Dielectric behaviour of Gelatin solutions and Gels

 Sol and Gel state properties of aqueous gelatin solutions of concentrations 4%, 6%, 8% and 10% (w/v) have been investigated through dielectric relaxation studies done at various temperatures in the range T=20–60 °C carried out over a frequency range f=20 Hz–10 MHz and no relaxation of any nature was observed. The sharp transition observed at the gelation temperature T _gel provided an excellent matching with the same measured through differential scanning calorimetry (DSC). The capacitance (C _p) values above f=100 kHz became increasingly negative as the gel was melted to the sol state. However, in the gel state C _p was found to be almost independent of temperature for frequencies above 100 kHz. At frequencies lower than 10 kHz, C _p measured was ∼10⁵ F, implying pronounced interfacial polarization either due to electro-chemical reaction or because of ions getting trapped at some interface within the bulk. Received: 10 February 1997 Accepted: 2 September 1997     

An effective cross-linking of polymer layer on monodisperse, poly(maleic anhydride-styrene)-modified colloidal silica particles and properties of the composite

Summary  The cross-linkings of the surface polymer layer on mono disperse, poly(maleic anhydride-styrene)-modified silica particles by the reaction with diisocyanate were studied. The extent of cross-linking was estimated by the weight decrease by immersing the particles in the buffer solution of pH 2.0, 4.0 and 9.0 at a room temperature for 24 h. The reaction of the polymer-modified silica with 1,6-diisocyanatohexane afforded relatively stable composite particles which lost less than 5 wt% of the polymer in aqueous solution in the pH range 2.0–9.0. The diisocyanate was a preferable cross-linker to 2,4-diisocyanatotoluene in terms of stability in acidic or basic aqueous solution. The flexibility of the cross-linker molecule possibly plays an important role in the cross-linking reaction. The carboxyl and amino groups were formed by treating the cross-linked composite particles with diluted HCl solution; 5–6 and 0.5–1.1 μmol g^-1, respectively. The cross-linked composite particles exhibited the characteristic property of ζ-potential, −44 to −47 mV and −102 to −107 mV in a neutral aqueous solution and ethanol, respectively. Received: 26 May 1997 Accepted: 4 August 1997     

Experimental determination of the static permittivity of extremely thin liquid layers of water dependent on their thickness

 A new method for determining the static permittivity (dielectric constant) of extremely thin liquid interlayers is illustrated. A special condenser, which can be filled with a test liquid, is constructed. Both condenser plates – one planar, the other spherically curved – are made of vitreous carbon and are supplied with a high-grade politure. In order to adjust plate separation distances from 10 nm up to about some μm the planar plate can be easily shifted by a piezoelectric translation stage; the plate separation is monitored by an optical system supported by displacement transducers. The measuring frequency was kept constant at 1 MHz. Water was chosen as the test liquid. At 19.8 °C thin water layers having thickness smaller than 0.3 μm exhibit a decrease of the dielectric constant. The experimental data are consistent with a decay length α^-1 of the order of 1 nm which in view of the underlying crude model must be regarded as approximative. Received: 28 May 1996 Accepted: 30 September 1996     

Production of micron-sized monodispersed core/shell polymethyl methacrylate/polystyrene particles by seeded dispersion polymerization

 Micron-sized monodispersed polymethyl methacrylate (PMMA)/polystyrene (PS) (PMMA/PS=2/1, wt ratio) composite particles consisting of PMMA-core and PS-shell were successfully produced by seeded dispersion polymerization of styrene in a methanol/water medium in the presence of about 2 μm-sized monodispersed PMMA particles. From the view point of thermodynamic equilibrium, such a morphology is difficult to form by usual seeded polymerization in a polar medium such as water. It is concluded that seeded dispersion polymerization in which almost all monomers and initiators exist in the medium has an advantage to produce core/shell polymer particles in which polymer layers accumulate in their order of the production regardless of the hydrophobicity of polymers, because of high viscosity in polymerizing particles. Received: 9 December 1996 Accepted: 26 February 1997     

