Alkyl Chains Of Molecules Research Articles

Effect of Chemical Bond Type on Electron Transport in GaAs−Chemical Bond−Alkyl/Hg Junctions

How alkyl chain molecules are bound chemically to GaAs directly affects current transport through GaAs/Alkyl/Hg junctions. We used two different binding groups, thiols that form an As−S bond and phosphonates with the much stronger Ga−O (actually Ga−O−P) bond. Analyzing transport through the junctions as tunneling through a dielectric medium of defined thickness, characterized by one barrier and the effective mass of the electronic carrier, we find the main difference in the electronic properties between the two systems to be the effective mass, 1.5−1.6 me with thiols and 0.3 me with phosphonates. The latter value is similar to that found with, or predicted for, other systems. We ascribe this difference primarily to less scattering of carriers by the Ga−O than by the As−S interface.

Molecular Structure of a Partly Deuterated Chiral Smectic Liquid Crystal Studied by Polarized FTIR Spectroscopy

The molecular structure of a partly deuterated chiral smectic liquid crystal 4-(1-methylpentyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (acronym M5POBC-d 25 ) in a homogeneously aligned sample is studied using polarized Fourier-transform infrared (FTIR) spectroscopy. The relative orientations of the chiral and deuterated achiral alkyl chains of molecules with respect to their long axes and some properties of the orientational distributions of the carbonyl groups are quantitatively analysed for the ferroelectric SmC* phase with a field unwound structure. The analysis is carried out in terms of the absorption anisotropy of several bands related to stretching vibrations of the phenyl C 6 H 4 fragment, the deuterated biphenyl C 12 D 8 fragment, two carbonyl groups situated in different parts of a molecule, the methylene groups CH 2 and CD 2 . It has been found that the chiral and achiral alkyl chains make substantially different angles (∼60° and ∼36°, respectively) with the molecular long axis. The rotational orientational distribution of the carbonyl groups about the molecular long axes has been found to have a comparatively low polar biasing.

Pressure-Temperature Phase Diagrams of Ferroelectric Liquid Crystals

The pressure-temperature phase diagrams of the ferroelectric liquid crystals FLC-031 and FLC-032 were obtained from the complex dielectric constant. As the pressure increases, all the transition temperatures increase. On heating, the temperature width of the smectic phase is almost independent of the pressure, while the width of the ferroelectric chiral smectic C (SmC * ) phases reduces with pressure and disappears at 300 MPa and 125°C for FLC-031 and 160 MPa and 102°C for FLC-032. On the other hand, the width of the paraelectric smectic A (SmA) phase increases with increasing pressure. Therefore, this indicates that, for FLC-031 and FLC-032, the application of pressure reduces the stability of the SmC * phase, while the stability of the SmA phase increases. Moreover, there is no systematic relation between the length of the non-chiral alkyl chain in molecules and the decreasing rate of the width of the SmC * phase with pressure.

Adlayer structure of 1‐C18H37 SH molecules: scanning tunnelling microscopy study

AbstractThe adlayer structure of 1‐octadecanethiol (1‐C18H37SH) is investigated by scanning tunnelling microscopy (STM) with bare and chemically (I and S) modified Pt/Ir tips. The molecules were observed to form a well‐ordered self‐assembly monolayer on highly oriented pyrolytic graphite. The alkyl chains of molecules were clearly imaged by bare tips. The S‐modified tip was found to be effective to reveal the structural details of thiol groups (SH). Based on the STM results and theoretical calculations, a model for a 1‐octadecanethiol adlayer is proposed. Copyright © 2001 John Wiley & Sons, Ltd.

Dielectric Relaxation in Liquid Crystals with (CH2) Spacers in Molecules

The dielectric properties of solutions of the liquid crystals with (CH2) spacers and nitrile groups in molecules in weakly polar nematic diluent were studied. In the frequency range of a.c. electric field (102−108)Hz the dispersion of complex dielectric permitivities ε| in nematic phase were investigated. From the dielectric spectra of ε|, the relaxation frequencies fRwere obtained for each of the investigated substances. It was shown that fR increases with the (CH2) spacers length of the central alkyl chains of molecules.

