Rotational diffusion of anisotropic-shaped colloidal particles in microgravity as studied by free-fall experiments

Rotational relaxation times (τ) and diffusion coefficients of ellipsoidal colloids of tungstic acid are studied in aqueous suspension in microgravity by free-fall experiments. τ values are evaluated from the relaxation traces of the optical transmittance of the suspension using the stopped-flow technique. Experimental errors at 0 G are small compared with those at 1 G, which is ascribed to lack of the movement of impurities in suspension such as quite small dust and bubbles and the convection of the suspension in microgravity. The limiting slopes of the relaxation curves in the plots of the transmittance against time at 1 G depend on the flow direction of the suspension in the flow cell, whereas those at 0 G remain zero irrespective of the flow direction. For more reliable diffusion coefficients are obtained in microgravity; however, the diffusion coefficients themselves are quite insensitive to gravity. Received: 10 November 1998 Accepted in revised form: 22 December 1998     

Rotational diffusion of tungstic acid colloids in microgravity as studied by aircraft experiments

 Rotational relaxation times (τ) of anisotropic tungstic acid colloids (3.24 μm in major axis) in aqueous suspension are measured in microgravity (0G), normal gravity (1G) and at 2G. The measurements at 0G and 2G are achieved by parabolic and circular flights, respectively. The limiting slopes of the relaxation curves in the plots of the transmitted light intensity against time are close to zero at 0G irrespective of the flow directions in the flow cell, whereas those at 1G and especially at 2G depend on the flow direction by the convection of the suspension and particle sedimentation. Experimental errors at the τ values at 0G are small compared with those at 1G and 2G, which is ascribed to the lack of movement of impurities in the suspension such as quite small air bubbles, which cannot be recognized with the naked eye, and the convection of the suspension in microgravity. More reliable rotational relaxation times are obtained in microgravity; however, the relaxation times themselves are quite insensitive to gravity. Theτ values observed are larger than those calculated from the particle size, which indicates the important contribution of the electrical double layers formed around the colloidal particles. Received: 22 February 2001 Accepted: 13 June 2001     

Microgravity experiments on colloidal crystallization of silica spheres in the presence of sodium chloride

 Rate coefficients (k) in the colloidal crystallization of monodispersed silica spheres in the presence of sodium chloride are studied in microgravity achieved by parabolic flights of an aircraft. Time-resolved reflection spectroscopy is made with a continuous circulating-type stopped-flow cell system. The k values decrease as the salt concentration increases both at 0 and 1 G and those in microgravity are smaller than those in normal gravity by 16% (maximum), especially in water and in the presence of a small amount of the salt lower than 2 × 10⁻⁶ mol/l. The rates in flight at 1 G are larger by 15% (maximum) compared with those at 1 G on the ground. The k values obtained at 0 G, 1 G in flight and 1 G on the ground agree excellently with each other for the suspensions with 3 × 10⁻⁶ and 4 × 10⁻⁶ mol/l sodium chloride. Disappearance of the downward diffusion of spheres and no convection of the suspensions are important for retardation in microgravity. Received: 20 January 2000 Accepted: 9 March 2000     

Reentrant behavior of poly(vinyl alcohol)–borax semidilute aqueous solutions

 The reentrant behavior of Poly(vinyl alcohol) (PVA)–borax aqueous semidilute solutions with a PVA concentration of 20 g/l and borax concentrations varies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS) and dynamic viscoelastic measurements. Two (fast and slow modes) and three (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solutions without and with the presence of borax, respectively, were observed from DLS measurements. The fast and middle relaxation modes were q ²-dependent (q is the scattering vector) characteristic of diffusive behavior; however, the slow modes were q ³-dependent, characteristic of intraparticle dynamics. The experimental results showed that the slow relaxation mode dominates the DLS relaxation. The DLS slow mode relaxation time, τ_s, and the viscoelastic modulus G′(ω) and G′′(ω) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased from 0.0 to 0.20 M, i.e. τ_s, G′(ω), and G′′(ω) fluctuated with increasing borax concentration. The excluded-volume effect of polymers, charge repulsion among borate ions bound on PVA molecules, and intermolecular cross-linking didiol–borate complexation caused an expansion of the polymer chain; however, the screening effect of free Na⁺ ions on the negative charge of the borate ions bound on PVA and intramolecular cross-linking didiol–borate complexation led to a shrinkage of the polymer chain. The reentrant behavior was the consequence of the balance between expansion and shrinkage of the PVA–borate complex. Received: 26 March 1999/Accepted in revised form: 3 September 1999     

