Dependence of thermal diffusion effects in liquids on the physical properties of the dispersing phase

A generalization of the radiation-pressure theory of thermal diffusion in liquids explains the genesis of the forces acting in a condensed phase when heat flows through it. The analytical expressions obtained make it possible to connect such forces originated by the radiation pressure of thermal waves with the transport of matter taking place in solutions or suspensions of particles and also with the ultimate result of this transport, that is, the steady-state concentrations of the dispersed phase in the hot and in the cold regions of the nonisothermal solution. The form in which the theoretical results are laid down lends itself to direct and unambiguous experimental verification. The confrontation with a few data found in the literature lends support to the theory.     

The application of the perrin–agishev model for calculation of the translational diffusion coefficients of liquid dichloroalkanes

The article presents the translational diffusion coefficients calculated for dichloroalkanes series C _n H_{2 n} Cl₂ (where n =?6, 8, 10, 12) in the liquid state, with the use of the Perrin and Agishev model. It has been shown that the molecules of dichloroalkanes assume a mutually parallel arrangement in three possible coordinations. The model of arrangement, orientations and packing of the molecules has been presented. The activation parameters of the compounds studied have been discussed. The physical and structural properties of the liquids studied (macroscopic density, electron density and molecular weight) are correctly described within the van der Waals model predicting their orientations and packing. The formulae linking the diffusion, viscosity and temperature for the liquids have been presented. The assumption that each molecule can be approximated by an ellipsoid of the semiaxes lengths a, b and c has been justified. The translations become slower with increasing volume and weight of the molecule. The diffusion coefficients decrease with increasing molecular weight.     

Liquid Structure and Thermodynamics of Alkane Mixtures

Spectroscopic investigations and light scattering experiments with saturated, liquid hydrocarbons and their mixtures indicate a specific and distinct influence of the constitution, conformation, and flexibility of the molecule on the structure and macroscopic behavior of such liquids. Orientational order present in pure liquid n-alkanes, for example, characteristically affects the thermodynamic mixing properties, such as the enthalpy of mixing ΔH_M and the entropy of mixing ΔS_M , when these liquids are mixed with each other, or with other liquids. Nowadays it is possible to determine thermodynamic mixing properties experimentally with such precision that systematic investigations of these properties allow the behavior of liquids to be studied qualitatively and–with molecular theories of liquids–to some extent also quantitatively. The latest results in this respect, exemplified by mixtures of alkanes, are discussed. These results not only demonstrate the progress made in understanding the relations between molecular (microscopic) and macroscopic properties, but are also of importance for industrial applications (e.g. separation processes) in which mixtures of hydrocarbons are involved.     

Mobility of Spherical Probe Objects in Polymer Liquids

We use scaling theory to derive the time dependence of the mean-square displacement ?Δr(2)? of a spherical probe particle of size d experiencing thermal motion in polymer solutions and melts. Particles with size smaller than solution correlation length ξ undergo ordinary diffusion (?Δr(2) (t)? ~ t) with diffusion coefficient similar to that in pure solvent. The motion of particles of intermediate size (ξ < d < a), where a is the tube diameter for entangled polymer liquids, is sub-diffusive (?Δr(2) (t)? ~ t(1/2)) at short time scales since their motion is affected by sub-sections of polymer chains. At long time scales the motion of these particles is diffusive and their diffusion coefficient is determined by the effective viscosity of a polymer liquid with chains of size comparable to the particle diameter d. The motion of particles larger than the tube diameter a at time scales shorter than the relaxation time τ(e) of an entanglement strand is similar to the motion of particles of intermediate size. At longer time scales (t > τ(e)) large particles (d > a) are trapped by entanglement mesh and to move further they have to wait for the surrounding polymer chains to relax at the reptation time scale τ(rep). At longer times t > τ(rep), the motion of such large particles (d > a) is diffusive with diffusion coefficient determined by the bulk viscosity of the entangled polymer liquids. Our predictions are in agreement with the results of experiments and computer simulations.     

