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While writing up a lab for biology, I realized that the dialysis tubing for sale online seemed to retain large molecules (12-14,000 Da) but didn't specify an upper bound. I learned that the tubing is impermeable to Sucrose, molecular weight 342 Da, because it's too big. This doesn't make sense to me; the site says the tubing "retains >12,000 Da molecules", but Sucrose is clearly smaller than that.

So either A) Sucrose does indeed pass through dialysis tubing and I misheard/mis-learned or B) my class used tubing with much smaller pores.

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  • $\begingroup$ So it seems that we used tubing with smaller pores and there is a discrepancy between the metric I was using to compare and the size of the molecules. Thank you to both answers. $\endgroup$ – Maddy Byahoo Oct 13 '12 at 4:06
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There exist many types of semi-permeable membranes (the ones used for dialysis tubing), with various pore sizes. One of the very common lab experiment on the topic of osmosis is using a sucrose solution (sucrose is cheap) and small-pores membrane, such that water and small ions (typically Na+ and Cl) can pass, but not sucrose. The one which you link to simply happens to be a variety with much wider pores.

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Dialysis tubing (and most semi-permeable membranes) operate based on differences of size and not molecular weight. While the size of a molecule or ion does increase as its molecular weight increases, there is some nonintuitive size-mass discrepancies that can occur.

Many of these discrepancies are well known to polymer chemists. Most of the ways that we attempt to measure the average molecular weight of a polymer sample are actually measures of average molecular size. The shape that a molecule adopts in solution greatly impacts its apparent size. A polymer of MW in the 40 kDa range that adopts a rigid-rod conformation in solution will usually have a larger apparent size than a polymer of MW in the 100 kDa range that folds up into a tight globular conformation (spheres have a smaller surface area to volume ratio than cylinders).

Another factor in the "size" of a molecule is the number of solvent molecules that get dragged along with it. If the molecule is dissolved in a good solvent (for example sucrose in water), it will always have a number os solvent molecules associated with it. The solvodynamic radius of a molecule may be much greater that the molecule's actual size.

If indeed you had dialysis tubing rated to retain only greater than 12,000 Da (and like @F'x, I'm not sure that you did) it is still possible for sucrose to not pass through if the sucrose is trying to drag too many water molecules (from hydrogen bonding) with it.

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  • $\begingroup$ It's a very valid point… but in the case presented, it would be one or two orders of magnitude difference between weight and size. $\endgroup$ – F'x Oct 12 '12 at 12:56

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