Influence of water on the dielectric behaviour of chitosan films

 The dielectric properties of chitosan films with a degree of deacetylation of 86% have been investigated in the frequency range of 10^3–10⁶ Hz covering a broad range of temperatures from −150 to 150 °C. The variation of the dielectric pro-perties with temperature has been associated with two dielectric processes: (a) a local relaxation attributed to the presence of hydrogen-bonded water appearing at low temperatures (b) a conduction process related to water molecules which becomes desorbed upon heating at T>80 °C. Isothermal dielectric experiments have been performed in order to follow, in real time, the occurrence of both, the water sorption and desorption processes. Received: 20 June 1996 Accepted: 19 November 1996     

Application of factor analysis in EDXRF

 The application of factor analysis in EDXRF as a method for evaluation of elemental concentrations and calculation of net peak areas is demonstrated in two examples. A quantitative PCR-analysis of sulphur interfered with by molybdenum could be performed by using the XRF-spectra as input with a standard error of analysis (SEA) below 0.2 mg/g for aqueous and 3.6 mg/g for solid samples. By using PCR-predicted net peak areas as input for the empirical evaluation method developed by Lucas-Tooth and Price, the SEA for solid samples could be decreased to 2.3 mg/g. The comparison of PCR to PLS showed no significant difference. Received: 25 October 1996/Revised: 6 March 1997/Accepted: 10 March 1997     

Emulsifier-free emulsion polymerization of the comonomer system styrene/tetrahydrofurfuryl metacrylate

 The emulsifier-free emulsion copolymerization of styrene and tetrahydrofurfuryl methacrylate (TMA) in aqueous phase is described. Monodisperse latex particles with diameters from about 280 to 620 nm are obtained consisting of a hydro-phobic polystyrene core and a hydrophilic poly-TMA shell. The influence of a variation of TMA, styrene and initiator (potassium persulfate) concentration in the original emulsion on particle size, molecular weight and composition of the copolymer is described. The concentration of TMA and initiator affects the number of primary particles but not the size of the final particles, whereas the styrene concentration strongly influences the particle diameter, a large size being favored by a high styrene concentration. The molecular weights of the polymers are between 6.2×10⁴ and 7.0×10⁵ g/mole. Size exclusion chromatography of polymer solutions in tetra-hydrofuran shows that high molecular weights are especially found in large particles, which are preferentially formed in emulsions with a high concentration of styrene. ¹H-NMR spectroscopy of the polymer shows that only about 50% of the initial TMA concentration are polymerized in the particles. Thus the copolymers prepared at increasing styrene concentration and constant initiator concentration of the emulsion show an increasing polystyrene content and are formed in particles of increasing size. Received: 4 June 1997 Accepted: 19 August 1997     

Dielectric measurement of a single sub-millimeter size microcapsule

A new technique is described for measuring dielectric dispersion of a single microcapsule in suspension over a frequency range from 1 kHz to 10 MHz. It was applied to polystyrene microcapsules which showed a two-step dielectric dispersion, that is, a superposition of two Debye type dispersions. The dielectric dispersion was analyzed by an electrical model in which a spherical core covered with a shell is immersed in a continuous phase, yielding the phase parameters related to the microcapsule: the wall thickness, the permittivity and conductivity of the core phase. The advantage of this technique is that it can characterize individual microcapsules, whereas the conventional method provides average properties of many microcapsules. Hence, the technique enables us to directly determine the distributions of the phase parameters and to exactly examine the relationship between the dielectric behavior and the microcapsule structure simultaneously observed by microscopy.     