Computerized measurement of viscoelastic properties of macromolecular solutions: Frequency dependence over and extended range of solvent viscosity

The modified Birnboim transducer and a computerized data acquisition and processing system (DAPS) for the measurement of viscoelastic properties of macromolecular solutions are described. The apparatus has a continuous frequency range from 0.01 to ca. 700 Hz and a viscosity range from 2 to ca. 30,000 poise (sample volume 1 to 1.5 cm²). Sample temperature is controlled to within 0.002°C from ?30°C to +80°C. Working displacements are 10² to 10⁴ Å. The DAPS is designed for precise determinations of the magnitudes and relative phasing of two sinusoidally time-varying electrical signals (0.02%, 0.02°, respectively, for signals > 2 V peak) from 10^?2 to 10⁵ Hz. Cross-correlation techniques are used for noise rejection. For frequencies below 30 Hz values of the storage (G') and loss (G″) components of the complex shear modulus (G^*) of 1 dyne/cm² are determined to within 10% and 4%, respectively, for liquids of moderately low viscosity. Proportionately higher measurement accuracies for typical values of G' and G″ and the wide frequency and viscosity ranges permit extrapolation to infinite dilution and studies of limited molecular flexibility for many polymer—solvent systems.     

Some Emulsion Features

Traditionally, emulsions have been defined as consisting of two liquids, of which one is dispersed in the other as macroscopic drops, stabilized by mono‐molecular layer of surfactant at the interface. However, a large majority of commercial emulsions are more complex than so and the added elements are essential for the properties of the emulsions including their stability.

With this in mind, this treatment of emulsions is divided into emulsions with mono‐layers and multilayers at the interface. In addition, additional elements in emulsions are described; such as lyotropic liquid crystals, vesicles, microemulsion droplets and solid particles, and their potential influence on the emulsion properties is indicated.     

Mass Distribution and Diffusion of [1‐Butyl‐3‐methylimidazolium][Y] Ionic Liquids Adsorbed on the Graphite Surface at 300–800 K

The structure and diffusion behavior of 1‐butyl‐3‐methylimidazolium ([bmim]⁺) ionic liquids with [Cl]^?, [PF₆]^?, and [Tf₂N]^? counterions near a hydrophobic graphite surface are investigated by molecular dynamics simulation over the temperature range of 300–800 K. Near the graphite surface the structure of the ionic liquid differs from that in the bulk and it forms a well‐ordered region extending over 30 Å from the surface. The bottom layer of the ionic liquid is stable over the investigated temperature range due to the inherent slow dynamics of the ionic liquid and the strong Coulombic interactions between cation and anion. In the bottom layer, diffusion is strongly anisotropic and predominantly occurs along the graphite surface. Diffusion perpendicular to the interface (interfacial mass transfer rate k_t) is very slow due to strong ion–substrate interaction. The diffusion behaviors of the three ionic liquids in the two directions all follow an Arrhenius relation, and the activation barrier increases with decreasing anion size. Such an Arrhenius relation is applied to surface‐adsorbed ionic liquids for the first time. The ion size and the surface electrical charge density of the anions are the major factors determining the diffusion behavior of the ionic liquid adjacent to the graphite surface.     

Experimental Determination of Third Derivative of the Gibbs Free Energy, <Emphasis Type="Italic">G</Emphasis> II: Differential Pressure Perturbation Calorimetry

We have been evaluating third derivative quantities of the Gibbs free energy, G, by graphically differentiating the second derivatives that are accessible experimentally, and demonstrated their power in elucidating the mixing schemes in aqueous solutions. Here we determine directly one of the third derivatives of G, the partial molar entropy-volume cross fluctuation density of 2-butoxyethanol (BE) in the BE–H₂O system, ^SV δ _BE . The difference of the heats of compression were directly determined using two identical cells and applying the same pressure change to both cells concurrently. Both cells are filled with sample solutions having a small appropriate difference in mole fraction. The results indicated that this method is feasible with the prior knowledge of the thermal expansivity of the solution to within a few per cent accuracy. If the volumes of the two cells are identical within the order of 0.01%, the method provides the required results to within 0.1% without the thermal expansivity data. This success opens a possibility of evaluating the fourth derivative graphically, which is expected to provide much more detailed information about the molecular processes in aqueous solutions.     

Collective drift of molecules in liquids according to the incoherent thermal-neutron scattering

The collective transport of molecules in liquids is studied. The main attention is given to determining the corresponding radius r _L of a Lagrangian particle, which is the most important element of the Lagrange theory of thermal hydrodynamic fluctuations and serves a basis for describing the collective drift in liquids. To determine r _L, we use the incoherent thermal-neutron scattering and a direct calculation method. It is found that the results of three independent methods for the determination of r _L are in good agreement with each other and confirm theoretical concepts of the nature of collective transport in liquids.     

