Variable Pressure/Speed Experimental Plan
Back when I first joined the lab I did some experiments where I varied the pressure driving a gold filtration experiment. The critical plot was this:
It seems unlikely that the sieving coefficient could change so dramatically with applied pressure, so I would like to repeat the experiments.
Because the larger gold particles on average interact with the sides of the pore more, they travel at a slower speed than the filtrate. This difference in speed leads to an increase in concentration of gold on the feed side of the membrane (called concentration polarization) that in turn slows down the flow of solvent due to osmotic pressure and the increased hydraulic resistance. In contrast to protein filtration, where protein-protein interactions mean that the concentration polarization causes the irreversible formation of a cake layer, the surface charge of the gold keeps the molecules from binding, and no cake is formed.