Packing Of Alkyl Chains Research Articles

In Situ Studies of Phase Transitions in Thin Discotic Films

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.

Solid-state phase behaviour of dodecylglycosides

The solid-state phase behaviour of lyophilised n-dodecyl-β- d-glucoside (β-C 12G 1), n-dodecyl-β- d-maltoside (β-C 12G 2) and n-dodecyl-β- d-maltotrioside (β-C 12G 3) has been investigated by differential scanning calorimetry (DSC) and X-ray techniques. For β-C 12G 1, lyophilisation results in a formation of a crystalline anhydrate. The lamellar spacing (37 Å) is consistent with an alkyl chain packing in which the chains are not interdigitated. At 80 °C, the material melts into a lamellar liquid crystal with a lamellar spacing of 32 Å, which suggests that the non-interdigitated chain packing of the crystalline state is retained in the liquid crystal. In contrast, lyophilisation of β-C 12G 2 and β-C 12G 3 results in the formation of a glassy state, best described as a frozen version of the lamellar liquid crystal. For β-C 12G 2, the lamellar spacing in the glass and liquid crystal suggests interdigitation of the alkyl chains. The glass transition temperature was found to be 65 °C for β-C 12G 2 and 100 °C for β-C 12G 3, which compares favourably with the glass transition of the parent carbohydrates. A second crystalline modification of β-C 12G 1 was prepared by precipitation from an aqueous solution at temperatures below the Krafft point (38 °C). For this modification, the lamellar distance (24 Å) is consistent with interdigitated alkyl chains. At 50 °C, the crystalline material melts into a liquid crystalline phase. The material also readily loses water and rapidly re-crystallises to the anhydrate. The amount of water lost upon drying is consistent with the idea that the material is a monohydrate of β-C 12G 1. The drying and re-crystallisation processes give rise to ‘pre-transitions’ in the DSC thermograms and illustrate the importance of careful control of water in any analysis of the phase behaviour of alkylglycosides.

Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

The conformational behaviour of non-deuterated and selectively deuterated alkyl modified silica gels in the dry state is examined by variable temperature FT IR spectroscopy. In the present study, three systems are considered, which are distinguished by the length of the tethered alkyl chains (C 9H 19 , C 18H 37 , C 22H 45 ). The desired information is obtained by the analysis of various conformational-sensitive IR bands, including CH 2 wagging, CD 2 stretching and CD 2 rocking bands. The analysis of the CH 2 wagging bands provides the relative amounts (i.e., integral numbers over the whole chain) of the kink/ gauche– trans– gauche, double- gauche and end- gauche conformers in the tethered alkyl chains. From the analysis of the CD 2 stretching and CD 2 rocking bands information about the conformational order at a specific deuterated methylene segment is available. Here, the CD 2 rocking band data are used to determine the amount of gauche conformers at the deuterated carbon positions C-4 and C-6, and C-12. It is found that the conformational order critically depends on the actual alkyl chain length, chain position and sample temperature. Particular emphasis is given to the impact of the external pressure during sample preparation on the alkyl chain conformations, about which so far no information is available. It is observed that the samples prepared as KBr pellets, which experienced a pressure of about 10 kbar, are characterised by a lower amount of gauche conformers. This substantial increase of conformational order is attributed to better alkyl chain packing along with a gain of intermolecular chain interactions.

Gas permeation properties of polyethylene-layered silicate nanocomposites

The inorganic cations of clays with different cation exchange capacities (CECs) have been exchanged with alkyl ammonium ions, in which 1–4 octadecyl chains are attached to the nitrogen atom. In this way, organo-montmorillonites (OMs) with different surface coverage and alkyl chain packing density were obtained. Nanocomposites of the OMs and high-density polyethylene (HDPE) were prepared and the influence of the filler surface coverage and chain packing density on the exfoliation as well as on the composite properties was studied. The morphology of the nanocomposites was investigated by wide-angle X-ray diffraction (WAX) and transmission electron microscopy (TEM). Increased packing density of the alkyl chains led to larger d-spacing (up to 4 nm) and augmenting exfoliation. Identical chemical structure of the organic monolayer and the polymer as well as full surface coverage did not lead to complete exfoliation, but to partial delamination and a mixed morphology (exfoliated layers + OM tactoids) resulted. Partial exfoliation was achieved, although no polymer intercalation was observed. The oxygen permeability of 2.8 vol% composites decreased to almost half that of the neat HDPE. Films of the nanocomposites retained the optical homogeneity of the polymer and are useful for food and drug packaging. The gas permeability was measured as a function of the filler loading and was correlated to the impermeable inorganic volume fraction of the composite. The microscopically observed orientation of the inclusions was a function of the observation scale. Comparison of the measured permeability with numerical predictions for composites of parallel oriented or misaligned disk-shaped inclusions allowed the estimation of a macroscopic average aspect ratio for the inclusions.

