Tail aggregation and domain diffusion in ionic liquids

An extended multiscale coarse-graining model for ionic liquids is used to investigate the liquid crystal-like phase in certain ionic liquids. The tail groups of the cations with a sufficient side-chain length are found to aggregate, forming spatially heterogeneous domains, due to the competition between the electrostatic interactions between the charged head groups and the anions and the collective short-range interactions between the neutral tail groups. With a sufficiently long alkyl chain at a low enough temperature, the tail domains remain relatively stable, despite the diffusion of individual ions in the liquid phase. With increasing temperature, the average tail domains begin to diffuse, while beyond a transition temperature, their average density has an almost uniform distribution, although the tail groups still form instantaneous domains.     

Effect of the tail‐group length and degree of polymerization on the order parameter of side‐chain liquid‐crystalline polymethacrylates containing phenylbenzoate mesogenic groups

The orientation of the side‐chain liquid‐crystalline polymers (LCP) containing phenylbenzoate mesogenic groups in the magnetic field was examined with ²H NMR spectroscopy. The influence of the degree of polymerization as well as the length of the methylene tail group (n = 1–4) have been established. The decrease of the order parameter S of the LCPs with an increased length of the tail group was found. The order parameter S of LCPs does not depend on the degree of polymerization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2044–2048, 2002     

Photonic superdiffusive motion in resonance radiation trapping

In this work we consider the relation between the jump length probability density function and the line shape function in resonance radiation trapping in atomic vapors. The two-sided jump length probability density function suitable for a unidimensional formulation of radiative transfer is also derived. As a side result, a procedure to obtain the Maxwell distribution of velocities from the Maxwell-Boltzmann distribution of speeds was obtained. General relations that give the asymptotic jump length behavior and the Levy flight parameter mu for any line shape are obtained. The results are applied to generalized Doppler, generalized Lorentz, and Voigt line shape functions. It is concluded that the lighter the tail of the line shape function, the less heavy the tail of the jump length probability density function, although this tail is always heavy, with mu < or =1.     

Effect of chain length and asymmetry on material properties of bilayer membranes

Dissipative particle dynamics is used to extract the material parameters (bending and area stretch moduli) of a bilayer membrane patch. Some experiments indicate that the area stretch modulus of lipid vesicles varies little as the chain length of the lipids composing the bilayer increases. Here we show that making the interactions between the hydrophilic head groups of the model amphiphiles proportional to the hydrophobic tail length reproduces the above result for the area stretch modulus. We also show that the area stretch modulus of bilayers composed of amphiphiles with the same number of tail beads but with asymmetric chains is less than that of bilayers with symmetric chains. The effects on the bilayer density and lateral stress profiles of changes to the amphiphile architecture are also presented.     

Two-dimensional packing patterns of amino acid surfactant and higher alcohols in an aqueous phase and their associated packing parameters

