Single-Walled Carbon Nanotube/Lyotropic Liquid Crystal Hybrid Materials by a Phase Separation Method in the Presence of Polyelectrolyte

X. Xin, H. Li, E. Kalwarczyk, A. Kelm, M. Fiałkowski, E. Gorecka, D. Pociecha and R. Hołyst

Langmuir 2010, 26, 11, 8821–8828

We present a detailed study on the incorporation of single-walled carbon nanotubes (SWNTs) into lyotropic liquid crystals (LLC) by phase separation in the presence of polyelectrolytes. Two cases were studied in this work: (i) incorporation of SWNTs into the LLC phase formed by an anionic surfactant sodium dodecyl sulfate (SDS) in the presence of an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSS); (ii) incorporation of SWNTs into the LLC phase formed by a cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of a cationic polyelectrolyte poly(diallydimethylammonium chloride) (PDADMAC). The SWNTs/LLC composites were characterized by polarized optical microscopy (POM) observations and small-angle X-ray scattering (SAXS) measurements. In both systems, the surfactant phase was condensed into a hexagonal lattice by the polyelectrolyte within the investigated concentration range. Several factors that can influence the property of SWNTs/LLC composite were examined, including concentration of surfactants and polyelectrolytes and temperature. Aggregated SWNTs were not observed, indicating that SWNTs were well dispersed in the LLC phases. SAXS measurements showed the lattice parameter of the host LLC phase changed upon varying the mixing ratio of polyelectrolyte to ionic surfactant. The SWNTs/LLC hybrids showed considerable stability against temperature rise in both systems, and desorption of surfactant from SWNTs was not observed at higher temperature.

Size and Shape of Micelles Studied by Means of SANS, PCS, and FCS

J. Gapiński, J. Szymański, A. Wilk, J. Kohlbrecher, A. Patkowski and R. Hołyst

Langmuir 2010, 26, 12, 9304–9314

The hexaethylene glycol monododecyl ether (C12E6) micelles at concentrations up to 10% have been studied in their isotropic phase (10−48 °C) by means of small angle neutron scattering (SANS) and photon correlation spectroscopy (PCS). The SANS data obtained at low temperatures could be unequivocally interpreted as a result of scattering from a suspension of compact globular micelles with the shape of a triaxial ellipsoid or a short end-capped elliptical rod. Different models have been applied to analyze the SANS data obtained at higher temperatures: (i) elongated rod-like micelles with purely sterical interactions, (ii) compact globular micelles with a weak attractive potential, and (iii) globular micelles influenced by the critical phenomena in the whole temperature range studied. The good quality of the experimental data indicated model (i) as the best fit for our data. The diffusion coefficients obtained from the PCS measurements have been compared to the diffusion coefficients calculated for the rod-like micelles—results of the SANS data analysis. A good agreement was achieved using the solvent viscosity, in agreement with the theoretical predictions for sterically interacting globular colloidal particles. Finally, the SANS results obtained at 24 °C were compared to the micelle self-diffusion coefficients previously measured by means of fluorescence correlation spectroscopy (FCS) at this temperature. The good agreement obtained after scaling the data with solution viscosity supports the validity of the generalized Stokes−Einstein relation in sterically interacting systems: the product of the colloidal particle self-diffusion coefficient and the macroscopic viscosity remains constant in a broad range of concentrations. It has been concluded that the FCS technique in combination with simple viscosity measurements might serve as a tool for estimating the micellar size and shape.

Evaluation of Ligand-Selector Interaction from Effective Diffusion Coefficient

A. Bielejewska, A. Bylina, K. Duszczyk, M. Fiałkowski and R. Hołyst

Anal. Chem. 2010, 82, 13, 5463–5469

We present an analytical technique for determination of ligand-selector equilibrium binding constants. The method is based on the measurements of effective molecular diffusion coefficient of the ligand during Poiseuille flow through a long (approximately 25 m), thin (0.254 mm ± 0.05 mm ID) capillary with and without the selector. The data are analyzed using the Taylor dispersion theory. Bovine Serum Albumin (BSA) and cyclodextrin (CD) were taken as model selectors. We have tested our method on the following selector-ligand complexes: BSA with warfarin, propranolol, noscapine, salicylic acid, and riboflavin, and cyclodextrin with 4-nitrophenol. The results are in good agreement with data from the literature and with our own results obtained within classical chromatography. This method works equally well for uncharged and charged compounds.

