Friction is resistance. In this case, it tells you how hard it is for something to get through the membrane. If you design a membrane that has less water resistance, and further resistance to salt or whatever else you want to remove, you get a cleaner product with potentially less work.
But that model was shelved in 1965, when another group introduced a simpler model. model. He assumed that the plastic polymer of the membrane was dense and had no pores through which water could flow. He also did not hold that friction played a role. Instead, he assumed that water molecules in a saltwater solution would dissolve in the plastic and diffuse out the other side. For that reason, this is called the “solution-diffusion” model.
Diffusion is the flow of a chemical from where it is most concentrated to where it is least concentrated. Think of a drop of dye spilled into a glass of water or the smell of garlic wafting from a kitchen. It keeps moving toward equilibrium until its concentration is the same everywhere and does not depend on a pressure difference, like suction sucking water through a straw.
The model stood, but Elimelech always suspected that he was wrong. To him, accepting that water diffuses through the membrane implied something strange: that water dispersed into individual molecules as it passed. “How can it be?” asks Elimelech. Breaking down groups of water molecules requires a ton of energy. “You almost need to evaporate the water to get it into the membrane.”
Still, says Hoek, “20 years ago it was anathema to suggest that it was wrong.” Hoek didn’t even dare to use the word “pores” when he was talking about reverse osmosis membranes, since the mainstream model didn’t recognize them. “For many, many years,” he says wryly, “I’ve been calling them ‘interconnected free volume elements’.”
Over the past 20 years, images taken with advanced microscopes have reinforced Hoek and Elimelech’s doubts. researchers discovered that the plastic polymers used in desalination membranes are not so dense and poreless after all. They actually contain interconnected tunnels, though they are utterly minuscule, maxing out at around 5 angstroms in diameter, or half a nanometer. Still, a water molecule is about 1.5 angstroms long, so that’s enough space for small groups of water molecules to squeeze through these cavities, rather than having to go one at a time.
About two years ago, Elimelech felt the time was right to end the solution diffusion model. He worked with a team: Li Wang, a postdoc in Elimelech’s lab, examined the flow of fluids through tiny membranes to take actual measurements. Jinlong He, from the University of Wisconsin-Madison, tinkered with a computer model that simulates what happens on a molecular scale when pressure pushes salty water through a membrane.
Predictions based on a solution diffusion model would say that the water pressure should be the same on both sides of the membrane. But in this experiment, the team found that the pressure at the entrance and exit of the membrane differed. This suggested that pressure drives the flow of water across the membrane, rather than simple diffusion.
—————————————————-
Source link