as to the fundamental processes in the system. For instance, an anomalous behavior in the viscosity of AOT microemulsions has been reported by Huang [18], Bergenholtz et al. [12], and Batra et al. [19]. They attribute the viscosity maximum to increased attraction among microemulsion droplets, either deriving from surfactant tail overlap [12, 13] or fluctuations in droplet charge [19]. Neither explanation, though, appears satisfactory [19]. Marignan and Cabos in little cited work [3] found discrepancies from the normal scattering behavior. Furthermore, they speculate that local lamellar structures exist [3] in what has been reported as a one-phase microemulsion region [20, 21], raising fundamental questions about the microstructure and interactions in a composition region that has been subjected relatively little to scientific study. The aim of this work is to find explanations for this unusual behavior and to contribute to a better understanding of the fundamental system processes. As the detected anomaly is likely caused by changes in structure and/or interactions, we probe these by methods such as rheology, phase diagram determinations, as well as static and dynamic light scattering (SLS and DLS). In addition, it is attempted to locate a low-temperature phase boundary in the system where excess water is expelled. Previous work suggests that nonionic surfactant systems behave as a dispersion of hard-sphere particles along this phase boundary [4]. Provided such a phase boundary can be located in the present systems, it could serve as an ideal reference point for systematic studies of droplet interactions, droplet shape fluctuations, and percolation phenomena in AOT systems. In short, it is shown that the viscosity anomaly is directly correlated to the existence of vesicles, which is supported by DLS and SLS experiments, as well as microscopy. The general interactions of droplets and the basic theories and experimental backgrounds of the used methods such as DLS, SLS, and rheology will be reviewed. After presenting results and discussing their interpretation conclusions are drawn and some suggestions for future work are made.

2 Theory

2.1 Microemulsions

Microemulsions are solutions of water and oil stabilized by a surfactant. These solutions, which are composed of either droplets, cylindrical, or lamellar structures, are thermodynamically stable and isotropic[1]. The thermodynamic stability is correlated with the interfacial tension between oil and water being low enough to compensate for the dispersion entropy ( < 10-[2] mN/m)[22]. In many systems, however, the used surfactant is not able to decrease the interfacial tension below 1 mN/m. Therefore, cosurfactants are frequently used. Surfactants are amphiphilic molecules with a hydrophilic head group and a hydrophobic tail[23]. The hydrophilic head group is either polar or ionic and interacts strongly and preferentially with polar solvents like water. The hydrophobic part, often a hydrocarbon chain, has a high affinity for non-polar solvents like oils. Amphiphiles often enrich at water interfaces where they form a monolayer. This behavior is due to the dual, hydrophilic and hydrophobic, character of the molecules.

illustration not visible in this excerpt

Fig. 2.1: (left) Formula of sodium bis-(2-ethylhexyl)-sulphosuccinate (AOT)[24]. (right) Structure of a reversed micelle, with l the length of surfactant tail, aH2O the radius of the water droplet, and am the radius of the micelle[24].

As oil and water in a mixture usually are not stable, surfactants are added as stabilizing agents, so that stable droplets are formed with the oil molecules emulsified within the water phase. These droplets become coated with surfactant molecules, which have dissolved hydrophobic parts in oil and dissolved hydrophilic parts in water. The hydrophilic part of the surfactant can either be ionic or non-ionic[23]. In inverted microemulsions, oil instead of water acts as the continuous phase and the dispersed droplets contain water.

In this study an anionic surfactant is used, which forms reversed, swollen waterin-oil micelles with oil as the continuous phase, as is illustrated in figure 2.1. The assayed microemulsions consisted of sodium bis-(2-ethylhexyl)-sulphosuccinate (AOT) as the ionic surfactant (figure 2.1), D2O, and decane or heptane.

