Dialysis is a procedure employed in a number of cases when a change in the concentration or composition of solutes is necessary. In the biochemical practice, dialysis is often used to alter the concentration of salts and/or small molecules in protein solutions—usually aimed at decreasing the concentration of these solutes. However, the composition of the solution can also be changed in additional ways.
Dialysis is based on diffusion during which the mobility of solute particles between two liquid spaces is restricted, mostly according to their size. (In rarely used versions of dialysis, restriction of diffusion via polarity or charge is also possible.) Size restriction is achieved by using a porous material, usually a semi-permeable membrane called dialysis membrane. This membrane is permeable only for particles below a certain size. In the biochemical laboratory, this membrane is mostly a hose made from transparent material (also called dialysis bag) that can be tightly closed (tied) at its ends (Figure 2.2). The solution to be dialysed (with a volume V1) is loaded into the dialysis bag. The dialysis bag is then placed into a dialysis solution (with a volume V2) that is stirred slowly to aid the diffusion of the subset of solutes that can be released through the bag membrane, in order to achieve equilibrium between solute concentrations in the two liquid spaces. If the difference in volume between the two spaces is large (V2 >> V1, e.g. V2 = 10 L and V1 = 0.1 L, a 100-fold difference), the onset of the equilibrium will lead to a very significant dilution of the small solutes that were initially inside the bag (their concentration will change by a factor V1/(V1+V2), in this case << 1), with only a slight change in the concentration of small solutes in the outside solution (by a factor V2/(V1+V2) ≈ 1), whereas the concentration of the molecules inside the bag that cannot penetrate the membrane remains almost completely unchanged (see in details below).
The efficiency of dialysis, i.e. the extent to which the concentration and composition of the inside solution can be changed, is an important aspect. It follows from the above description of dialysis that the efficiency of dialysis largely depends on the difference between the volumes of the inside and outside liquid spaces. This is why we generally seek to use as large volume (V2) of the dialysing solution as possible. However, the efficiency of dialysis can be further increased by performing multi-step dialysis by exchanging the outer solution after the equilibrium has been reached. In this case, the attainable dilution of the inside solution will be [V1/(V1+V2)]n where n is the number of steps. It is easy to see that efficiency that can be achieved by applying a two-step dialysis at a 50-fold volume difference is much higher than the efficiency of a single-step dialysis at a 100-fold volume difference.
The speed of dialysis can be increased not only by stirring the outside solution but also by increasing the surface/volume ratio of the inside solution, as the flux of diffusion is linearly proportional to the cross-section. It is, therefore, more practical to choose a narrower and longer tube than a wider and shorter one.
The semi-permeable membrane can be crossed not only by salts and small molecules but also by solvent particles (in most cases, water). The direction and extent of the net solvent flow is determined by the difference between the total concentration of solutes in the inside and outside solutions such that the solvent migrates from the less to the more concentrated solution (with regard to solutes). This way the equilibrium concentration of the solute(s) of the inside solution that cannot cross the membrane will be influenced also by the diffusion of the solvent. As the solute(s) that cannot cross the membrane also contribute to the total concentration of the inside solution, the net direction of solvent migration will almost always point towards the inside solution. Therefore, the volume of the inside solution will increase, thereby selectively decreasing the concentration of the membrane-impermeable solute(s)—but not that of the membrane-permeable ones, even if the relative increase in the volume is large. However, the relative increase in the volume is generally not large because (i) the concentration of the large impermeable solutes is low (much lower than that of the small permeable ones) (ii) the dialysis tube is largely unable to increase its volume. The occasional small (5-20 %) volume increase of the inside solution is associated with the compression of air above the liquid phase that was originally enclosed in the bag. Taken together, the decrease in the concentration of the large solutes (proteins) is usually negligibly small. The increase in the volume of the inside solution is remarkable from a technical point of view because it is accompanied by a (sometimes substantial) elevation of the pressure. Therefore, if there is a hidden “weakness” somewhere in the material of the membrane, the elevation of pressure may lead to bursting of the bag and, as a consequence, the complete loss of the dialysed material (e.g. protein preparation). To avoid this “catastrophe”, it is recommended to perform a pressure test on the bag in its water-filled state. The other risk associated with pressure elevation occurs during the opening of the bag after completion of dialysis. In the absence of necessary care, the pressurised inside solution can sprinkle out, causing loss of material.
In the biochemical laboratory practice, solutions of proteins are generally dialysed following fractioned ammonium sulfate precipitation (detailed in Chapter 5) as well as before or after ion exchange chromatography (detailed in Chapter 6). A size selectivity (size exclusion or cut-off) specified as 4 or 11 kDa means that the pores of the dialysis membrane are impermeable for particles larger than 4 or 11 kDa, respectively.
Besides the biochemical laboratory, dialysis is utilised in the field of life sciences also for therapeutic purposes during haemodialysis, i.e. in artificial kidneys. The principal difference between these two applications is that, in the artificial kidney, dialysis is executed under continuous counter-flow of the two solution spaces: both the inside solution (the blood of the patient) and the outside solution are pumped. Thus, in such a setting, also the inside liquid space is “open”: it is not in a “bag” but flows inside a tube. Moreover, in order to increase the flux of diffusion, a large number of capillary tubes are employed in a bundle (which is actually the artificial kidney) by which the surface/volume ratio is increased enormously. The composition of the outside dialysing solution is very special as it must meet special requirements. In addition, the artificial kidney equipment is a very special apparatus because it must be able to ensure the appropriate pressure and temperature while the blood entering the body of the patient must be free of entrapped air bubbles that could lead to lethal consequences.