Alkane Tail Research Articles

Nanoparticle-Assisted Liquid Crystal Droplet Sensors Enable Analysis of Low-Concentration Species in Aqueous Medium.

We introduce nanoparticle-assisted liquid crystal (LC) droplet-based sensors that allow determination of low-level concentrations of aqueous soluble species. The silica nanoparticles functionalized with mixed monolayers composed of two distinct groups, hydrophobic alkane tail- and charged group-terminated silanes, facilitated ternary physical interactions between the model analytes (methylene blue (MB) or methyl orange (MO)) and the nematic mesogens 5CB (4-cyano-4'-pentylbiphenyl), and the interfacial species of the nanoparticle. The response of the LC droplets was measured upon nanoparticle adsorption as a function of analyte concentration, which was characterized by the optical determination of the configuration distributions of the LC droplets. We highlight the importance of the charging and the composition of the nanoparticle interfaces for analytical purposes that allow accurate determination of the concentration of the analytes on the order of 0.01 ppb. Such a low concentration corresponds to a low interfacial coverage of nanoparticles, indicating the promisingly high sensitivity of the sensor platform to target analytes. Distinct from the past examples of the LC-based sensors, the nanoparticle-assisted LC sensors allow detection of the species that do not directly cause an ordering transition at the LC-water interfaces, which allow a broader range of analytical targets. The sensor platform that we report herein can be easily tunable for a range of target molecules and will find use in the determination of a wide range of micropollutants in aqueous environments.

Phospholipid Epitaxial Assembly Behavior on a Hydrophobic Highly Ordered Pyrolytic Graphite Surface.

Supported lipid bilayers (SLBs) are excellent models of cell membranes. However, most SLBs exist in the form of phospholipid molecules standing on a substrate, making it difficult to have a side view of the phospholipid membranes. In this study, the phospholipid striped lamella with the arrangement of their alkane tails lying on highly ordered pyrolytic graphite (HOPG) was constructed by a spin coating method. Atomic force microscopy and molecular dynamics simulations are utilized to study the self-assembly of phospholipids on HOPG. Results show that various phospholipids with different packing parameters and electrical property are able to epitaxially adsorb on HOPG. 0.1 mg/mL Plasm PC (0.1 mg/mL) could form a striped monolayer with a width of 5.93 ± 0.21 nm and form relatively stable four striped layers with the concentration increasing to 1 mg/mL. The width of the DOPS multilayer is more than that of electroneutral lipids due to the static electrical repulsion force. This universal strategy sheds light on direct observation of the membrane structure from the side view and modification of 2D materials with amphiphilic biomolecules.

What promotes smectic order: Applying mean-field theory to the ends.

Not every particle that forms a nematic liquid crystal makes a smectic. The particle tip is critical for this behavior. Ellipsoids do not make a smectic, but spherocylinders do. Similarly, only those N-CB alkylcyanobiphenyls with sufficiently long (N≥8 carbons) alkane tails form smectics. We understand the role of the particle tip in the smectic transition by means of a simple two-dimensional model. We model spherocylinders by "boubas" with rounded tips, and ellipsoids by "kikis" with pointed tips. The N-CB molecules are modeled by a small body with a polymer tail. We find that rounded tips and longer polymer tails lead to a smectic at lower densities by making the space between layers less accessible, destabilizing the nematic.

Diffusion of hydrophilic to hydrophobic forms of Nile red in aqueous C12EO10 gels by variable area fluorescence correlation spectroscopy.

Solute diffusion within lyotropic liquid crystal gels prepared from a series of water and decaethylene glycol monododecyl ether (C12EO10) mixtures was explored by variable area fluorescence correlation spectroscopy. Aqueous C12EO10 gels were prepared in concentrations ranging from 55 : 45 to 70 : 30 wt% of surfactant and water. Small angle X-ray scattering revealed that these gels comprise hexagonal mesophases of cylindrical micelles. Micelle spacing was found to decrease with increasing C12EO10 concentration. Three different Nile red (NR) dyes were employed as model solutes and were separately doped into the gels at nanomolar levels. These include a hydrophilic form of NR incorporating an anionic sulfonate group (NRSO3-), a hydrophobic form incorporating a fourteen-carbon alkane tail (NRC14), and commercial NR as an intermediate case. FCS data acquired from the gels revealed that NRSO3- diffused primarily in 3D. Its diffusion coefficient exhibited a monotonic decrease with increasing gel concentration and micelle packing density, consistent with confinement of its motions by its exclusion from the micelle cores. NRC14 exhibited the smallest diffusion coefficient, most likely due to its larger size and enhanced interactions with the micelle cores. NR yielded an intermediate diffusion coefficient and the most anomalous behavior of the three dyes, attributable to its facile partitioning between core and corona regions, and greater participation by 1D diffusion. The results of these studies afford an improved understanding of molecular mass transport through soft-matter nanomaterials like those being developed for use in drug delivery and membrane based chemical separations.

