Specific structure, morphology, and properties of polyacrylonitrile (PAN) membranes prepared by needleless electrospinning; Forming hollow fibers.

Polyacrylonitrile (PAN) membranes have been prepared using needleless electrospinning with wire electrode and characterized by a series of methods HRSEM, XRD, air permeability and area weight measurements in dependence of high voltage and electrode distance. HRSEM analysis revealed the tendency to longitudinal rolling of strip-shaped PAN fibers forming hollow fibers. Combination of XRD analysis and molecular modeling explains this phenomenon as the consequence of the specific crystal structure of PAN fibers, where the isotactic PAN chains are arranged in layers forming belt shaped nanofibers with the strong tendency to roll up longitudinally forming hollow fibers. This effect offers the possibility to create hollow nanofibers by electrospinning with the appropriate choice of structure of polymer chains.

Stability of antibacterial modification of nanofibrous PA6/DTAB membrane during air filtration.

Stable antimicrobial nanofibrous membrane for air filtration based on polyamide 6 (hereafter PA6) modified by 1-dodecyltrimethylammonium bromide (DTAB) has been prepared by electrospinning using one-step technology, i.e. with modifying antimicrobial agent dissolved in spinning solution. Stability of antibacterial membrane function has been tested by air-blowing test to prove the permanency of chemical composition and antibacterial activity. X-ray diffraction, high-resolution scanning electron microscopy (HRSEM) revealed the effect of modifying agent on structure and morphology of PA6 nanofibres. X-ray photoelectron spectroscopy, electrokinetic analysis and antibacterial tests proved the stability of chemical composition and antibacterial activity after air-blowing tests. Special air-blowing device has been constructed for this purpose. The results prove the applicability so prepared membrane for a long-term air-conditioning.

A simple molecular modeling method for the characterization of polymeric drug carriers.

A simple molecular modeling method for the characterization of polymeric drug carriers is presented. Six biodegradable polymers have been investigated as drug carriers using molecular simulations: l-polylactide, d-polylactide, chitosan, polyglycolic acid, polyethylene glycol and cellulose. Cyclosporine A has been chosen as a model drug substance. Classical molecular dynamics and docking calculations were employed to model and predict polymer-drug interactions. These interactions have been analyzed by non-bond interaction energy and interaction parameter calculated using Flory-Huggins theory. Flexibility of polymer chains has been characterized by the change of gyration radius along the molecular dynamics trajectory. The relationship between mixing energy, chain length and chain flexibility has been revealed for each polymer/drug system.

Structure and stability of kaolinite/TiO2 nanocomposite: DFT and MM computations.

The adhesion of TiO(2) (anatase structure) nanoparticles to kaolinite substrate was investigated using molecular modeling. Universal force field computation, density function theory computation, and a combination of both two approaches were used. This study enabled the adhesion energy for the TiO(2)/kaolinite nanocomposite to be estimated, and revealed the preferred orientation of the TiO(2) nanoparticles on the kaolinite substrate. The results of all three levels of computation were compared in order to show that the accuracy of universal force field computations is sufficient in this context. The role of nanoparticle size and the importance of the nanoparticle-substrate bonding contribution are presented here and discussed. A comparison of the molecular modeling results with scanning electron microscopy observations showed that the results of the modeling were consistent with the experimental data, and that this approach can be used to help characterize nanocomposites of the nanoparticle/phyllosilicate substrate type.

Molecular modeling of gel nanoparticles with cyclosporine A for oral drug delivery.

Structure and behavior of amphiphilogel nanoparticles as a drug carriers for cyclosporine A (CsA) have been studied by the molecular modeling using empirical force field. Five atomistic models of a gel-based emulsions (GEM) with various gel compositions have been investigated in order to find a system most similar to a sixth atomistic model of self-microemulsifying drug delivery system (SMEDDS) taken as an exemplar of CsA delivery system. Structural parameters and energy characteristics (i.e. non-bond interaction energy between CsA and whole remaining components of a gel nanoparticle, CsA/gel nanoparticle intermolecular non-bond interaction energy, CsA-gel molecule pair interaction energy, volume fraction, concentration profiles and number of pervaded water molecules) of these six models in a waterless form and in a water containing form have been studied in dependence on the composition. The Flory-Huggins theory as implemented in the Accelrys Materials Studio 4.2 modeling environment was used to study the pair interactions of cyclosporine A with various types of surfactants. Structural parameters and energy characteristics of all systems have been compared and one composition was selected as a very promising for further experimental study.

