38 research outputs found
Correlation between specific ion adsorption at the air/water interface and long-range interactions in colloidal systems
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugĂ€nglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Specific ion effects are of high impact in colloid science and dominate processes in aqueous systems from protein folding or precipitation to ordering of particles or macromolecules in bulk solutions. Due to the large internal interface of colloidal systems especially interfacial ion effects are of importance. This paper presents a new insight into the specific ion effects at the air/water interface of monovalent electrolyte solutions and their consequences for long-range interactions in colloidal systems. Solely, in an asymmetric film (i.e. wetting film) one can determine the sign and precise value of the surface potential of the free air/water surface. It is shown that the all over charges of the interfacial region, which are affected by the type of ion, dominate the interfacial forces even over several tens of nm. This is of interest for tailoring the stability of colloidal systems. It is clearly shown that the air/water interface is negatively charged and that both anions and cations affect the surface potential even at very low electrolyte concentrations (10â4 M).DFG, SPP 1273, Kolloidverfahrenstechni
Pickering emulsions stabilized by stacked catanionic micro-crystals controlled by charge regulation
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In this paper the mechanism behind the stabilization of Pickering emulsions by stacked catanionic micro-crystals is described. A temperature-quench of mixtures of oppositely charged surfactants (catanionics) and tetradecane from above the chain melting temperature to room temperature produces stable oil-in-water (o/w) Pickering emulsions in the absence of Ostwald ripening. The oil droplets are decorated by stacks of crystalline discs. The stacking of these discs is controlled by charge regulation as derived from conductivity, scattering and zeta potential measurements. Catanionic nanodiscs are ideal solid particles to stabilize Pickering emulsions since they present no density difference and a structural surface charge which is controlled by the molar ratio between anionic and cationic components. The contact angle of catanionic nanodiscs at a water/oil interface is also controlled by the non-stoichiometry of the components. The resulting energy of adhesion and the repulsion between droplets is much larger than kT. As a consequence of these unique properties of nanodiscs, this type of emulsions presents an extremely high resistance towards coalescence and creaming, even in the presence of salt
Ranking the efficiency of gas hydrate anti agglomerants through molecular dynamic simulations
Using both computational and experimental methods, the capacity of four
different surfactant molecules to inhibit the agglomeration of sII hydrate
particles was assessed. The computational simulations were carried out using
both steered and non-steered Molecular Dynamics (MD), simulating the
coalescence process of a hydrate slab and a water droplet, both covered with
surfactant molecules. The experimental work was based on rocking cell
measurements, determining the minimum effective dose necessary to inhibit
agglomeration. Overall, good agreement was obtained between the performance
predicted by the simulations and the experimental measurements. Moreover, the
simulations allowed to gain additional insights that are not directly
accessible via experiments, such as an analysis of the mass density profiles,
the diffusion coefficients, or the orientations of the long tails
Pickering emulsions stabilized by stacked catanionic micro-crystals controlled by charge regulation
Ion-Specific Long-Range Correlations on Interfacial Water Driven by Hydrogen Bond Fluctuations
Spezifischer Einfluss von Ionen in Schaum- und Benetzungsfilmen
Innerhalb der vorliegenden Doktorarbeit wurde der Einfluss verschiedener Ionen auf Benetzungs- und Schaumfilme untersucht. Die Untersuchungen von Benetzungsfilmen der reinen Elektrolytlösungen auf Siliziumwafern zeigten, dass in AbhĂ€ngigkeit der IonengröĂe, sowohl Kationen wie auch Anionen an der Wasser/Luft GrenzflĂ€che adsorbieren können. Die Adsorption von Kationen fĂŒhrt zu einer Absenkung der OberflĂ€chenladungsdichte und somit zu instabileren Filmen, wohingegen die Adsorption von Anionen zur Erhöhung der OberflĂ€chenladungsdichte und somit zu stabileren Filmen fĂŒhrt. Die vorliegenden Ergebnisse sind ein Beweise fĂŒr: (i) eine negative Ladung der Wasser/Luft GrenzflĂ€che, eine Thematik, die immer noch kontrovers diskutiert wird und (ii) dafĂŒr, dass Ionen spezifische Wechselwirkungen elektrostatische Wechselwirkungen dominieren, insofern als das zusĂ€tzliche negative Ladungen an eine negativ geladene GrenzflĂ€che adsorbieren. Die Dominanz der Ionen spezifischen Wechselwirkungen wurde anhand von Untersuchungen von Schaumfilmen bestehend aus anionischen Tensid/Salz Gemischen bestĂ€tigt. Sowohl bei Benetzungsfilmen der reinen Elektrolytlösungen als auch bei Schaumfilmen stabilisiert mit anionischen Tensiden gibt es eine starke Korrelation zwischen FilmstabilitĂ€t so wie âdicke und OberflĂ€chenkrĂ€ften. Als nĂ€chstes wurde der Einfluss des Vorzeichens der OberflĂ€chenladung auf spezifische Ioneneffekte in Schaumfilmen untersucht. Interessanter Weise ist die Abfolge der StabilitĂ€ten in AbhĂ€ngigkeit der IonengröĂe fĂŒr Schaumfilme aus anionischen oder kationischen Tensiden gleich, ein Indiz, dass das Vorzeichen er OberflĂ€chenladung keinen Einfluss auf die spezifischen Ioneneffekte hat. Jedoch haben ergĂ€nzende Messungen ergeben, dass die Stabilisierung der Filme unterschiedliche Ursachen hat. WĂ€hrend im Falle von anionischen Tensid/Salz Gemischen die FilmstabilitĂ€ten von OberflĂ€chenkrĂ€ften abhĂ€ngen, so wird die StabilitĂ€t von Filmen aus kationischen Tensid/Salz Gemischen von der ElastizitĂ€t des Filmes bestimmt. Durch die Variation