Preparative free-flow isoelectric focusing: modeling and experiments

Free-flow isoelectric focusing was adapted to preparative scale separations and chemical engineering methods were used to describe the main mechanisms operating in the apparatus. A mixture of human serum albumin (pI 4.6) and beta-lactoglobulin (pI 5.22) was separated in pH gradients, generated with carrier ampholytes of different origin and covering the pH ranges 4-6.5, 3.5-5, 4-5.5 and 4.5-5.0. Best results were obtained in the pH 4-5.5 range. The experimental results have validated the results obtained with a numerical model.     

Kinetics of gold nanoparticle aggregation: experiments and modeling

We investigate the aggregation kinetics of gold nanoparticles using both experimental techniques (i.e., quasi-elastic light scattering, UV-visible spectroscopy, and transmission electron microscopy) and mathematical modeling (i.e., constant-number Monte Carlo). Aggregation of gold nanoparticles is induced by replacing the surface citrate groups with benzyl mercaptan. We show that the experimental results can be well described by the model in which interparticle interactions are described by the classical DLVO theory. We find that final gold nanoparticle aggregates have a fractal structure with a mass fractal dimension of 2.1-2.2. Aggregation of approximately 11 initial gold nanoparticles appears to be responsible for the initial color change of suspension. This kinetic study can be used to predict the time required for the initial color change of a gold nanoparticle suspension and should provide insights into the design and optimization of colorimetric sensors that utilize aggregation of gold nanoparticles.     

Structural ordering and phase behavior of charged microgels

Recent theoretical phase diagrams for loosely cross-linked ionic microgels with a low monomer volume fraction (Gottwald; et al. Phys. Rev. Lett. 2004, 92 , 068301 ) have predicted a re-entrant order-disorder transition (i.e., fluid-FCC-BCC-fluid) as a function of concentration and so far there has been no experimental verifications of these theoretical predictions. Here, we present experimental results on phase behavior of loosely cross-linked charged poly(N-isopropylacrylamide co acrylic acid) (PNIPAm-co-AAc) microgesls with a low monomer volume fraction (approximately 0.003) for a wide range of concentrations (0.02-0.6 wt %) using static and dynamic light scattering methods. These microgel dispersions exhibit a short-range liquid order at low concentration (<0.03 wt %), a FCC crystalline order at intermediate concentrations (0.03- 0.3 wt %). In addition, we suggested a possible coexistence of BCC and FCC phases at higher concentration crystalline suspension (approximately 0.34 wt %). These results clearly demonstrate the experimental verification of above theoretical prediction below the overlap concentration and also reveal that the interaction potential between the microgel particles is of screened Coulomb repulsive type within these concentration ranges. At further higher concentration (approximately 0.57 wt %), we once again observed a disordered state and this disordered state from dynamic light scattering was confirmed to be a glass. These initial results are discussed in the light of previously reported results on the phase behavior of ionic microgel colloidal dispersions.     

Molecular modeling of adsorption and ordering at solid interfaces

Interfaces between liquids and solid surfaces are of considerable scientific as well as technological interest, in particular in the context of the adsorption and organization of molecular films. In recent years the direct observation of the molecular structure and often even the dynamics of ordered monolayers at such hidden interfaces has been made possible by the rapid development in scanning probe microscopy. Nevertheless, there is still a lack of understanding with respect to the formation and organization of such films and their interaction with the experimental apparatus. Here computer modeling plays an increasing role as both the complexity of the interfaces and the available computer power increase. This article addresses the application of phenomenological molecular modeling to physisorption at solid surfaces with a special emphasis on the liquid-solid interface. The paper presents an overview over different modeling approaches and illustrates their application in a series of examples ranging from the simulation of adsorption isotherms of small molecules to the prediction of the structure of physisorbed layers for larger molecules.     

Autoignition of heptanes; experiments and modeling

There is much interest in determining the influence of molecular structure on the rate of combustion of hydrocarbons; the C₇H₁₆ isomers of heptane have been selected here as they exemplify all the different structural elements present in aliphatic, noncyclic hydrocarbons. With the exception of n‐heptane itself, no autoignition studies have been carried out to date on the other isomers of heptane at high temperatures. Therefore, ignition delay times were measured for the oxidation of four isomers—n‐heptane, 2,2‐dimethylpentane, 2,3‐dimethylpentane, and 2,2,3‐trimethylbutane—under stoichiometric conditions at a reflected shock pressure of 2 atm, within the temperature range of 1150–1650 K. Measurements under identical conditions reveal that they all have essentially the same ignition delay time; this confirms earlier theoretical predictions based purely on detailed chemical kinetic modeling. The variation of ignition delay times for n‐heptane with changing oxygen concentrations and reflected shock pressure was determined and shown to follow expected trends. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 728–736, 2005     

Structural ordering in glass forming tellurium-germanium melts

The character of reorganization of the short-range order and the scale structure during the ordering of glass forming melts is elaborated based on the results of diffraction and microscopic investigations of the ordered and glassy alloys in tellurium-germanium systems. For this purpose cooling curves reflecting the formation processes of structural levels in real time are analyzed and the effective activation energies are estimated.     

