Dielectric relaxation study of formamide–propylene glycol using time domain reflectometry

Using picoseconds time domain reflectometry, dielectric relaxation studies have been carried out for formamide (FMD)–propylene glycol (PLG) mixtures over the frequency range from 10?MHz to 20?GHz at various temperatures. The dielectric parameters, i.e. static dielectric constant (ε ₀) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The excess dielectric properties and Kirkwood correlation factor of the mixtures have also been determined. The Kirkwood angular correlation factor is greater than one (g ^eff?>?1) in FMD-rich region and less than one (g ^eff?<?1) in PLG-rich region, which indicates that in the mixture the dipole pairs have been formed in such a way that their orientation is parallel in FMD-rich region and antiparallel in the PLG-rich region.     

Dielectric relaxation study of glycine–water mixtures using time domain reflectometry technique

The complex permittivity of glycine in water mixture for various temperatures and concentrations have been measured as a function of frequency between 10?MHz and 30?GHz using time domain reflectometry technique. Dielectric parameters, i.e. static dielectric constant and relaxation time were obtained from the complex permittivity spectra using nonlinear least square fit method. The dielectric relaxation parameter increases with an increase in molar concentration of glycine due to the formation of hydrogen bond groups by glycine molecule in an aqueous solution medium. The activation entropy, activation enthalpy and Kirkwood correlation factor have also been determined for glycine–water mixtures.     

Dielectric relaxation study of aqueous α-amylase using time domain reflectometry technique

Time domain reflectometry technique has been used to study the complex permittivity spectra of aqueous α-amylase solution at different pH. Static dielectric constant and relaxation time were obtained from the complex permittivity spectra using nonlinear least square fit method. The dielectric relaxation parameter increases with an increase in molar concentration of α-amylase in water due to the formation of hydrogen bonding. The hydration numbers were also determined from the static dielectric constant.     

Dielectric relaxation study of hexamethylphosphoramide–1,4-dioxane mixtures using time domain reflectometry (TDR) technique

Dielectric relaxation study on hexamethylphosphoramide–1,4-dioxane binary mixtures has been carried out at 16 concentrations over the frequency range of 10?MHz to 30?GHz at different temperatures using time domain reflectometry technique. The mixtures exhibit a principal dispersion of the Debye relaxation type at microwave frequencies. The concentration-dependent static dielectric constant, excess dielectric properties and thermodynamic parameters have been determined. The hydrogen-bonded theory is applied to compute the Kirkwood correlation factors for the mixture. The average numbers of the hydrogen bonds between hexamethylphosphoramide–hexamethylphosphoramide and hexamethylphosphoramide–1,4-dioxane pairs are estimated from the dielectric constant.     

Temperature-dependent relaxation study of tertiary butyl alcohol–water mixtures using TDR technique

Using time-domain reflectometry (TDR) technique, we have measured the complex permittivity of tertiary butyl alcohol (TBA)–water mixtures in the frequency range of 10 MHz–30GHz, at temperatures 15°C, 20°C and 25°C. The complex permittivity of TBA–water mixture shows Debye-type behaviour. The dielectric parameters such as dielectric constant and relaxation time were obtained from the complex permittivity spectra. The Kirkwood correlation factor and Bruggeman factor have also been determined to investigate inter- and intramolecular interaction among associating liquids.     

Experimental study on rheological behavior of water–ethylene glycol mixture in the presence of functionalized multi-walled carbon nanotubes

Journal of Thermal Analysis and Calorimetry - This paper examines the rheological behavior of water (60%vol.)–ethylene glycol (40%vol.) mixture in the presence of functionalized multi-walled...     

Cellulose–water interactions during enzymatic hydrolysis as studied by time domain NMR

The different states and locations of water within the cellulose matrix can be studied by the use of time domain low field NMR. In this work we show how the state and location of water associated with cellulose in filter paper fibers are affected by enzymatic hydrolysis. Three locations of water were identified in the filter paper; (1) bound water associated with the microfibril surfaces and (2) water in the cell wall or cellulose matrix and (3) capillary water in the lumens and between fibers. The different mechanisms of cellulase enzymes can be seen in their effect on the cellulose–water interactions and the synergistic effects between endo- and exo enzymes can be easily detected by time domain NMR. An interesting observation is that it is possible to link the state and location of water within the cellulose fiber with structural changes upon enzymatic hydrolysis.     

Thermo-physical properties of pure ethylene glycol and water–ethylene glycol mixture-based boron nitride nanofluids

Journal of Thermal Analysis and Calorimetry - Nanofluids are suspensions of solid nanoparticles in conventional heat transfer fluids, and they often exhibit improved heat transfer characteristics....     

Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol–water binary mixtures

The aggregation behaviour of tetradecyltrimethylammonium bromide in ethylene glycol–water mixtures across a range of temperatures has been investigated by electrical conductivity measurements. The critical micelle concentration (cmc) and the degree of counterion dissociation of micelles were obtained at each temperature from plots of differential conductivity, (κ/c) _{T , P} , versus the square root of the total concentration of the surfactant. This procedure not only enables us to determine the cmc values more precisely than the conventional method, based on plots of conductivity against total concentration of surfactant, but also allows straightforward determination of the limiting molar conductance and the molar conductance of micellar species. The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters of micellization. Only small differences have been observed in the standard molar Gibbs free energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, and it showed a strong dependence on temperature in the ethylene glycol poor solvent system. An enthalpy–entropy compensation effect was observed for all the systems, but whereas the micellization of the surfactant in the solvent system with 20 wt% ethylene glycol seems to occur under the same structural conditions as in pure water, in ethylene glycol rich mixtures the results suggest that the lower aggregation of the surfactant is due to the minor cohesive energy of the solvent system in relation to water. Received: 13 December 1998 Accepted in revised form: 25 February 1999     

Potential of magnesium salt–monoalkylpolyethylene glycol–water systems for use in micellar extraction

Solubilities in MgCl₂ (MgSO₄, Mg(NO₃)₂)–syntanol DS-10 (syntanol ALM-10)–water systems have been studied by the visual-polythermal method and the isothermal section method. The liquid–liquid phase separation region was shown to change its topology depending on temperature and the salting-out agent. The salting-out agent anion was shown to influence the cloud point of syntanol solutions. The effect of structure on the ability of syntanol to be salted out by magnesium salts was studied. Optimal temperature and concentration parameters of boron extraction in the studied systems were determined.     

Vitrification and crystallization of low-temperature amorphous condensates of water and methane—water mixture

Low-temperature condensates of water and water-methane mixture are studied in the temperature range of 65–200 K. Amorphous samples are obtained by molecular beam deposition under vacuum conditions on the substrate cooled with liquid nitrogen. The vitrification and crystallization temperatures are determined from the changes in the dielectric properties of the condensates upon heating. The kinetics of crystallization of amorphous water layers is studied by differential thermal analysis. The temperature conditions for the growth of thick methane crystalline hydrate layers during the low-temperature condensation of molecular water-gas mixture beams are found.     

At-line SPE–GC–MS of micropollutants in water using the PrepStation

An automated at-line SPE–GC–MS system for the determination of micropollutants in aqueous samples, which is based on the PrepStation and uses large-volume on-column injections, has been redesigned. A cartridge made from stainless steel and polychlorotrifluoroethylene and a 2-needle system was constructed which allow the determination of micropollutants at the low ng/L level without interferences from impurities extracted from the septa of the vials or the commercial cartridges. No time-consuming pre-cleaning of the cartridges or septa is required. The SPE sample extract (300 μL) is transferred from the sample preparation module to the autosampler of the GC–MS and 50 or 100 μL are injected. The analytical characteristics of the integrated procedure such as analyte recovery (typically 80–105%) and repeatability (RSDs, 2–9%), were satisfactory. Several micropollutants were detected in (unfiltered) river water at the 0.2–400 ng/L level using full-scan MS acquisition. The system proved to be robust during the analysis of more than 100 tap and river water samples.     

Polarization orientation dependence of the far infrared spectra of oriented single crystals of 1,3,5-trinitro-S-triazine (RDX) using terahertz time–domain spectroscopy

The far infrared spectra of (100), (010), and (001)-oriented RDX single crystals were measured as the crystal was rotated about the axis perpendicular to the polarization plane of the incident radiation. Absorption measurements were taken at temperatures of both 20 K and 295 K for all rotations using terahertz time–domain spectroscopy. A number of discrete absorptions were found ranging from 10–100 cm⁻¹ (0.3–3 THz). The absorptions are highly dependent on the orientation of the terahertz polarization with respect to crystallographic axes.     

Viscometric studies of divalent transition metal sulphates in mixtures of water–diethylene glycol at 298.15–318.15 K

Relative viscosities of divalent transition metal sulphates solutions, viz. manganese, cobalt, nickel, copper, zinc and magnesium sulphate has been determined in water–diethylene glycol mixtures. Effect of temperature on the viscosities at 298.15–318.15 K has been studied and B coefficients of Jones–Dole equation are determined for these solutions. The obtained parameters have been interpreted in terms of ion–ion and ion–solvent interactions. Magnesium sulphate is taken as the reference electrolyte to see the change in the behavior of divalent ions when we shift from divalent transition metal ions to some other divalent ions in these solutions. Here these transition metal and magnesium sulphates behave in the same manner i.e structure makers in both water and in DEG + water mixtures.     

