Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy.

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved NMR spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the C­H and C­D stretching bands in Raman spectra. Thus, a concentration gradient in the evaporating droplet was established. Spatially resolved NMR spectroscopy revealed the composition at different positions of a droplet evaporating in the NMR tube, an environment in which air exchange is less pronounced. While not being perfectly comparable, both methods­confocal Raman and spatially resolved NMR experiments­show the presence of a vertical concentration gradient as 1-butanol/1-hexanol droplets evaporate.

Evaporation of Sessile Binary Mixture Droplets: Time Dependence of Droplet Shape and Concentration Profile from One-Dimensional Magnetic Resonance Microscopy.

Many technological applications like inkjet printing, coating, or cooling processes rely on the evaporation of sessile droplets. Regarding liquid mixtures, the understanding of the underlying physics is still incomplete and process optimization requires trial and error. Our main goal is to establish a novel method in this field, one-dimensional magnetic resonance microscopy, to investigate the evaporation of sessile binary mixture droplets in the microliter range. It allows us not only to determine the droplet volume and shape, including contact angle, but also to measure concentration profiles with a spatial resolution of a few micrometers. These capabilities are demonstrated for a mixture of 1-octanol (OCT) and pentadecafluoro-1-octanol (F-OCT) by combining spatially resolved 1H and 19F nuclear magnetic resonance measurements. We clearly observe three evaporation regimes for the OCT/F-OCT mixture. The first and second regimes are characterized by the predominant evaporation of F-OCT and are separated by a depinning event. The third regime starts when no F-OCT is left and, thus, features the evaporation of a pure OCT droplet. During all stages, concentration gradients perpendicular to the substrate are weak in the studied binary droplet.

Trifunctional Silyl Groups as Anchoring Units in the Preparation of Luminescent Phosphole-Silica Hybrids.

A synthetic strategy to ß-silylphospholes with three methoxy, ethoxy, chloro, hydrido, or phenyl substituents at silicon has been developed, starting from trimethoxy, triethoxy, or triphenyl silyl substituted phenyl phosphanides and 1,4-diphenyl-1,3-butadiyne. These trifunctional silylphospholes were attached to the surface of uniform spheric silica particles (15 µm) and, for comparison, to a polyhedral silsesquioxane (POSS)-trisilanol as a molecular model to explore their luminescent properties in comparison with the free phospholes. Density functional theory calculations were performed to investigate any electronic perturbation of the phosphole system by the trifunctional silyl anchoring unit. For the immobilized phospholes, cross-polarization magic-angle-spinning NMR measurements (13C, 29Si, and 31P) were carried out to explore the bonding situation to the silica surface. Thermogravimetric analysis and X-ray photoelectron spectroscopy measurements were performed to approximate the amount of phospholes covering the silica surface. Identity and purity of all novel phospholes have been established with standard techniques (multinuclear NMR, mass spectrometry, and elemental analysis) and X-ray diffraction for the POSS derivative.

A novel strategy for site selective spin-labeling to investigate bioactive entities by DNP and EPR spectroscopy.

A novel specific spin-labeling strategy for bioactive molecules is presented for eptifibatide (integrilin) an antiplatelet aggregation inhibitor, which derives from the venom of certain rattlesnakes. By specifically labeling the disulfide bridge this molecule becomes accessible for analytical techniques such as Electron Paramagnetic Resonance (EPR) and solid state Dynamic Nuclear Polarization (DNP). The necessary spin-label was synthesized and inserted into the disulfide bridge of eptifibatide via reductive followed by insertion by a double Michael addition under physiological conditions. This procedure is universally applicable for disulfide containing biomolecules and is expected to preserve their tertiary structure with minimal change due to the small size of the label and restoring of the previous disulfide connection. HPLC and MS analysis show the successful introduction of the spin label and EPR spectroscopy confirms its activity. DNP-enhanced solid state NMR experiments show signal enhancement factors of up to 19 in 13C CP MAS experiments which corresponds to time saving factors of up to 361. This clearly shows the high potential of our new spin labeling strategy for the introduction of site selective radical spin labels into biomolecules and biosolids without compromising its conformational integrity for structural investigations employing solid-state DNP or advanced EPR techniques.