Viscoelastic modeling of nanoindentation experiments: A multicurve method

Nanoindentation is an increasingly used method of extracting surface mechanical properties of viscoelastic materials, especially polymers. Recently, Hutcheson and McKenna used a viscoelastic contact mechanics model to analyze the contact problem between a nanosphere and polystyrene surface. In nanoindentation experiments, the ramp loading test is a similar problem to the particle embedment experiment except that the indentation load function differs. The motivation in this work is to expand the Hutcheson and McKenna analysis to the nanoindentation problem. In particular, we illustrate the limitations of analyzing only a single load‐indentation curve, which does not provide enough information to determine the full range of the viscoelastic response of a polymer, and we show that performing a test sequence that includes multiple loading rates or indentation rates spanning two or more orders of magnitude greatly improves the extracted viscoelastic properties. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 633–639     

Structural ordering transitions in ionic liquids mixtures

Differential capacitance curves for the electrical double layer (EDL) of mixtures of imidazolium-based ionic liquids (ILs) with a common cation (1-ethyl-3-methylimidazolium, [C₂MIM]⁺) and two different anions (bis(trifuoromethylsulfonyl)imide, [Tf₂N]⁻) and tris(pentafluoroethyl)trifluorophosphate [FAP]⁻) were obtained. Sharp peaks in the differential capacitance curves were observed for a small range of mixtures compositions at positive charge densities. The appearance and position on the potential scale of the peaks were found to be dependent on the mixture composition and temperature. The occurrence of these phenomena is interpreted as corresponding to an abrupt change in the EDL structure arrangement as a result of a complex interplay of electrostatic interactions and steric effects. The use of the non-structured mercury electrode allowed to decouple the eventual potential induced restructuring occurring at the double layer from the well-known surface reconstruction effects often reported for ionic liquids in contact with single crystal face electrodes.     

Thermal decomposition of monomethylhydrazine: Shock tube experiments and kinetic modeling

The thermal decomposition of gaseous monomethylhydrazine (MMH) was studied by recording MMH absorption at 220 nm of the reacting gas behind a reflected shock wave at temperatures of 900–1370 K, pressures of 140–450 kPa, and in mixtures containing 97.5–99 mol% argon. Based on previous work (Sun and Law; J Phys Chem A 2007, 111(19), 3748–3760), a kinetic mechanism was developed over extended temperature and pressure ranges to model these experimental data. Specifically, the temperature and pressure dependence of the unimolecular rate coefficients on the dissociation of MMH and the associated radicals were calculated by the QRRK/Master equation analysis at temperatures of 300–2000 K and pressures of 1–100 atm based on published thermochemical and kinetic parameters. They were then fitted using the Troe formalism and incorporated in the kinetic model. This unadjusted model was then used to predict the MMH decomposition profiles at different temperatures and pressures for seven groups of MMH/Ar mixtures and the half‐life decomposition times from shock tube experiments. Good agreement was observed below 940 K and above 1150 K for the diluted MMH/Ar mixtures. The model predictions further show that the overall MMH decomposition rate follows first‐order kinetics, and that the N–N bond scission is the most sensitive reaction path for the modeling of the homogeneous decomposition of MMH at elevated pressures. However, the model predictions deviate from the experimental data with the incubation period of ca. 100 μs observed in the 1030–1090 K temperature range, and it also predicts longer ignition delays for highly concentrated MMH/Ar mixtures. The discrepancy between the model predictions and experimental data at these special conditions of MMH decomposition was analyzed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 176–186, 2009     

Competitive adsorption of a polydisperse polymer during emulsification: experiments and modeling

In this paper we study the selective adsorption of a high molar mass polymer, OSA-starch, at the cyclohexane/water interface during emulsification. This was made possible through the use of AsFlFFF-MALS-RI which enables us to characterize the size and molar mass of polydisperse ultrahigh molar mass polymers. The results show that the high molar mass components in the molar mass distribution of the polymer were selectively adsorbed. The selective adsorption is most likely due to convective mass transport in the turbulent flow fields, during formation of the emulsion, which favors transport to the interface of the high molar mass polymers in the sample. The rapid adsorption that occurs during emulsification is likely to give rise to nonequilibrium effects and jamming at the interface. Furthermore, we describe the adsorption process and illustrate its selective nature through a turbulent flow collision model.     

Eu(III) adsorption on rutile: Batch experiments and modeling

Eu(III) adsorption on rutile was investigated as a function of contact time, pH, ionic strength and Eu(III) concentration by using a batch experimental method. The effects of carbonate, sulfate, and phosphate were also studied. It was found that the kinetics of Eu(III) adsorption on rutile could be described by a pseudo-second-order model. The adsorption of Eu(III) on rutile is strongly pH-dependent, but relatively insensitive to ionic strength. A double layer model (DLM) with two inner-sphere Eu(III) surface complexes was applied to quantitatively interpret the adsorption of Eu(III) on rutile. There were no apparent effects of carbonate and sulfate on Eu(III) adsorption, whereas the presence of phosphate promoted Eu(III) adsorption on rutile. The surface complexes of Eu(III) on rutile were evidenced by X-ray photoelectron spectroscopy (XPS).     

