@phdthesis{Ulaganathan2016,
  author    = {Ulaganathan, Vamseekrishna},
  title     = {Molecular fundamentals of foam fractionation},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-94263},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ix, 136},
  year      = {2016},
  abstract  = {Foam fractionation of surfactant and protein solutions is a process dedicated to separate surface active molecules from each other due to their differences in surface activities. The process is based on forming bubbles in a certain mixed solution followed by detachment and rising of bubbles through a certain volume of this solution, and consequently on the formation of a foam layer on top of the solution column. Therefore, systematic analysis of this whole process comprises of at first investigations dedicated to the formation and growth of single bubbles in solutions, which is equivalent to the main principles of the well-known bubble pressure tensiometry. The second stage of the fractionation process includes the detachment of a single bubble from a pore or capillary tip and its rising in a respective aqueous solution. The third and final stage of the process is the formation and stabilization of the foam created by these bubbles, which contains the adsorption layers formed at the growing bubble surface, carried up and gets modified during the bubble rising and finally ends up as part of the foam layer. Bubble pressure tensiometry and bubble profile analysis tensiometry experiments were performed with protein solutions at different bulk concentrations, solution pH and ionic strength in order to describe the process of accumulation of protein and surfactant molecules at the bubble surface. The results obtained from the two complementary methods allow understanding the mechanism of adsorption, which is mainly governed by the diffusional transport of the adsorbing protein molecules to the bubble surface. This mechanism is the same as generally discussed for surfactant molecules. However, interesting peculiarities have been observed for protein adsorption kinetics at sufficiently short adsorption times. First of all, at short adsorption times the surface tension remains constant for a while before it decreases as expected due to the adsorption of proteins at the surface. This time interval is called induction time and it becomes shorter with increasing protein bulk concentration. Moreover, under special conditions, the surface tension does not stay constant but even increases over a certain period of time. This so-called negative surface pressure was observed for BCS and BLG and discussed for the first time in terms of changes in the surface conformation of the adsorbing protein molecules. Usually, a negative surface pressure would correspond to a negative adsorption, which is of course impossible for the studied protein solutions. The phenomenon, which amounts to some mN/m, was rather explained by simultaneous changes in the molar area required by the adsorbed proteins and the non-ideality of entropy of the interfacial layer. It is a transient phenomenon and exists only under dynamic conditions. The experiments dedicated to the local velocity of rising air bubbles in solutions were performed in a broad range of BLG concentration, pH and ionic strength. Additionally, rising bubble experiments were done for surfactant solutions in order to validate the functionality of the instrument. It turns out that the velocity of a rising bubble is much more sensitive to adsorbing molecules than classical dynamic surface tension measurements. At very low BLG or surfactant concentrations, for example, the measured local velocity profile of an air bubble is changing dramatically in time scales of seconds while dynamic surface tensions still do not show any measurable changes at this time scale. The solution's pH and ionic strength are important parameters that govern the measured rising velocity for protein solutions. A general theoretical description of rising bubbles in surfactant and protein solutions is not available at present due to the complex situation of the adsorption process at a bubble surface in a liquid flow field with simultaneous Marangoni effects. However, instead of modelling the complete velocity profile, new theoretical work has been started to evaluate the maximum values in the profile as characteristic parameter for dynamic adsorption layers at the bubble surface more quantitatively. The studies with protein-surfactant mixtures demonstrate in an impressive way that the complexes formed by the two compounds change the surface activity as compared to the original native protein molecules and therefore lead to a completely different retardation behavior of rising bubbles. Changes in the velocity profile can be interpreted qualitatively in terms of increased or decreased surface activity of the formed protein-surfactant complexes. It was also observed that the pH and ionic strength of a protein solution have strong effects on the surface activity of the protein molecules, which however, could be different on the rising bubble velocity and the equilibrium adsorption isotherms. These differences are not fully understood yet but give rise to discussions about the structure of protein adsorption layer under dynamic conditions or in the equilibrium state. The third main stage of the discussed process of fractionation is the formation and characterization of protein foams from BLG solutions at different pH and ionic strength. Of course a minimum BLG concentration is required to form foams. This minimum protein concentration is a function again of solution pH and ionic strength, i.e. of the surface activity of the protein molecules. Although