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.

Three-dimensional space partition based on the first Laplacian eigenvalues in cells

O. Cybulski and R. Hołyst

Phys. Rev. E 2008, 77, 056101

We determine a partition of three-dimensional space into cells by minimization of the sum of the first Laplacian eigenvalues over the cells. This partitioning scheme emerges as a stationary state of a reaction-diffusion process taking place in a system of n different species which mutually annihilate, and simultaneously are duplicated in an autocatalytic reaction, so that the number of particles is kept constant and equal for each species. The system is considered in the limit of strong reactivity, so that the species separate each other into cells with well-defined, sharp boundaries. For a given n and fixed sizes of a periodic simulation box, this partition minimizes the aforementioned sum of eigenvalues. Further minimization is done by changing n and the side ratio of the periodic box. The global minimum is obtained for the structure with A15 symmetry, similar to the Weaire-Phelan foam. Depending on n and the side ratio, there are also many local minima, in particular: hcp (hexagonal close packed), fcc (face centered cubic), the Kelvin structure, and Frank-Kasper sigma phase.

Late Stage of the Phase-Separation Process: Coalescence-Induced Coalescence, Gravitational Sedimentation, and Collective Evaporation Mechanisms

T. Kalwarczyk, N. Ziebacz, M. Fiałkowski and R. Hołyst

Langmuir 2008, 24, 13, 6433–6440

We study the separation in the binary and ternary mixtures of the water/surfactant C12E5/polymer PEG system. The phase separation in the mixtures at late stages is governed by two distinct mechanisms: the coalescence-induced coalescence and the droplet evaporation mechanism. We show that when the coalescence-induced coalescence process is globally terminated in the sample consisting of a dense system of domains, another mechanism, which we call the collective droplet evaporation, starts to dominate. It manifests itself as a front of “evaporating” domains, which propagates at constant speed in the system. We show that the collective evaporation is induced by the gravitational drift of large droplets.

Dynamics of Phase Separation in Polymer Blends Revisited: Morphology, Spinodal, Noise, and Nucleation

M. Fiałkowski and R. Hołyst

Macromolecular Theory and Simulations, 2008, 17, 6, 263-273

The properties of the model B of mesoscopic dynamic with the Flory–Huggins free energy for the homopolymer blend are discussed. We focus on the rescaling of the spatial coordinates in the model and demonstrate that the commonly used rescaling of the spatial coordinates by the function vanishing at the spinodal leads to the unphysical pinning of the domains. The proper rescaling function for the spatial variables which avoids the unphysical pinning of the domain growth at the spinodal is proposed. We discuss in detail the evolution of morphological measures during separation in homopolymer blends and the problem of nucleation close to the spinodal.

Collective Rotations of Ferroelectric Liquid Crystals at the Air/Water Interface

P. Milczarczyk-Piwowarczyk, A. Żywociński, K. Noworyta and R. Hołyst

Langmuir 2008, 24, 21, 12354–12363

We study the Langmuir monolayers of four different ferroelectric liquid crystals on water surface. Two of them are attached to water surface by their polar groups, and the chiral groups, at the opposite ends of the elongated molecules, remain well above the interface. The other two ferroelectrics have both groups (polar and chiral) at close proximity, and therefore the chiral group is also attached to the surface or even submerged in water. We demonstrate that only when the chiral group of the ferroelectric liquid crystal in Langmuir monolayer is not attached to the interface and stays in the air does the system exhibit the collective rotations induced by evaporation of water (described for the first time by: Tabe, Y.; Yokoyama, H. Nat. Mater20032, 806). The isotherms of surface pressure and surface potential versus molecular area of four compounds were measured with simultaneous observations using Brewster angle microscopy. Experimental data of the compression isotherms are described with a van der Waals model with very good accuracy, and the fitted parameters were used for calculations of compressibility coefficients for different phases found in the compounds under investigations. The ability of the two compounds for rotation and the disability of the two others is discussed in a context of thermodynamic properties of the monolayers.

Hydrogen storage in nanoporous carbon materials: myth and facts

P. Kowalczyk, R. Hołyst, M. Terrones and H. Terrones

Phys. Chem. Chem. Phys., 2007,9, 1786-1792

We used Grand canonical Monte Carlo simulation to model the hydrogen storage in the primitive, gyroid, diamond, and quasi-periodic icosahedral nanoporous carbon materials and in carbon nanotubes. We found that none of the investigated nanoporous carbon materials satisfy the US Department of Energy goal of volumetric density and mass storage for automotive application (6 wt% and 45 kg H2 m−3) at considered storage condition. Our calculations indicate that quasi-periodic icosahedral nanoporous carbon material can reach the 6 wt% at 3.8 MPa and 77 K, but the volumetric density does not exceed 24 kg H2 m−3. The bundle of single-walled carbon nanotubes can store only up to 4.5 wt%, but with high volumetric density of 42 kg H2 m−3. All investigated nanoporous carbon materials are not effective against compression above 20 MPa at 77 K because the adsorbed density approaches the density of the bulk fluid. It follows from this work that geometry of carbon surfaces can enhance the storage capacity only to a limited extent. Only a combination of the most effective structure with appropriate additives (metals) can provide an efficient storage medium for hydrogen in the quest for a source of “clean” energy.