Isothermal pressure dependence of the solubility of some triazines in supercritical CO2

 The solubility of two related series of each three triazine compounds has been determined in supercritical CO₂, at 40 °C and pressures between 80 and 220⋅10⁵ Pa. A high pressure small volume cell with adjustable optical path and built-in circulation pump has been constructed to be used with quartz windows in a Perkin Elmer UV-VIS spectral photometer. At 220 bar solubilities are between 0.3 and 25 g/L, dependent on the specific triazine structure. The isothermal pressure dependence of the solubility could be modelled using the pressure dependent dielectric constant of CO₂ as the only variable. Received: 30 October 1996/Revised: 3 April 1997/Accepted: 13 April 1997     

Enhanced recovery of chlorophenols from surface waters using polymer based extraction cartridges

 The effectiveness of a new polymer based solid phase extraction cartridge (SDB 1) to recover phenolic compounds from aqueous samples has been evaluated by comparison of performance against conventional silica based cartridges. The polymer based material is found to be much more retentive, requiring a larger volume of solvent to achieve maximum recovery. Recovery efficiency is enhanced if the recovery solvent (methanol) is acidified to 0.1% with trifluoroacetic acid. The optimised extraction procedure has been applied to surface water samples and yields quantitative recovery at the 10 ng ml^-1 level of all nine phenols studied. Received: 26 November 1996/Revised: 21 March 1997/Accepted: 30 March 1997     

Degradation of hyaluronic acid at the metal surface

 When the viscosity of hyaluronic acid (HA) solution was measured with a rotational visco-meter, depolymerization occurred only by leaving the sample in the cup of the viscometer, while the solution in a glass test tube at the same temperature, 37 °C, was practically stable during the entire period of investigation. As the parts where the sample solution had contact with the viscometer are stainless steel, the participation of metal ions in the depolymerization was suspected. Therefore, experiments were carried out with stainless-steel beads in the HA solution. The results show that the molecular weight of HA decreased on exposure to the beads under aerobic conditions. The addition of 10 beads to 2 ml of a 0.1% HA solution prepared with 0.2 M NaCl reduced the molecular weight to about 75% of the initial value after 4 h. The degradation rate was proportional to the surface area of the beads. However, no dissolution of metal ion was detected, and therefore, this reaction was thought to proceed at the surface of the metal. Received: 26 September 1997 Accepted: 27 November 1997     

Estimation of kinetic rate constants by turbidity and nephelometry techniques in a homocoagulation process with different model colloids

 In this work turbidimetric and nephelometric techniques have been used to study the homocoagulation of aqueous dispersions of uniform spherical particles of surfactant-free latexes. Cationic and anionic latexes of similar particle sizes (361 and 370 nm) and different surface charge densities (+16.4 and −3.6 μC/cm²) were used throughout. The kinetic constants which control the aggregation processes when the electrical repulsion disappears were estimated by both techniques at different particle concentration and wavelength in order to establish the experimental conditions which provided reliable and similar values for the coagulation rate constant. Both experimental techniques (turbidity and nephelometry) and two ways of fitting the data have been used with both latexes. For the first method, the initial slope of turbidity or total scattered intensity versus time curves were used to calculate the kinetic constants. In the second method, the whole turbidity or total scattered intensity versus time curves were fitted and the kinetic constants calculated. An unambiguous experimental value for the doublet rate constant in diffusion conditions is obtained by turbidity and nephelometry techniques. By nephelometry both data treatments have permitted a distinction between the doublet rate constant and the global rate constant in diffusion conditions. Received: 2 June 1997 Accepted: 14 August 1997     

Determination of As, Cd, and Pb in epicuticular waxes of pine and spruce needles by ETAAS

 Waxes were separated from needles by chloroform washing, drying and dissolving the residue in tetrahydrofuran (THF). Both a THF solution of the wax and an aqueous solution of the Pd-Mg matrix modifier were sampled directly into the graphite tube. The reaction conditions were optimised for the wax concentration of 45 mg ml^-1 in THF and the Pd:Mg ratio of 1:1 in the matrix modifier. The atomisation and charring temperatures for As, Cd and Pb were the same as those recommended for analysis of aqueous solutions except the charring temperature of Cd takes 200 °C lower. The characteristic masses for As (26 pg), Cd (0.5 pg) and pb (9 pg) are also comparable to published data. The parameters of calibration curves were identical for all three elements dissolved either in clear THF or in the wax solution. Elemental concentrations were found to range from 0.1 to 1.0 mg g^-1, increasing with needle age for As, Pb but not Cd. Received: 20 September 1996 / Revised: 14 November 1996 / Accepted: 13 December 1996     