Microwave dielectric properties of potassium hydroxide aqueous solutions

Microwave dielectric properties of potassium hydroxide aqueous solutions were studied at frequencies of 13 to 25 GHz over a wide range of concentrations and temperatures of 288 to 308 K. Experimental electrical conductivity data were used to determine ionic losses at high frequencies. The static dielectric permittivity σ_s, dielectric relaxation time τ, and thermodynamic activation parameters of dielectric relaxation (ΔH _ɛ⁺⁺, ΔG _ɛ⁺⁺, and ΔS _ɛ⁺⁺) were calculated. These quantities decrease in transfer from water to solutions. Thereby we demonstrate that potassium hydroxide has a disturbing effect on the initial hydrogen-bond network of water.     

Theoretical Study of the Interactions between Isolated <Emphasis Type="Italic">DNA</Emphasis> Bases and Various Groups IA and IIA Metal Ions by <Emphasis Type="Italic">Ab Initio</Emphasis> Calculations

Summary. Interactions of the DNA bases adenine (A), guanine (G), cytosine (C), and thymine (T) with various metal ions (M) of groups IA and IIA of the periodic table of the elements were studied at the HF, MP2, and DFT levels of theory. The structures and thermodynamic stabilities of these species were studied at the gas phase. The calculations uphold that there exist two active sites in G and one in A, C, and T. The calculations also show that the O² atom in T is a more active site for metal ion bindings than that in C. The stability energies for G … M complexes are larger than those for A … M complexes and the stability energies for T … M complexes are larger than those for C … M complexes. As z/r ratio for the metal ion increases, the interaction energy for the complex increases systematically. Thermodynamic quantities such as ΔH, ΔG, ΔS, and ln K were determined for each complexation reaction, [Base+M ⁿ⁺ →(Base … M) ⁿ⁺]. A, G, and C complexation reactions except for C … Rb⁺ are exothermic. The situation is quite different for T complexation reactions and all except for T … Be²⁺ and T … Mg²⁺ are endothermic.     

Theoretical Study of the Interactions between Isolated DNA Bases and Various Groups IA and IIA Metal Ions by Ab Initio Calculations

Interactions of the DNA bases adenine (A), guanine (G), cytosine (C), and thymine (T) with various metal ions (M) of groups IA and IIA of the periodic table of the elements were studied at the HF, MP2, and DFT levels of theory. The structures and thermodynamic stabilities of these species were studied at the gas phase. The calculations uphold that there exist two active sites in G and one in A, C, and T. The calculations also show that the O² atom in T is a more active site for metal ion bindings than that in C. The stability energies for G … M complexes are larger than those for A … M complexes and the stability energies for T … M complexes are larger than those for C … M complexes. As z/r ratio for the metal ion increases, the interaction energy for the complex increases systematically. Thermodynamic quantities such as ΔH, ΔG, ΔS, and ln K were determined for each complexation reaction, [Base+M ⁿ⁺ →(Base … M) ⁿ⁺]. A, G, and C complexation reactions except for C … Rb⁺ are exothermic. The situation is quite different for T complexation reactions and all except for T … Be²⁺ and T … Mg²⁺ are endothermic.     

Enthalpy relaxation in an epoxy-cycloaliphatic amine resin

Enthalpy relaxation in a system containing the diglycidyl ether of bisphenol A (DGEBA) resin and a diamine, 1,3-bisaminomethylcyclohexane (1,3-BAC) as curing agent, has been investigated by differential scanning calorimetry (DSC). Samples fully cured were annealed at temperature T_g–15 °C for periods of time from 1 h to a maximum of 168 h. The enthalpy relaxation is analyzed by the peak shift method, in which the sample is heated at 10 °C/min following cooling at various rates through the glass transition region. The key parameters of structural relaxation determined were the non-linearity parameter x=0.47 ± 0.02, the apparent activation energy Δh*=1264 ± 48 kJ/mol or Δh*/R=152 ± 6 kK and the non-exponentiality parameter β ≈ 0.3. The results, obtained by the same method, were compared with those for other systems based on fully cured DGEBA. The correlations among these parameters with the peak shift model should be considered with caution. However, the results show that a correlation between crosslink lengths and the value of Δh* can be considered. The relaxation process for DGEBA/1,3-BAC proves to be highly cooperative. Received: 28 June 2000 Accepted: 6 September 2000     

