What solute should I use with dialysis tubing if I want a 100% retention rate? I purchased dialysis tubing from the Carolina company.

The molecular cut off is 12,000-14,000 Daltons and I've been using table sugar for my experiments with it to raise columns of water. While the sugar does work it slowly leaks out of the membrane over time if it's left to sit for an hour or so. What should I use if I want near 100% of the solute to stay within the dialysis tubing? Preferably something cheap and non-toxic that will also dissolve in water. Or maybe a different type of membrane I could purchase that would retain the sugar better?
Polyethylene oxide, AKA polyethylene glycol or PEG has MW > 100,000. You can find higher-MW PEG in disposable diapers and similar absorbent products.

You could also use agar, pectin or gelatin, available in groceries and from lab supply companies.

One of my favorite demos was to make an agar solution with a bit of phenolphthalein added and put it in sausage casing (AKA dialysis tubing). Holding the hotdog over an open bottle of ammonium hydroxide turned it red, and over hydrochloric acid, back to a clear frank.