Research Progress on the Synthesis of Different Types of Gemini Surfactants with a Functionalized Hydrophobic Moiety and Spacer

AbstractGemini surfactants are advantageous over conventional surfactants as they find applications as antimicrobial agents, corrosion inhibitors, enhanced oil recovery, capping agents for nanoparticle synthesis, drug delivery agents, etc. This article provides a detailed overview of synthetic procedures for various types of geminis and the fundamental physicochemical parameters of these systems. This includes a discussion on the cationic geminis based on various groups such as biphenyl, amide (with/without alkane tail), binol, quaternary ammonium (with/without hydroxyl group as a spacer), morpholinium, pyridinium, amide (with/without alkane tail), ester quaternary ammonium, and imidazolium (spacer with/without hydroxyl group and thioether/ester) types. Further, it is focused on anionic geminis with carboxylate, borate, and sulfonate head groups. Zwitterionic geminis such as betaine surfactants, cocogem surfactants based on dodecylisopropylol amine and dicarboxylic acid, diester type, sulphobetaine gemini based on S‐triazine, zwitterionic hetero gemini containing ammonium and carboxylate head groups, zwitterionic gemini containing quaternary ammonium and a sulfate group, gemini containing phosphodiester anion and quaternary ammonium salt are also reviewed. Lastly, synthesis of nonionic geminis from sunflower oil, amide‐based nonionic, sulfonamide nonionic, N,N’‐diethylaminedialkyldiamide nonionic gemini, sugar‐based gemini, and nonionic gemini with sulfonate spacer have been discussed.

On the role of alkanethiol Au complex in the formation of gold deposits; an in-silico approach

The association of organic matter with ore minerals has been observed for many years. Over decades, many speculations have been proposed to define the possible roles that organic matter might have played in the ore genesis. In this paper, we performed a series of electronic calculations, and molecular dynamic simulations to investigate the behaviour of organic matter in ore-forming environments.Ab initio calculations revealed that variation in the length of alkane tails has a limited impact on the geometry of the Au-ligand, and the strength of AuS bonds. MD and metadynamics simulations proved that the organic Au-ligand could be dissolved in the organic phase but it mostly pose at the interface of the organic-inorganic phase, in a way that its ionic head place in the aqueous solution and its alkane tails dissolved in organic matter. These observations suggest the organic-gold ions (and organic ligands of some other metals) are preferentially accumulated in the narrow interface of the aqueous/petroleum phase. Overall, although experimental results suggest that organic matter might have a role in gold transport, this study proves that organic gold ligand is concentrated only at the interface between the organic and aqueous phases and therefor, the organic phase alone cannot carry gold. This finding is also consistent with field observations, which suggest that gold concentrations in oil are relatively low.

Interactions of boron nitride nanosheet with amino acids of differential polarity

Free amino acids represent a category of different biomolecules in the blood plasma, which bond together to make up larger organic molecules such as peptides and proteins. Their interactions with biocompatible nanoparticles are especially important for plasma-related biomedical applications. Among the various nanomaterials, the applications of carbon and boron nitride-based nanotubes/nanosheets have shown a huge increase in recent years. The effect of molecular polarity on the interaction between a boron nitride nanosheet (BNNS) and amino acids is investigated with quantum mechanical calculations by density functional theory (DFT), classical MD simulations, and well-tempered metadynamics simulations. Four representative amino acids, namely, alanine (Ala), a nonpolar amino acid, and aspartic acid (Asp), lysine (Lys) and serine (Ser), three polar amino acids are considered for their interactions with BNNS. In DFT calculations, the values of the adsorption energies for Lys-BNNS and Ser-BNNS complexes are − 48.32 and − 32.89 kJ/mol, respectively, which are more stable than the other cases. Besides, the adsorption energy calculated confirms the exergonic reactions for all investigated systems; it implied that the interaction is favorable electronically. The MD results show that the LYS molecules have a higher attraction toward BNNS because of its alkane tail in its side chain, and the ASP revealed the repulsion force originating from its COO– group. All the results are confirmed by free energy analyzes in which the LYS showed the highest adsorption free energy at a relatively farther distance than other complexes. In fact, our results revealed the contribution of functional groups and backbone of the amino acids in the adsorption or repulsion features of the studied systems.