Liquid–liquid equilibrium behavior of tetrabutylammonium bromide,benzene, and water mixture

Tetrabutylammonium bromide (QBr) is a weak organic salt and used as a phase-transfer catalyst in phase-transfer catalytic reaction producing the desired product of benzyl bromide in the organic phase. The distribution of QBr between the organic phase (benzene is the organic phase solvent) and the aqueous phase is the important factor influencing benzyl bromide yield. In this study, the liquid–liquid equilibrium of benzene, water, and QBr ternary mixture were measured at 318.15, 328.15, and 338.15 K, respectively. The experimental results exhibited that the solubility of QBr is very large in this heterogeneous liquid mixture and the amount of aqueous phase increases whenever more QBr was added at constant temperature. The concentration of QBr in aqueous phase decreased with the increasing temperature. The organic phase composition did not vary obviously since the solubilities of water and QBr in benzene are very low. An empirical parameter was introduced to represent the degree of dissociation of QBr in solvent while the experimental data were correlated since it is very difficult to measure the degree of dissociation of QBr attributed to the partial dissociation of a weak organic salt. Finally, the experimental data were correlated with the NRTL model of Renon and Prausnitz taking account ion–molecule and molecule–molecule interactions.     

Direct electrochemistry and electrochemical catalysis of immobilized hemoglobin in an ethanol–water mixture

Hemoglobin (Hb) was immobilized on a glassy carbon electrode (GCE) surface by konjac glucomannan (KGM). KGM hydrogel films on GCE have relatively high stabilities in aqueous–ethanol mixtures. The entrapped hemoglobin undergoes fast direct electron transfer reactions in aqueous–organic solvent mixtures. The peak current is bigger, the peak-to-peak separation smaller and the formal potential observed in the cyclic voltammogram is more negative for Hb–KGM/GCE in ethanol–PBS compared to Hb–KGM/GCE in PBS. The electrochemical properties of the Hb in aqueous–organic solution are almost unchanged from with those observed for the purely aqueous solution, suggesting that water pools in the KGM hydrogel play an important role in preventing changes in conformation and making proteins unreactive with polar organic solvents. The immobilized Hb was able to catalyze the reduction of nitric oxide, peroxides (hydrogen peroxide, cumene hydroperoxide, t-butyl hydroperoxide, 2-butanone peroxide), and the dehalogenation of haloethanes (hexachloroethane, pentachloroethane, tetrachloroethane, etc.). The stability and reproducibility of the modified electrode meant that it could be used to determine these substances.      

Phase equilibrium in the ternary system water–ethylene glycol–dimethyl sulfoxide

The ternary system water–ethylene glycol–dimethyl sulfoxide (H₂O–EG–DMSO) was investigated by differential scanning calorimetry in the temperature range of 188–298 K. In the concentration range from ∼10 to ∼50 mol% DMSO, crystallization or glass formation are not observed when the temperature is lowered to 188 K. Significant supercooling of the solution in this composition range is explained by the existence of spatial networks of H₂O and EG.     

Thermal analysis of a binary base fluid in pool boiling system of glycol–water alumina nano-suspension

Journal of Thermal Analysis and Calorimetry - The main aim of the present research is to measure the nucleate boiling heat transfer coefficient (BHTC) of aqueous glycol nano-suspension as a coolant...     

Rheological characterization of polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer (Soluplus®) water dispersions

Soluplus® is a graft copolymer, with PEG and vinylcaprolactam/vinyl acetate side chains, recently available as excipient used to promote fast drug release in pharmaceutical dosage forms and as solubility enhancer. Despite this copolymer is reported to be able to act as a thickening additive and even as gelling agent as a function of temperature, there is a lack of information about the physical–chemical properties of its water dispersions. Thus, the aim of this paper is to investigate the influence of Soluplus® concentration and experimental temperature on the modification of the rheological properties of Soluplus® water dispersions. The results clearly indicated the influence of both the studied parameters and of their interactions on the Soluplus® thickening ability. Although some systems appear gel by human perception at 37 °C, the mechanical spectra demonstrated the lack of the formation of a tridimensional network structure. Overall, in all the analyzed temperatures and concentrations, the systems always behave as a “rheological” dilute or semidilute polymer solution.     

Extraction of pesticides in water samples using vortex-assisted liquid–liquid microextraction

A simple solvent microextraction method termed vortex-assisted liquid–liquid microextraction (VALLME) coupled with gas chromatography micro electron-capture detector (GC-μECD) has been developed and used for the pesticide residue analysis in water samples. In the VALLME method, aliquots of 30 μL toluene used as extraction solvent were directly injected into a 25 mL volumetric flask containing the water sample. The extraction solvent was dispersed into the water phase under vigorously shaking with the vortex. The parameters affecting the extraction efficiency of the proposed VALLME such as extraction solvent, vortex time, volumes of extraction solvent and salt addition were investigated. Under the optimum condition, enrichment factors (EFs) in a range of 835–1115 and limits of detection below 0.010 μg L⁻¹ were obtained for the determination of target pesticides in water. The calculated calibration curves provide high levels of linearity yielding correlation coefficients (r²) greater than 0.9958 with the concentration level ranged from 0.05 to 2.5 μg L⁻¹. Finally, the proposed method has been successfully applied to the determination of pesticides from real water samples and acceptable recoveries over the range of 72–106.3% were obtained.