No abstract

The solid-liquid interface formed by single terminated muscovite mica in contact with two different ionic solutions is analyzed using surface X-ray diffraction. Specular and nonspecular crystal truncation rods of freshly cleaved mica immersed in CsCl or RbBr aqueous solution were measured. The half monolayer of the surface potassium ions present after the cleavage is completely replaced by the positive ions (Cs or Rb) from the solution. These ions are located in the ditrigonal surface cavities with small outward relaxations with respect to the bulk potassium position. We find evidence for the presence of a partly ordered hydration shell around the surface Cs or Rb ions and partly ordered negative ions in the solution. The lateral liquid ordering induced by the crystalline surface vanishes at distances larger than 5 Å from the surface.

The crystalline sponge method entails the elucidation of the (absolute) structure of molecules from a solution phase using single‐crystal X‐ray diffraction and eliminates the need for crystals of the target compound. An important limitation for the application of the crystalline sponge method is the instability of the available crystalline sponges that can act as host crystals. The host crystal that is most often used decomposes in protic or nucleophilic solvents, or when guest molecules with Lewis basic substituents are introduced. Here a new class of (water) stable host crystals based on f‐block metals is disclosed. It can be shown that these hosts not only increase the scope of the crystalline sponge method to a wider array of solvents and guests, but that they can even be applied to aqueous solutions containing hydrophilic guest molecules, thereby extending the crystalline sponge method to the important field of water‐based chemistry.

The surface potassium ions of muscovite mica were exchanged for several different metal ions from aqueous solution (Ag, Ca, V, Mn, Fe, Ni, Cu, Zn, Co, and Cd). The surfaces were rinsed in water, dried under nitrogen atmosphere, and subsequently analysed using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and, for half the systems, surface X-ray diffraction (SXRD). XPS and SXRD confirmed the presence of the different metal ions at the muscovite mica surface, with a partial monolayer of the monovalent and divalent ions present on the surface. No counter ions from the used salts were detected. AFM revealed that Ni-, and Fe-terminated muscovite mica surfaces were partially covered by nanoparticles, most likely consisting of metal (hydr)oxide. The exchanged ions remained on the surface after rinsing with ultra pure water three times. SXRD showed that Cd and Ag have a lower affinity for the muscovite mica surface than Cu, Ca, and Mn.

Surfaces with controllable topography and chemistry were prepared to act as substrates for protein crystallization, in order to investigate the influence of these surface properties on the protein crystallization outcome. Three different methods were investigated to deposit 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) on a muscovite mica substrate to find the best route for controlled topography. Of these three, sublimation worked best. Contact angle measurements revealed that the surfaces with short exposure to the TCDB vapor (20 min or less) are hydrophilic, while surfaces exposed for 30 min or longer are hydrophobic. The hydrophilic surfaces are flat with low steps, while the hydrophobic surfaces contain macrosteps. Four model proteins were used for crystallization on the surfaces with controlled topography and chemistry. Hen egg white lysozyme crystals were less numerous on the surface with macrosteps than on smoother surfaces. On the other hand, insulin nucleated faster on the hydrophobic surfaces with macrosteps, and therefore, the crystals were more abundant and smaller. Bovine serum albumin and talin protein crystals were more numerous on all TCDB functionalized surfaces, compared to the reference clean muscovite mica surfaces. Overall, this shows that surface topography and chemistry is an important factor that partly determines the outcome in a protein crystallization experiment.

Stable layers of crown ethers were grown on muscovite mica using the potassium-crown ether interaction. The multilayers were grown from solution and from the vapor phase and were analyzed with atomic force microscopy (AFM), matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and surface X-ray diffraction (SXRD). The results show that the first molecular layer of the three investigated dibenzo crown ethers is more rigid than the second because of the strong interaction of the first molecular layer with the potassium ions on the surface of muscovite mica. SXRD measurements revealed that for all of the investigated dibenzo crown ethers the first molecule lies relatively flat whereas the second lies more upright. The SXRD measurements further revealed that the molecules of the first layer of dibenzo-15-crown-5 are on top of a potassium atom, showing that the binding mechanism of this layer is indeed of the coordination complex form. The AFM and SXRD data are in good agreement, and the combination of these techniques is therefore a powerful way to determine the molecular orientation at surfaces.

The use of an achiral metal–organic framework for structure determination of chiral compounds is demonstrated for camphene and pinene. The structure of enantiopure β-pinene can be resolved using the crystalline sponge method. However, α-pinene cannot be resolved using enantiopure material alone because no ordering of guest molecules takes place in that case. Interestingly, enantiomeric pairs order inside the channels of the host framework when impure (+)-camphene is offered to the host, which is also the case when a racemic mixture of α-pinene is used. A mixture of (+)-α-pinene and (−)-β-pinene also leads to ordered incorporation in the host, showing the influence of the presence of an inversion center in the host framework. We further show that powder X-ray diffraction provides a direct view on incorporation of ordered guest molecules. This technique, therefore, provides a way to determine the optimal and/or minimal soaking time. In contrast, color change of the crystal only demonstrates guest uptake, not ordering. Moreover, we show that color change can also be caused by guest-induced host degradation.

The structure of the solid–liquid interface formed by muscovite mica in contact with two divalent ionic solutions (SrCl2 and BaCl2) is determined using in situ surface X-ray diffraction using both specular and non-specular crystal truncation rods. The 0.5 monolayer of monovalent potassium present at the surface after cleavage is replaced by approximately 0.25 monolayer of divalent ions, closely corresponding to ideal charge compensation within the Stern layer in both cases. The adsorption site of the divalent ions is determined to be in the surface ditrigonal cavities with minor out-of-plane relaxations that are consistent with their ionic radii. The divalent ions are adsorbed in a partly hydrated state (partial solvation sphere). The liquid ordering induced by the presence of the highly ordered crystalline mica is limited to the first 8–10 Å from the topmost crystalline surface layer. These results partly agree with previous studies in terms of interface composition, but there are significant differences regarding the structural details of these interfaces.