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.

Tensile properties of polyethylene-layered silicate nanocomposites

The sodium ions of clays with different cation exchange capacities (CEC) have been exchanged with alkyl ammonium ions, in which 1–4 octadecyl chains are attached to the nitrogen atom. Due to the different cation cross-sectional area to available area per cation ratio, the resulting organo-montmorillonites (OMs) have different organic surface coverage and alkyl chain packing density. Nanocomposites of these OMs and HDPE were prepared and the influence of the organic monolayer structure on the exfoliation of montmorillonite and the tensile properties of the composites was studied. A high cation cross-sectional area to available area ratio led to complete surface coverage and large d-spacing, favoring the dispersion of the filler. In spite of the identical chemical structure of the polymer and the organic monolayer, complete exfoliation was not attained. Partial exfoliation was achieved without a compatiblizer, which often adversely affect the mechanical properties of the composites. Enhanced exfoliation increased the elastic modulus and yield stress but decreased the yield strain and stress at break of the nanocomposites. Increased filler loading enhanced the elastic modulus but decreased all other tensile properties. The tensile properties were correlated to the volume fraction of the inorganic part of the OMs and not to the total volume of the OM. Fitting the elastic modulus data to the Halpin-Tsai equation showed that the fitting parameter in this equation is not only related to the aspect ratio of the inclusions.

The effect of fluorination: a crystallographic and computational study of mesogenic alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates.

The series of alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates (F8-n) shows a systematic change of crystal structures depending on the length of the alkyl chain: separate packing of perfluorooctyl (Rf) and alkyl (Rh) chains from each other for shorter (n=2) and longer (n=11) members, alternate packing of Rf and Rh chains for middle (n=6,7) members, and an intermediate type of packing for n=4. Semiempirical MO calculations show slightly repulsive interactions between the Rf chains, and attractive ones between Rf and Rh chains and between Rh and the core of a molecular pair. It is concluded that fluorination determines the molecular shape of the crystal structures by making the chain rigid. It is confirmed that the interactions between Rf chains are small compared with those between other moieties and that they are forced to aggregate owing to the exclusion from other moieties. Thus, the effect is dependent on the geometries and intermolecular interactions of the other moieties.

Phase behaviour and morphology of binary mixtures of DPPC with stearonitrile, stearic acid, and octadecanol at the air −water interface

The behaviour of dipalmitoylphosphatidylcholine (DPPC), mixed with stearonitrile (SN), was investigated at the air–water interface by surface pressure–area ( π– A) measurements and by direct visualisation of monolayers by Brewster angle microscopy (BAM). The π– A– X diagram of system DPPC/SN was compared with the corresponding diagrams of systems DPPC/stearic acid (SA) and DPPC/octadecanol (OD) at 20 °C. Monolayers of the three systems reach the closest packing of alkyl chains in the 0.4−0.6 range of X DPPC. Thermodynamic analysis indicates miscibility in the three binary systems with negative deviations from the ideal behaviour. Morphological features of system DPPC/SN change significantly with X DPPC and temperature in the range 10–30 °C. At 10 and 20 °C mixed monolayers form condensed states from low π all over the composition range. At 30 °C, the liquid-expanded (LE)−liquid-condensed (LC) phase transition occurs at increasing π with X DPPC. The shape and size of condensed domains change with X DPPC and π. Contrarily to the behaviour of pure components, mixed monolayers of DPPC/SN exhibit orientational order in the 0.2−0.6 mol fraction range of DPPC. BAM observation confirmed the partial miscibility indicated by G E data in a limited range of compositions at 30 °C.

Grazing Incidence Synchrotron X-ray Diffraction of Polymerizing Langmuir Monolayers

Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a Langmuir trough. Utilizing the high-intensity synchrotron X-ray radiation at the Advanced Photon Source at Argonne National Laboratory, the kinetics of film growth of OTMS as a function of the subphase pH was studied. In situ grazing incidence X-ray diffraction experiments at the air/water interface revealed that OTMS self-assembles into small domains and that the number of these ordered domains increases as OTMS undergoes hydrolysis/condensation reactions. The hexagonal packing of alkyl chains found in the OTMS monolayer is consistent with the ordering found in bulk OTMS by powder X-ray diffraction.

