This title appears in the Scientific Report : 2014 

Investigation of the thermodiffusion behavior of complex fluids and development of new methods
Naumann, Philipp (Corresponding Author)
Weiche Materie ; ICS-3
Düsseldorf Universitätsbibliothek Düsseldorf 2014
104 p.
HHU Düsseldorf, Diss., 2014
Dissertation / PhD Thesis
Soft Matter Composites
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Thermodiffusion or thermophoresis occurs when a fluid mixture is exposed to a temperature gradient, leading to partial separation of the different components. It is still an unresolved non-equilibrium problem in physical chemistry. Thermodiffusion can be used for large scale separation and polymer characterization and furthermore it has been related to the origin of life occurring at deep sea regions with strong temperature gradients caused by hot vents or other volcano activities. The basic parameter is referred to as the Soret coeffcient which is defined as the ratio of the thermal diffusion coeffcient DT and the mass diffusion coefficientD . Despite a long time of research there is still a lack of a microscopic understanding of thermodiffusion. In order to study the origin of thermodiffusion, it is required to use well suited model systems and reliable methods. The model systems need to be accessible with theoretical concepts and show simplified geometries such as spheres. Additionally the model system must allow a systematic variation of properties such as size, mass or charge. The requirements for experimental methods are a small sample volume to be able to use rare materials and a well shaped experimental geometry to be accessible by theoretical models, for example well characterized walls, edges and interfaces. Furthermore measurements need to be contact free in order to minimize artificial distortions like the sample-removal geometry through an outlet. This work contains both aspects, the development of a new experimental method and the systematic investigation of microemulsion droplets, which can be regarded as a tunable colloidal model system.A well suited experimental method employs the classical so-called thermogravitational columns (TGs), which were one of the first devices using thermodiffusion for separation, and which rely on sample extraction and additional measurements to determine the concentration and thereby the thermodiffusion properties. One main aspect of this work was the development of a classical TG combined with an optical detection method which requires only very small sample volumes. This project was accomplished in cooperation with the group of M. M. Bou-Ali at the Mondragon-University. The thermogravitational micro column (m-TG) was constructed with a small sample volume of 50 mL for investigation of very rare or expensive samples such as biological samples. The dimensions are chosen to achieve a parabolic, laminar flow field inside the column, which is required for theoretical modeling. We chose a gap width of only around 500 mm and a height of 3 cm. The m-TG is operated contact free by using an interferometrical detection method to determine the concentration differences at two different heights of the column. This optical method allows sensitive and time resolved measurements of the concentration difference. Although, the analysis of the time dependent concentration profile was not yet possible with existing theoretical models which assume infinite short rising times of the temperature gradient. We used an active phase tracking procedure using a piezo actuator at one of the mirrors, which changes themirror position, leading to the phase difference. This robust method is independent of the intensity and contrast fluctuations. The m-TG has been validated by measuring three binary benchmark mixtures and by investigating the mixture of toluene and n-hexane. The obtained results agree within 5% with literature results. Additionally, measurements of a microemulsion system were performed, which allow a systematicinvestigation of the thermal diffusion behavior as function of the microemulsion droplet size and their interfacial tension. The size dependence of the thermodiffusion is controversially discussed. Theoretical studies propose the Soret coefficient a linear, quadratic and power laws of higher order. Experimentally a linear and quadratic size dependence of the Soret coefficient for hard and soft colloids has been found. Two different studies on the same sample system led to a linear and a quadratic dependence. One reason for this discrepancy might be the surface properties of the studied colloidal systems. Although hard spheres with different sizes can be ynthesized, the grafting density or charge may differ substantially. A phase transition temperature of colloidal systems stemming from different batches differ often by several 10 K. To overcome this drawback, we have hosen microemulsion droplets as model system, which can be tuned in size, shape and their interfacial tension over a wide range. They consist of a polar liquid such as water, a non-polar liquid such as an oil and a surfactant. By adjusting the appropriate concentration and temperature, a microemulsion (mE) forms discrete water/oil or oil/water aggregates, which are thermodynamically stable and can be used as a colloidal model system. In this study we avoid complications due to surface charge effects and use a non ionic surfactant.We chose a system of water, n-alkanes and the non-ionic surfactant C12E5 at the water-rich side of the phase diagram. All measurements have been performed in the one-phase region between the upper near critical boundary (ncb) and the lower emulsification failure boundary (efb). The droplets have been characterized by Dynamic Light Scattering (DLS) and Small Angle Neutron Scattering (SANS). The results show that the shape varies strongly with temperature, from network like (close to the ncb) to elongated to spherical droplets at lower temperature close to the efb. The thermal diffusion behavior of the mE-droplets hasbeen investigated by the Infrared Thermal Diffusion Forced Rayleigh Scattering technique. With this method we obtain the mass diffusion coefficient D, the thermal diffusion coefficientDT and the Soret coefficient §T. In a first study we used only n-decane as oil and investigated the behavior along three different paths. We varied the temperature between the ncb and the efb and could relate the results to the influence of the shape of the droplets. Close to the efb the results for low temperatures and thus spherical droplets are compared for different volume fractions of droplets and different oil content and thereby size. We found only a very small influence of the volume fraction below 10% on the thermodiffusion coefficients. The comparison of different sizes showed an increase of the Soret coefficient with radius but the measurement range was too small to differentiate between a linear and quadratic behavior. Additionally we compared our results with a model of Parola and Piazza, who proposed that the Soret coefficient depends linearly on the radius and on the temperature derivative of the product of the interfacial tension and some characteristic length l. This length is a measure for thickness of the interfacial layer, which is influenced by the temperature gradient. The interfacial tension measurements were conducted by the group of Strey at the University of Cologne. The results of the thermodiffusion measurements and the calculated values show fairly good agreement for spherical droplets. The drawback of this study was a shift in the efb temperature accompanied by increasing the oil content and thereby the size. To access a wider parameter space for size and interfacial tension, we used different n-alkanes, so it was possible to measure different droplet sizes at the same temperature. Also, as before the micro emulsions were characterized by DLS and SANS measurements to obtain thestructure and size close to the efb. The Soret coefficient increases linearly with the droplet radius for spherical particles. Furthermore we were able to extend the investigation of the thermodiffusion behavior in relation with interfacial tension. We determined the characteristic length l between 1 and 2 A. Although we used different oils in our studies, we assumed that the droplets core does not influence the thermodiffusion behavior but only the shell and the interface interact with the surrounding. We found that for our sample system, the interfacial tension dependence is dominated by the particle size due to a small derivative ofthe interfacial tension by temperature, which was proposed in the theory. Thereby we gavea first insight into this model applied to soft colloidal particles.