An amino acid surfactant, monosodium N-stearoyl-L-glutamate (MSSG), was assumed to associate with higher alcohols (HAs) in 1:1 molar ratio in an aqueous phase, and the packing parameters of the 1/1 MSSG/HA associates, Pa, were calculated by molecular dynamics (MD). Pa is defined as At1(l(t1)+l(t2))/a(h)l(t1), where At1 stands for the cross-sectional area of the MSSG tail, l(t1) for the length of the MSSG tail, l(t2) for the length of the HA tail, and a(h) for the area of the MSSG head group. The Pa value increased from 0.83 to 1.02 as the HA tail length (l(t2)) increased from C14 to C22. Associates of HAs having longer tails with MSSG are likely to fit better into bilayers because Pa is closer to 1 when l(t2) is longer. In the graphical results of the MD simulation for the association, however, a steric hindrance was found between the head groups of MSSG/HA when l(t2) was > or = C19. Based on this result, HA was classified into short-chain HA (SCHA, l(t2) < C19) and long-chain HA (LCHA, l(t2) > or = C19), and several possible packing units, composed of compositional combinations of MSSG, SCHA, and LCHA, were proposed. The packing unit is a building block which could constitute bilayers, and it is composed of variable compositional combinations of MSSG/HA. Assuming that SCHA associates with MSSG, the packing parameter, Punit, was calculated in a water box by MD for each packing unit. Punit is the packing parameter of a packing unit and it is defined as Vu/l(u)a(u), where Vu is the tail volume of the packing unit, l(u) is the chain length, and a(u) is the head area. For the calculation, stearyl alcohol (C18-OH, SA) was chosen as a SCHA and behenyl alcohol (C22-OH, BA) as a LCHA. When the compositional ratio MSSG:SCHA:LCHA was 1:1:1, Punit was around 1. The packing unit having Punit of around 1 formed a colloidally stable suspension for 30 days and its aggregate was a lamellar structure. However, the other packing units, for which Punit deviates significantly from 1, precipitated out in their suspensions and showed no evidence of a lamellar structure. According to the graphical MD simulations for the compositional MSSG/SCHA/LCHA associations in bilayers, vertical steric hindrance was found between LCHAs when Punit deviated significantly from 1. The steric hindrance would prevent the packing units from forming a stable bilayer and induce precipitation in the suspensions. Therefore, a proper combinational ratio of MSSG:LCHA:SCHA would play a major role in forming a lamellar structure.     

Enhanced thermal response of the SAd1 layer thickness in highly fluorinated thermotropic liquid crystals

X-ray studies of a homologous series of rod-shaped liquid crystal molecules with one tail perfluorinated and the other protonated, reveal large decreases in the smectic A layer spacing with increasing temperature. These materials form unique dimer phases in which the smectic layer spacing is dependent on the length of the perfluorinated tail and independent of the length of the protonated tail. The chain statistics of the perfluorinated tail significantly influence the thermal expansion coefficient since the length of the fluorinated tail defines the smectic layer spacing. Thermal expansion coefficients for the layer spacing observed here are negative and nearly an order of magnitude greater than for typical protonated rod-shaped thermotropic liquid crystals in the SA phase.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB[6] and CB[7] can form [2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB[6] at guest molecules changed from the tail to the middle part, while CB[7] remained located over the tail chain. As CB[6] and CB[7] were added in HBPB-8 aqueous solution, a [3]pseudorotaxane was formed by the inclusion of the internal middle site in CB[6] and the tail chain in CB[7].     

Isocratic mixed mode liquid chromatographic separation of phospholipids with octadecylsilane-silica stationary phases

Molecular species of phosphatidylcholine, phosphatidylethanolamine and phosphatadic acid were resolved by isocratic reversed phase high performance liquid chromatography (HPLC) using mobile phases of methanol-isopropanol containing para-toluenesulfonic acid (p-tsa). Separation by both non-polar fatty acid chain length and by polar head group functionality was achieved concurrently upon a commercially available octadecylsilane (C18) column endcapped with trimethylsilane (C1) groups. Using a mobile phase of 97.5:2.5 methanol:isopropanol with 7OmMpara-toluenesulfonic acid (p-tsa) at a pH of approximately 1, twelve phospholipid species comprised of four tail group classes (dilauroyl-,dimyristoyl-, dipamitoyl- and distearoyl-) and three head group speciations (phosphatidylcholine, phosphatidylethanolamine and phosphatadic acid) were separated. The column was then exposed to the acidic mobile phase for 48 hours continuously during which the bound phase underwent severe acid-induced hydrolysis, after which the separation of the twelve analytes resulted in the separation of the phospholipid species by non-polar tail group alone. The experimental results are discussed in terms of potential separation mechanisms including dependency of the separation on adsorption of the counter ion into the stationary phase, residual acidic silanol group interactions, and potential interactions of the surface active phospholipids with C1 groups.     

