Solute transport experiments

Models that are used to predict transport of chemicals in soil can be grouped into two main categories those based on an assumed or empirical distribution of pore water velocities, and those derived from a particular geometric representation of the pore space. Velocity-based models are currently the most widely used predictive tools. However, they are unsatisfactory because their parameters generally cannot be measured independently and often depend upon the scale at which the transport experiment is conducted. The focus of this chapter is on pore geometry models for chemical transport. These models are not widely used today. However, recent advances in the characterization of complex pore structures means that they could provide an alternative to velocity based-models in the future. They are particularly attractive because their input parameters can be estimated from independent measurements of pore characteristics. They may also provide a method of inversely estimating pore characteristics from solute transport experiments. 

If membrane porosity is calculated from solute transport experiments, only those pores connecting both membrane surfaces are taken into consideration. However, if gas adsorption-desorption measurements are made, the whole membrane free volume will be detected, including dead-end pores. 

For membrane transport experiments, the relevant membrane is sandwiched between two solutions a donor typicaUy at constant dmg concentration, C = Cg, and a receiver at zero concentration, C = 0. The dmg concentration in the receiver is monitored as a function of time and the cumulative amount transported, has a linear asymptote with time where M is the area,/ is the steady-state flux, /is the time, and / is the time lag. 

Mikroreaktoren sind so klein wie ein Fingerhut, Handdsblatt, May 1998 Steep progress in microelectronics, sensor and analytical techniques in the past transport intensification for catalysis first catalytic micro reactors available partial oxidation to acrolein partial hydrogenation to cyclododecene anodically oxidized catalyst supports as alternatives to non-porous supports study group on micro reactors at Dechema safety, selectivity, high pressure exclusion of using particle solutions limited experience with lifetime of micro reactors [236],... 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Several examples have already been pointed out in which the properties of the solute itself can impact on the results obtained from a transport experiment. Metabolic instability and propensity for nonspecific adsorption are problems which can frequently be encountered and must be considered any time a new solute is to be studied. In addition to these problems, there are several other solute-related factors which must be considered in the design and interpretation of transport studies. 

Here, we briefly describe the automated Caco-2 assay used at the research site in AstraZeneca R D Molndal. The solubility of the test compounds is measured (or theoretically predicted) before they are run in the Caco-2 assay. In order to be able to make correct determinations of the permeability coefficient, the substance must be dissolved when added to cell monolayer in the transport experiment. Compounds with insufficient solubility are therefore not tested. We generally apply a test concentration of 10 pM, but in specific projects or under certain circumstances a concentration of only 1 pM is applied. The test compounds are first prepared in DM SO solution (1 mM) on a parent plate and are then diluted in transport buffer to give a final drug concentration of 10 pM (solution containing 1% DMSO) when added to the cell monolayers. 

Curtis GP, Roberts PV, Reinhard M. 1986. A natural gradient experiment on solute transport in a sand aquifer 4. sorption of organic solutes and its influence on mobility. Water Resources Research 22 2059-2067. 

Distinguishing between adsorption on to the cell surface and the actual transfer across the cell membrane into the cell may be difficult, since both processes are very fast (a few seconds or less). For fish gills, this is further complicated by the need to confirm transcellular solute transport (or its absence) by measuring the appearance of solutes in the blood over seconds or a few minutes. At such short time intervals, apparent blood solute concentrations are not at equilibrium with those in the entire extracellular space, and will need correcting for plasma volume and circulation time in relation to the time taken to collect the blood sample [30]. Nonetheless, Handy and Eddy [30] developed a series of rapid solution dipping experiments to estimate radiolabelled Na+... 

Either Transwell inserts or side-by-side diffusion chambers can be used for transport studies. Bode et al. have provided an excellent review on this subject [60], Briefly, cells are incubated for 30-60 min with a buffer solution. To initiate the transport study, a transport buffer containing the drug under investigation is added to either the apical or the basal chamber depending on the transport direction of interest. At predetermined time points, the respective receiver chamber is sampled and the withdrawn volume is replaced with the same volume of fresh buffer. The permeability coefficient (Papp) is calculated and the ratio of /apP in the basolateral-to-apical direction versus that in the apical-to-basolateral direction gives the efflux ratio. These sort of transport experiments are well suited to determine if drugs/xenobiotics are substrates of the placental efflux proteins. 

To study the nature of this rapid polymer transport in detail, this section will be concerned with a series of experimental measurements on one particular system, namely a solution of dextran T10 (N5W 10 ) with a uniform concentration of 135 kg m-3 and an imposed gradient of a linear, flexible polyvinylpyrrolidone) (NTn 3 x 10s) (PVP 360). This gradient initially extended from 5 kg m3 to zero concentration. The choice of using the polymers at this concentration was based upon our earlier work441 in which it was shown that nearly maximal transport rates of PVP 360 occur in such a system. This system will be referred to as the standard system. The phase diagram of this PVP 360/dextran T10 mixture clearly demonstrates that the transport experiments were performed within the one-phase region 47). 