Some electrophysical properties of polyphthalocyanines under the action of high pressure and shear deformation

The effect of high pressure or the combined action of high pressure and shear deformation on the conductivity G and capacitance C of metal-free polyphthalocyanines (PPhCs), differing in macromolecular size, order, and degree of crosslinking, has been studied. It has been found that both G and C increase with compression. A similar phenomenon is observed under the combined action of high pressure and shear deformation on PPhC, but in this case the critical pressure needed for change in G is lower than that observed in uniaxial compression experiments. Spectroscopic investigations show that the effects of pressure and deformation on the electrophysical properties of samples under load is not connected with changes in the chemical structure of PPhC, but appear to be due to the change in the average distance between the regions of continuous conjugation. The sizes of such regions may be increased by preliminary thermal treatment.     

Dispersed platinum and tin polyaniline film electrodes for the anodes of the direct methanol fuel cell

To develop better and cheaper electrocatalysts for the oxidation of methanol in direct methanol fuel cells, several combinations of a conductive polymer polyaniline (PANI) and dispersed metal particles such as Pt and Sn were examined. The anodic current for the methanol oxidation (i _MeOH) showing the electrocatalytic activity of Pt particles was remarkably enhanced when the particles were dispersed on PANI films that should provide higher surface areas for the dispersed particles. The activity strongly depended on the morphology and the electric conductivity of the PANI films electropolymerized in five different acid solutions: H₂SO₄, HNO₃, HClO₄, HBF₄, and HCl. The highest activity was achieved using the dispersed Pt particle on PANI film electropolymerized from H₂SO₄ polymerizing solution. In order to reduce the dispersed amount of the expensive Pt particles, other metal particles were pre-dispersed on the PANI film prepared from the H₂SO₄ polymerizing solution, and then Pt particles were dispersed on the film. Among the pre-dispersed metal particles attempted here (Sn, Cu, Cr, Ni, In, Co, Sb, Bi, Pb, and Mn), the highest activity was obtained with Sn particles. When the ratio of dispersed Pt to Sn particles ranges from 32:68 to 100:0, i _MeOH is higher than that measured with the dispersed Pt particle on PANI films without the Sn particles. This means that the dispersed amount of the Pt particles could be reduced by utilizing dispersed Sn particles.     

On the retention mechanisms and secondary effects in microthermal field-flow fractionation of particles

The behavior of nanometer or micrometer-sized particles, dispersed in liquid phase and exposed to temperature gradient, is a complex and not yet well understood phenomenon. Thermal field-flow fractionation (TFFF), using conventional-size channels, played an important role in the studies of this phenomenon. In addition to thermal diffusion (thermophoresis) and molecular diffusion or Brownian movement, several secondary effects such as particle–particle and/or particle–wall interactions, chemical equilibria with the components of the carrier liquid, buoyant and lift forces, etc., may contribute to the retention and complicate the understanding of the relations between the thermal diffusion and the characteristics of the retained particles. Microthermal FFF is a new high-performance technique allowing much easier manipulation and control of the operational parameters within an extended range of experimental conditions in comparison with conventional TFFF. Consequently, in combination with various other methods, it is well suited for a detailed investigation of the mentioned effects. In this work, some contradictory published results concerning the thermal diffusion of the colloidal particles, studied by TFFF but also by other methods, are analyzed and compared with our experimental findings.     

Stability and magnetic characterization of oleate-covered magnetite ferrofluids in different nonpolar carriers

This work describes the preparation and stability evaluation of suspensions consisting of hydrophobic magnetite nanoparticles dispersed in different organic solvents. The ferrite particles are covered by a shell of chemisorbed oleate ions following a procedure that is described in detail. The oleate-covered particles were dispersed in different organic solvents with dielectric constants, epsilon(r), ranging between 1.8 and 9, and the centrifugal field strength needed to remove particle aggregates formed during the synthesis was determined for the different liquid carriers used. A thermodynamic analysis demonstrated that the observed stability of the suspensions in liquids with epsilon(r) < 5 is well correlated with the very low lyophobic attraction between the particles. This can easily be surmounted by thermal agitation, since the van der Waals attraction is negligible. In contrast, for liquids with epsilon(r) > 9, the suspensions become unstable because of the combined action of the van der Waals and lyophobic attractions, the latter being dominant for very polar solvents. Finally, a complete magnetic characterization of the oleate-magnetite powder, as well as of several stable ferrofluids prepared with it, was carried out. From this characterization, the magnetic diameters and magnetic moments of the particles immersed in the different liquid carriers were estimated and compared to those corresponding to the dry magnetic particles. This made it possible to estimate the thickness of the nonmagnetic layer on the particles.     