Reverse Vesicles from a Salt-Free Catanionic Surfactant System: A Confocal Fluorescence Microscopy Study

H. Li, X. Xin, T. Kalwarczyk, E. Kalwarczyk, P. Niton, R. Hołyst and J. Hao

Langmuir 2010, 26, 19, 15210–15218

We give a detailed confocal fluorescence microscopy study on reverse vesicles from a salt-free catanionic surfactant system. When tetradecyltrimethylammonium laurate (TTAL) and lauric acid (LA) are mixed in cyclohexane at the presence of a small amount of water, stable reverse vesicular phases form spontaneously. The reverse vesicular phases can be easily labeled with dyes of varying molecular size and hydrophobicity while the dyes are nearly insoluble in cyclohexane without reverse vesicles. This indicates the reverse vesicular phases can be good candidates to host guest molecules. With the help of a fluorescence microscope combined a confocal method, the features of these interesting reverse supramolecular self-assemblies were revealed for the first time. Because of the absence of electrostatic repulsions and hydration forces between adjacent vesicles, the reverse vesicles have a strong propensity to aggregate with each other and form three-dimensional clusters. The size distributions of both individual reverse vesicles and clusters are polydisperse. Huge multilamellar reverse vesicles with closely stacked thick walls (giant reverse onions) were observed. Besides the spherical reverse vesicles and onions, other supramolecular structures such as tubes have also been detected and structural evolutions between different structures were noticed. These interesting supramolecular self-assemblies form in a nonpolar organic solvent may serve as ideal micro- or nanoreaction centers for biological reactions and synthesis of inorganic nanomaterials.

Binary and graded evolution in time in a simple model of gene induction

M. Tabaka and R. Hołyst

Phys. Rev. E 2010, 82, 052902

We solve analytically the model of gene expression induction which consists of three steps: gene activation, gene products synthesis, and product degradation. The solution is given as a time-dependent probability distribution for gene products. Following the distribution in time from the inactive state of the gene to the stationary state we observe binary or graded response depending solely on the ratio r of the gene activation rate to the rate of the gene product degradation. If r1 the response is binary and the continuous transition from binary to graded response occurs between r=0.1 and r=1. Therefore, if binary response is observed during relaxation to steady state, then the activation rate constant must be smaller than the degradation rate constant.

Dynamic charge separation in a liquid crystalline meniscus

T. Szymborski, O. Cybulski, I. Bownik, A. Żywociński, S.A. Wieczorek, M. Fiałkowski, R. Hołyst and P.Garstecki

Soft Matter, 2009,5, 2352-2360

Oscillating electric fields can sustain a macroscopic and steady separation of electrostatic charges. The control over the dynamic charge separation (dyCHASE) is presented for the example of circular menisci of thin, free standing smectic films. These films are subject to an in-plane, alternating radial electric field. The boundaries of the menisci become charged and unstable in the electric field and deform into pulsating, flower-like shapes. This instability ensues only at frequencies of the electric field that are lower than a critical one. The critical frequency is a linear function of the strength of the electric field. Since the speed of electrophoretic drift of ions is also linearly related to the strength of the field, the linear relation between critical frequency and the amplitude of the field sets a characteristic length scale in the system. We postulate that dyCHASE is due to (i) electrophoretic motion of ions in the liquid crystalline (LC) film, (ii) microscopic separation of charges over distances similar in magnitude to the Debye screening length, and (iii) further, macroscopic separation of charges through an electro-hydrodynamic instability. Interestingly, the electrophoretic motion of ions couples with the macroscopic motion of the LC material that can be observed with the use of simple optical microscopy.

Scaling form of viscosity at all length-scales in poly(ethylene glycol) solutions studied by fluorescence correlation spectroscopy and capillary electrophoresis

R. Holyst, A. Bielejewska, J. Szymański, A. Wilk, A. Patkowski, J. Gapiński, A. Żywociński, T. Kalwarczyk, E. Kalwarczyk, M. Tabaka, N. Ziębacza and S. A. Wieczoreka