2.1.1 Droplet Interactions

Instead of considering droplet microemulsions as molecular systems they can equally well be looked at as systems of colloidal particles as the formed droplets are of colloidal size (~ 100 Å, micelles ~15 Å[13] ). This colloidal droplet structure even exists, at least in AOT-based systems, when the solution segregates into two coexisting microemulsion phases at the lower critical solution temperature (LCST)[21]. Therefore many phenomena in microemulsion systems can be related to droplet interactions, but a proper account of shape changes is often needed as well. In the colloid literature many such theories exist [19, 25, 26].

Close to a lower temperature phase boundary, where preferred and actual droplet radii are equal, hard sphere behavior has been observed [4]. Far away from the critical point and phase boundaries attractive interactions appear to cause deviations from the hard-sphere behavior in AOT water-in-oil microemulsions [12, 27, 28]. According to the source of those attractions, three main theories can be found in literature. Lemaire et. al. suggested that those interactions could be explained by a desolvation and interpenetration of alkyl segments on the surfactant tails of neighboring aggregates [2, 25]. In this model the strength of the attraction grows as the microemulsion droplet size increases. Using the same overlap model, it has been argued that molecular rearrangement of the surfactant can lead to attractions that vary in range in a non-monotonic fashion with increasing droplet size [12]. Furthermore, Denkov has argued that as droplets approach one another they deform under the influence of van der Waals forces. The deformation away from a spherical shape leads to a stronger van der Waals attraction[26]. The third model by Batra et. al. traces the source of attractive interactions to the exchange of ions between droplets and a resulting charge fluctuation[19].

2.2 Phase Diagram

In order to make a reasonable thermodynamic description of colloidal systems such as microemulsions, an investigation of the phase diagram is of great interest [17, 29]. Phase diagrams depict the phase behavior of a system and summarize large data sets in an easily visualized chart [1].

An analysis of the phase behavior permits links to be drawn between macroscopic behavior and intermolecular interactions, as interactions need to be strong enough to promote macroscopic ordering to lead to formation of a phase transition. Major changes in macroscopic properties generally follow changes of phase, which are oftentimes triggered by slight variations in physicochemical conditions. Therefore phase behavior is one of the most important factors in thermodynamics and structure-property relations and they often provide essential clues to macroscopic behavior.

2.2.1 Droplet Structures

There are many different structures that can occur in a microemulsion phase diagram. Most frequent are droplet structures, either of water in oil or oil in water, each surrounded by a surfactant layer.

Figure 2.2 shows schematically the different structures occurring in a ternary phase diagram of water-oil-surfactant mixtures such as microemulsions.

illustration not visible in this excerpt

Fig. 2.2: Schematic ternary phase diagram of water-oil-surfactant mixtures representing the Winsor classification and probable internal structures. L1: one phase region of oil-in- water micelles (o/w), L2: one phase region of reverse swollen water-in-oil micelles (w/o) and : anisotropic lamellar liquid crystalline phase. Oil is marked with O, water with W, and the microemulsion with[11].

Often these detected equilibria between phases at low surfactant concentration are referred to as Winsor phases[30]. Winsor I (W I) shows two phases: the upper excess oil phase in equilibrium with the lower (oil/water) microemulsion phase. Equilibrium between excess water and the upper microemulsion phase (water/oil) appears at Winsor II (W II). Winsor III (W III) indicates three different phases, a middle microemulsion phase (oil/water and water/oil, referred to as bicontinuous) in equilibrium with an upper excess oil and a lower excess water phase. The one-phase microemulsion region[20] consisting of water, oil and surfactant in homogeneously mixed proportions is named as Winsor IV (W IV). By adjusting proportions of the constituents, conversions among these phases can be achieved. The possibility of simultaneous presence of two microemulsion phases also exists, where one is in contact with water and one with oil[31]. This behavior is considered as an extension of the Winsor classification to a fifth category[11].