Preparation of Hierarchically Porous Metal-Organic Frameworks via Slow Chemical Vapor Etching for CO2 Cycloaddition.

Hierarchically porous metal-organic frameworks (HP-MOFs) are a class of promising functional material with micropores, mesopores, and/or macropores, which can address the issue of slow mass transfer and less exposed active sites for primitive microporous MOFs. Despite many attempts that have been achieved through a variety of techniques to date, there is still a myriad of spaces that urgently need to be exploited. In this work, we report the novel synthesis of HP-MOFs via slow chemical steam etching. The preparation process can be subtly achieved using water vapor as an etchant; meanwhile, the addition of ethanol into the vapor atmosphere is carried out because it can stabilize the MOF framework well with its hydrophobic alkane tails, thereby slowing the etching rate toward MOFs, successfully realizing the controllable etching manner of MOF components. Furthermore, the joint influence of the water content and etching temperature on the MOF backbone structure etched has thus been investigated in detail. Impressively, we can harvest desired HP-MOFs with the retained crystalline structure at a water content of 50% and an etching temperature of 120 °C. The resulting HK-120/50 product etched exhibits excellent catalytic activity and stability in [2 + 3] cycloaddition of CO2 than pristine MOF, which can be attributed to the more exposure of active sites and the acceleration of mass transportation across the entire MOF skeleton. Noteworthy, the strategy proposed in this study may be extended to other HP-MOF construction systems due to the lability of most MOFs toward the chemical water vapor.

Ultrafast excited state dynamics of provitamin D3 and analogs in solution and in lipid bilayers.

The photochemical ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D3) is responsible for the light-initiated formation of vitamin D3 in mammalian skin membranes. Visible transient absorption spectroscopy was used to explore the excited state dynamics of DHC and two analogs: ergosterol (provitamin D2) and DHC acetate free in solution and confined to lipid bilayers chosen to model the biological cell membrane. In solution, the excited state dynamics of the three compounds are nearly identical. However, when confined to lipid bilayers, the heterogeneity of the lipid membrane and packing forces imposed on the molecule by the lipid alter the excited state dynamics of these compounds. When confined to lipid bilayers in liposomes formed using DPPC, two solvation environments are identified. The excited state dynamics for DHC and analogs in fluid-like regions of the liposome membrane undergo internal conversion and ring-opening on 1 ps-2 ps time scales, similar to those observed in isotropic solution. In contrast, the excited state lifetime of a subpopulation in regions of lower fluidity is 7 ps-12 ps. The long decay component is unique to these liposomes and results from the structural properties of the lipid bilayer. Additional measurements in liposomes prepared with lipids having slightly longer or shorter alkane tails support this conclusion. In the lipid environments studied, the longest lifetimes are observed for DHC. The unsaturated sterol tail of ergosterol and the acetate group of DHC acetate disrupt the packing around the molecule and permit faster internal conversion and relaxation back to the ground state.

A Parameterization of Cholesterol for Mixed Lipid Bilayer Simulation within the Amber Lipid14 Force Field.

The Amber Lipid14 force field is expanded to include cholesterol parameters for all-atom cholesterol and lipid bilayer molecular dynamics simulations. The General Amber and Lipid14 force fields are used as a basis for assigning atom types and basic parameters. A new RESP charge derivation for cholesterol is presented, and tail parameters are adapted from Lipid14 alkane tails. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers are simulated at a range of cholesterol contents. Experimental bilayer structural properties are compared with bilayer simulations and are found to be in good agreement. With this parameterization, another component of complex membranes is available for molecular dynamics with the Amber Lipid14 force field.

Solubility of Carbon Dioxide in Pentaerythritol Hexanoate: Molecular Dynamics Simulation of a Refrigerant-Lubricant Oil System.