Phys. Chem. Chem. Phys. 2009,11, 9025-9032

We measured the viscosity of poly(ethylene glycol) (PEG 6000, 12 000, 20 000) in water using capillary electrophoresis and fluorescence correlation spectroscopy with nanoscopic probes of different diameters (from 1.7 to 114 nm). For a probe of diameter smaller than the radius of gyration of PEG (e.g.rhodamine B or lyzozyme) the measured nanoviscosity was orders of magnitude smaller than the macroviscosity. For sizes equal to (or larger than) the polymer radius of gyration, macroscopic value of viscosity was measured. A mathematical relation for macro and nanoviscosity was found as a function of PEG radius of gyration, Rg, correlation length in semi-dilute solution, ξ, and probe size, R. For R < Rg, the nanoviscosity (normalized by water viscosity) is given by exp(b(R/ξ)a), and for R > Rg, both nano and macroviscosity follow the same curve, exp(b(R/ξ)a), where a and b are two constants close to unity. This mathematical relation was shown to equally well describe rhodamine (of size 1.7 nm) in PEG 20 000 and the macroviscosity of PEG 8 000 000, whose radius of gyration exceeds 200 nm. Additionally, for the smallest probes (rhodamine B and lysozyme) we have verified, using capillary electrophoresis and fluorescence correlation spectroscopy, that the Stokes–Einstein (SE) relation holds, providing that we use a size-dependent viscosity in the formula. The SE relation is correct even in PEG solutions of very high viscosity (three orders of magnitude larger than that of water).

Evaporation into vacuum: Mass flux from momentum flux and the Hertz-Knudsen relation revisited

R. Hołyst and M. Litniewski

J. Chem. Phys. 2009, 130, 074707

We performed molecular dynamics simulations of liquid film evaporation into vacuum for two cases: free evaporation without external supply of energy and evaporation at constant average liquid temperature. In both cases we found that the pressure inside a liquid film was constant, while temperature decreased and density increased as a function of distance from the middle of the film. The momentum flux in the vapor far from the liquid was equal to the liquid pressure in the evaporating film. Moreover the pseudopressure (stagnation pressure) was found to be constant in the evaporating vapor and equal to the liquid pressure. The momentum flux and its relation to the pressure determined the number of evaporating molecules per unit time and as a consequence the mass evaporation flux. We found a simple formula for the evaporation flux, which much better describes simulation results than the commonly used Hertz–Knudsen relation.

Challenges in thermodynamics: Irreversible processes, nonextensive entropies, and systems without equilibrium states

R. Hołyst

Pure and Applied Chemistry 2009, 81, 10, 1719-1726

Recent works on evaporation and condensation demonstrate that even these sim-plest irreversible processes, studied for over 100 years, are not well understood. In the caseof a liquid evaporating into its vapor, the liquid temperature is constant during evaporationand the evaporation flux is governed by the heat transfer from the hotter vapor into the colderliquid. Whether liquid evaporates into its own vapor or into the vacuum, the irreversible path-way in the process goes through a number of steps which quickly lead to the steady-state con-ditions with mechanical equilibrium in most parts of the system—the fact overlooked in allprevious studies. Even less is known about general rules which govern systems far from equi-librium. Recently, it has been demonstrated that a work done in an irreversible process canbe related to the free energy difference between equilibrium states joined by the process.Finally, a real challenge in thermodynamics is a description of living systems since they donot have equilibrium states, are nonextensive, (i.e., they cannot be divided into subsystems),and cannot be isolated. Thus, their proper description requires new paradigms in thermo -dynamics.

From complex structures to complex processes: Percolation theory applied to the formation of a city

A. Bitner, R. Hołyst and M. Fiałkowski

Phys. Rev. E 2009, 80, 037102

We investigate the morphology of the spatial pattern resulting from the division of land into the parcels that is observed in the centers of the cities, by analyzing the distribution function of the parcel areas. A simple model based on a two-dimensional bond percolation is employed to mimic the process of the formation of the city. The model reproduces the empirical distribution of the parcel areas that is found to exhibit the power law with the exponent τ=2.0. We argue that the city emerges from a collection of separated settlements in a process that can be described as a structural phase transition.

Thousand-Fold Acceleration of Phase Decomposition in Polymer/Liquid Crystal Blends

N. Ziębacz, S. A. Wieczorek, T. Szymborski, P. Garstecki, R. Hołyst

CHEMPHYSCHEM 2009, 10, 2620-2622

Pulling apart: The authors demonstrate experimentally that oscillating electric fields can be used to accelerate the rate of phase separation by up to three orders of magnitude (see picture), and to change the character of the phase‐separation process from a power to exponential evolution of the mean size of the domains.

