The Dialysis Process

The dialysis process of a solute occurs at the membrane surface, in context to the concentration differences of two solutions on either side of the membrane. This process is affected by the variables of temperature, viscosity and mixing rate of a solution.

The movement of a solute across a semipermeable membrane is the result of random molecular motion. As the solute molecules in a solution move, they will collide from time to time with the membrane until they diffuse. (See diagram 1.)

Diagram 1The permeation of a given solute (Y), from a solution on the left (L) of the membrane to the right (R), and back again, will depend upon the frequency of collisions between Y molecules on either side of the membrane.

For example, if the concentration of solute-Y, in the solution L is 100 mM, and solution R is 10 mM, the probability of Y-solute colliding with the Y-solute in the solution L, will be much higher than the chance of the solute-Y colliding with the R side of the membrane. Therefore, the net rate of transfer of a given solute, (at a certain temperature, viscosity and mixing rate) will increase with greater concentration differences between the two solutions.

Permeation Rate

The rate of transport across a semipermeable membrane depends mainly on the shape, charge, and size of the solute. Speed and size are only two out of numerous interacting factors. The flux, or permeation across a semipermeable membrane of solutes in solution, is inversely related to the molecular weight.

Small molecules collide more often with the membrane, thus, their rate of molecular migration through the membrane will be high. Large molecules, moving at low velocities collide infrequently with the membrane. Therefore, their rate of migration through the semi-permeable membrane will be low (even those that fit through the membrane pores).

The size of a solute correlates highly with the molecular weight. As the molecular size approaches and exceeds the size of the membrane pores (MWCO), passage of solutes will completely or partially be prevented.

Influence on Permeation Rate

  • Parameter
  • Concentration Gradient
  • Molecular Size
  • Temperature
  • Wall Thickness
  • Membrane Surface Area
  • Increased
  • Large Concentration Differential
  • Small Spherical Molecules
  • High Temperature
  • Thin (<20μm)
  • Large
  • Decreased
  • Small Concentration Differential
  • Large Fibrous Molecules
  • Low Temperature
  • Thick (>20μm)
  • Small

Dialysis Membrane Materials

Synthetic and natural membranes are commonly used for filtration applications. Membrane materials most often used include regenerated cellulose, cellulose acetate, polysulfone, polycarbonate, polyethylene, polyolefin, polypropylene, and polyvinylidene fluoride.

The regenerated cellulose in CelluĀ·Sep® is derived from cotton: Cotton linters are dissolved in a solution and spread into flat sheets or extruded into tubes. The material is treated with glycerin to prevent the pores from collapsing and air dried at a certain temperature and pressure to form a rigid membrane. CelluĀ·Sep regenerated cellulose membrane has a symmetric pore structure which allows small molecules to migrate in either direction, making it ideal for experimental purposes.