Effects of DPH on DPPC-cholesterol membranes with varying concentrations of cholesterol: from local perturbations to limitations in fluorescence anisotropy experiments.

We have used atomistic molecular dynamics simulations to consider 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probes in a fluid dipalmitoylphosphatidylcholine bilayer with 5 and 20 mol % cholesterol (CHOL). We show that while DPH affects a number of membrane properties, the perturbations induced by DPH depend on the concentration of cholesterol in a membrane. For example, we find DPH to influence the mass density distribution of lipids across the membrane and to promote the ordering of acyl chains around the probe. Yet, these perturbations get relatively weaker for increasing cholesterol concentration. Meanwhile, we also find that the commonly used analysis in terms of the Brownian rotational diffusion (BRD) model with Legendre polynomials to interpret fluorescence anisotropy (FA) experiments is sensitive to the amount of cholesterol. For small concentrations of cholesterol, the analysis of FA turns out to yield a relatively good approximation of the correct orientational distribution of DPH. However, for a CHOL concentration of 20 mol %, we find that the FA analysis fails to yield the true orientational distribution of DPH, the disagreement being substantial. The results suggest that in highly ordered membrane domains, the view given by FA analysis using the BRD model is likely reliable in a qualitative sense, but the quantitative description deviates substantially from the correct one. The present results imply that FA studies for the orientational distribution of DPH should be interpreted with great care.

Molecular modeling of surface modification of Wyoming and Cheto montmorillonite by methylene blue.

The surface area of various types of montmorillonites (MMT) with different values of layer charge plays a very important role in surface arrangement of methylene blue cations (MB). Photoluminescence measurements can be strongly or partially influenced by this surface arrangement of cations. For these reasons and on the basis of our previous results, molecular simulations were performed for various types of montmorillonites covered with methylene blue cations. Adsorption of methylene blue cations on Na-Wyoming MMT surface is different from Ca-Cheto MMT. In the case of Wyoming with a lower layer charge, MB cations lie parallel to the silicate layer for all investigated samples. On the other hand, Cheto surface is covered with a higher amount of MB cations. The results obtained from molecular modeling indicate that MB lies parallel to low loading case and become tilted with respect to layer for a higher loading. Moreover, a higher amount of MB cations covering the silicate layer are much less energy-stable. A higher loading of MB cations leads to aggregates but at low loading MB cations degrade to monomers.

Surface and interlayer structure of vermiculite intercalated with methyl viologen.

Molecular modeling using empirical force field revealed the differences between the surface and interlayer arrangement of the dye guest molecules in vermiculite intercalated with the divalent methyl viologen cation (MV(2+)). Conformation and anchoring of MV(2+) cations on the silicate layer in the interlayer space of vermiculite host structure is different from that on the crystal surface. A preferential position has been found for the anchoring of guests on the silicate layer. Anyway the arrangement of guests in the interlayer space as well as on the crystal surface exhibits a high degree of disorder due to a certain flexibility in guest molecules arrangement and first of all due to the presence of water molecules in the interlayer space. The presence of water disturbs not only the regularity in guest positions and orientations but also in conformation of guest molecules in the interlayer space of the host structure.

Silver nanoparticles/montmorillonite composites prepared using nitrating reagent at water and glycerol.

Three procedures (P) were applied to prepare silver nanoparticles on natural Ca-montmorillonite (MT). The intercalation of the montmorillonite with silver nitrate in aqueous solution (P1), the intercalation of the montmorillonite with silver nitrate in glycerol (P2) and the successive combination of both P1 and P2 methods resulted to P3 method. X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Fourier Transform Infrared (FTIR) spectroscopy and the molecular modeling were employed to characterize silver nanoparticles and montmorillonite nanocomposite. The P1 produced MT-1 composite with 2.3 wt% Ag and the partially collapsed layered structure. Nanoparticles of silver larger than 20 nm with a lot of planar defects were randomly distributed on the MT-1 surface; nanoparticles smaller than 20 nm were oriented to the montmorillonite substrate. The MT-2 composite from P2 contained only 1 wt% of Ag. The molecular simulation model of MT-2 showed the interlayer space with the exchangeable cations and metallic silver atoms arrangement within the glycerol bilayer. The P3 produced composite MT-3 that contained 2.4 wt% Ag. The nanoparticles > 20 nm size had a well-defined geometry, very small nanoparticles were amorphous. The modeled structure showed the exchangeable cations, Ag+ and Ag0 located close to the silicate layers and monolayer of glycerol molecules in the interlayer space.