Effect of Flow on Human Serum Albumin Adsorption to Self-Assembled Monolayers of Varying Packing Density

Adsorption of human serum albumin (HSA) to self-assembled monolayers (SAMs) of different packing densities under dynamic conditions was investigated in situ using optical reflectometry and neutron reflectometry. When optical reflectometry was used, it was observed that HSA has a greater apparent steady-state surface concentration and higher initial rate of adsorption on a less densely packed SAM than on a densely packed SAM. These results are consistent with the contention that HSA specifically binds to the loosely packed chains. Changes in the morphology of an adsorbed protein layer with the imposition of continuing flow during adsorption were studied for the first time using neutron reflectometry. HSA interpenetrates the less densely packed SAM but sits on top of the densely packed SAM when adsorbing under persistent flow or without persistent flow. However, the orientation of the adsorbed molecules changes with the details of the flow. In the absence of persistent flow, a side-on orientation prevails. The adsorbed layers are thicker when adsorbed under persistent flow, consistent with a reasonably uniform tilting of the molecules up away from the surface. The variation in the adsorption tenacity on the SAMs with an alkyl chain packing density is greater when the adsorbed layer structure is equilibrated under quiescent conditions than when the adsorbed layer is subjected to continuing flow.

Aromatic Side Chain-Functionalized Long Chain Acid Salts: Structural Factors Influencing Their Lyotropic Liquid-Crystalline Behavior

The structural factors influencing the self-assembly of aromatic side chain functionalized long chain acid salts into lyotropic liquid crystal (LLC) phases were examined. Two sets of amphiphiles were synthesized containing two different ionic headgroups (i.e., sodium carboxylate and sodium sulfonate) and four different aromatic side chains (i.e., 4-styryl ether, 4-ethylphenyl ether, 1-naphthyl ether, and 4-phenylazophenyl ether) positioned at various locations along the amphiphile backbone. Systematic variation of the type of the aromatic side chain and its relative position on the amphiphile as a function of the two headgroups revealed that the position of the side chain on the amphiphile is the most important structural factor influencing the type and stability of the LLC phases formed. The nature of the aromatic side chain and the nature of the anionic headgroup play much more subtle roles in the LLC behavior of this particular amphiphile structural motif. Noncovalent aromatic−aromatic interactions are known to play important roles in the aggregation behavior of amphiphiles containing aromatic units along the alkyl chains; however, such interactions could not be detected spectroscopically in the LLC phases of our aromatic side chain amphiphiles. We speculate that the differences in alkyl chain interdigitation and packing efficiency arising from having the aromatic unit as a side chain rather than along the alkyl chains are responsible for this discrepancy. Control experiments with aliphatic ether side chain amphiphiles also corroborate the fact that aromatic interactions do not play a significant role in the aggregation behavior of these side chain amphiphiles. Side chain amphiphiles lacking an aromatic unit are still able to form the inverted hexagonal LLC phase, so long as the extent of branching and volume of the aliphatic side chain are sufficient.

X-ray reflectivity and diffraction studies on lipid and lipopolymer Langmuir-Blodgett films under controlled humidity.

Synchrotron X-ray reflectivity and grazing incidence diffraction were applied to study the structures of phospholipid Langmuir-Blodgett films under controlled humidity. Distearoylphosphatidylethanolamine (DSPE) and DSPE-PEO lipopolymers, in which a poly(ethylene oxide) chain with n = 8, 17, or 45 EO units is covalently linked to the polar headgroup of DSPE, were used. When the relative humidity was changed from below 2 to 97%, the phosphate headgroup in a DSPE film was hydrated with a concomitant thickness increase of 4.2 A and an electron density decrease. The swelling of the monolayer was found to be a reversible process without any significant roughening of the film. Subtle differences in the film thickness could be detected for DSPE monolayers transferred under various lateral pressures. The degree of lateral ordering of the alkyl chains in DSPE monolayers increased considerably when the humidity was raised. In the case of DSPE-PEO with eight EO units, the hydration of the monolayer was also found to be reversible, but the water uptake was larger due to the presence of the hydrophilic polymer interlayer, which is located between the substrate and the DSPE moieties. Under high humidity, the lipopolymer monolayer with n = 17 exhibited a well-defined, layered structure similar to that of DSPE-PEO with n = 8. In the case of n = 45, however, there are indications for a significant intermixing of the polymer with the DSPE moieties, probably resulting in the formation of hydrophobic nanodomains of closely packed alkyl chains that are immersed into the hydrated polymer.

