Cationic Head Groups Research Articles

Dynamics and scission of rodlike cationic surfactant micelles in shear flow.

Flow-induced configuration dynamics and scission of rodlike micelles are studied for the first time using molecular dynamics simulations in the presence of an explicit solvent and salt. Predicted dependence of tumbling frequency and orientation distribution on shear rate S agrees with mesoscopic theories. However, micelle stretching increases the distance between the cationic head groups and adsorbed counterions, which reduces electrostatic screening and increases the overall energy Φ linearly with micelle length. Micelle scission occurs when Φ exceeds a threshold value, independent of S.

Composition dependent structural organization in trihexyl(tetradecyl)phosphonium chloride ionic liquid-methanol mixtures

This article reports results from the molecular dynamics simulations on the structural arrangement of the ions and molecules in the mixtures of trihexyl(tetradecyl)phosphonium chloride ([P666,14 (+)][Cl(-)]) ionic liquid (IL) and methanol (MeOH) over the entire composition range. Effects of composition on the charge and polarity orderings have been investigated via computation of X-ray scattering structure function, S(q), and by using a partitioning scheme proposed for such multi-component mixtures. Except for the neat methanol liquid, the total S(q) shows two peaks in its intermolecular region for all the mole-fractions. The lowest q peak is dominated primarily by anion-anion, cation-anion, and methanol-anion correlations. Our results signify that the methanol bulk structure, which predominantly has short-distance characteristic correlations and is governed by polar group of methanol, is retained for xIL ≤ 0.1. Then, the mixture goes through gradual structural changes from methanol-like to the IL-like for 0.1 < xIL ≤ 0.7. The dipolar interaction between methanol molecules weakens in this range, and the structural landscape of the mixture is steered by strong ion-ion, anion-methanol, and nonpolar interactions. The IL-like structural arrangement is virtually recovered for xIL > 0.7. At all the compositions studied, while the cation head groups are predominantly solvated by anions and subsequently by methanol molecules, the polar hydroxyl group of methanol is preferentially solvated by the anions. The radial distribution functions of selected pair of atomic species have also confirmed these observations.

Physicochemical and biological characterization of 1,2-dialkoylamidopropane-based lipoplexes for gene delivery

Elucidation of the molecular and formulation requirements for efficient lipofection is a prerequisite to enhance the biological activity of cationic lipid-mediated gene delivery systems. To this end, the in vitro lipofection activity of the ionizable asymmetric 1,2-dialkoylamidopropane-based derivatives bearing a single primary amine group as the cationic head group was evaluated. The electrostatic interactions of these cationic lipids with plasmid DNA in serum-free medium were investigated by means of gel electrophoresis retardation and Eth-Br quenching assays. The effect of the inclusion of the helper lipid DOPE in the formulation on these interactions was also considered. The physicochemical properties of these lipids in terms of bilayer fluidity and extent of ionization were investigated using fluorescence anisotropy and surface potential techniques, respectively. The results showed that only the active lipid, 1,2lmp[5], existed in a liquid crystalline state at physiological temperature. Moreover, the extent of ionization of this lipid in assemblies was significantly higher that it's saturated analogues. Inclusion of the helper lipid DOPE improved the encapsulation and association between 1,2lmp[5] and plasmid DNA, which was reflected by the significant boost of lipofection activity of the 1,2lmp[5]/DOPE formulation as compared to the lipid alone. In conclusion, membrane fluidity and sufficient protonation of ionizable cationic lipid are required for efficient association and encapsulation of plasmid DNA and elicit of improved in vitro lipofection activity.

In situ growth of ZnO nanoparticles in precursor-insensitive water-in-oil microemulsion as soft nanoreactors.

Zinc oxide (ZnO) nanostructures of uniform shapes and sizes (spherical, needle-like, and acicular) were directly synthesized using a relatively precursor-insensitive water-in-n-heptane microemulsion system stabilized by a mixture of cationic and non-ionic surfactants. With this colloidal system, the synthesized ZnO possesses the highest reported surface area (76 m2 g−1) among the published reports utilizing other microemulsion systems. Such precursor insensitivity allowed studying the effect of Zn precursor:precipitating agent molar ratio (as high as 1:8) on the particle size, specific surface area, porosity, and morphology of the synthesized nanoparticles. The interaction of the cationic surfactant head groups and their Br− counter ions with Zn2+ and OH− ions is believed to play a major role in controlling the ZnO characteristics. Due to such interactions, it is believed that the nucleation processes are retarded while the growth is more dominating if compared with other microemulsion systems.