We have investigated the solubility and the solvation structure between a refrigerant (carbon dioxide, CO2) and a lubricant oil (pentaerythritol hexanoate, PEC6) by molecular dynamics simulations. First, to investigate the solubility, we calculated the vapor-liquid equilibrium pressure. The chemical potential of the liquid phase and the gas phase were calculated, and the equilibrium state was obtained from the crossing point of these chemical potentials. The equilibrium pressures agreed well with experimental data over a wide range of temperatures and mole fractions of CO2. Second, the solvation structure was also investigated on a molecular scale. We found the following characteristics. First, the tails of the lubricant oil are relatively rigid inside the ester groups but flexible beyond. Second, CO2 molecules barely enter the lubricant core as delimited by the ester groups. Third, the double-bonded oxygen atoms of the ester groups are good sorption sites for CO2. Fourth, only a few CO2 molecules are attached to more than one carbonyl oxygen simultaneously. Finally, there is also significant unspecific sorption of CO2 in the alkane tail region. These results indicate that increasing the size of the rigid lubricant core would probably decrease the solubility, whereas increasing the number of polar groups would increase it.

Ionic Liquid Structure in Thin Films

Room Temperature Ionic liquids present an exciting opportunity to examine surface confined thin fluid films and extended interfacial systems that may prove useful for electrochemical applications. This work explores the behavior of thin (1-5000 nm) films and bulk RTIL interfaces in contact with air and silver metal surfaces. Thin films are introduced by a highly controlled dynamic wetting process. Bulk interfaces are prepared by immersing the metal electrode into an IL solution. Data from ellipsometry, vibrational spectroscopy, contact angle, and electrochemical measurements are reported for a series of Cn-3-methylimidazolium based liquids with varying length alkane tails. Results correlate chemical nature of the liquid and substrates with resulting molecular structure of the interface and behavior of the wetting film. Descriptors include chemical interactions i.e. hydrophobic forces, hydrogen bonding, van der Waals, electrostatics, etc.

Supramolecular Precursors for the Synthesis of Anisotropic Cu2S Nanocrystals

Copper alkanethiolates are organometallic precursors that have been used to form Cu2S nanodisks upon thermal decomposition. Here, we demonstrate that molecular assembly of Cu alkanethiolates into an ordered liquid crystalline mesophase plays an essential role in templating the disk morphology of the solid-state product. To examine this templating effect, we synthesize Cu alkanethiolate precursors with alkane tails of varying chain length and sterics. We demonstrate that short chain precursors produce two-dimensional (2D) nanosheets of Cu2S, while longer-chained variants produce Cu2S nanodisks exclusively. This work provides new insights into the use of liquid crystalline phases as templates for nanocrystal synthesis and as a potential route for achieving highly anisotropic inorganic nanostructures.

Balance of Forces in Self-Assembled Monolayers

Self-assembly represents a typical example of bottom-up nanotechnology where the formation of macroscopic structures is driven by forces operating at the molecular level. Under situations where several forces of comparable strength are competing with one another, the stable structure achieved depends on the fine balance of all the forces. A good example is the formation of self-assembled monolayers (SAMs) of non-branched alkanethiol molecules, CH3–(CH2)n–SH, on the surface of gold, where the interaction between the S headgroup and the gold substrate competes with the force acting between the alkane tails. Here we demonstrate that, if the tail is short, n ≤ 2, the S–Au interaction dictates the structure of the SAM. As the tail becomes longer than n = 2, the enhanced tail–tail interaction is able to displace the S headgroup. Our finding highlights the importance of the “weak” van der Waals interaction for molecular assembly even in the presence of chemical bonding.

Steered Molecular Dynamics Studies of the Potential of Mean Force for Peptide Amphiphile Self-Assembly into Cylindrical Nanofibers

Steered molecular dynamics (SMD) simulations were applied to determine the potential of mean force for the self-assembly of peptide amphiphile (PA) nanofibers, specifically considering a single PA adding to a growing cylindrical nanofiber at 310 K. It is found that the free energy, enthalpy, and entropy differences for this assembly process are -67 kcal/mol, -71.5 kcal/ml, and -14.5 cal/(mol K), respectively, and therefore that enthalpy provides the driving force for self-assembly to form a fiber. A pairwise interaction analysis shows that both electrostatic and van der Waals interactions play important roles in the self-assembly process, with the van der Waals interaction being the larger effect. The mechanistic picture that emerges from this work is that as the PA is pulled from the fiber, the interaction evolves through three stages: (1) initially electrostatic interactions between the charged head of the pulled PA and other PAs, and between the pulled PA and solvent are dominant, (2) after the charged head emerges, the rest of the peptide comes out, with both PA-solvent electrostatic interactions and van der Waals interactions being significant, and (3) in the last step, the alkane tail emerges, dominated by van der Waals interactions with either peptide or solvent.