Dielectric relaxation of sulfolane in benzene solution from microwave absorption data

The electric constant (ɛ′) and dielectric loss (ɛ″) for dilute solutions of sulfolane in benzene solution has been measured at 9.885 GHz at different temperatures (25, 30, 35, and 40°C) by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (τ) and dipole moment (μ) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. Based on the above studies, monomer structure of sulfolane in benzene has been inferred. The presence of solute-solvent associations in benzene solution has been proposed. Energy parameters (ΔH _ɛ, ΔF _ɛ, ΔS _ɛ) for dielectric relaxation process of sulfolane in benzene at 25, 30, 35, and 40°C have been calculated and compared with the corresponding energy parameters (ΔH _η, ΔF _η, ΔS _η) for the viscous flow.     

Stress-induced compaction of concentrated dispersions of gel particles

 When submitted to successive shear stress steps, the elastic shear modulus of a concentrated dispersion of soft gel particles shows an exponential increase from 50 to 110 Pa. A slow relaxation time (τ _r ≃500 s) attributed to the mobility of the gel particles within their free volume is obtained. The amplitude of the relaxation time distribution decreases with the number of shear stress sequences, indicating a progressive decrease in the free volume available per particle. The results are explained by an increase in the packing density as grains rearrange under the external constraint. A rate constant is determined from the evolution of the dispersion's elastic modulus (K≃4 × 10⁻⁴ s⁻¹). The rate of compaction shows a logarithmic decrease when the initial particle's packing fraction is increased. Received: 5 October 1999 In revised form: 21 December 1999 Accepted: 21 January 2000     

Slow salt-induced aggregation of citrate-covered silver particles in aqueous solutions of cellulose derivatives

In this work, the salt-induced aggregation of bare and polymer-covered silver particles has been studied with the aid of light scattering and UV-visible spectroscopy. Light scattering on a suspension of bare silver particles at a low salt concentration shows that the cluster fractal dimension d _f changes from 1.6 to 2 in the course of the aggregation process, whereas no restructuring of the clusters is observed at a higher salinity where d _f ≈ 1.6. The growth of the clusters over time can be described by a power law R _h ∝ t ^α , where R _h is the apparent hydrodynamic radius. The UV-visible experiments revealed that increasing the size of the bare silver particles lead to a significant broadening and red-shift of the absorbance band, whereas for salt-induced growth of Ag clusters, a blue-shift and broadening was observed. Addition of salt to a suspension of silver particles and hydroxyethylcellulose divulged a slower broadening of the surface plasmon peak than without polymer.     

Effect of organic salts on micellar growth and structure studied by rheology

We report a rheological study on the effect of adding organic salts [sodium tosylate (NaTos) and benzoic acid potassium salt (BaPs)] on the micellar growth and structure of aqueous solutions of cethyltrimethylammonium chloride (CTAC) at a constant molar concentration ratio [salt]/[CTAC]. The rheological data show two well-defined domains of growth characterized by scaling laws for the surfactant concentration. The addition of NaTos leads to an unusual maximum in the viscosity-surfactant concentration curve. Before the maximum (domain 1), the analysis of the data (η₀, τ_R and G ₀) suggests the presence of branched micelles (connections). After the maximum (domain 2), however, the exponents of the scaling laws do not reflect either the relaxation of this branched structure or that of an entangled transient network structure. A faster mechanism of relaxation, not yet elucidated governs their dynamics. The exponents of the power laws in the presence of the BaPs are found, however, to be in accordance with the theory of equilibrium polymers. Received: 15 April 1998 Accepted in revised form: 20 October 1998     

An ab initio Molecular Orbital Study of the Conformational Properties of all-( Z )-Cyclododeca-1,4,7,10-tetraene

 Ab initio HF/6-31G^* and MP2/6-31G^*//HF/6-31G^* methods were used to calculate the structure optimization and conformational interconversion pathways for all-(Z )-cyclododeca-1,4,7,10-tetraene. This compound adopts the symmetrical crown (C _4v) conformation. Ring inversion takes place via symmetrical intermediates, such as boat-chair (BC, C _s) and twist (C _2h) conformers and requires about 22.3 kJ · mol⁻¹. The calculated strain energies for BC and twist conformers are 5.9 and 13.5 kJ · mol⁻¹. The results of semiempirical AM1 calculations for structural parameters and relative energies of the important geometries of the title compound are in good agreement with the results of ab initio methods.     

