Search results for "dynamics"
showing 10 items of 9782 documents
Pullulan and dextran: uncommon composition dependent Flory-Huggins interaction parameters of their aqueous solutions.
2008
Vapor pressure measurements were performed for aqueous solutions of pullulan ( M w 280 kg/mol) and dextran ( M w 60 and 2100 kg/mol, respectively) at 25, 37.5, and 50 degrees C. The Flory-Huggins interaction parameters obtained from these measurements, plus information on dilute solutions taken from the literature, show that water is a better solvent for pullulan than for dextran. Furthermore, they evince uncommon composition dependencies, including the concurrent appearance of two extrema, a minimum at moderate polymer concentration and a maximum at high polymer concentration. To model these findings, a previously established approach, subdividing the mixing process into two clearly separa…
Channel flow at an immobilised liquid|liquid interface
2000
A novel rectangular channel flow electrochemical cell for the study of liquid | liquid interfaces is presented. The organic phase is immobilised by the use of a gelling agent, while the aqueous phase flows past the interface. This creates an asymmetric setup that allows us to establish diagnostic criteria to determine, for example, the direction of the ion transfer. The effects of varying flow rate and sweep rate have been considered both theoretically and experimentally. By comparison with two-dimensional simulations, it is demonstrated that a simple one-dimensional theory can be used to describe the cyclic voltammetry response of the channel flow cell.
Volumes and compressibilities of pentanol in aqueous dodecyltrimethylammonium bromide solutions at 15, 25 and 35°C
1990
Ultrasonic velocities and densities of the water-dodecyltrimethylammonium bromide (DTAB)-pentanol (PentOH) ternary system were measured at 15, 25 and 35°C as a function of the surfactant and alcohol concentrations. The apparent molar volumes and isentropic compressibilities of PentOH were calculated. The standard partial molar volumes increase with surfactant concentration continuously whereas the standard partial molar isentropic compressibilities show sharp changes in slope at about 0.25 mol-kg−1 DTAB, which can be ascribed to a micellar structural transition. The volume data for alcohol in micellar solutions were treated by a model reported for the distribution of polar additives between…
Solution Properties of Polyelectrolytes. VI. Secondary Effects in Aqueous Size-Exclusion Chromatography
1990
Abstract An independent analysis of different operational variables in aqueous size exclusion chromatography of poly electrolytes has been carried out using a silica-based support. The effect of polyion concentration, pH and ionic strength on sodium polystyrenesulfonate calibration plots has been investigated. Finally, a novel semi-empirical model has been developed from thermodynamic considerations which relates the support effective pore volume to the polyelectrolyte molecular weight and qualitatively describes secondary effects.
Influence of the addition of modifiers on solute-micelle interaction in hybrid micellar liquid chromatography
1998
In reversed-phase micellar liquid chromatography (MLC) organic modifiers are usually added to the mobile phase to modify the eluent strength and to increase the efficiency of the chromatographic peaks. The effect of the modifiers methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran on the critical micelle concentration (cmc) of the anionic surfactant sodium dodecylsulphate (SDS) has been studied to enable understanding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased thecmc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the…
Synchronized gradient elution in capillary liquid chromatography
2013
Abstract The synchronization of injection valve operation and gradient elution in capillary liquid chromatography (cHPLC) is studied. Focus is placed on the cHPLC systems which rely on the splitting of a primary flow to provide the much smaller secondary flow required at the injection device and analytical column. Owing to the tiny secondary flow rates, synchronization is necessary to achieve proper optimization of gradient elution methods. Otherwise, there is a risk of having the analytes totally or partially eluted in the initial isocratic conditions, and there is no control on the actual gradient profile reaching the column. Synchronization is first achieved by switching back the valve t…
A New Simple Static Method for the Determination of Solubilities of Condensed Compounds in Supercritical Fluids
2002
Abstract A simple static method based on gravimetric measurements has been adopted to measure the solubility of benzenecarboxylic acid, 2-hydroxy-benzenecarboxylic acid, 4-hydroxy-benzenecarboxylic acid and 1,8-dihydroxyanthraquinone along several isotherms in the temperature range 318–357 K and the pressure interval 7–23 MPa. Solubility data have been correlated with the Zieger–Eckert approach, using the Fedor atomic and group contribution method to estimate the solubility parameter and the molar volume of the solutes. A good agreement was obtained between data reported in the literature and those determined using the proposed method which allows one to measure solubility mole fraction as …
On the measurement of consistent long-term retention factor values in micellar liquid chromatography
2007
Abstract In the field of the quantitative structure–retention and retention–activity relationships (QRAR and QSRR) is crucial to obtain consistent retention factors (k). For this purpose, two unbiased approaches to estimate k are used: (i) the IUPAC approach (based on the extra-column time correction) and (ii) the ‘2-references’ approach (based on the k estimation respect to two prefixed reference k values). Three reference chemicals were selected attending to their retention time, chemical stability and non-ionic character. Consistent retention factor values for these references were estimated for C18 chromatographic columns and Brij35 solutions as mobile phases after statistical analysis.…
Novel general expressions that describe the behavior of the height equivalent of a theoretical plate in chromatographic systems involving electricall…
2002
Novel general expressions are constructed and presented that describe the behavior of the height equivalent of a theoretical plate (plate height), H, as a function of the linear velocity, Vx, along the axis, x, of the column and the kinetic parameters that characterize the mass transfer and adsorption mechanisms in chromatographic columns. Open tube capillaries as well as columns packed with either non-porous or porous particles are studied. The porous particles could have unimodal or bimodal pore-size distributions and intraparticle convective fluid flow and pore diffusion are considered. The expressions for the plate height, H, presented in this work could be applicable to high-performanc…
Packing technology, column bed structure and chromatographic performance of 1-2-μm non-porous silicas in high-performance liquid chromatography
1989
This work is aimed at further elucidating the aggregation behaviour of micron- and submicron-size non-porous silicas and the column performance of 1–2-μm C18 silicas in reversed-phase high-performance liquid chromatography of low-molecular weight compounds. It is demonstrated that highly ordered, dense, porous aggregates of such silica beads were obtained by gravity settling and centrifugation. The slurry techniques applied at constant flow-rate and a pressure up to 50 MPa provided less-ordered aggregates, but generated an acceptable performance of columns when 1–2-μm C18 silica beads were employed. To operate columns of 53 mm × 4.6 mm I.D., the maximum flow-rate needs to be ca. 2.5 ml/min …