Fluorescence and structure of methyl red-clay nanocomposites.

Two types of clay minerals-montmorillonite and vermiculite have been chosen as a host matrix for the intercalation of methyl red (MR) in order to investigate a possible fluorescence tuning via dye-clay interactions. The effect of silicate layer charge on the structure and fluorescence of dye-clay intercalated hybrid nanostructures was investigated using combination of molecular modeling with experiment. Structure of both intercalates MR-vermiculite (MR-VER) and MR-montmorillonite (MR-MMT) exhibits high degree of structural disorder resulting in broaden emission band. The fluorescence wavelength range of MR intercalated in clays is shifted to lower wavelengths compared with the pristine MR polycrystalline sample (800 nm). Results showed the strong dependence of fluorescence band maximum on the silicate layer charge, lambda(max) = 565 nm for MR-MMT, 645 nm for MR-VER and 800 nm for the methyl red fine crystalline powder, whereas the structural disorder in the arrangement of dye molecules affects the emission band broadening.

Layered double hydroxide intercalated with p-methylbenzoate and p-bromobenzoate: molecular simulations and XRD analysis.

Samples of Mg4Al2 layered double hydroxide (LDH) intercalated with p-methylbenzoate and p-bromobenzoate anions were prepared by reconstruction of calcined LDH. The interlayer arrangement of guests was investigated by molecular modeling combined with X-ray powder diffraction and thermogravimetry. Molecular modeling was carried out in a Cerius2 modeling environment. In both structures the guest anions adopt a nearly perpendicular arrangement of their long axis with respect to the host layers and they are anchored to the OH groups of the layers through COO* groups via electrostatic interactions. Molecular modeling revealed that both structures of the intercalates exhibit a certain disorder of guest anions in the interlayer space. In the case of LDH-p-methylbenzoate intercalate the anions tend to be situated in disordered rows, and the LDH-p-bromobenzoate intercalate exhibits a total disorientation of guest anions. A good agreement between calculated and measured X-ray diffraction patterns and between experimental and calculated basal spacings was obtained. In the LDH-p-methylbenzoate intercalate d exp=16.96 A and d calc=16.97 A, and in the case of LDH-p-bromobenzoate intercalate d exp=17.19 A and d calc=17.40 A.

Structural ordering of organovermiculite: experiments and modeling.

The ordering of three different sizes of quaternary ammonium salts (QUATs) has been studied with respect to concentration of guests in the host's interlayer gallery. From the modeling, we could verify that small molecules of n-butylammonium salt build a monolayer structure in the vermiculite gallery without reference to concentration. On the other hand, the larger molecules of dodecyltrimethylammonium and dioctadecyldimethylammonium salts are responsive to the numbers of their molecules in the interlayer space of the host, building mono- or bilayered structures. Supersaturated structure of both QUATs keep an arrangement of alkyl chains nearly perpendicular to silicate layers, while only saturated samples exhibit tilted alkyl chains in the gallery. The ordering changes bring out the calculation of mean crystallite size. Low values of the nonbond energy of supersaturated forms predict that those organovermiculites will readily exfoliate, e.g., in polymer/clay nanocomposite.

Free pyrene probes in gel and fluid membranes: perspective through atomistic simulations.