Theory of broadband dispersion of permittivity of biological cell suspensions

The classical Pauly-Schwan electrical model of the biological cell is generalized by considering the diffusion processes that occur due to the selective (with respect to ions of different signes of charges) conductivity of surface cell structures (cytoplasmic membrane and electrical double layer). The analytical theory of the dispersion of the permittivity of biological cell suspensions that cover a broad frequency band that includes three typical (for these systems) dispersion regions α, β, and γ is constructed using the generalized model, whereas the classical model describes only β and γ regions. Very large values of permittivity and their complete stipulation by the ionic selectivity of surface structures, which are characteristic for the region of α dispersion, predetermine the unique sensitivity of low-frequency dielectric spectrum to the effective conductivity of the membrane of a living cell, which can find important applications in biology and medicine.     

Rheology of concentrated multi-sized poly(St/BA/MMA) latices

 Four monodisperse latices with particle size of 75, 135, 340 and 477 nm were synthesized. The rheo-logical properties of mixture of the monodisperse latices were inves-tigated as a function of blending ratio, and compared with those of multi-modal latices, in a range of shear rate from 0.2 to 100 s^-1. The maximum packing (highest solid contents) was observed at a weight fraction 80% of large particles with respect to total solids contents for both bimodal and trimodal latices, and the lowest viscosity was obtained when the ratio of large to medium to small particles was approximately 80/10/10 (by wt.). It was also demonstrated that this minimum in the viscosity is not strongly dependent on the actual size of each kind of particles present in the multimodal latices. Received: 22 November 1996 Accepted: 26 November 1997     

Styrene and methylmethacrylate oil-in-water microemulsions

 A simple procedure for the preparation of styrene-in-water and methylmethacrylate (o/w) micro-emulsions was established. This consisted of the preparation of a w/o emulsion using a low HLB number surfactant (Synperonic NP4, nonyl phenyl with 4 mol ethylene oxide) and a small amount of an anionic surfactant (Aerosol OT, diethyl hexyl sulphosuccinate, or sodium dodecyl benzene sulphonate). The w/o emulsion was then titrated with an aqueous solution of a high HLB number surfactant (Synperonic NP15, nonyl phenyl with 15 mol ethylene oxide). The droplet size and poly-dispersity were determined using photon correlation spectroscopy (PCS). The temperature range within which a microemulsion remained stable decreased with increase in the concentration of styrene or methylmethacrylate and this could be explained in terms of the phase diagram of the microemulsion system. Conductivity measurements as a function of temperature showed that the systems are oil-in-water microemulsions. Received: 20 December 1996 Accepted: 5 March 1997     

Comparison of liquid chromatography-mass spectrometry interfaces for the analysis of polar metabolites of benz[a]pyrene

 The performance of two liquid chromatography-mass spectrometry (LC/MS) interfacing techniques, thermospray (TSP) and atmospheric pressure chemical ionization (APCI), for the analysis of benzo[a]pyrene (BaP) metabolites (hydroxy, epoxy and quinone derivatives) was compared. Interface and detection parameters such as source temperature, eluent composition or flow rate were optimized using negative ion mode. In TSP, the main ions are mostly [M]^-, [M−H₂O]^- or [M+CH₃COO]^-, whereas APCI gives mainly the [M]^- and [M−H]^- ions. Quantification was carried out by flow injection. Calibration graphs were linear in the range of 10 ng to 1000 ng in TSP and 0.1 ng to 10 ng in APCI. Detection limits were in the range of 1 ng to 20 ng in TSP and 0.002 ng to 0.2 ng in APCI. The presence of BaP-1,6-dione, BaP-3,6-dione, and BaP-6,12-dione was confirmed in environmental samples of air particulate matter. Received: 6 January 1997/Accepted: 18 April 1997     