Charge Transport in Device Configuration Versus Charge Transfer in Solution Chemistry Experiments... 

Separation into chemical and electrical terms is possible with gradients but not with quantities, i.e., p and < >, themselves. The reason is simple. The electrochemical potential p was only conceptually separated into a chemical term p and an electrical term z F< >. The conceptual separation was based on thought experiments in practice, no experimental arrangement can be devised to correspond to the thought experiment described in Section 6.3.13.1, Thus, e.g., one cannot switch off the charges and dipole layer at the surface of a solution as one can switch off the externally applied field in a transport experiment Only the combined effect of lj and ZjFij) can be determined. 

Fesch, C., W. Simon, S. B. Haderlein, P. Reichert, and R. P. Schwarzenbach, Nonlinear sorption and nonequilibrium solute transport in aggregated porous media Experiments, process identification and modeling , J. Contam. Hydrol., 31, 373-407 (1998). 

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

To form the membranes, the desired monomer components were combined and hexyl alcohol was added to the mixture until it comprised 20-50 weight percent of the total. The hexyl alcohol was added to monomer mixtures formed with VBTAC to solvate the liquid monomers (VBC and divinylbenzene) with the VBTAC monomer. Hexyl alcohol was also added to monomer mixtures which did not contain VBTAC, not for solvation, but for consistency in preparation schemes. The photoinitiator was added to the hexyl alcohol / monomer solutions so that it comprised approximately 2 weight percent. Divinylbenzene was included in the monomer mixture in amounts from 1.2 to 1.4 mole percent of monomers so that the polymers would be lightly crosslinked to prevent solubilization of functionalized polymers during transport experiments. 

The functionalized membranes were tested in ion transport experiments to evaluate the effect of type and level of functionalization8. The membranes were contacted on one side with a feed solution containing 1 mM concentrations of sodium, zinc, ferric and/or neodymium nitrate salts. The receiving solution in contact with the other side of the... 

Fig. 25. Schematic representation of the transport experiment in which aqueous NaCl solution is added to a preparation of liposomes containing bouquet molecules in the membrane in aqueous LiCl solution, creating opposing gradients in Na+ and Li+ ion concentrations. The entry of Na+ ions, initially found only in the external volume, into liposomes is followed by 23Na NMR spectroscopy the exit of Li+ may also be followed by 7Li NMR spectroscopy.

Diffusion in the interstitial space is closely related to the volume fraction of space that is available to fluid and solute transport in tissues. The volume fraction of tumor interstitial fluid space varies from 15% in human gliomas up to 60% in a rat fibrosarcoma 4956 (Jain, 1987). The available volume fraction (KAV) of solutes is a measure of the steady state ratio of drag concentrations between tissues and the plasma (Krol et al., 1999). Thus, drug and gene delivery can be significantly improved through increasing KAV. In ex vivo experiments, KAV determines the ratio of concentrations between tissues and external solutions at the equilibrium state. KAV has been studied extensively in normal tissues but poorly in tumor tissues (Table 20.1) (Krol el al., 1999). KAV depends on the size of solutes and the dependence is determined by both the size and the connectedness of pores (Yuan et al., 2001). 

Here, P s and /<. s are the cesium permeability in the ith transport experiment and in the first run, respectively, and R = (Vfeed+ Vslnp)/VSLM, Vfeed, Vstrip, and VSLM are the volumes of feed and stripping aqueous phases and the volume of organic phase in the membrane, respectively. Kp is the apparent partition constant of the carrier between the SLM and both aqueous and stripping solutions. Values of this apparent partition constant can be deduced via linear regression of log PCs versus (i-1). They are equal to 128,100, 29,100, 106,700, and 295,000, respectively, for decyl-benzo-21-crown-7, calix[4]arene-bis(crown-6), calix[4]arene-bis(benzo-crown-6), and calix[4]arene-bis(naphtho-crown-6). Calix[4]arene-bis(crown-6) rapidly leaked off the membrane... 

Goltz, M. N., and Roberts, P. V. (1986). "Interpreting organic solute transport data from a field experiment using physical nonequilibrium models." J. Contam. Hydrol., 1(1), 77-93. 

Abstract A permeameter was developed for measurement of coupled flow phenomena in clayey materials. Results are presented on streaming potentials in a Na-bentonite induced by hydraulic flow of electrolyte solutions. Transport coefficients are derived from the experiments, assuming the theory of irreversible thermodynamics to be applicable. Hydraulic and electro-osmotic conductivities are consistent with data reported elsewhere. However the electrical conductivity of the clay is substantially lower. This is ascribed to the high compaction of the clay resulting in overlap of double layers... 

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