Phenol and phenolate sorption differently affect the bilayer properties of cationic DODAC vesicles

Abstract Interactions between bilayer membranes and solute molecules are important in various scientific disciplines, such as biochemistry, pharmacology and product formulation. In this contribution, the interaction between phenol (either in the neutral or the anionic phenolate form) and cationic DODAC vesicles was investigated as a function of pH and temperature. Whereas only a minor fraction (i.e. 25–40%) of the dissolved phenol was sorbed by DODAC vesicles at neutral pH, nearly complete sorption was obtained at pH conditions whereby phenol became dissociated into the phenolate anion provided that a molar exces of DODAC was applied. Interestingly, this observed pH effect was reversible and hence most sorbed phenolate was set free upon subsequent acidification. Whereas the surface charge density of the cationic DODAC vesicles was decreased by phenolate sorption at alkaline pH, still neutralization or charge reversal could not be obtained, which was in line with the extrapolated molar sorption maximum from Langmuir isotherm fitting. Moreover, the sorption efficiency was found to increase with temperature, with a maximum around the gel to liquid crystalline phase transition temperature. Diffusion NMR data revealed that this pronounced temperature effect was much more pronounced for extruded, as compared to sonicated DODAC vesicles. Whereas phenol incorporation at neutral pH decreased the phase transition temperature, the opposite effect was observed at alkaline pH. Apparently, the reduced repulsion by phenolate binding to the cationic headgroups overruled the effect of phenol or phenolate intercalation in between the octadecyl chains. Overall, these data clearly show that the sorption of an ionizable solute by DODAC vesicles is not only affected by the surface charge and fluidity of the bilayer, but that in return the latter characteristics are also affected by solute sorption. Hereby, the effect was strongly dependent on the ionic form of the solute: whereas the neutral phenol made the membranes less rigid, the opposite was observed upon sorption of the anionic phenolate.

New ester based gemini surfactants: the effect of different cationic headgroups on micellization properties and viscosity of aqueous micellar solution

A new series of ester functionalized cationic gemini surfactants having different cationic headgroups (i.e. piperidinium, pyrrolidinium, morpholinium and quaternary ammonium) have been synthesized and characterized using NMR and Mass spectroscopy. These new gemini surfactants were investigated for their micellization and viscosity properties using surface tension, conductivity, fluorescence and rheology thechniques. The physicochemical properties of the aqueous surfactant system were influenced by polarity, size and the nature of cationic headgroups as the surface, thermodynamic and viscosity properties of these gemini surfactants were found to be dependent on the type of cationic headgroup. The current research finding establishes the structure-property relationship of the surfactant molecule specifically taking into account the dominant role displayed by the nature of the cationic headgroup.

A delocalizable cationic headgroup together with an oligo-oxyethylene spacer in gemini cationic lipids improves their biological activity as vectors of plasmid DNA.

