Dialysis concentration gradients

Dialysis Concentration gradient < 5 nm Treatment of renal failure... 

Dialysis Concentration gradient Difference in diffusion rate Ions and low MW organics Symmetrical porous/ nonporous Separation of high and low MW compounds, water desalination... 

The technique consists of a microdialysis probe, a thin hollow tube made of a semi-permeable membrane usually around 200-500 /xm in diameter, which is implanted into the skin and perfused with a receiver solution that recovers the unbound permeant from the local area. In principle, the driving force of dialysis is the concentration gradient existing between two compartments separated by a semi-permeable membrane. For skin under in vivo conditions, these compartments represent the dermal or subcutaneous extracellular fluid (depending on the probe position) and an artificial physiological solution inside the probe [36-38],... 

In vivo microdialysis is based on the principle of dialysis, the process whereby concentration gradients drive the movement of small molecules and water through a semipermeable membrane. In vivo microdialysis involves the insertion of a small semipermeable membrane into a specific region of a living animal, such as the brain. The assembly that contains this semipermeable membrane is called a probe, which is composed of an inlet and an outlet compartment surrounded by a semipermeable membrane (see O Figure 9-1). Using a microinfusion pump set at a low flow rate (0.2-3 /rL/min), an aqueous solution known as the perfusate is pumped into the inlet compartment of the microdialysis probe. Ideally, the... 

The dialysis membrane employed is usually hydrophilic and isolates two aqueous solutions in a static or dynamic regime depending on the particular purpose. While these sensors are formally similar to those discussed in the previous section, it is molecules or ions that are separated (by virtue of a concentration gradient), the process being aided both by the dynamic character of the acceptor solution and the reaction involved, which removes the species transferred across the membrane. 

Dialysis may be described as the fractional diffusion of solids from one side of a semi-permeable membrane to the other side under a concentration gradient Electrolysis is die process of local or spatial separation of the ions of an electrolyte and the transfer of their respective charges, ie the decompn of a compd by an elec current... 

Dialysis is particularly useful for removing small dissolved molecules from colloidal solutions or dispersions - e.g. extraneous electrolyte such as KNO3 from Agl sol. The process is hastened by stirring so as to maintain a high concentration gradient of diffusible molecules across the membrane and by renewing the outer liquid from time to time (Figure 1.5). 

Dialysis is the simplest application of the solution-diffusion model because only concentration gradients are involved. In dialysis, a membrane separates two solutions of different compositions. The concentration gradient across the membrane causes a flow of solute and solvent from one side of the membrane to the other. 

Figure 4.1 Concentration gradients formed when a dialysis membrane separates two solutions of different concentrations...

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

As mentioned above, Edelhauser (7) showed that the concentration gradient across the dialysis membrane must exceed a critical value to make the dialysis proceed at a practical rate. In contrast, Ottweill and Shaw (12) found from electrophoretic mobility measurements and desorption of radioactive emulsifier that all of the emulsifier was removed by dialysis or at least that a constant surface charge was obtained. 

In addition to the processes discussed so far there are two more electromembrane separation processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Diffusion dialysis is utilizing anion- or cation-exchange membranes only to separate acids and bases from mixtures with salts. Donnan dialysis can be used to exchange ions between to solutions separated by an ion-exchange membrane. Both processes have so far gained only limited practical relevance [4] and will not be discussed in this chapter. 

Dialysis has been applied to the preparation of a wide range of sample types, ranging from foodstuffs to physiological fluids. Membrane-based sample preparations for chromatography have been reviewed by Van de Merbel et al.60 In ordinary dialysis, solutes are transferred from a concentrated to a more dilute solution as a consequence of the concentration gradient. 

In electrodialysis, an applied electric field rather than a concentration gradient is used to draw ions across the membrane. Because it is faster than ordinary dialysis, electrodialysis is often used in biochemical analyses for purposes of fractionation, concentration, and desalting. Reverse osmosis (RO) is a process that uses semipermeable membranes to allow water permeation however, the membranes act as a barrier to the passage of dissolved and suspended particles. Typically, RO membranes are used to extract pure water from aqueous solutions of dissolved salts, such as seawater. The particle size cutoff is typically 0.0001 yum with driving pressures of 200 to 800 psi (1.4 to 5.5 MPa).61... 

Activated carbon is commonly administered as an antidote to reduce poisoning by the adsorption mechanism. In addition, because drug adsorbs on activated carbon, a concentration gradient between the tissue and gastrointestinal fluids exists, and thus unwanted toxic substances diffuse out of the tissues (i.e., gastrointestinal dialysis). Ionic adsorbates adsorb much less to activated carbon due to the presence of electrostatic repulsion and polarity than their neutral counterpart molecules do. 

For a given diffusion constant (D), the velocity is proportional to the surface area (q) and the concentration gradient (dc/dx), from which it follows that it is advantageous to use dialysis tubing of narrow bore (and hence greater surface area) and to change the dialysis buffer periodically to accelerate the process. Usually, dialysis should be complete in 12-24 h to ascertain whether dialysis is complete the conductivity of the sample should be compared with that of the dialysis buffer. For... 

Peritoneal dialysis involves instilling appropriate fluid into the peritoneal cavity. Poison in the blood diffuses into the dialysis fluid down the concentration gradient. The fluid is then drained and replaced. The technique requires little equipment but is one-half to one-third as effective as haemodialysis it may be worth using for lithium and methanol poisoning. 

---