Correlation of structure and phase behaviour for a series of modular, chiral liquid crystal diacrylates based on lactic acid

The mesogenic properties of a family of chiral liquid crystal (LC) diacrylates based on a 4-[4-(1R-methyl-2-hydroxyethoxy)phenyl]phenyl 4-hydroxybenzoate core were studied as a function of different tail lengths. In general, this family of LCs was found to exhibit a strong preference for adopting the chiral smectic A phase. Systematic variation of the alkyl spacer lengths on either side of the chiral core revealed that the onset of smectic A behaviour is highly sensitive to the length of the tail adjacent to the chiral unit. However, no correlation between phase transition temperatures and the length of the spacer on the other side of the core was observed. With a fixed spacer length on the chiral side of the core, systematic changes in the length of the other tail resulted in the formation of a monotropic smectic B phase and an increased tendency to supercool.     

Correlation of structure and phase behaviour for a series of modular,chiral liquid crystal diacrylates based on lactic acid

The mesogenic properties of a family of chiral liquid crystal (LC) diacrylates based on a 4-[4-(1R-methyl-2-hydroxyethoxy)phenyl]phenyl 4-hydroxybenzoate core were studied as a function of different tail lengths. In general, this family of LCs was found to exhibit a strong preference for adopting the chiral smectic A phase. Systematic variation of the alkyl spacer lengths on either side of the chiral core revealed that the onset of smectic A behaviour is highly sensitive to the length of the tail adjacent to the chiral unit. However, no correlation between phase transition temperatures and the length of the spacer on the other side of the core was observed. With a fixed spacer length on the chiral side of the core, systematic changes in the length of the other tail resulted in the formation of a monotropic smectic B phase and an increased tendency to supercool.     

High-performance affinity chromatography of messenger RNA.

A 50-mer of thymidylic acid, (dT)50, was coupled to silica inside prepacked columns using the N-hydroxysuccinimide chemistry. The resulting (dT)50-silica columns were used to resolve oligomers of adenylic acid, (dA)19-24, and to separate poly(A) mRNA (messenger RNA) from Saccharomyces. Oligomers which differed in length by a single nucleotide base were readily resolved. Using either (dT)50- or (dT)18-silica, poly(A) mRNA could be purified in as little as 8 min. The poly(A) mRNA isolated appeared to be full length and could be used directly for T4 RNA ligase and RNAse A and T1 enzymatic reactions. The (dT)50-silica column was used to fractionate total poly(A) mRNA by tail length. While the separation was primarily due to poly(A) tail length, most fractions appeared to contain multiple tail lengths. Whether this represents an intrinsic feature of the RNA or a limitation of the method is discussed. These studies show that polynucleotides in the kilobase size range can be separated rapidly and with good resolution on DNA-silica.     

模型高分子尾型链的构象统计理论

基于金刚石格点上受限于半空间的随机行走，借概率论中的反射原理推导出了模型高分子尾型链的构象分布．得到了链长为Ｎ的模型尾型链的允许构象数Ｃ１（Ｎ）和末端距分布函数．在四选择模型中，发现Ｃ１（Ｎ）／４Ｎ与Ｎ－１／２成正比．模型尾型链在与壁正交方向上的均方末端距分布与平行方向比较扩张至２倍，而后者与自由链对应的分量相等．这些解析结果得到了精确计数和ＭｏｎｔｅＣａｒｌｏ模拟计算机“实验”的支持．     

The effect of spacer length on the polymerization of diacetylenes in sams on gold surfaces

The influence of the molecular architecture of diacetylene disulfides on the structure and polymerizability of self assembled monolayers on gold was investigated. The position of the diacetylene group was varied systematically by adjusting the length of the tail and the spacer. For the structures studied, the longer the tail, the higher the akyl chain crystallinity as sensed by IR spectroscopy, and the higher the polymerization efficiency as probed by resonance Raman spectroscopy. The influence of the spacer on the polymerizability is more complicated. It is conjectured that the spacer reduces the lattice strain due to the misfit of the arrangement of the anchor groups and the arrangement of the diacetylene units and the polydiacetylene backbone, respectively. Simple modeling reveals that the number of the methylene units (odd or even) and the overall length both influence the arrangement. The highest polymerization efficiency is found for the longest, odd numbered spacer.     