Lipoplex nano-aggregates constituted of plasmid DNA (pDNA) pEGFP-C3 and mixed cationic liposomes, consisting of several percentages of a gemini cationic lipid (GCL) of the 1,2-bis(hexadecyl imidazolium) oxyethylene series, referred to as (C16Im)2(C2O)n, with oxyethylene spacers (n = 1, 2 or 3) between the imidazolium cationic groups and the DOPE zwitterionic helper lipid, have been characterized by various biophysical and biological approaches carried out at several GCL compositions (α), and either the mass or the effective charge ratio of the lipoplex. The electrochemical study by ζ-potential confirms that the three GCLs yield a 10% lower effective charge than the nominal one, while compacted pDNA yields only a 25% effective negative charge. The SAXS study reveals, irrespective of the spacer length (n) and effective charge ratio (ρeff), the presence of two lamellar structures, i.e., one (Lα,main) in the whole GCL composition and another (Lα,DOPE,rich) with higher periodicity values that coexists with the previous one at low GCL composition (α = 0.2). The cryo-TEM analysis shows two types of multilamellar structures consisting of cationic lipidic bilayers with pDNA sandwiched between them: a cluster-type (C-type) at low α = 0.2 and a fingerprint-type (FP-type) at α≥ 0.5, both with similar interlamellar spacing (d) in agreement with the Lα,main structure determined by SAXS. Transfection efficacies (TEs) of each lipid mixture were determined in four different cell lines (HEK293T, HeLa, Caco-2 and A549) at several α and ρeff values in the absence and presence of serum (FBS). The optimized formulations (α = 0.2 and ρeff = 2.0) substantially transfect cells much better than a commercial transfection reagent, Lipofectamine 2000 and previously studied efficient lipoplexes containing other cationic head groups or spacers both in the absence and presence of serum. The activity of optimized formulations may be attributed to the combination of several factors, such as: (a) the fusogenic character of DOPE which results in higher fluidity of the lipoplexes at α = 0.2, (b) the coexistence of two lamellar structures at α = 0.2 that synergizes the TE of these lipid vectors, and mainly (c) the higher biocompatibility of the GCLs reported in this work due to the presence of two imidazolium cationic groups together with an oligo-oxyethylene spacer. The length of the spacer in the GCL seems to have less impact, although (C16Im)2(C2O)n/DOPE-pDNA lipoplexes with n = 1 and 3 show higher gene transfection than n = 2. All the optimum formulations reported herein are all highly efficient with negligible levels of toxicity, and thus, may be considered as very promising gene vectors for in vivo applications.

Influence of N-alkylpyridinium halide based ionic liquids on micellization of P123 in aqueous solutions: a SANS, DLS, and NMR study.

The isotropic micellar state of Pluronic P123 in the presence and absence of N-alkylpyridinium halide ionic liquids (ILs) is investigated using SANS, DLS, and (1)H NMR studies. The micellar structural parameters are obtained as a function of variation in alkyl chain length, anions, and concentrations of ILs by fitting the SANS scattering data with a model composed of core-shell form factor and a hard sphere structure factor of interaction. Addition of ILs decreases the micellar core, aggregation number, and hard sphere radius of P123 micelles. From quantitative analysis, we determined the amount of solvent (D2O + IL) present inside the core and the core-shell interface along with cationic head groups. This is further supported by monitoring interaction between ILs and polymer micelle using (1)H NMR spectroscopy. The results are discussed and explained as a function of concentration of C8PyCl, alkyl chain length, and anions of N-alkylpyridinium halides.

Toxicity assessment of ionic liquids with Vibrio fischeri: An alternative fully automated methodology

A fully automated Vibrio fischeri methodology based on sequential injection analysis (SIA) has been developed. The methodology was based on the aspiration of 75μL of bacteria and 50μL of inhibitor followed by measurement of the luminescence of bacteria. The assays were conducted for contact times of 5, 15, and 30min, by means of three mixing chambers that ensured adequate mixing conditions. The optimized methodology provided a precise control of the reaction conditions which is an asset for the analysis of a large number of samples.The developed methodology was applied to the evaluation of the impact of a set of ionic liquids (ILs) on V. fischeri and the results were compared with those provided by a conventional assay kit (Biotox®).The collected data evidenced the influence of different cation head groups and anion moieties on the toxicity of ILs. Generally, aromatic cations and fluorine-containing anions displayed higher impact on V. fischeri, evidenced by lower EC50. The proposed methodology was validated through statistical analysis which demonstrated a strong positive correlation (P>0.98) between assays. It is expected that the automated methodology can be tested for more classes of compounds and used as alternative to microplate based V. fischeri assay kits.

Ionic liquids entrapped in reverse micelles as nanoreactors for bimolecular nucleophilic substitution reaction. Effect of the confinement on the chloride ion availability.