We consider the properties of free pyrene probes inside gel- and fluidlike phospholipid membranes and unravel their influence on membrane properties. For this purpose, we employ atomic-scale molecular dynamics simulations at several temperatures for varying pyrene concentrations. Molecular dynamics simulations show that free pyrene molecules prefer to be located in the hydrophobic acyl chain region close to the glycerol group of lipid molecules. Their orientation is shown to depend on the phase of the membrane. In the fluid phase, pyrenes favor orientations where they are standing upright in parallel to the membrane normal, while, in the gel phase, the orientation is affected by the tilt of lipid acyl chains. Pyrenes are found to locally perturb membrane structure, while the nature of perturbations in the gel and fluid phases is completely different. In the gel phase, pyrenes break the local packing of lipids and decrease the ordering of lipid acyl chains around them, while, in the fluid phase, pyrenes increase the ordering of nearby acyl chains, thus having an opposite effect. Interestingly, this proposes a similarity to effects induced by cholesterol on structural membrane properties above and below the gel-fluid transition temperature. Further studies express a view that the orientational ordering of pyrene is not a particularly good measure of the acyl chain ordering of lipids. While pyrene ordering provides the correct qualitative behavior of acyl chain ordering in the fluid phase, its capability to predict the correct temperature dependence is limited.

Preparation of vermiculite nanoparticles using thermal hydrogen peroxide treatment.

Powdered natural Mg-vermiculite (Letovice, Czech Republic), with the formula (Mg0.35K0.02Ca0.01) (Mg2.39Fe0.51(3+)Fe0.02(2+)Al0.08) (Si2.64Al1.33Ti0.03) O10(OH)2 x 4.97H2O and particle size < 5 microm, was used for the investigation of exfoliation after hydrogen peroxide and/or microwave treatment (600 W). A sample heated in the microwave oven for 40 min exhibits a 11% mass loss and reduction of the 001 peak intensity in the X-ray diffraction pattern. The basal 001 peak intensity of untreated Mg-vermiculite sample (/001 = 100%) drops to 35% in the microwave treated sample. Only the sample treated for 5 h at 80 degrees C fully rehydrated after 120 min at room temperature. A more pronounced reduction of the 001 peak intensity (to 8%) was observed after hydrogen peroxide treatment of the sample at 25 degrees C. The combination of a five-hour hydrogen peroxide treatment at 80 degrees C and subsequent microwave heating leads to an effective extinction of the 001 diffraction in the XRD pattern. The 001 diffraction profile becomes very diffuse with peak intensity less than 1%. The degree of reduction of the 001 diffraction intensity also depends on the time and temperature of hydrogen peroxide treatment and on the peroxide concentration. An even more pronounced reduction of the peak intensity is caused by exfoliation of particles to nano-domains coupled with a randomization of the c-axes.

Influence of DPH on the structure and dynamics of a DPPC bilayer.

We have conducted extensive molecular dynamics (MD) simulations together with differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments to quantify the influence of free 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probes on the structure and dynamics of a dipalmitoylphosphatidylcholine bilayer. Atomistic MD simulations show that in the membrane-water interface the influence of DPH is minor, whereas in the acyl-chain region DPH gives rise to major perturbations. In the latter case, DPH is found to influence a wide range of membrane properties, such as the packing and ordering of hydrocarbon tails and the lateral diffusion of lipid molecules. The effects are prominent but of local nature, i.e., the changes observed in the properties of lipid molecules are significant in the vicinity of DPH, but reduce rapidly as the distance from the probe increases. Long-range perturbations due to DPH are hence not expected. Detailed DSC and (2)H NMR measurements support this view. DSC shows only subtle perturbation to the cooperative behavior of the membrane system in the presence of DPH, and (2)H NMR shows that DPH gives rise to a slight increase in the lipid chain order, in agreement with MD simulations. Potential effects of other probes such as pyrene are briefly discussed.

Effect of surface and interlayer structure on the fluorescence of rhodamine B-montmorillonite: modeling and experiment.

The surface and interlayer structure of rhodamine B (RhB)-montmorillonite for various guest concentrations has been studied using a combination of X-ray powder diffraction and molecular modeling (molecular mechanics and molecular dynamics) in the Cerius(2) modeling environment. The joint effect of surface and interlayer structure on the fluorescence spectrum has been observed and discussed in relation to the position and orientation of RhB(+) cations with respect to the silicate layer. Structural analysis showed that the surface and interlayer structures are different as to the arrangement of RhB(+) cations, and both of them strongly depend on the guest concentration in the intercalation solution and on the method of preparation. The repeated intercalation of montmorillonite by rhodamine B used in the present work allowed obtaining RhB-montmorillonite in the maximum degree of ion exchange for every sample.

Characterization of molecular structures and properties of polyurethanes using molecular dynamics simulations.

