Solute permeability coefficient

Other important factors dictated by the solute are solubility and ionization state. If the compound has very limited solubility either intrinsically or at the experimental pH, it is frequently possible to do a quick calculation to determine if the experiment is even possible. That is, if the donor concentration is very dilute, one can estimate the receiver concentration which would be obtained for a given solute permeability coefficient and determine if it is within the limits of detection of the assay. 

The transport of both solute and solvent can be described by an alternative approach that is based on the laws of irreversible thermodynamics. The fundamental concepts and equations for biological systems were described by Kedem and Katchalsky [6] and those for artificial membranes by Ginsburg and Katchal-sky [7], In this approach the transport process is defined in terms of three phenomenological coefficients, namely, the filtration coefficient LP, the reflection coefficient o, and the solute permeability coefficient to. 

Some of the parameters used to evaluate membranes are the diffusion coefficient, D the permeability coefficient, P the solubility constant, S the filtration coefficient, L, the solute permeability coefficient, to and the reflection coefficient, o. The diffusion coefficient can be obtained by the time lag method illustrated in Figure 35-9. 

The diffusion coefficient, the permeability coefficient, the solubility constant, the filtration coefficient, the solute permeability coefficient, and the reflection coefficient. 

Once the average pore size of the membrane has been determined using Eq. (21.1-7), the permeability of any other solute of known radius through the same membrane can be calculated from simultaneous solution of Eqs. (21.1-5) and (21.1-6). fn the absence of specific solute-membrane interactions, such as charge or hydrophobic bonding, ihis model is useful for predicting solute permeability coefficients through a characterized membrane, for values of q less ihan 0.6. 

Here, 6 is the solute permeability coefficient, Ac is the difference in solute concentrations immediately adjacent to the membrane (wall) on the feed and product sides, is the concentration on the feed side, and is the solute rejection coefficient, which is usually taken to be constant. 

Membrane performance is a trade-offbetween membrane selectivity and membrane productivity. Membrane selectivity, a (=A/B), is defined by the ratio of permeability of components through the membrane where A is the water permeability coefficient and B is the solute permeability coefficient. In the case of RO and NF membranes, water/NaCl selectivity for seawater RO membranes is about 10,000. The higher the selectivity, the lower the permeate flux or productivity. This relationship for various RO membranes used with dilute NaCl solution is shown in Figure 1.6. The shaded regions refer to different feed concentrations and to different types of membranes. The data is fairly independent of the feed concentration but is a function of the physical and chemical properties of the membrane. 

Schematic drawing to obtain solute permeability coefficient to and reflection coefficient

Reverse osmosis can be used for the separation of ions om an aqueous solution. Neutral membranes are mainly used for such processes and the transport of ions is determined by their solubility and diffusivity in the membrane (as expressed by the solute permeability coefficient, see eq V 162). The driving force for ion transport is the concentration difference, but if charged membranes or ion-exchange membranes are used instead of neutral membranes ion transport is also affected by the presence of the fixed charge. Teoreil [45] and Meyer and Sievers [46] have used a fixed charge theory to describe ionic transport through these type of systems. This theory is based on two principles the Nemst-Planck equation and Dorman equilibrium. 

The hydraulic or water permeability coefficient (L ) can be determined from a simple permeation experiment Assume for a given membrane a Lp value of 5 lO m/hr. bar. The membrane has a rejection coefficient of 95% for NaCl and of 99.8% for NajSO< at 40 bar and 10000 ppm salt Calculate the solute permeability coefficient for both salts. 

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