In this work was explored how the confinement of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), inside toluene/benzyl-n-hexadecyldimethylammonium chloride (BHDC) reverse micelles (RMs) affects the Cl(-) nucleophilicity on the bimolecular nucleophilic substitution (SN2) reaction between this anion and dimethyl-4-nitrophenylsulfonium trifluoromethanesulfonate. The results obtained show that, upon confinement, the ionic interactions between the ILs with the cationic surfactant polar head group and the surfactant counterion modify substantially the performance of both ILs as solvents. In toluene/BHDC/bmimCl RMs, the Cl(-) interacts strongly with bmim(+) (and/or BHD(+)) in such a way that its nucleophilicity is reduced in comparison with neat IL. In toluene/BHDC/bmimBF4 RMs, an ionic exchange equilibrium produces segregation of bmim(+) and BF4(-) ions, changing the composition of the RMs interface and affecting dramatically the Cl(-) availability. These results show the versatility of this kind of organized system to alter the ionic organization and influence on reaction rate when used as nanoreactors.

Complexation behavior of mixed monolayer/mixed micelle formation between cationic noble surfactant-nonionic conventional surfactant in the presence of biocompatible polymer

Herein, we are reporting the interaction of biocompatible polymer polyvinylpyrrolidone (PVP), with gemini, pentamethylene-1,5-bis(hexadecyldimethylammonium bromide) (G5) and TX-114 surfactant mixture. Various thermodynamic parameters are evaluated with the help of regular solution theory (RST) and pseudo-phase separation model. By the addition of PVP, the critical aggregation concentration (cac) value of G5 as well as mixture decreases. A decrease in the cac value mainly credited to the interactions between the polymer and surfactant. The partial negative charged oxygen atoms, present in the amide group of PVP, were believed to be responsible for interaction with cationic head group of gemini surfactant.

Molecular dynamics simulations of DPPC/CTAB monolayers at the air/water interface.

An atomistic-level understanding of cationic lipid monolayers is essential for development of gene delivery agents based on cationic micelle-like structures. We employ molecular dynamics (MD) simulations for a detailed atomistic study of lipid monolayers composed of both pure zwitterionic dipalmitoylphosphatidylcholine (DPPC) and a mixture of DPPC and cationic cetyltrimethylammonium bromide (CTAB) at the air/water interface. We aim to investigate how the composition of the DPPC/CTAB monolayers affects their structural and electrostatic properties in the liquid-expanded phase. By varying the molar fraction of CTAB, we found the cationic CTAB lipids have significant condensing effect on the DPPC/CTAB monolayers, i.e., at the same surface tension or surface pressure, monolayers with higher CTAB molar fraction have smaller area per lipid. The DPPC/CTAB monolayers are also able to achieve negative surface tension without introducing buckling into the monolayer structure. We also found the condensing effect is caused by the interplay between the cationic CTAB headgroups and the zwitterionic phosphatidylcholine (PC) headgroups which has electrostatic origin. With CTAB in its vicinity, the P-N vector of PC headgroups reorients from being parallel to the monolayer plane to a more vertical orientation. Moreover, detailed analysis of the structural properties of the monolayers, such as the density profile analysis, hydrogen bonding analysis, chain order parameter calculations, and radial distribution function calculations were also performed for better understanding of cationic DPPC/CTAB monolayers.

Effect of ionic liquids on microstructures of micellar aggregates formed by PEO–PPO–PEO block copolymer in aqueous solution

Effect of ionic liquids (ILs) viz. pyridinium, picolinium and imidazolium halide on the micellization and the structure of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer, F127 in aqueous solution has been studied by small angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements. The interaction information between ILs and PEO/PPO has been explored by using 1H NMR and viscosity measurements. The micellar structural parameters are obtained as a function of variation in alkyl chain length, cationic head group and concentrations of ILs by fitting the SANS data with model composed of core–shell form factor and a hard sphere structure factor of interaction. Addition of ILs is found to decrease the micellar core radius, aggregation number and hard sphere radius of F127 micelles. The effect of concentration, chain length and head groups of ILs also have been studied.

