(University of Cologne - Thomas Sottmann)
The main topic of the network of excellence Soft Matter Composites is the development of new nanoscale functional materials using the manifold of structures of self-organizing polymeric, amphiphilic and colloidal systems. One track to these new nanoscale functional materials is the formulation of various components to form nanoscaled systems, which are in general rather complex. Here often hydrophobic (oleic) components have to be solubilized in a hydrophilic (water-rich) system or vice versa. Smart molecules, which accomplish this task are typically amphiphilic molecules.
In general if we add an amphiphile to a water-oil mixture it has, in principle, three options: It can dissolve in the water phase forming a variety of oil-in-water (o/w) structures (fig. 1, top: centre-left), it can dissolve in the oil phase forming w/ostructures (fig. 1, top: right) or it can make up its own bicontinuous structured phase (fig. 1, top: centre-right). Thereby, the surfactant rich phase is called microemulsion. Within our formulation cluster we provide the knowlegde to select the components to achieve a microemulsion with the desired structure and property.
 | | Fig. 1: Approaching microemulsions at room temperature ( T = 25°C) Top: test tubes of, left: equal volumes of water and oil; centre-left: water – oil - hydrophilic surfactant provides an oil-in-water (o/w) microemulsion in coexistence with an excess oil phase (Winsor I, 2); right: water – oil - hydrophobic surfactant provides a water-in-oil (w/o) microemulsion in coexistence with an excess water phase (Winsor II, 2 ); centre-right: water – oil - balanced surfactant, bicontinuous microemulsion in coexistence with both excess water and oil Winsor III (3). Bottom: Isothermal Gibbs triangles of the three systems at T = 25°C. |
The phase behaviour or more precisely the phase diagram may be viewed as the road map for an effective formulation. For example, the Gibbs triangle on the left hand side of fig. 1 indicates that at intermediate temperatures a hydrophilic surfactant system is over wide composition regions in the Winsor I (2) - state. The Gibbs triangle on the right hand side of fig. 1 shows a hydrophobic surfactant system displaying a Winsor II ( 2) - behaviour. The Gibbs triangle in the centre of fig. 1 shows the extended threephase triangle typical for the Winsor III (3) state of a balanced surfactant system. The selection of the components and their concentrations but also of the state variables of temperature T and pressure p , is crucial for the type of micromulsion that is obtained and its dilution properties.
In order to study the temperature dependence of the phase behaviour of microemulsions and other polymeric, amphiphilic and colloidal systems a cluster of 25 water baths is available. Fig. 2 shows the typical setup used. The procedure is the following:
All components are weighed into a test tube S, which is sealed with a PE stopper. The samples are homogenized by shaking and / or stirring, in some cases at elevated temperatures. Then the samples are placed in a thermostated water bath W with temperature control up to 0.01 °C. While the temperature is regulated the sample has to be stirred (using the magnetic stirrer M), when temperature equilibrium is reached the stirrer is turned off and the number and kind of coexisting phases is determined by visual inspection in both transmitted and scattered light, using crossed polarizers (P1 and P2) to recognize anisotropic phases. At constant composition the temperature is varied. After recording all appearing phases and phase transition temperatures the composition of the sample is changed and the process is repeated. In general the phases can be distinguished by their optical appearance: The one- phase selforganizing system is isotropic and transparent in transmitted light, it appears slightly bluish in scattered light. In the case of large structures the sample can appear red to dark red in transmitted light and milky white in scattered light. Transferring the sample into a flat cell with short optical path length can facilitate the optical inspection. Multi-phase regions are turbid and are difficult to distinguish immediately. Thus phase separation has to be awaited to determine the respective phase state. The most important, i.e. most often appearing, mesophase is the lamellar phase. It is optically anisotropic and can thus be identified by the birefringence exhibited under crossed polarizers.
 | Fig. 2: Test tube S containing the complex fluid F in a water bath W with a thermostat Th and a cooling coil C, which is connected to a cryostat (not shown). The complex fluid F is illuminated by a microscopy lamp L and can be stirred via a magnetic stirrer M. The temperature is determined by the temperature probe Tp and shown on the display Td. If required the anisotropy can be detected using the
two polarisers P1 and P2. |
In order to study the pressure dependence of the phase behaviour we have developed very reverently a high pressure viewing cell. This cell is shown in Fig. 3. Apparently, the main component of this setup is the sapphire cylinder, which offers an optical investigation of the whole sample volume of 3 ml. Pressures up to 300 bar with an accuracy of 1 bar can be generated by turning a piston down into the sapphire cylinder. Thereby the pressure is measured by a pressure probe, placed in the metal block at the bottom. A magnetic stirring bar provides sufficient mixing. As for the set up in fig. 2 the number and kind of coexisting phases is determined by visual inspection in both transmitted and scattered light, using crossed polarizers to recognize anisotropic phases. In order to control the temperature with an accuracy of 0.01°C the whole pressure cell can be placed in the set-up shown in fig.2. Components, which are gaseous at atmospheric pressure can be filled into the cell as liquefied gas under pressure via an capillary drill hole in the metal block at the bottom of the cell.
 | | Fig. 3: High pressure viewing cell to study the pressure dependence of the phase behaviour. The sapphire cylinder located in the middle, allows the optical investigation of the pressurized sample in transmitted light. Turning the piston down into the sapphire cylinder a pressure of up to 300 bar can be generated. Thereby the pressure is measured by a probe which is implemented in the metal block at the bottom. |
We (Schlumberger) have not done so yet, but are willing to share samples of a surfactant EHAC (erucyl bis(hyroxyethyl)methylammonium chloride) that forms wormlike micelles up to reasonable temperatures (80C) and has been the basis of several publications to date. We would propose it as a standard for such systems for further work on the pure fluid or composites (eg foams, added particles).
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