2.3 Light Scattering

2.3.1 Dynamic Light Scattering (DLS)

In DLS a beam of light passes through a sample, for instance a colloidal dispersion, and it is scattered by the particles in all directions[32]. If the particles are small in comparison to the wavelength of the light, the scattered light intensity is uniform and independent of the angle of detection. Here the Rayleigh-Gans- Debye theory works well. For particles larger than ~ 250 nm Mie theory is more accurate. In scattering from intermediate-size particles the angular dependence arises from constructive and destructive interference of light scattered from different positions in the same particle, as shown in figure 2.3[33].

illustration not visible in this excerpt

Fig. 2.3: Schematic representation of scattered light by particles, with k0 as the incident wave vector and ks as the scattered wave vector[34].

If light scattering takes place at an accumulation of point-like particles with position coordinates ri and rj, under a scattering angle , the detected intensity is unlike the incident one. The reason for these different intensities is a phase shift of the initial coherent electric field, as in figure 2.3 the upper path of the light is longer than the lower way by an amount [illustration not visible in this excerpt] is the wave vector. Therefore constructive and destructive interference is detected and directly correlated to the origin of the scattering and the relative position of the scattering particles [34, 35].

Suspended particles cause a random diffraction pattern, shown in figure 2.4. These so called “speckles” consist of a bright spot, where constructive interference occurs and a dark surrounding area, which is the result of destructive interference from diffraction.

illustration not visible in this excerpt

Fig. 2.4: Principle geometry for a dynamic light scattering experiment. The detector is schematically replaced by a screen to illustrate the speckle pattern in addition to the detector pinhole [34, 36].

A change in these diffraction patterns can be explored when particles diffuse due to collisions between the large mass of the Brownian particles relative to the minor mass of the solvent molecules[34], i.e. the Brownian motion. The electric field autocorrelation function, gE(q, t), describes the resulting intensity fluctuations and is dependent of the wave number q and time t. The time dependence is due to changes in configuration as a result of particle diffusion. For short time intervals the initial decay can be described as

illustration not visible in this excerpt

where D(q) is the wave number dependent diffusion function, with q defined as

illustration not visible in this excerpt

The wave number q is dependent on the laser wavelength , the refractive index of the solvent n0, and the detection angle of the scattered light. For monodisperse spherical particles D(q) can be expressed as

illustration not visible in this excerpt

The Stokes-Einstein diffusion coefficient D0 is given here in terms of the solvent viscosity 0, the particle radius a, the Boltzman constant kB, and the absolute temperature T. H(q) is the hydrodynamic factor and it arises from hydrodynamic interactions that affect the particle diffusion. S(q) reflects the effect of thermodynamic interactions between particles, which are e.g. caused by particle charge or excluded-volume interactions[37]. If the dispersion is strongly diluted both H(q) and S(q) reduce to unity. This leads to a simplification of (2.1) to

illustration not visible in this excerpt

where D0 can be extracted from the linear slope of ln(gE(q, t)) versus time and the particle radius can accordingly be determined. This leads to the possibility to use DLS for measuring the particle radius in dilute dispersions.

As the scattered intensity is proportional to a[6], comparatively high particle concentrations are needed to compensate for the low intensity scattered from systems containing small particles. In this case H(q) and S(q) are no longer equal to one and the diffusivity varies with q. It has two limits, however, where it is independent of q:

illustration not visible in this excerpt

where DS is the self- and DC the collective diffusion coefficient. Both are independent of q but different from D0 for non-diluted dispersions. The collective diffusion coefficient is equal to

illustration not visible in this excerpt

It can either be an increasing or decreasing function of concentration, depending on the relative magnitude of H(q=0) and S(q=0) [38]. If data are extrapolated to a concentration of zero, both hydrodynamic and thermodynamic factors can be neglected and the particle size can also in this way be determined for non-dilute dispersions.

