Vibrational sum-frequency generation (SFG) spectroscopy has become an established technique for in situ surface analysis. While spectral recording procedures and hardware have been optimized, unique data analysis routines have yet to be established. The SFG intensity is related to probing geometries and properties of the system under investigation such as the absolute square of the second-order susceptibility χ((2)) (2). A conventional SFG intensity measurement does not grant access to the complex parts of χ((2)) unless further assumptions have been made. It is therefore difficult, sometimes impossible, to establish a unique fitting solution for SFG intensity spectra. Recently, interferometric phase-sensitive SFG or heterodyne detection methods have been introduced to measure real and imaginary parts of χ((2)) experimentally. Here, we demonstrate that iterative phase-matching between complex spectra retrieved from maximum entropy method analysis and fitting of intensity SFG spectra (iMEMfit) leads to a unique solution for the complex parts of χ((2)) and enables quantitative analysis of SFG intensity spectra. A comparison between complex parts retrieved by iMEMfit applied to intensity spectra and phase sensitive experimental data shows excellent agreement between the two methods.
Schlagworte:Calciumphosphat-Knochenzement -mechanische Festigkeit -Porositätnobis Calciumphosphat-Zemente sind Gegenstand aktueller Forschung als biokompatible und am Anwendungsort frei-formbarer Knochenersatzwerkstoffe. Die Angaben zur Festigkeit der Werkstoffe schwankt in weiten Bereichen, trotz vielfach ähnlicher Zusammensetzungen der Werkstoffe. Festigkeitsbeeinflussende Parameter sind neben der Zusammensetzung auch die Herstellung und die angewandten Prüfungsmethoden der Zemente bzw. ausgehärteten Zementmatrix, insbesondere die Prüfungsmethoden erfolgen vielfach unter wenig anwendungsnahen Bedingungen. In dieser Arbeit wurde für zwei unterschiedliche Zementtypen die mechanischen Eigenschaften verglichen; die Herstellung von Prüfkörpern erfolgte hierbei sowohl durch Kompression der Zementpaste als auch anwendungsnah druckfrei. Während ein Hydroxylapatit bildender Zement aus Tetracalciumphosphat/Calciumhydrogenphosphat eine Druckfestigkeit von zrika 20 MPa aufweist, wird für einen Bruschit-Zement eine Druckfestigkeit von etwa 10 MPa nach kompressionsfreier Aushärtung erreicht. Die Vorverdichtung der Zementpaste führt in beiden Fällen zu einer deutlichen Steigerung der Festigkeit, was für den klinischen Anwender allerdings wenig bedeutsam ist, sondern lediglich zum validierten Vergleich einzelner Zementchargen oder zur Herstellung biologisch modifizierter Keramiken nützlich ist.
Beetles of the species Stenus comma live and hunt close to ponds and rivers, where they occasionally fall on the water surface. To escape this jeopardized state, the beetle developed a strategy relying on the excretion of a secretion containing the substances stenusine and norstenusine. They reduce surface tension and propel the bug to the saving river bank. These substances were synthesized and analyzed with respect to their equilibrium and dynamic adsorption properties at the air-water interface (pH 7, 23 ± 1 °C). The surface dilatational rheological characteristics in a frequency range from 2 to 500 Hz at molar bulk concentrations of 20.6 mmol L(-1) were studied using the oscillating bubble technique. Both alkaloids formed surface viscoelastic adsorption layers. The frequency dependence of the surface dilatational modulus E could successfully be described by the extended Lucassen-van den Tempel model accounting for a nonzero intrinsic surface viscosity κ. The findings confirmed a dual purpose of the spreading alkaloids in the escape mechanism of the Stenus beetle. Next to generating a surface pressure, a transition to surface viscoelastic behavior of the adsorbed layers was observed.
Analyzing molecules at aqueous interfaces in situ, in vitro, or even in vivo without the need for labels and/or disruptive sample preparation is crucial for the understanding and optimization of material's interactions with its surrounding. In this context, a central theme is the ability to differentiate between molecules in the respective bulk phases and those that are located at the interface. Here we introduce vibrational sum-frequency generation (SFG) spectroscopy, a nonlinear optical technique that is capable to selectively probe molecules at interfaces. SFG spectroscopy can be applied under ex vacuo conditions and allows to record vibrational spectra from molecules at interfaces. Though this technique holds great potential in research themes involving aqueous interfaces, the data analysis of SFG spectra can get quite complex and often requires a comprehensive understanding of the underlying nonlinear optical processes. This chapter introduces experimental and theoretical aspects of SFG spectroscopy with a strong focus on data analysis. It is meant for scientists new to the field of SFG spectroscopy who like to explore its applicability and theoretical background or are starting to apply SFG spectroscopy in their own research.
The relation between the complex surface dilatational modulus E of aqueous surfactant solutions and the splashing behavior of their drops on liquid surfaces was investigated. The surface dilatational modulus E of selected surfactant systems has been determined in the frequency range of 3 to 500 Hz by means of the oscillating bubble technique. According to the functional dependence of the phase ϕ of the complex modulus E(ω, c)exp[iϕ(ω, c)] at higher frequencies, adsorption layers can be classified as surface elastic or surface viscoelastic. Each behavior shows pronounced differences in drop splashing experiments. The impact of a drop on the liquid was monitored with a high-speed camera. The splash of a drop is a rather complex phenomenon, so the focus of this article is to establish a relationship between the imaginary part of the surface dilatational modulus E and the height of the drop rebound. These findings may be of importance for formulations in crop protection, introducing a chemical way to influence the impact of drops on solid and liquid interfaces.
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