Thermotropic polyurethanes with mesogenic groups in side chains were prepared from two diisocyanates and four diols with stoichiometric ratios of reactive isocyanate (NCO) and hydroxy (OH) groups. Their thermal behavior was determined by differential scanning calorimetry. The effect of structure modifications of the diisocyanates and diols, in particular changes in the mesogen, were investigated. Introduction of mesogenic segments into the polymers suppresses the ordering. Stiff end substituents (phenyl and alkoxy groups) of the mesogens stabilize the mesophases to such an extent that the negative influence of long polymer chains is compensated and the liquid-crystalline properties are recovered. All-atom molecular dynamics simulations in the Cerius2 modeling environment were carried out to characterize the structures of the polymers. Analysis of the dynamic trajectories at 20, 100, 120 and 170 degrees C revealed changes in conformation of macromolecules, which correlate with DSC measurements.

An algorithm to filter out packing arrangements based on steric clashes.

This document outlines the use of an algorithm to filter out impossible crystal-packing arrangements based on steric considerations. Within an exhaustive grid search frame, the space sample is reduced by analysis of spherical areas where atom pairs from different rigid units might clash. This technique finds areas in the state space where the global energy minimum might lie. The minimum can then be found by the usual methods of molecular modeling restricted to these particular areas. Only a tiny fraction of atom pair distances need to be tested; usually a single quantity on average per one state of model space! For example, a crystal of three rigid molecules, each containing 12 atoms, has 3x12x12=432 atom pairs just in one unit cell but our method needs to test on average 1 to 4 atom pairs per state. Using modern computers, about 10(12-15) models can be tested within several hours or days. For example, a crystal model with six rotational degrees of freedom (two rigid molecules in the unit cell) each with step 3 degrees can be tested in a few hours on a 1-GHz x86 processor-based machine. The method presented here has been implemented in the SUPRAMOL program.

Combination of modeling and experiment in structure analysis of intercalated layer silicates.

A strategy for the structure analysis of intercalated layer silicates based on a combination of modeling (i.e. force field calculations) and experiment is presented. Modeling in conjunction with experiment enables us to analyze the disordered intercalated structures of layer silicates where conventional diffraction analysis fails. Experiment plays a key role in the modeling strategy and in corroboration of the modeling results. X-ray powder diffraction and IR spectroscopy were found to be very useful complementary experiments to molecular modeling. Molecular mechanics and molecular dynamics simulations were carried out in the Cerius2 and Materials Studio modeling environments. An overview is given of the structures of layer silicates, especially smectites intercalated with various inorganic and organic guest species. Special attention is paid to the ordering of guests in the interlayer space, as it is important for the practical applications of these intercalates, where the interlayer porosity, photofunctions, etc. must be controlled. Figure Structure of montmorillonite intercalated with octadecylamine via ion-dipole interaction with the maximum concentration of guests corresponding to the monolayer arrangement of guests with basal spacing 33.3 A. The Na cations remaining in the interlayer are visualized as pink balls

Structure analysis of montmorillonite intercalated with rhodamine B: modeling and experiment.

The intercalation process and the structure of montmorillonite intercalated with [rhodamine B]+ cations have been investigated using molecular modeling (molecular mechanics and molecular dynamics simulations), X-ray powder diffraction and IR spectroscopy. The structure of the intercalate depends strongly on the concentration of rhodamine B in the intercalation solution. The presence of two phases in the intercalated structure was revealed by modeling and X-ray powder diffraction: (i) phase with basal spacing 18 A and with bilayer arrangement of guests and (ii) phase with average basal spacing 23 A and with monolayer arrangement of guests. In both phases the monomeric and dimeric arrangement can coexist in the interlayer space. Three types of dimers in the interlayer structure have been found by modeling: (i) H-dimer (head-to-head arrangement) present in the 18 A phase, (ii) sandwich type of the head-to-tail arrangement (present in the 23 A phase) and (iii) J-dimer (head-to-tail arrangement) present in the 23 A phase. Figure Montmorillonite intercalated with rhodamine B cations. On the left: phase 18 A, bilayer dimeric arrangement of guests (H-dimers). On the right: phase 23 A, monolayer arrangement of guests prepared using intercalation solution with a low concentration of rhodamine B