2.3.2 Static Light Scattering (SLS)

SLS is a non-invasive technique for characterization of particles in a dispersion or dissolved polymers. Unlike DLS, which measures the time-dependent fluctuations in the scattering intensity, SLS uses the time-averaged intensity of the scattered light[33]. The scattered intensity varies with the size and structures of the different particles in the investigated samples and is in general a function of the detector angle. The angular dependence is pronounced for dispersions of large particles [39, 40].

The angular dependence of light scattered by particles much smaller than the wavelength (a < /20) can be predicted by an equation developed by Rayleigh, Gans and Debye[35]:

illustration not visible in this excerpt

where R is the Rayleigh ratio, which is essentially defined as the ratio between scattered and incident light of the sample, is the wavelength of the laser, and n is the number density of particles. With P(q), known as form factor, and S(q), known as structure factor, (2.8) shows two correction factors to Rayleigh’s result. The form factor derives from the interference of the scattering originating in the same particle or macromolecule (see, for instance, figure 2.4). The structure factor comes from the interference of the scattering in different particles or macromolecules. The factor C characterizes the contrast between particles and solvent and is defined as:

illustration not visible in this excerpt

where np and n0 are the refractive index for the particle and for the solvent. The refractive index for the whole sample is denoted by n. If the sample concentration is low ( n 0), equation (2.8) can be simplified, as S(q) 1 [35] and the form factor can be calculated.

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Fig. 2.5: The form factor for optically homogeneous spheres ( ) and vesicles ( ) as a function of qa, where q is the wave number and a the particle radius. The straight lines illustrate the proportionality to 1/(qa)[2] for vesicles and 1/(qa)[4] for spheres in this region [34].

Figure 2.5 shows that the form factor is equal to zero for certain scattering angles. This is caused by destructive interference. A comparison between homogeneous spheres and vesicles shows a shift in the asymptotic scattering behavior, which is equal to 1/(qa)[4] for spheres and 1/(qa)[2] for vesicles. The difference in the positions of the minima is a result of the bilayer thickness of the vesicles as the contrast in the system changes. Size polydispersity tends to smear the sharp minima.

2.4 Rheology

According to a term Eugene Bingham coined, rheology is “the deformation and flow of matter under the influence of an applied stress”[41].

A simple application within this subject is illustrated in figure 2.6. Two parallel plates, each with the area A are separated by a distance z. A fluid fills the space inbetween them. When the plates move with a relative velocity u, the fluid is sheared and the force per area (F/A) is proportional to the velocity gradient, u/z, for a Newtonian fluid. The proportionality constant is known as the viscosity . u

illustration not visible in this excerpt

Fig. 2.6: Shear strain

For lots of fluids the viscosity is a function of the shear rate (or shear-stress), which is one example of non-Newtonian behavior (figure 2.7). One distinguishes between various different non-Newtonian effects. Fluids that exhibit an increasing viscosity with increasing shear rate are said to be shear thickening or dilatant. Fluids for which the viscosity decreases with increasing shear rate are called shear thinning.

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Fig. 2.7: (left) Classification of fluids; (right) Thixotropy[42]

Fluids can also show a time effect, where the measured viscosity decreases with shearing time. If it is followed by a recovery in structure after the stress has been removed, the fluid is thixotropic (figure 2.7). Often complex fluids exhibit more than one of these non-Newtonian effects.

To get information about particle interactions and particle geometry of colloidal dispersions, such as microemulsions, rheology may be a good choice. The flow behavior of colloidal dispersions depends on intermolecular forces like interactions between particles, Brownian forces on small particles (< 1 μm), and the viscous hydrodynamic forces on particles.


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The anomalous viscometric behavior of AOT water-in-oil microemulsions
University Karlsruhe (TH)  (Mechanische Verfahrenstechnik und Mechanik)
Physikalische Chemie, Verfahrenstechnik
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Petra Kudla (Author), 2007, The anomalous viscometric behavior of AOT water-in-oil microemulsions, Munich, GRIN Verlag,


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