The influence of 1-butanol and electrolytic background on the properties of CTAB micelles as examined using a set of indicator dyes

The ionic equilibrium of six indicator dyes, bromophenol blue, thymol blue, sulfonefluorescein, n-decyl ester and methyl ether of fluorescein, and N,N′-di-n-octadecylrhodamine was studied in CTAB micellar solutions. It was revealed that the values of the apparent ionization constants, Kaapp, of the indicators are substantially influenced by additions of 1-butanol either at low or high bulk ionic strength created by NaBr additions. Whereas the pKaapp values at ionic strength of 0.05M as a rule decrease to more or less extent as compared to the corresponding pKas in water, the penetration of the alcohol into the micellar pseudophase results in either increase or decrease in pKaapp. The dependence of pKaapp vs. log[Br−] at 0.80M n-C4H9OH allows estimating the degree of counter-ion binding under such conditions: β=0.40±0.01. The distinct deviation of the last quantity from the corresponding value, β=0.76±0.02 registered in butanol-free CTAB micellar solution is caused by the decrease in the interfacial concentration of the cationic head groups within the Stern layer. The molar fraction of 1-butanol in micelles and the partition constant of this alcohol between the aqueous phase and micelles were estimated.

Novel Cationic Gemini Surfactants and Methods for Determination of Their Antimicrobial Activity – Review

Abstract Newly series of Gemini surfactants have emerged which are composed of two hydrophobic tails and two cationic head groups linked by spacer group. Today, these cationic Gemini surfactants are attracting attention of the researchers as these surfactants are more efficient in lowering surface tension as the conventional ones. Cationic Gemini surfactants have lower critical micelle concentrations (CMC) and higher surface tension reduction as compared to those of corresponding monoalkyl ammonium salts. They have wide industrial and household applications. They can be used as emulsifiers, dispersing and anti-foaming agents etc. Cationic Gemini surfactants show good antimicrobial activity against gram-positive and gram-negative bacteria. Antimicrobial activity has also been studied against fungi and yeast species. These cationic Gemini surfactants can be used as antibacterial, antifungal, antiviral agents. This review paper deals with synthesis, antimicrobial activity, methods for determining antimicrobial activity (MIC means minimum inhibitory concentration) and applications of cationic Gemini surfactants.

Surface Modification of Citrate-Capped Gold Nanoparticles Using CTAB Micelles

The surface of nanoparticles governs the overall properties of the nanoparticles because nanoparticles have a large surface-to-volume ratio. Charge, crystallinity, and chemical properties (such as hydrophilicity) of surfaces determine the stability, structure, and reactivity of nanoparticles. In this respect, it is essential to have a capability to control and modify the surfaces of nanoparticles. Among many nanoparticles, gold nanoparticles (AuNPs) are fascinating because of their unique plasmonic properties. Surface plasmons, collective oscillation of conduction electrons, permit a wide variety of applications of AuNPs in spectroscopy, imaging, energy, and biomedicine. AuNPs are typically synthesized using citrate-reduction of Au ions in an aqueous solution, which results in the AuNP surfaces covered with citrate anions. The citrate anions on the AuNP surface are weakly bound and thus easy to replace with more strongly binding thiol ligands. Changing the surface functionality of AuNPs from citrate anions (negative charge) to carboxylate (negative charge) or hydroxyl group (hydrophilic group) has been achieved using the corresponding organofunctional thiol compounds. More challenging in modifying the surface of citratecapped AuNPs is to impart positive charges to the surface. Simple ligand exchange using thiol with cationic functional groups (e.g., NH3 ) generally causes irreversible agglomeration of AuNPs due to multiple electrostatic crosslinking. Another formidable task is to conjugate hydrophobic materials to the AuNPs prepared in an aqueous phase. In this study, we aim to resolve these two issues, which will help us expand our ability to control and modify the surface of citratecapped AuNPs for further advanced applications. The molecule of choice for converting the surface charge from negative to positive was cetyltrimethylammonium bromide (CTAB), a cationic surfactant with a positive ammonium head group and a long hydrocarbon tail. CTAB is a commonly used surface ligand in nanochemistry, particularly for the synthesis of gold nanorods or nanocubes. CTAB stabilizes Au nanorods by forming a stable bilayer on the surfaces with the cationic head groups exposed to the outside. When we added a CTAB solution to a solution of citratecapped AuNPs, we observed an anomalous concentrationdependent response of the AuNPs. Addition of a small amount of CTAB expectedly resulted in aggregation of AuNPs. The cationic CTAB head group binds to the negatively charged citrate-capped AuNPs via electrostatic interaction. Consequently, either surface charge neutralization or hydrophobic interaction causes the AuNPs to aggregate in an aqueous solution. When we increased the concentration of added CTAB, we obtained surprisingly unexpected results. Addition of a large amount of CTAB stabilized the AuNPs rather