Macromolecular reaction and interdiffusion in a compatible polymer blend, 2. The role of H‐bonding

A theory describing slow macromolecular reaction and interdiffusion in a compatible polymer blend is extended to consider H‐bonding. The known treatments of H‐bonding influence on the free energy of mixing and chains' mobilities are combined to calculate mutual diffusion coefficients in the framework of linear non‐equilibrium thermodynamics. Numerical calculations are performed for a blend of two random copolymers AC and BC to reveal the effect of H‐bonding (between A and B, B and B units) on the interdiffusion profiles. Then, the transformation of A units into B ones is included and the reaction‐diffusion equations are solved with the parameters corresponding to the blend of poly(tert‐butyl acrylate‐co‐styrene) with poly‐(acrylic acid‐co‐styrene) in which the thermal decomposition of tert‐butyl acrylate units takes place. The numerical calculations show that this system is suitable for the experimental verification of theoretical predictions concerning the interplay between macromolecular reaction and interdiffusion in polymer blends.     

Preparation of Nylon-6 Particles in Ionic Liquids

Summary: Utilizing the thermal stability of ionic liquid, micrometer-sized Nylon-6 particles were successfully prepared by hydrolytic polymerization of ε-caprolactam at high temperature with polyvinyl pyrrolidone as stabilizer in ionic liquids, 1-butyl-3-methyl imidazolium tetrafluoroborate, [Bmim][BF₄] and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoro-methanesulfonyl)amide, [DEME][TFSA]. The obtained particles had a unique shape because Nylon-6 is a crystalline polymer. Viscosity-average molecular weights of Nylon-6 prepared in [Bmim][BF₄] and [DEME][TFSA] at 180 °C for 48 h were 4200 and 2200, respectively.     

Review article: some transport phenomena in classical liquids and neutron scattering

Abstract

The sound velocity in toluene has been measured up to 2634 bar and at temperatures from 173 to 320 K using the pulse-echo overlap method. The sound velocity in n-heptane has been measured up to 2634 bar and at temperatures from 185 to 310 K by the phase comparison pulse-echo method. The density, the isothermal compressibility, the isobaric thermal expansion and the specific heat at constant pressure of both liquids have been evaluated from the measured sound velocity, following the method of Davis and Gordon. From a comparison of the calculated densities with those obtained previously from direct measurements, it is concluded that the method is very suitable for precise determination of liquid densities under elevated pressures.     

Temperature dependence of the phase separation in ethyl cyanoethyl cellulose/poly(acrylic acid)/4′‐n‐pentyl‐4‐cyano‐biphenyl composite films

A novel polymer‐dispersed liquid‐crystal film consisting of micrometer‐scale liquid‐crystal droplets in ultraviolet‐cured polymer composite matrices with cholesteric order was prepared and the influence of cure temperature on the phase separation was studied. The existence and pitch of the ethyl cyanoethyl cellulose cholesteric liquid‐crystalline phase were influenced by the existence of low molecular weight liquid crystals. The macromolecular cholesteric phase disappeared when the 4′‐n‐pentyl‐4‐cyano‐biphenyl concentration was over 40 wt %, and 4′‐n‐pentyl‐4‐cyano‐biphenyl domains were dispersed in the isotropic matrix of the polymer composite. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1334–1341, 2002     

Modeling and Spectroscopic Investigations on the Evaporation of Zirconia in a Thermal rf Plasma

The evaporation process of zirconia powders injected in a thermal rf plasma is investigated. Both model calculations and optical emission spectroscopy are used to study the evaporation behavior. Gas temperatures and velocity distributions are determined numerically from conservation laws and Maxwell equations. The influence of plasma and particle parameters on the thermal history of entrained particles is discussed. Asymmetric Abel inversion is applied to detect asymmetric emission profiles in the plasma source. Spectroscopic measurements reveal that evaporated zirconium is concentrated near the axis of the plasma. Numerical calculations show that line-integrated emission profiles can be used to distinguish the cases of complete and incomplete evaporation. Axial emission profiles confirm that the evaporation zone is shifted upstream of the plasma when smaller precursor particles are used.