Preparation of Dendritic Multisulfides and Their Assembly on Air/Water Interfaces and Gold Surfaces

Poly(propylene imine) (PPI) dendrimers of generations 1−5 have been derivatized with dialkyl sulfide chains. On water, these dendrimers form stable LB films with densely packed alkyl chains pointing to the air and cores exposed to the aqueous solution. Self-assembly of the same dendrimers from solution onto a gold surface leads to a flattened orientation of the molecules on the surface, as shown by high contact angle hystereses, high electrochemical capacitances, and low charge transfer resistances. X-ray photoelectron spectroscopy data indicate that the formation of the flattened structure can be attributed to adsorption to the gold surface of not only part of the sulfide moieties in the alkyl chains, but also part of the tertiary amines in the core. The electrochemically determined surface coverages of ferrocene-modified dendritic wedges (with similar molecular structures as those of the dialkyl sulfide PPI dendrimers) confirm the proposed flattened, surface-spread dendritic structure. In contrast, LB films of the sulfide PPI dendrimers that were transferred onto gold appear to largely remain their original, densely packed state, as for these films smaller contact angle hystereses and higher charge transfer resistances are measured than those observed for the monolayers prepared by self-assembly from solution. Additionally, the electrochemical capacitances of the transferred LB films point to a layer thickness comparable to the thickness of a densely packed didecylsulfide monolayer. The results in this study demonstrate that different surface architectures can be prepared of the same compounds by solely varying the preparation procedure.

Langmuir and Langmuir-Blodgett films of amphiphilic hexa-peri-hexabenzocoronene: new phase transitions and electronic properties controlled by pressure.

We present the synthesis as well as the structural and electronic properties of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC), which contains one alkyl substituent that is terminated with a carboxylic acid group. The molecules form well-defined Langmuir films when spread from a solution at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal two crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (zeta = 200-400 A) pi-stacked lamellae of HBC molecules tilted approximately 45 degrees relative to the surface normal is observed in the low-pressure phase. In this phase, the HBC molecules pack in a rectangular two-dimensional unit cell with a = 22.95 A and b = 4.94 A. In the high-pressure phase, coherence from the pi stack is lost. This is a consequence of stress induced by the crystallization of the substituent alkyl chains into a hexagonal lattice, which has a trimerized superstructure in one direction: a = 3 x b = 15.78 A, b = 5.26 A, gamma = 120 degrees, A = 71.9 A2 = 3 x 23.9 A2. Thin monolayer films can be transferred to solid supports by the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) with atomic resolution reveals the crystalline packing of alkyl chains in the high-pressure phase. Kelvin force microscopy (KFM) shows a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in the highly coherent low-pressure phase, which is in contrast to the localized molecular orbitals present in the high-pressure phase. The highly coherent pi stack is expected to sustain a very high charge-carrier mobility.

Changes in Protein Adsorption on Self-Assembled Monolayers with Monolayer Order: Comparison of Human Serum Albumin and Human Gamma Globulin

The role that alkyl chain packing density in a self-assembled monolayer (SAM) plays in the adsorption of protein to the SAM surface was investigated using in situ neutron reflectivity (NR) and total internal reflection fluorescence (TIRF) measurements of the adsorption behavior of human serum albumin (HSA) and human immunoglobulin G (HGG). The proteins differ particularly in the fact that HSA has specific binding pockets for alkyl chains while HGG does not. NR results show that HSA adsorbs from a 1.0 mg/mL solution as a single layer on the SAMs, with the protein interpenetrating into a less densely packed SAM, but not into a more densely packed SAM. Likewise, the kinetics of the HSA adsorption to the SAMs varies markedly with the alkyl chain packing. In contrast, both the structure of the adsorbed layer and adsorption kinetics vary little with the alkyl chain packing density in the case of HGG adsorbing from solution. HGG also does not penetrate into loosely packed SAMs. NR results reveal that the HGG adsorbs as two layers, with the layer closest to the SAM being more tightly packed. When HSA and HGG compete for adsorption sites on a SAM, HGG effectively displaces HSA from a tightly packed SAM, but does not compete effectively with HSA adsorbed tenaciously onto a loosely packed SAM.

