In dialysis, what happens when a hypertonic solution is used?
Dialysis demonstrations use cellophane or cellulose acetate bags with pore diameters permitting water and small molecules under few nanometers while excluding colloids like starch or proteins. When bag containing dilute salt or dye solution is immersed into beaker with concentrated solution hypertonic relative to bag contents, solute concentration inside lower than outside, water potential inside higher. Water activity gradient drives net efflux from interior to exterior attempting to equalize potentials and dilute outer compartment. Membrane remains impermeable to large solutes, so water movement dominates, volume of dialysis bag decreases, weight loss measurable, outer solution level rises. If situation reversed hypotonic outside, bag would gain water and swell possibly bursting. This behavior governed by second law favoring entropy maximizing dilution, quantified by van't Hoff osmotic pressure. Addition of hydrostatic pressure could oppose or reverse flow as in hemofiltration or reverse osmosis, but without pressure driver is osmosis. Importantly ATP hydrolysis unnecessary because passive diffusion supplies driving force. Thus hypertonic external bath causes water to leave dialysis bag.
Ref: Lodish et al., Molecular Cell Biology, Chapter: Dialysis and Osmotic Water Movement from Bags.