The effect of alkane tail length of C iE 8 surfactants on transport to the silicone oil–water interface

Detailed surfactant transport studies have typically been restricted to the air–water interface. This is mainly due to the lack of experimental devices and techniques available to study liquid–liquid interfaces. As a result, there is a lack of relevant data and understanding of surfactant behavior in microfluidic studies and emulsion applications. Using a novel shape fitting algorithm for a pendant drop capable of handling fluids of similar densities, i.e. low Bond numbers, we measure the dynamic surface tension as a function of bulk surfactant concentration at the silicone oil–water interface for a homologous series of C i E 8 nonionic surfactants. We show that the isotherms governing equilibrium at the oil–water and air–water interfaces are very different. Using a scaling analysis comparing two governing mass transport timescales, we demonstrate that there exists a transition from diffusion-limited to kinetic-limited dynamics at the silicone oil–water interface. Adsorption rate constants are determined from a one parameter nonlinear fit to dynamic surface tension data. These results demonstrate that the dynamics of interfacial transport are highly dependent on the immiscible fluids that form the interface.

A Novel Phase Transformation Phenomenon in Mesostructured Aluminophosphate

A novel phase transformation phenomenon that involves two successive phase transformation events was found for the first time in the synthesis of mesostructured aluminophosphate and studied by XRD, TEM, and multinuclear solid-state NMR techniques. The results showed that a hexagonal phase Hex and two lamellar phases, L1 and L2, were formed after hydrothermal treatment for 1, 3, and 50 h, respectively. The status of the surfactant was found to be arrayed interdigitated in a bilayer with a tilt angle in L1 phase but upright in L2 phase. A mechanism that the exciting of the alkane tail of the surfactant together with the condensation of aluminophosphate cooperatively promoted the phase transformation was proposed for the observed phenomenon. Additionally, a ZON microporous structure was found for the first time existing in the framework of the mesostructured aluminophosphate.

Modification of the CHARMM force field for DMPC lipid bilayer

The CHARMM force field for DMPC lipids was modified in order to improve agreement with experiment for a number of important properties of hydrated lipid bilayer. The modification consists in introduction of a scaling factor 0.83 for 1-4 electrostatic interactions (between atoms separated by three covalent bonds), which provides correct transgauche ratio in the alkane tails, and recalculation of the headgroup charges on the basis of HF/6-311(d,p) ab-initio computations. Both rigid TIP3P and flexible SPC water models were used with the new lipid model, showing similar results. The new model in a 75 ns simulation has shown a correct value of the area per lipid at zero surface tension, as well as good agreement with the experiment for the electron density, structure factor, and order parameters, including those in the headgroup part of lipids.

Sequence-specific purification of DNA oligomers in hydrophobic interaction chromatography using peptide nucleic acid amphiphiles: extended dynamic range.

We present improvements on a previously reported method (Vernille JP, Schneider JW. 2004. Biotechnol Prog 20(6):1776-1782) to purify DNA oligomers by attachment of peptide nucleic acid amphiphiles (PNAA) to particular sequences on the oligomers, followed by their separation from unbound oligomers using hydrophobic interaction chromatography (HIC). Use of alkyl-modified HIC media (butyl and octyl sepharose) over phenyl-modified media (phenyl sepharose) reduced the elution time of unbound DNA while not affecting the elution time of the PNAA/DNA complex. Modifying the alkane tail length for PNAA from C(12) to C(18) increased slightly the retention of PNAA/DNA duplexes. By combining these two refinements, we show that sequence-specific purifications of DNA oligomers 60 bases in length or more can be achieved with high resolution, even when the PNAA alkane is attached to the center of the target strand. The insensitivity of the PNAA/DNA duplex binding to choice of HIC media appears to be due to a surface-induced aggregation phenomenon that does not occur in the case of untagged DNA. We also report on the use of batch HIC as an adequate predictor of elution profiles in linear gradient HIC, and its potential to considerably reduce purification times by applying step gradients.

Molecular Dynamics Simulations of Surfactant Self-Organization at a Solid−Liquid Interface

Self-organization of aqueous surfactants at a planar graphite-like surface is studied by means of coarse-grain molecular dynamics simulations. The nonionic surfactant, n-alkyl poly(ethylene oxide), and water are both represented by coarse-grain models while an implicit representation is used for the graphite surface. The observed morphology of the aggregated surfactants depends on the alkyl chain length. Surfactants with a short chain form a monolayer on the graphite surface with a thickness roughly equal to that of the alkane tail. On the other hand, longer-tail surfactants form continuous hemicylinders on the surface with diameter approximately 5.0 +/- 0.5 nm, in good agreement with experimental AFM data.