der Gegenionen zweier Tensidarten konnte gezeigt werden, dass die Wechselwirkung zwischen Tensidrumpf und Gegenion einen starken Einfluss auf die StabilitĂ€t von Schaumfilmen hat und somit durch die Wahl eines geeigneten Gegenions eine Verbesserung der Schaumfilmeingenschaften erzeugt werden kann. Der Ăbergang von anorganischen zu organischen Salzen (Alkanoaten) hat gezeigt, dass diese je nach HydrophobizitĂ€t wie organisches Salz, Cotensid oder Tenside wirken. Die Wirkung der HydrophbozitĂ€t sowie der Konzentration der Alkanoate auf Schaumfilm-eingenschaften wurde an Gemischen aus C14TAB und Alkanoaten untersucht. Die C14TAB Konzentration war konstant wĂ€hrend die Menge der Alkanoate variierte. Die Auswirkung von Alkanoaten mit salzartigem bzw. tensidischem Verhalten auf Schaumfilme ist deutlich anders. Im Falle von organischem Salz tritt eine Abschirmung senkrecht zu den FilmgrenzflĂ€chen auf. Dies fĂŒhrt zu einer kontinuierlichen Abnahme der StabilitĂ€t mit Erhöhung der Konzentration des organischen Salzes. Im Gegensatz dazu fĂŒhrt die Bildung von Komplexen bei organischen Salzen mit tensidischem Charakter zu einer Abschirmung sowohl (i) innerhalb der FilmgrenzflĂ€che als auch (ii) senkrecht zu den FilmgrenzflĂ€chen. Dies hat zur Folge, dass bei höheren Konzentrationen des organischen Salzes mit tensidischem Charakter eine erneute Stabilisierung der Filme erreicht werden kann. Die abschlieĂenden Untersuchungen der Auswirkung der Ladungsdichte der Kopfgruppe auf Schaumfilme aus Mischungen von entgegengesetzt geladenen Tensiden zeigte, dass Mischungen aus Tensiden mit gleichen Kopfgruppenladungsdichten Schaumfilme effizienter stabilisieren. Generell sind Tensidgemische sehr vielversprechende Systeme hinsichtlich der Stabilisierung von Schaumfilmen, da geringe Mengen ausreichen um sehr hohe SchaumfilmstabilitĂ€ten zu erhalten. Hinsichtlich aller untersuchten dĂŒnnen Filme ist zu bemerken, dass diese Systeme sehr sensibel auf Ionen und IonenspezifitĂ€t reagieren. Somit konnten spezifische Ioneneffekte schon bei geringen Salzkonzentrationen beobachtet werden. Dies wurde bisher fĂŒr unmöglich erachtet.Within this thesis the influence of various ions on wetting and foam films has been investigated. The investigation of wetting films of pure electrolyte solutions on bare silicon showed that as a function of the ion size anions as well as cations can adsorb at the air/ water interface. The adsorption of cations results in a decrease in surface charge density and thus to less stable films. In contrast to this, the anion adsorption increases the surface charge density which is reflected in more stable films. The presented results are a proof for (i) a negatively charged air/ water interface, a topic which is still controversially discussed in science and (ii) that the ion specificity overwhelms the electrostatic repulsion in such a way that anions can adsorb at a negatively charged air/ water interface. The dominating effect of the ion specificity has been affirmed by measuring the effect of anions on the properties of foam films stabilized by anionic surfactants. In both cases, wetting films of aqueous electrolyte solutions and foam films stabilized by anionic surfactants, a strong correlation between film stability and thickness and surface forces is found. In the following the impact of the sign of the surface charges on ion specific effects in foam films has been considered. Interestingly, the order of the observed stabilities as a function of the ion size is similar for foam films stabilized by both anionic and cationic surfactants. This indicates that the sign of the interfacial charges has no influence on specific ion effects. However, further measurements showed that the mechanism of stabilization depends on the sign of the interfacial charges. In case of anionic surfactants the decisive parameter for the film stability is the surface forces, while in case of foam films stabilized by cationic surfactants, the stability can be explained by the surface elasticity. Due to variation of the counter-ion of two different types of surfactants it could be demonstrated that the interaction between the headgroup of the surfactant and the counter-ion has a strong impact on the foam film stability. Due to the choice of the counter-ion it is possible to tailor the film properties in a certain way. The step from inorganic to organic electrolytes (alkanoates) showed that organic electrolytes as a function of their hydrophobicity can act as salts, co-surfactant or surfactant. The influence of increasing chain length and amount of organic electrolytes has been investigated by measuring mixtures of C14TAB/ alkanoates with chain lengths varied between C4 and C12. The concentration of C14TAB was fixed while the amount of alkanoates has been varied. In case of alkanoates acting as inorganic salt screening perpendicular to the film interfaces is observed leading to a continuously decrease in stability with rising amount of salt. In contrast to this, the formation of complexes as in case of alkanoates behaving as an anionic surfactant results in screening (i) within a film interface and (ii) perpendicular to the film interfaces. This leads to an increase in stability at higher concentrations of such alkanoates. The finial investigation of the influence of the surface charge density of surfactantâ headgroups on the properties of foam films stabilized by mixtures of oppositely charges surfactants demonstrated that mixtures of surfactants with similar surface charge densities are more efficient in stabilizing foam films. In general mixtures of surfactants are promising systems with regard to the stabilization of foam films since even small amounts of stabilizing agents are sufficient to obtain long lasting foam films. In respect of all investigated systems it is important to realize that thin liquid films are highly sensitive to ion specific effects which enables to detected ion specificities at very low electrolyte concentration which was thought to be impossible until yet