Effect of surfactant type on platinum nanoparticle size of composite Pt/α-Al2O3 catalysts synthesized by a microemulsion method

A facile method for the generation of platinum-on-alumina hybrid materials with high-surface area is presented, employing a microemulsion-based synthesis of metal nanoparticles. Pt nanoparticles (NPs) supported on α-Al2O3 were prepared by the reduction of metal ions in water-in-oil microemulsion systems stabilized by a range of different surfactants with cationic, anionic and nonionic headgroups, namely AOT, CTAB, Tween80 and TX-100. The synthesized materials were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is demonstrated that choice of surfactant can be used to tailor the size of the generated Pt nanoparticles, and seen that surfactant charge has a determining role in this process. Pt NPs formed in microemulsion systems based on charged surfactants (AOT and CTAB) are smaller than those prepared in nonionic microemulsion systems (TX-100 and Tween80). A solvent-induced demixing process was used to cleanly obtain the hybrid materials from the reaction medium at low energy cost.

Thermodynamic study on the interaction of imidazolium salts and POE-type nonionic surfactant in the adsorbed film

The composition of the adsorbed film and the excess Gibbs energy of adsorption $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were evaluated from thermodynamic analysis of surface tensions for the 1-decyl-3-methylimidazoulium bromide (C10mimBr)–tetraethylene glycol monooctyl ether (C8E4) and 1-decyl-3-methyl-imidazolium tetrafluorobrorate (C10mimBF4)–C8E4 systems, where the counter anion of imidazolium salts is different from each other. The higher miscibility of two components compared to an ideal mixing and thus negative $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were observed in the former, which comes from the ion–dipole interaction between imidazolium cation and the oxyethylene group of C8E4. On the other hand, the lower miscibility and thus positive $$ {\widehat{g}}^{\mathrm{H},\mathrm{E}} $$ were observed for the latter. Such differences were attributed to that BF4 − forms two hydrogen bonds and has stronger affinity with the cationic head group of C10mim+ than Br−. This results in that the ion–dipole interaction between C8E4 and C10mim+ cation is diminished in the C10mimBF4–C8E4 system.

Specific Ion Effects in Amphiphile Hydration and Interface Stabilization

Specific ion effects can influence many processes in aqueous solutions: protein folding, enzyme activity, self-assembly, and interface stabilization. Ionic amphiphiles are known to stabilize the oil/water interface, presumably by dipping their hydrophobic tails into the oil phase while sticking their hydrophilic head groups in water. However, we find that anionic and cationic amphiphiles adopt strikingly different structures at liquid hydrophobic/water interfaces, linked to the different specific interactions between water and the amphiphile head groups, both at the interface and in the bulk. Vibrational sum frequency scattering measurements show that dodecylsulfate (DS(-)) ions do not detectably perturb the oil phase while dodecyltrimethylammonium (DTA(+)) ions do. Raman solvation shell spectroscopy and second harmonic scattering (SHS) show that the respective hydration-shells and the interfacial water structure are also very different. Our work suggests that specific interactions with water play a key role in driving the anionic head group toward the water phase and the cationic head group toward the oil phase, thus also implying a quite different surface stabilization mechanism.

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

A series of phenylguanidinium salts 3·X, which are linked via an alkoxy spacer either to a 4-decyloxy- or 4-cyano-substituted biphenyl mesogen, was prepared and the mesomorphism studied. A decyloxybiphenyl core and a spacer of at least C6 chain length were required for mesophase formation. Replacement of the chloride counterion by other anions like bromide or tetrafluoroborate improved the thermal stability of the mesophase. A comparison of substitution pattern (meta v. para) on the phenyl ring revealed decreased melting and clearing points for the bent cationic head group. All guanidinium ionic liquid crystals 3 displayed only smectic A (SmA) phases. A packing model is assumed where the molecules in a bilayer stack over each other in opposite direction with interdigitated terminal decyloxy groups and spacers.