The molecular structure of thiol-monolayers on gold imaged with an Atomic Force Microscope

Thiols spontaneously form monolayers on gold. An important question is which factors determine the structure of the thiols. The two extreme positions are: (a) the structure of the underlying gold determines the packing of the thiol molecules; (b) the geometry of the thiol molecules and the interaction between thiols determine their two-dimensional structure.To help answer this question two series of thiol molecules on gold (111) were imaged by atomic force microscopy. In order to reduce the force between tip and sample, and to prevent distortion of the thiol monolayer, all images were taken in ethanol at forces between 0.5 nN and 3 nN.The first series consisted of several azobenzene-thiols:Although these molecules possess a terminal alkyl chain, their lattice is different from the well known hexagonal (√3 × √3) R 30° lattice (with respect to the underlying gold (111) lattice) of alkane-thiols. Instead, we found two stable rectangular lattices which could both be described by lattice constants of 0.61 nm and 0.79 nm, and an angle of 89°. These structures were also found on polycrystalline gold. Also, varying the length of the alkyl chain (molecules (i) and (ii)) or using a disulfide instead of a thiol (molecules (ii) and (iii)) did not change the lattice constants. Thus we conclude that the lattice for azobenzenethiols is end-group dominated.

Non-Chiral Compounds Exhibiting Alternating Tilt Smectic Phases

Abstract In this article we present an investigation of the liquid crystal properties of non-chiral and racemic branched alkyl chain (swallow-tailed) molecules and some chiral analogues based on a cinnamic acid core group. It was found in these systems that increasing the extent of the branching of the terminal alkyl chain increased the incidence of alternating tilt smectic phases, whereas smectic Cα and smectic Cγ phases were suppressed. X-ray studies of the effect on the layer spacing in the smectic A phase of increasing the long alkyl branch of one series of compounds are included. Results of polarisation studies on some chiral analogues are also reported.

Monolayers having large in-plane dipole moments: characterization of sulfone-containing self-assembled monolayers of alkanethiols on gold by Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and wetting

Monolayers were formed from alkyl chain molecules having large in-plane dipole moments. The molecular species introduced was a sulfone moiety, and it was found that its incorporation strongly affected the molecular conformation within the monolayer. The effect of varying the position of the sulfone group in an otherwise straight alkyl chain, while constant molecular length waa maintained, was studied, and the results were compared to those found for an octadecanethiol (ODT). The monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy and by their wetting properties.

Is there a ‘cut-off’ in the adsorption of long chain amphipathic molecules into lipid membranes?

Electrokinetic mobilites of lipid vesicles have been measured in solutions of n-alkyltrimethylammonium ions. The electrokinetic (zeta) potentials calculated from the mobilities have been used to estimate the adsorption of the alkyl chain ions. No significant difference was found in the adsorption to egg phosphatidylcholine or egg phosphatidylcholine + cholesterol (2: 1 mole ratio). For n-octyl- to n-octadecyltrimethylammonium, the standard free energy of adsorption increased on average by approx. 3 kJ•mol -1 from one homologue to the next, and there was no suggestion of a ‘cut-off’ in adsorption as the chain length increased. These results are considered in relation to recent reports of the action of alkyl chain molecules on biological membranes.

Insertion of various long alkyl chain molecules in brush-type grafted monolayers : Chromatographic study of the resulting materials

A monolayer was prepared on the surface of macroporous silica gels by the insertion of various long alkyl chain molecules in a densely grafted C 22 alkyl-silica. The original properties of these layers are briefly described, together with inverse gas chromatographic and some differential scanning calorimetric and NMR spectroscopic experiments. The feasibility of using these materials in reversed-phase high-performance liquid chromatography is exemplified by the separation of three classical mixtures. The advantages of these layers are discussed.