Reaction rates of heavy metal ions at goethite: relaxation experiments and modeling

In the present paper we extend our theory that calculates the fastest reaction step observable in suspensions containing charged microcrystals and heavy metal cations. The calculation requires the solution of the nonlinear Poisson-Boltzmann equation for nonsymmetric electrolytes plus the Nernst-Planck equation for transport of ions in electric fields. We find that the diffusional transport of ions to and from the surface is the rate-limiting process for our experimentally observed maximum rates. At low pH and low metal ion concentration the diffusion of metal ions is the rate-limiting step, whereas for high pH and high metal ion concentration the diffusion of the solvated protons controls the overall relaxation rate. The validity of this theory is checked for the reactions of Pb2+ and Cd2+ with goethite by means of pressure jump relaxation experiments over a wide range of temperature and pH. In all cases we observe fast processes (relaxation in the range of 10(3) s(-1)) in quantitative agreement with the theory, followed by slower processes, most probably caused by diffusion into the interior of the porous microcrystals.     

Structural ordering and spectral properties of smectic A with biaxial solute molecules

The influence of (i) orientational-translational ordering of solvent and solute molecules, (ii) anisotropic intermolecular solute-solvent interactions and (iii) the features of the electronic structure of biaxial solute molecules dissolved in a smectic A phase on the spectral position of polarized bands of a solute electronic absorption has been investigated. Equations for the positional-orientational pseudopotential in a pure smectic A doped with biaxial solute molecules have been obtained within the framework of the molecular statistical approach. The question about the correlation of contributions of partial orientational and translational molecular ordering to the spectral properties of a molecular system has been answered.     

Plasmonic properties of single multispiked gold nanostars: correlating modeling with experiments

Gold nanostars, possessing multiple sharp spikes, have emerged as promising plasmonic particles in the field of ultrasensitive sensing. We have developed a water-based method for high-yield synthesis of size-tunable anisotropic gold nanoparticles with a varying number of spiky surface protrusions, and performed systematic experimental and theoretical analyses of the optical properties of the single gold nanostars by characterizing them simultaneously with scanning electron microscopy and dark-field scattering spectroscopy. The morphologies and corresponding scattering spectra of the individual gold nanostars have been compared with electromagnetic simulations of the plasmonic resonances utilizing the finite-difference time-domain (FDTD) method. The study provides a correlation between the experimental and calculated scattering spectra and charge distributions of the different plasmon modes in the individual gold nanostars with varying numbers and relative orientations of surface protrusions. Our results provide guidelines for choosing gold nanostars with a proper number of spikes and appropriate dimensions of the core and arms for particular plasmonic applications as well as for further developing preparation methods of multispiked metal nanoparticles.     

Structural investigations and modeling of cavities in clathrates

Summary A molecular-graphics study has been performed in order to build and visualize the shape of cavities within different clathrates from X-ray diffraction data [e.g. Dianin's compound, Werner complexes Ni(SCN)₂(3-methylpyridine)₄, Fe(acetylacetonate)₃ and Ni(ethylxanthate)₂(4,4-dimethyl-2,2-bipyridyl) complexes]. The algorithm of the solvent-accessible surfaces representation has been applied to a part of the whole crystal structure rather than to isolated host molecules, by using the MOLCAD molecular modeling package. This type of modelization has been found very efficient both to study the shape properties of the host cavities (cage or channel types) and to approach the structural features of the host/guest interactions.     

Orthotropic elastic behaviors and yield strength of fused deposition modeling materials: Theory and experiments

This work is devoted to study the mechanical behaviors of polylactic acid (PLA) materials from additive manufacturing, and an orthotropic model is established to predict the mechanical properties under arbitrary printing orientation. Firstly, the morphology of PLA material is analyzed by using scanning electron microscopy, from which the orthotropic behavior of PLA material is obtained. Three printing planes are adopted, and on each printing plane different printing angles may be selected. The mechanical parameters, including Young's modulus, yielding stress, and Poisson's ratio, for material under different printing directions are determined via quasi-static experiments. Secondly, the orthotropic constitutive model of PLA materials under different printing angles is thus obtained, and the prediction method of orthotropic mechanical properties is built based on the coordinate transformation matrix, where the orthotropic coordinate transformation matrix is acquired by attitude angles (i.e., Euler angle, the rotation angle of the local coordinate system relative to the global coordinate system). Finally, the theoretical prediction method was verified, and high-quality printing methods were recommended. In addition, the obtained results of the model show that: for PLA material, the orthotropic hypothesis model is superior to the transverse isotropic hypothesis one. This present method is not only suitable for predicting the constitutive model of printed specimens in any direction but also for other materials of fused deposition modeling.