Quasi-thermodynamics of Viscous Flow of Electrolyte Solutions in Aqueous, Non-aqueous and Mixed Aqueous Solvents

Viscosity B-coefficients for cesium chloride and lithium sulfate in methanol + water mixtures at 25 and 35 °C are reported. A general treatment of the quasi-thermodynamics of viscous flow of electrolyte solutions is described. ΔG ₃ ^Θ (1→1′), the contribution made to the Gibbs energy of activation of the solution by the influence of the solute on the solvent, is a function of solute–solvent interactions only; but, ΔH ₃ ^Θ (1→1′) and ΔS ₃ ^Θ (1→1′) also reflect the solvent–solvent interactions. In aqueous solution all alkali-metal ions except Li⁺ are sterically unsaturated, having solvent co-ordination numbers n<n _max, the maximum allowed sterically. Such complexes exchange molecules with the solvent more readily than saturated ones and have energy–reaction co-ordinate diagrams in forms that explain the negative B or ΔG ₃ ^Θ (1→1′) values found in aqueous solution. Saturated complexes are the norm in non-aqueous solvents, and the ΔG ₃ ^Θ (1→1′) values are determined mainly by the secondary solvation. Behavior in mixed solvents reflects the transition from aqueous to non-aqueous behavior across the range of solvent composition.     

Conformational Properties of (Z,Z)-, (E,Z)-, and (E,E)-Cycloocta-1,4-dienes

Summary.  Ab initio calculations at the HF/6-31G^* level of theory for geometry optimization and the MP2/6-31G^*//HF/6-31G^* level for a single point total energy calculation are reported for (Z,Z)-, (E,Z)-, and (E,E)-cycloocta-1,4-dienes. The C ₂-symmetric twist-boat conformation of (Z,Z)-cycloocta-1,4-diene was calculated to be by 3.6 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form; the calculated energy barrier for ring inversion of the twist-boat conformation via the C _S-symmetric boat-boat geometry is 19.1 kJ·mol⁻¹. Interconversion between twist-boat and boat-chair conformations takes place via a half-chair (C ₁) transition state which is 43.5 kJ·mol⁻¹ above the twist-boat form. The unsymmetrical twist-boat-chair conformation of (E,Z)-cycloocta-1,4-diene was calculated to be by 18.7 kJ·mol⁻¹ more stable than the unsymmetrical boat-chair form. The calculated energy barrier for the interconversion of twist-boat-chair and boat-chair is 69.5 kJ·mol⁻¹, whereas the barrier for swiveling of the trans-double bond through the bridge is 172.6 kJ·mol⁻¹. The C _S symmetric crown conformation of the parallel family of (E,E)-cycloocta-1,4-diene was calculated to be by 16.5 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form. Interconversion of crown and boat-chair takes place via a chair (C _S) transition state which is 37.2 kJ·mol⁻¹ above the crown conformation. The axial- symmetrical twist geometry of the crossed family of (E,E)-cycloocta-1,4-diene is 5.9 kJ·mol⁻¹ less stable than the crown conformation. Corresponding author. E-mail: isayavar@yahoo.com Received March 25, 2002; accepted April 3, 2002     

An ab initio Molecular Orbital Study of the Conformational Properties of all-(Z)-Cyclododeca-1,4,7,10-tetraene

Summary.  Ab initio HF/6-31G^* and MP2/6-31G^*//HF/6-31G^* methods were used to calculate the structure optimization and conformational interconversion pathways for all-(Z )-cyclododeca-1,4,7,10-tetraene. This compound adopts the symmetrical crown (C _4v) conformation. Ring inversion takes place via symmetrical intermediates, such as boat-chair (BC, C _s) and twist (C _2h) conformers and requires about 22.3 kJ · mol⁻¹. The calculated strain energies for BC and twist conformers are 5.9 and 13.5 kJ · mol⁻¹. The results of semiempirical AM1 calculations for structural parameters and relative energies of the important geometries of the title compound are in good agreement with the results of ab initio methods. Received July 9, 2001. Accepted September 26, 2001     