Photocontrol of Ion Permeation through Bilayer Membranes Using an Amphiphilic Spiropyran

Addition of small amounts (≤2 mol %) of an amphiphilic spiropyran to dihexadecyl phosphate unilamellar vesicles caused rates of leakage of occluded K+ or Co(bpy)32+ to increase severalfold above those obtained in the absent of the dopant. Photoisomerization of the spiropyran to its merocyanine form by exposure to ultraviolet light caused the leak rates to revert to values nearly identical to the background levels. Subsequent exposure to visible light caused reformation of the spiro compound with an attendant increase in the leak rate. These effects were fully reversible over several photoisomerization cycles. Similar behavior was observed when egg phosphatidylcholine vesicles were used, although the magnitude of the effect at equivalent loadings was considerably less. Solvatochromic shifts observed by transient spectrophotometry revealed that, following rapid photocleavage of the spiro carbon ring, the medium surrounding the initially formed merocyanine became considerably more polar. Collectively, these results suggest that the spiro form is located relatively deeply within the aqueous/organic interface where it disrupts normal alkyl chain packing in the hydrocarbon microphase of the bilayer, thereby increasing electrolyte permeabilities. Conversion to the more polar merocyanine form is accompanied by relocation away from the membrane interior, decreasing side chain disorder and restoring the intrinsic ion impermeabilities of the bilayers.

Crystalline Polymorphism of Alkyl Chains in Supramolecular Complexes of Polyethyleneimine with Octadecanoic Acid

The supramolecular complexes of branched polyethyleneimine and octadecanoic acid (PEI(OA) x ) were prepared vie interfacial reaction and homogeneous solution complexation. The binding mode and the crystalline polymorphism of alkyl chains in PEI(OA) x were investigated by FTIR spectroscopy and WAXS. There are three kinds of OA exist in the complexes, the first one is bound by primary amines through proton transferring, the second one associated with PEI through hydrogen-bonding, and the third one free OA. The crystal structure of alkyl chains in PEI(OA) x is different from that of complexes of linear poly(4-vinylpyridine) and pentadecylphenol reported by G. ten Brinke group. Hexagonal and orthorhombic and triclinic packing of alkyl chains may occur in PEI(OA) x with 0.32 ≤ x ≤ 4.16 and the hexagonal packing still occurs at high content of OA. The ordered packing of alkyl chains is determined by the binding mode of OA in PEI(OA) x as well as the specimen-preparing process.

Melting of monolayer protected cluster superlattices

Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses.

Lipid chain dynamics in stratum corneum studied by spin label electron paramagnetic resonance

The lipid chain motions in stratum corneum (SC) membranes have been studied through electron paramagnetic resonance (EPR) spectroscopy of stearic acid spin-labeled at the 5th, 12th and 16th carbon atom positions of the acyl chain. Lipids have been extracted from SC with a series of chloroform/methanol mixtures, in order to compare the molecular dynamics and the thermotropic behavior in intact SC, lipid-depleted SC (containing covalently bound lipids of the corneocyte envelope) and dispersion of extracted SC lipids. The segmental motion of 5- and 12-doxylstearic acid (5- and 12-DSA) and the rotational correlation time of 16-doxylstearic acid (16-DSA) showed that the envelope lipids are more rigid and the extracted lipids are more fluid than the lipids of the intact SC over the range of temperature measured. The lower fluidity observed for the corneocyte envelope, that may be caused mainly due to lipid–protein interactions, suggests a major contribution of this lipid domain to the barrier function of SC. Changes in the activation energy for reorientational diffusion of the 16-DSA spin label showed apparent phase transitions around 54°C, for the three SC samples. Some lipid reorganization may occur in SC above 54°C, in agreement with results reported from studies with several other techniques. This reorganization is sensitive to the presence of the extractable intercellular lipids, being different in the lipid-depleted sample as compared to native SC and lipid dispersion. The results contribute to the understanding of alkyl chain packing and mobility in the SC membranes, which are involved in the mechanisms that control the permeability of different compounds through skin, suggesting an important involvement of the envelope in the skin barrier.

Controlled Functionalization and Multistep Chemical Manipulation of Covalently Modified Si(111) Surfaces1

A hydrogen-terminated Si(111) surface has been covalently modified by UV irradiation in the presence of ethyl undecylenate leading to a Si(111)-C10H20CO2Et surface. It is possible to carry out a diverse range of chemical manipulations of the ester group on the surface. For example, reduction with sodium borohydride provides a surface terminated with a primary alcohol. Reaction with an alkyl Grignard reagent gives a tertiary alcohol that can be acylated with acetyl chloride. Finally, hydrolysis of the ester leads to a carboxylic acid terminated surface that can be coupled to an amino acid using a standard solid phase amide coupling protocol. The surface density of the ester function can be controlled by dilution of the reacting ester with a long-chain alkene. This has the beneficial effects of minimizing the disruption of the alkyl chain packing in the monolayers and avoiding steric blocking of the ester group. It is expected that the ability to precisely control the average distance between large biomolec...