at the isoelectric point, at about pH 5 for BLG, the hydrophobicity and hence the surface activity should be the highest, the concentration and ionic strength effects on the rising velocity profile as well as on the foamability and foam stability do not show a maximum. This is another remarkable argument for the fact that the interfacial structure and behavior of BLG layers under dynamic conditions and at equilibrium are rather different. These differences are probably caused by the time required for BLG molecules to adapt respective conformations once they are adsorbed at the surface. All bubble studies described in this work refer to stages of the foam fractionation process. Experiments with different systems, mainly surfactant and protein solutions, were performed in order to form foams and finally recover a solution representing the foamed material. As foam consists to a large extent of foam lamella - two adsorption layers with a liquid core - the concentration in a foamate taken from foaming experiments should be enriched in the stabilizing molecules. For determining the concentration of the foamate, again the very sensitive bubble rising velocity profile method was applied, which works for any type of surface active materials. This also includes technical surfactants or protein isolates for which an accurate composition is unknown.},
  language  = {en}
}
@phdthesis{Koelsch2005,
  author    = {K{\"o}lsch, Patrick},
  title     = {Static and dynamic properties of soluble surfactants at the air/water interface},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5716},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Amphiphilic molecules contain a hydrophilic headgroup and a hydrophobic tail. The headgroup is polar or ionic and likes water, the tail is typically an aliphatic chain that cannot be accommodated in a polar environment. The prevailing molecular asymmetry leads to a spontaneous adsorption of amphiphiles at the air/water or oil/water interfaces. As a result, the surface tension and the surface rheology is changed. Amphiphiles are important tools to deliberately modify the interfacial properties of liquid interfaces and enable new phenomena such as foams which cannot be formed in a pure liquid. In this thesis we investigate the static and dynamic properties of adsorption layers of soluble amphiphiles at the air/water interface, the so called Gibbs monolayers. The classical way for an investigation of these systems is based on a thermodynamic analysis of the equilibrium surface tension as a function of the bulk composition in the framework of Gibbs theory. However, thermodynamics does not provide any structural information and several recent publications challenge even fundamental text book concepts. The experimental investigation faces difficulties imposed by the low surface coverage and the presence of dissolved amphiphiles in the adjacent bulk phase. In this thesis we used a suite of techniques with the sensitivity to detect less than a monolayer of molecules at the air-water interface. Some of these techniques are extremely complex such as infrared visible sum frequency generation (IR-VIS SFG) spectroscopy or second harmonic generation (SHG). Others are traditional techniques, such as ellipsometry employed in new ways and pushed to new limits. Each technique probes selectively different parts of the interface and the combination provides a profound picture of the interfacial architecture. The first part of the thesis is dedicated to the distribution of ions at interfaces. Adsorption layers of ionic amphiphiles serve as model systems allowing to produce a defined surface charge. The charge of the monolayer is compensated by the counterions. As a result of a complex zoo of interactions there will be a defined distribution of ions at the interface, however, its experimental determination is a big scientific challenge. We could demonstrate that a combination of linear and nonlinear techniques gives direct insights in the prevailing ion distribution. Our investigations reveal specific ion effects which cannot be described by classical Poisson-Boltzmann mean field type theories. Adsorption layer and bulk phase are in thermodynamic equilibrium, however, it is important to stress that there is a constant molecular exchange between adsorbed and dissolved species. This exchange process is a key element for the understanding of some of the thermodynamic properties. An excellent way to study Gibbs monolayers is to follow the relaxation from a non-equilibrium to an equilibrium state. Upon compression amphiphiles must leave the adsorption layer and dissolve in the adjacent bulk phase. Upon expansion amphiphiles must adsorb at the interface to restore the equilibrium coverage. Obviously the frequency of the expansion and compression cycles must match the molecular exchange processes. At too low frequencies the equilibrium is maintained at all times. If the frequency is too fast the system behaves as a monolayer of insoluble surfactants. In this thesis we describe an unique variant of an oscillating bubble technique that measures precisely the real and imaginary part of the complex dilational modulus E in a frequency range up to 500 Hz. The extension of about two decades in the time domain in comparison to the conventional method of an oscillating drop is a tremendous achievement. The imaginary part of the complex dilational modulus E is a consequence of a dissipative process which is interpreted as an intrinsic surface dilational viscosity. The IR-VIS SFG spectra of the interfacial water provide a molecular interpretation of the underlying dissipative process.},
  subject      = {Nichtlineare Optik},
  language  = {en}
}