Certification of the contents of five polychlorodibenzo-p-dioxins and six polychlorodibenzo furans in milk powder

 Preparation, homogeneity and stability studies of a natural milk powder material and subsequent certification of the mass fraction of five polychlorodi-benzo-p-dioxins (D48, D54, D66, D67, D70) and six polychlorodibenzo furans (F83, F94, F114, F118, F121, F130) are described. The material, CRM 607, is intended for use in the method validation and quality control of the complete analytical procedure for congener-specific determination of dioxins in milk powder and similar dairy products and was certified in an interlaboratory study for: 2,3,7,8-TCDD, 0.25 ng/kg; 1,2,3,7,8-PeCDD, 0.79 ng/kg; 1,2,3,4,7,8-HxCDD 0.42 ng/kg; 1,2,3,6,7,8-HxCDD, 0.98 ng/kg; 1,2,3,7,8,9-HxCDD, 0.34 ng/kg; 2,3,7,8-TCDF, 0.05 ng/kg; 1,2,3, 7,8-PeCDF, 0.054 ng/kg; 2,3,4,7,8-PeCDF, 1.81 ng/kg; 1,2,3,4,7, 8-HxCDF, 0.94 ng/kg; 1,2,3,6,7,8-HxCDF, 1.01 ng/kg and 2,3,4,6,7,8-HxCDF, 1.07 ng/kg. An indicative (non-certified) value was assigned to the mass fraction of the hepta- and octachlorinated congeners D73, D75, F131, F134, F135 and the HxCDF F124. Received: 26 November 1996 / Accepted: 5 February 1997     

Determination of cadmium in indium phosphide by electrothermal atomic absorption spectrometry and ion-selective electrode potentiometry

 Two independent methods for the determination of cadmium in cadmium-doped indium phosphide have been developed. Electrothermal atomic absorption spectrometry (ETAAS) utilized both platform atomization and a chemical modifier composed of magnesium nitrate and orthophosphoric acid. As the matrix mass was found to influence the cadmium sensitivity, matrix matched calibration standards were necessary. The detection limit (3s_B) is 0.20 μg/g for a 100 mg sample. The electrochemical method employed a solid-state cadmium sulfide-silver sulfide electrode as potentiometric sensor. An excess of indium (III) influenced the electrode response. A preliminary chelation-extraction of indium with acetylacetone at pH 5.0 in acetate buffer overcame the interference. The detection limit of the ISE-potentiometric method is 10 μg/g for a 200 mg sample. Two indium phosphide single crystals grown from melts doped with cadmium sulfide or cadmium telluride were analyzed for their cadmium content. Received: 27 August 1996/Revised: 27 January 1997/Accepted: 30 January 1997     

Comparison of liquid chromatography-mass spectrometry interfaces for the analysis of polar metabolites of benz[a]pyrene

 The performance of two liquid chromatography-mass spectrometry (LC/MS) interfacing techniques, thermospray (TSP) and atmospheric pressure chemical ionization (APCI), for the analysis of benzo[a]pyrene (BaP) metabolites (hydroxy, epoxy and quinone derivatives) was compared. Interface and detection parameters such as source temperature, eluent composition or flow rate were optimized using negative ion mode. In TSP, the main ions are mostly [M]^-, [M−H₂O]^- or [M+CH₃COO]^-, whereas APCI gives mainly the [M]^- and [M−H]^- ions. Quantification was carried out by flow injection. Calibration graphs were linear in the range of 10 ng to 1000 ng in TSP and 0.1 ng to 10 ng in APCI. Detection limits were in the range of 1 ng to 20 ng in TSP and 0.002 ng to 0.2 ng in APCI. The presence of BaP-1,6-dione, BaP-3,6-dione, and BaP-6,12-dione was confirmed in environmental samples of air particulate matter. Received: 6 January 1997/Accepted: 18 April 1997