Conductivity of flowing polyaniline suspensions in electric field

The formation of chain structures by polarized polyaniline (PANI) particles suspended in silicone oil in the electric field has been monitored by recording suspension conductivity in the course of time. For that purpose, three types of PANI particles differing in the conductivity (3.1 × 10⁻³, 1.7 × 10⁻¹, and 2.0 × 10⁻¹ S cm⁻¹) have been chosen out of a series of nine samples prepared by controlled protonation of PANI base in orthophosphoric acid solutions. Relaxation times reflecting this process and characterizing the rate of the response to the electric field decreased with particle conductivity, indicating a higher polarizability of particles. At the same time, the maximum conductivity of suspension increased as a consequence of the electric and shear forces acting on the particles. In the shear fields, shorter relaxation times appeared than at rest. The simultaneous measurement of the shear stress confirmed that the conductivity investigation can reliably characterize the development of electrorheological structures.     

Quasielastic light scattering study of chemically crosslinked gelatin solutions and gels

Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde (GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10⁻⁵ to 1×10⁻³ (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g ₂(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−D _f q ² t)+Bexp(−t/τ_c)^β, both in the sol (50 and 60 ^∘C) and gel states (25 and 40 ^∘C). The fast-mode diffusion coefficient, D _f showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)^1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched exponential factor exp(−t/τ_c)^β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β₀+β₁ T+β₂ T ². The slow-mode relaxation time, τ_c, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τ_c(c)=τ₀+τ₁ c+τ₂ c ². Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state. Received: 25 May 1999 /Accepted in revised form: 27 July 1999     

Microwave Exposure Affecting Reproductive System in Male Rats

The object of present study is to investigate the effects of 50 GHz microwave frequency electromagnetic fields on reproductive system of male rats. Male rats of Wistar strain were used in the study. Animals 60 days old were divided into two groups—group I sham exposed and group II experimental (microwave exposed). During exposure, rats were confined in Plexiglas cages with drilled ventilation holes for 2 h a day for 45 days continuously at a specified specific absorption rate of 8.0 × 10⁻⁴ W/kg. After the last exposure, the rats were sacrificed immediately and sperms were collected. Antioxidant enzyme (superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase), histone kinase, apoptosis, and cell cycle were analyzed in sperm cells. Result shows a significant decrease in the level of sperm GPx and SOD activity (p ≤ 0.05), whereas catalase shows significant increase in exposed group of sperm samples as compared with control (p < 0.02). We observed a statistically significant decrease in mean activity of histone kinase as compared to the control (p < 0.016). The percentage of cells dividing in a spermatogenesis was estimated by analyzing DNA per cell by flow cytometry. The percentage of apoptosis in electromagnetic field exposed group shows increased ratio as compared to sham exposed (p < 0.004). There were no significant differences in the G₀/G₁ phase; however, a significant decrease (p < 0.026) in S phase was obtained. Results also indicate a decrease in percentage of G₂/M transition phase of cell cycle in exposed group as compared to sham exposed (p < 0.019). We conclude that these radiations may have a significant effect on reproductive system of male rats, which may be an indication of male infertility.     

Multiplex ELISA in a single microfluidic channel

In this article, we demonstrate a novel approach to implementing multiplex enzyme-linked immunosorbent assay (ELISA) in a single microfluidic channel by exploiting the slow diffusion of the soluble enzyme reaction product across the different assay segments. The functionality of the reported device is realized by creating an array of ELISA regions within a straight conduit that are selectively patterned with chosen antibodies/antigens via a flow-based method. The different analytes are then captured in their respective assay segments by incubating a 5-μL aliquot of sample in the analysis channel for an hour under flow conditions. Once the ELISA surfaces have been prepared and the enzyme substrate introduced into the analysis channel, it is observed that the concentration of the soluble enzyme reaction product (resorufin) at the center of each assay region grows linearly with time. Further, the rate of resorufin generation at these locations is found to be proportional to the concentration of the analyte being assayed in that segment provided that the ELISA reaction time in the system (τ _R ) is kept much shorter than that required by the resorufin molecules to diffuse across an assay segment (τ _D ). Under the operating condition τ _R  << τ _D , the reported device has been shown to have a 35% lower limit of detection for the target analyte concentration compared with that on a commercial microtiter plate using only a twentieth of the sample volume.