Petra Göring

Tailoring thermotropic cubic mesophases: amphiphilic polyhydroxy derivatives

Novel amphiphilic polyhydroxy compounds [N-(3,4-dialkoxybenzoyl)-1-amino-1-deoxy-d-glucitols (glucamides), N-(3,4-dialkoxybenzoyl)-1-deoxy-1-methylamino-d-glucitols, N-(3,4,5-trialkoxybenzoyl)-1-deoxy-1-methylamino-d-glucitols (N-methylgucamides), 1-benzoylaminopropane-2,3-diols, 2-benzoylaminopropane-1,3-diols, 2-(3,4,5-tridodecyloxybenzoylamino)-2-(hydroxymethyl)propane-1,3-diol and (3,4,5-tridodecyloxybenzoyl)bis(2,3-dihydroxypropyl)amine] have been synthesized. Their thermotropic liquid crystalline phases were investigated by means of polarizing microscopy, differential scanning calorimetry and X-ray diffraction. Depending on the chain length, and the size of the hydrophilic polyhydroxy units, different mesophases have been found: smectic A phases (SA), inverted bicontinuous cubic phases (CubV2 , Ia3d), hexagonal columnar phases (ColH2) and micellar cubic mesophases (CubI2 , Pn3m or P43n). In strong analogy to lyotropic systems, the type of thermotropic mesophase depends on the ratio between the volume of the lipophilic moiety and the surface area of the hydrophilic moiety at the hydrophilic–lipophilic interface. The crossing from zero interface curvature (SA phase) to the finite negative curvature of the inverted cylindrical aggregates of the columnar mesophase takes place via bicontinuous cubic mesophases. The cylindrical aggregates of the columnar mesophase are stable over a rather broad range of variation of the structural parameter. At a certain degree of the size of the lipophilic moiety in respect to the surface area of the hydrophilic group, however, the transition from the hexagonal columnar to a micellar cubic mesophase takes place. On the basis of proton conductivity measurements and from packing considerations we propose that this cubic lattice is built up by eight closed micelles per unit cell which have a rod-like shape and represent small segments of extended columns. Therefrom we can propose a model for the transformations between these different thermotropic mesophases.

Amphiphilic N-benzoyl-1-amino-1-deoxy-D-glucitol derivatives forming thermotropic lamellar, columnar and different types of cubic mesophases

Novel amphiphilic glucamine derivatives have been synthesized. These are N-benzoyl-1-deoxy1-methylamino-D-glucitols and N-benzoyl-1-amino-1-deoxy-D-glucitols carrying one, two or three aliphatic chains (CnH2n 1O- with n 3, 6 and 12) grafted to the benzamido group. The thermotropic mesophases of these compounds were studied by thermal polarizing optical microscopy and differential scanning calorimetry, and some also by X-ray scattering. Depending on the number and the length of the alkyl chains lamellar, bicontinuous cubic, hexagonal columnar or inverted micellar cubic mesophases were detected by analogy with lyotropic systems. In the contact region between lamellar phases of the single chain amphiphiles and micellar cubic phases of the mesomorphic triple chain compounds, hexagonal columnar phases can be induced. A hexagonal columnar phase was also induced in the contact region between a bicontinuous and a micellar cubic mesophase. The lyotropic liquid crystalline behaviour of the dodecyloxy substituted N-...

Monodisperse metal nanowire arrays on Si by integration of template synthesis with silicon technology

Thin films of monodomain porous alumina with 2 µm thickness and adjustable monodisperse pore diameter are obtained via anodization of nanoimprinted Al evaporated on a Si wafer. By using these templates, hexagonally arranged monodisperse silver nanowires on Si are prepared by electrochemical deposition with a commercially available bath. In addition, free standing Ag nanowires on Si are prepared by selective dissolution of the alumina matrix.

In situ surface-enhanced Raman spectroscopy of monodisperse silver nanowire arrays

Highly ordered two-dimensional arrays of monodisperse coinage metal nanowires embedded in an alumina matrix have been prepared. When light is propagating in the direction of the long axis of the nanowire, plasmon-enhanced absorption and light guidance of the nanowire were observed by optical microspectroscopy and scanning near-field optical spectroscopy and compared to Mie scattering theory. By selectively dissolving the matrix at a constant etching rate, we detected in situ and ex situ the surface-enhanced Raman scattering (SERS) of organic dyes. In contrast to earlier publications, we find that the SERS signal is linearly proportional to the free-surface area of the nanowires that are in contact with the dye. We cannot detect any change in the enhancement factor due to the releasing of the nanowires from the host structure.

Formation of mesophases based on micro-segregation: columnar liquid-crystalline phases of first generation dendrimers with perfluorinated segments

Tetrahedral pentaerythritol tetrabenzoates with semi-fluorinated alkyl chains have columnar liquid-crystalline phases with dramatically enhanced mesophase stabilities compared to the parent hydrocarbon derivatives.

Surface-enhanced Raman spectroscopy employing monodisperse nickel nanowire arrays

We have prepared two-dimensional arrays of hexagonally arranged, monodisperse nickel nanowires embedded in an alumina template. The degree of template filling is nearly 100% using an improved electrochemical deposition technique. Optical transmission measurements in the direction parallel to the long axis of the nickel nanowires show a plasmon-enhanced absorption around 400nm. We observe for typically surface-enhanced Raman spectroscopy (SERS) inactive metals like nickel a strong, but locally strongly inhomogeneous SERS signal during in situ Raman microspectroscopy. Supported by our numerical modeling, we suggest that significant SERS enhancement factors are possible only when nanowires in bundles are touching each other.

High-temperature resistant, ordered gold nanoparticle arrays

Ordered gold nanoparticle arrays with high lateral density of 6.87 × 10 10 nanoparticles cm −2 ,w hichare stable up to temperatures of 600 ◦ C, were fabricated. To this end, nanoparticles formed by thermal vacuum evaporation of Au were immobilized within the pores of nanoporous silicon wafers prepared by block copolymer lithography coupled with dry plasma etching. Even after high-temperature treatment the degree of order imposed by the block copolymer template was retained. Optionally, a nanoporous silicon nitride mask can cover the nanoporous silicon.

Molecular design of amphitropic liquid crystalline carbohydrates–amphiphilic N-methyl-glucamides exhibiting lamellar, columnar or cubic mesophases

Amphiphilic 1-deoxy-1-(N-methylbenzamido)-D-glucitols display smectic, hexagonal columnar or discontinuous cubic thermotropic liquid crystalline phases depending on the number of alkyl chains attached to the benzamido group.

Phase transitions between smectic A phases as well as smectic C* phases and undulated structures in terminal non-polar compounds

Abstract Two types of smectic A and smectic C phases, respectively, have been proved by X-ray and DSC methods for compounds that are terminal non-polar, but consist of tuning-fork shaped molecules. The structural models of the phases are discussed on the basis of steric interaction and of a dense packing. The X-ray patterns of oriented samples point to an undulation of the smectic layers in the case of the smectic A and smectic B phase.

Design of non-anisometric and non-amphiphilic liquid crystals: liquid-crystalline tetrahedra

Tetrahedral pentaerythritol tetra(3,4-dialkoxybenzoates) and tetra(3,4,5-trialkoxybenzoates) can display columnar liquid-crystalline phases despite the fact that they are neither disc-like nor amphiphilic.