Synergistic interactions in the mixed micelles of cationic gemini with zwitterionic surfactants: fluorescence and Krafft temperature studies

Pyrene fluorescence and Krafft temperature measurements have been carried out for various combinations of cationic gemini (m-2-m) with zwitterionic surfactants by changing the length of the hydrophobic tail over the whole mixing range. The results have been evaluated by using the regular solution theory. All the mixtures of cationic gemini+zwitterionic surfactants indicate the presence of synergistic interactions which largely decrease at higher hydrophobicity of both components. A greater amount of gemini component in the mixed micelles induces stronger synergism which reduces with the increase in the length of hydrophobic tail of the gemini component. The Krafft temperature measurements also indicate the presence of strong synergistic interactions, which decrease with increase in the length of hydrophobic tail of both components.     

Effects of the chemical structure of surfactants on the stability of naphthenic oil-based metalworking fluids

In this study,various nonionic surfactants(NS) with different ethylene oxide(EO) numbers and tail lengths and its binary blends with anionic surfactants(AS) were used as emulsifiers for naphthenic oil to form the microemulsion metalworking fluids(MWFs),and the effects of them on the stability of the emulsion system were investigated by formulation triangle method.The results indicated that binary complex surfactants of NS and AS as emulsifiers exhibited better emulsifying effect than that of single NS.NS with different EO numbers and tail lengths presented various emulsifying effects.NS(EO=10)exhibited the greatest number of stable formulations,especially the TX-10,but no linear relationship existed between the number of stable formulations and the tail length of NS.In addition,aromatic primary alcohol ethoxy late(APAE) series surfactants containing benzene groups similar to the cycloalkanes in the naphthenic oil so that presented the best emulsifying affect and the greatest number of stable formulations.The co-surfactant of sodium dodecyl benzene sulfonate(SDBS) binary blends with NS exerted the best synergistic effect,and the stable formulations numbers were ranged from 5 to 7,next sodium stearate(SS) comes last followed by sodium dodecyl sulfate(SDS-1) and sodium dodecyl sulfonate(SDS-2).     

Persistence length of wormlike micelles composed of ionic surfactants: self-consistent-field predictions

The persistence length of a wormlike micelle composed of ionic surfactants C(n)E(m)X(k) in an aqueous solvent is predicted by means of the self-consistent-field theory where C(n)E(m) is the conventional nonionic surfactant and X(k) is an additional sequence of k weakly charged (pH-dependent) segments. By considering a toroidal micelle at infinitesimal curvature, we evaluate the bending modulus of the wormlike micelle that corresponds to the total persistence length, consisting of an elastic/intrinsic and an electrostatic contribution. The total persistence length increases with pH and decreases with increasing background salt concentration. We estimate that the electrostatic persistence length l(p,e)(0) scales with respect to the Debye length kappa(-1) as l(p,e)(0) approximately kappa(-p) where p approximately 1.98 for wormlike micelles consisting of C(20)E(10)X(1) surfactants and p approximately 1.54 for wormlike micelles consisting of C(20)E(10)X(2) surfactants. The total persistence length l(p,t)(0) is a weak function of the head group length m but scales with the tail length n as l(p,t)(0) approximately n(x) where x approximately 2-2.6, depending on the corresponding head group length. Interestingly, l(p,t)(0) varies nonmonotonically with the number of charged groups k due to the opposing trends in the electrostatic and elastic bending rigidities upon variation of k.     

Asymmetrical features of mass spectral peaks produced by quadrupole mass filters

The individual mass spectral peaks produced by a quadrupole mass spectrometer (QMS) are asymmetric; they exhibit a 'tail' on the low mass side. In some cases a definite structure is observed in the tail. We show that the tail structure is a consequence of the use of circular electrodes. An extreme case of an experimentally observed QMS mass peak with a distinct tail structure is shown and the general form is reproduced using our numerical model. The effect of instrument resolution, length, operating frequency, ion energy, mass and ion source aperture upon the tail structure are considered. Results show that extensive long tails originate mainly from ions that enter the mass filter at a relatively large distance from the QMS axis; also no significant tail is produced in the case of ideal hyperbolic form electrodes of finite length.     