Net charge and electrophoretic mobility of lysozyme charge ladders in solutions of nonionic surfactant

J. Szymański, E. Poboży, M. Trojanowicz, A. Wilk, P. Garstecki and R. Hołyst

J. Phys. Chem. B 2007, 111, 19, 5503–5510

We report on the electrophoretic mobility and on the thermal diffusion of lysozyme proteins dissolved in aqueous solutions of a nonionic surfactant (C12E6) at a wide range of concentrations of the surfactant (0−20% by weight). We want to estimate the influence of a dense network of elongated micelles of C12E6 on the effective charge of the proteins as observed in the capillary electrophoresis experiments. The possible mechanism leading to the change in the effective charge of protein could involve the deformation of the cloud of counterions around the protein when it squeezes through the narrow (of the order of a protein diameter) aqueous channels formed in the solution of elongated micelles. The combination of independent measurements of the electrophoretic mobility of a family of modified proteins (lysozyme charge ladder [Colton et al. J. Am. Chem. Soc1997119, 12701]), of the microviscosity of the solutions of surfactant (obtained via fluorescence correlation spectroscopy), and of the hydrodynamic radius of the proteins (photon correlation spectroscopy) allow us to conclude that the effective charge of the proteins is not affected by the presence of surfactant, even at high concentrations.

Influence of poly(ethylene glycol) molecular mass on separation and in solutions of CiEj nonionic surfactants: Depletion interactions and steric effects

S. Makulska, E. Chudy, K. Urbaniak, S. A. Wieczorek, A. Zywocinski and R. Holyst

J. Phys. Chem. B 2007, 111, 28, 7948–7953

We study ternary mixtures of nonionic surfactants CiEj (i = 12; j = 5, 6, 8) and poly(ethylene glycol) (PEG) in water. For sufficiently large molecular mass of PEG (M >Msep ∼ 600), we observe a lowering of phase separation temperature with an increase in polymer concentration. The value of Msep is consistent with the analysis based on depletion interactions between micelles induced by polymer chains. We also demonstrate that there is another critical molecular mass of PEG (M = M* ∼ 2000) necessary to induce ordering in the surfactant-rich phase. This critical molecular mass follows from two requirements:  (a) PEG has to reduce the separation temperature below a temperature of hexagonal−isotropic phase transition in a binary surfactant−water mixture and (b) the PEG radius of gyration has to be larger than the size of the water channels in the hexagonal phase.

Brownian motion with inert drift, but without flux: A model

K. Burdzy, R. Hołyst and Ł. Pruski

Physica A: Statistical Mechanics and its Applications 2007, 384, 2, 278-284

We study the motion of a Brownian particle which interacts with a stationary obstacle in two dimensions. The Brownian particle acquires drift proportionally to the time spent on the boundary of the obstacle. The system approaches equilibrium, and the equilibrium distribution for the location and drift magnitude has the product form. The distribution for the location is uniform, while the drift distribution depends on the shape of the obstacle, resembling a gamma function for the circular or elliptic obstacle.

Kinetics and dynamics of dissolution/mixing of a high-viscosity liquid phase in a low-viscosity solvent phase

T. Kalwarczyk, N. Ziebacz, S. A. Wieczorek and R. Holyst

J. Phys. Chem. B 2007, 111, 41, 11907–11914

We studied mixing in the initially separated binary mixture of polystyrene/5CB liquid crystal and ternary mixtures of water/surfactant C12E5/polymer PEG system. In both systems the domains of one phase were characterized by a much higher viscosity than the solvent matrix. We demonstrated experimentally that during mixing these domains decrease their size linearly with time without a visible change of the optical contrast (i.e., without a rapid change of their compositions). Computer simulations and a theoretical model explain quantitatively our experimental observations.

Kinetic Trapping of Large Amount of Long Polymers in Nanopores

A. Żywociński, A. Korda, J. Gosk, S. A. Wieczorek, A. Wilk and R. Hołyst

J. Am. Chem. Soc. 2007, 129, 44, 13398–13399

It is well-established that when polymer size exceeds the size of a confining space, the equilibrium partition of polymer between the confined space and the bulk solution is zero. Surprisingly, we found that the long water-soluble polymers with a small hydrophobic group can be efficiently trapped inside the water channels (nanopores) of the hexagonal phase of non-ionic surfactant.

(+48) 22 343-31-23

Kasprzaka 44/52, 01-224 Warsaw, Poland