Micro- and macro-shear viscosity in dispersed lamellar phases

J. Szymański, A.Wilk, R. Hołyst, G. Roberts, K. Sinclair and A. Kowalski

Journal of Non-Newtonian Fluid Mechanics 2008, 148, 1–3, 134-140

Surfactant phases, such as dispersed lamellar gels, are extremely useful in commercial products because they are very weight-effective at building viscosity. An enduring challenge is to determine the microstructural features responsible for the bulk rheology so that we can design products with improved performance. The samples described here have very different rheological profiles as exemplified by an order-of-magnitude difference in their zero-shear-rate viscosity, and infinite-shear-rate viscosities which differ by half an order of magnitude. As a first approximation we consider the dispersed lamellar system to be analogous to a high-internal-phase-volume emulsion which is described by the well-known Kreiger–Dougherty equation. This requires us to establish the value of a number of parameters of which the continuous phase viscosity is the one that defines the baseline viscosity. We measured this in situ by a micro-viscosity technique involving Fluorescence Correlation Spectroscopy using microscopic probes: viz. a fluorescent dye molecule (rhodamine) of size 0.85 nm; a lyzozyme protein of 2 nm size and a quantum dot of 12.5 nm size. We show that the continuous phase has a viscosity about twice that of water. Moreover, this viscosity is the same for the all three probes indicating that the system is quite uniform at the microscopic level investigated. Interestingly, this micro-viscosity was practically the same for all the samples and thus could not be correlated with zero-shear-rate viscosity or other rheological characteristics. We conclude that the macro-viscosity arises from structures much larger than 25 nm (twice the hydrodynamic diameter of the quantum dot). Our future intention is to use larger probes to establish the length-scale at which the microstructure begins to be apparent in the bulk rheology characteristics.

Heat transfer at the nanoscale: Evaporation of nanodroplets

R. Hołyst and M. Litniewski

Phys. Rev. Lett. 2008, 100, 055701

We demonstrate using molecular dynamics simulations of the Lennard-Jones fluid that the evaporation process of nanodroplets at the nanoscale is limited by the heat transfer. The temperature is continuous at the liquid-vapor interface if the liquid/vapor density ratio is small (of the order of 10) and discontinuous otherwise. The temperature in the vapor has a scaling form T(r,t)=T[r/R(t)], where R(t) is the radius of an evaporating droplet at time t and r is the distance from its center. Mechanical equilibrium establishes very quickly, and the pressure difference obeys the Laplace law during evaporation.

Accurate genetic switch in Escherichia coli: Novel mechanism of regulation by co-repressor

M. Tabaka, O. Cybulski and R. Hołyst

Journal of Molecular Biology 2008, 377, 4, 1002-1014

Understanding a biological module involves recognition of its structure and the dynamics of its principal components. In this report we present an analysis of the dynamics of the repression module within the regulation of the trp operon in Escherichia coli. We combine biochemical data for reaction rate constants for the trp repressor binding to trp operator and in vivo data of a number of tryptophan repressors (TrpRs) that bind to the operator. The model of repression presented in this report greatly differs from previous mathematical models. One, two or three TrpRs can bind to the operator and repress the transcription. Moreover, reaction rates for detachment of TrpRs from the operator strongly depend on tryptophan (Trp) concentration, since Trp can also bind to the repressor–operator complex and stabilize it. From the mathematical modeling and analysis of reaction rates and equilibrium constants emerges a high-quality, accurate and effective module of trp repression. This genetic switch responds accurately to fast consumption of Trp from the interior of a cell. It switches with minimal dispersion when the concentration of Trp drops below a thousand molecules per cell.

Efficient adsorption of super greenhouse gas (Tetrafluoromethane) in carbon nanotubes

P. Kowalczyk and R. Holyst

Environ. Sci. Technol. 2008, 42, 8, 2931–2936

Light membranes composed of single-walled carbon nanotubes (SWNTs) can serve as efficient nanoscale vessels for encapsulation of tetrafluoromethane at 300 K and operating external pressure of 1 bar. We use grand canonical Monte Carlo simulation for modeling of CF4 encapsulation at 300 K and pressures up to 2 bar. We find that the amount of adsorbed CF4 strongly depends on the pore size in nanotubes; at 1 bar the most efficient nanotubes for volumetric storage have size R = 0.68 nm. This size corresponds to the (10,10) armchair nanotubes produced nowadays in large quantities. For mass storage (i.e., weight %) the most efficient nanotubes have size R = 1.02 nm corresponding to (15,15) armchair nanotubes. They are better adsorbents than currently used activated carbons and zeolites, reaching ≈2.4 mol kg−1 of CF4, whereas, the best activated carbon Carbosieve G molecular sieve can adsorb 1.7 mol kg−1 of CF4 at 300 K and 1 bar. We demonstrate that the high enthalpy of adsorption cannot be used as an only measure of storage efficiency. The optimal balance between the binding energy (i.e., enthalpy of adsorption) and space available for the accommodation of molecules (i.e., presence of inaccessible pore volume) is a key for encapsulation of van der Walls molecules. Our systematic computational study gives the clear direction in the timely problem of control emission of CF4 and other perfluorocarbons into atmosphere.

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