Membrane that allows smaller particles to pass but not larger ones can separate larger substances from smaller ones {filtration}|. Filtering {ultrafiltration} can use pressure and membranes with smaller pores.
Porous-material membranes allow small solute and solvent molecules to pass {dialysis}| but stop larger molecules, such as proteins, starches, and nucleic acids. For example, cellophane membranes allow molecules less than 1000 daltons to pass.
purpose
Dialysis dilutes impurities from solutions. Dialysis concentrates large-molecule solutions.
dilution
For example, on one membrane side is salt and large-molecule solution. On other side is water. Net ion flow enters water, because concentration is higher on salt side, so more ions reach membrane each second from concentrated side. Net water flow enters salt solution, because water concentration is higher on water side, so more water molecules reach membrane each second from water side.
concentration
On one membrane side is large-molecule solution. On other side is concentrated-salt solution. Net salt-ion flow enters large-molecule solution, because concentration is higher on salt side, so more ions reach membrane each second from salt side. Net water flow enters salt solution, because water concentration is higher on large-molecule side, so more water molecules reach membrane each second from large-molecule side.
Molecules can separate by size, using agarose, Sephadex, or Biogel beads {gel filtration}. Beads have surface depressions. Same-size or smaller-size molecules stay in depressions and slow, but larger molecules flow past and so exit faster.
Porous-material sheets {membrane, filter}| allow smaller solute and solvent molecules to pass through but stop larger molecules, such as proteins, starches, and nucleic acids.
5-Chemistry-Analytical Chemistry
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Date Modified: 2022.0225