Nonionic surfactants with linear and branched hydrocarbon tails: compositional analysis, phase behavior, and film properties in bicontinuous microemulsions

Nonionic alcohol ethoxylates are widely used as surfactants in many different applications. They are available in a large number of structural varieties as technical grade products. This variety is mainly based on the use of different alcohols, which can be linear or branched and contain primary, secondary, or tertiary OH groups. Technical grade products are poorly defined as they are composed of alcohol mixtures being different in chain length and structure. On the other hand, monodisperse alcohol ethoxylates are commercially available; however, these surfactants exist only with primary and linear alcohols. In the field of microemulsion research the monodisperse alcohol ethoxylates are widely used. The phase behavior and film properties of these surfactants were studied intensively with respect to the size of the hydrophilic and hydrophobic moieties. Due to the lack of appropriate model surfactants until now, there is little information on how the structure of the hydrocarbon tail influences the microemulsion behavior. To examine structural influences, we synthesized a series of surfactants with the composition C10E5 and having different linear and branched hydrocarbon tails. The surfactants were monodisperse with respect to the hydrocarbon tail but polydisperse with respect to the ethoxylation degree. However, a detailed characterization showed that they were similar concerning the average ethoxylation degree and EO chain length distribution. The phase behavior was investigated for bicontinuous microemulsions, and the film properties were analyzed by small-angle neutron scattering (SANS). Our results show that the structure of the hydrocarbon tail strongly influences the microemulsion behavior. The most efficient surfactant is obtained if the hydrocarbon tail is linear and the hydrophilic group is attached in the C-1 position. Surfactants having the hydrophilic group bound to the C-2 or C-4 position or which contain a branched hydrocarbon tail are less efficient and exhibit in most cases visibly lower phase inversion temperatures. Both the efficiency and temperature behavior mainly can be explained on the basis of increased bulkiness of the branched structures compared to the fully linear version. The phase behavior results are largely confirmed by the SANS investigations. Those results show that the fully linear surfactant exhibits the most rigid interfacial film. In additional experiments this surfactant was compared with its monodisperse analogue. According to the phase diagrams, the surfactant having the polydisperse hydrophilic moiety is drastically more efficient although the film stiffnesses are almost identical.     

Synthesis and characterization of new ferroelectric liquid crystals containing oligomethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligomethylene spacers (n 3, 4, 6, 8) were synthesized and characterized. The chiral moieties are the (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry and optical polarized microscopy. Spontaneous polarization (P) of the liquid crystals was measured by an automated polarization tester over a wide temperature range. The mesomorphic properties are discussed as a function of spacer length and position of the chiral centre. It is found that the phase transition temperature decreases with increasing number of oligomethylene units. The phase behaviours of the LC with (S)-1-methylheptoxycarbonyl are more complex than those with (S)-2-methylbutoxycarbonyl, due to the flexible tail in the former. Moreover, the spontaneous polarization depends mainly on the position of the chiral centre, with Ps 15 nC cm 2 and Ps 60 123 nC cm 2 for the series s with (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups, respectively.     

Side-on fixed polysiloxanes and 'diluted' copolysiloxanes with nematic and smectic C phases

The influence of the molecular parameters of side-on fixed liquid crystal polymers (LCP), namely the spacer length, the aliphatic tail length and the proportion of mesogenic groups ('dilution' effect), on the mesomorphic, thermodynamic and structural properties of the nematic and smectic C phases has been studied. This shows in particular the stabilization of the SmC phase on increasing the terminal chain lengths. Moreover, this paper provides evidence for an original behaviour in the SmC organization of side-on LCP with a nonstructural monotonous evolution of the thermodynamic and properties with the 'dilution'. Finally, a comparison with side-end LCP can be established : the question of the participation of the polymer backbone in the smectic organization in side-on LCP is raised.