How do dialysis and ultrafiltration use membranes to separate substances?

Dialysis and ultrafiltration are both membrane-based separation techniques that rely on the physical properties of semipermeable membranes to filter substances based on particle size and pressure gradients.12

Dialysis

Dialysis separates substances primarily through the process of diffusion. When a mixture is separated by a porous membrane, smaller molecules and ions (crystalloids) move through the membrane pores from an area of higher concentration to one of lower concentration. Larger particles, such as proteins or colloidal particles, are unable to pass through the pores and remain on the original side. This process is typically slow and depends on the size and diffusion rates of the particles involved.32

Ultrafiltration

Ultrafiltration is a separation method that accelerates the movement of substances through a membrane by applying a pressure difference. Unlike simple dialysis, where diffusion is the primary driver, ultrafiltration uses this pressure to force water and smaller dissolved solutes through the membrane. This technique is frequently used to remove excess fluid or to concentrate specific components within a solution.12