Friction is the drag force. In this case, it tells you how hard it is for something to cross the membrane. If you design a membrane with less resistance to water, and than resistance to salt or anything else you want to remove, you get a cleaner product with less effort.
But that model was shelved in 1965, when another group introduced a simpler model. model. This assumption assumes that the flexible polymer of the film is dense and has no pores through which water can flow. Nor does it assume that friction plays a role. Instead, it is assumed that the water molecules in the saltwater solution will dissolve into the plastic and diffuse out to the other side. For that reason, this is called the “solution diffusion” model.
Diffusion is the flow of a chemical from an area of higher concentration to an area of lower concentration. Think of a drop of dye spreading across a glass of water, or the faint smell of garlic in the kitchen. It keeps moving towards equilibrium until its concentration is the same everywhere and it does not depend on the pressure difference, just like the force of attraction pulls water through a straw.
The model stuck, but Elimelech always suspected it was wrong. To him, accepting that water diffuses across a membrane implies something strange: water disperses into individual molecules as it passes. “How is that possible?” Elimelech asked. Breaking up clusters of water molecules requires a ton of energy. “You almost need to evaporate the water to get it into the membrane.”
However, Hoek said, “20 years ago, people thought that was wrong.” Hoek doesn’t even dare to use the word “pores” when talking about reverse osmosis membranes, because the dominant model doesn’t acknowledge them. “For many, many years,” he said wryly, “I called them ‘connected free-volume elements.’”
Over the past 20 years, images taken with advanced microscopes have reinforced Hoek and Elimelech’s suspicions. Researchers discovered that the plastic polymers used in the desalination film are not too dense and do not have voids. They actually contain interconnected tunnels—although they are extremely small, peaking at about 5 angstroms, or half a nanometer, in diameter. However, a water molecule is about 1.5 angstroms long, so there is enough room for small clusters of water molecules to squeeze through these holes, rather than having to go one by one.
About two years ago, Elimelech felt it was time to remove 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 make actual measurements. Jinlong He, at the University of Wisconsin-Madison, tinkered with a computer model that simulated what happens at the molecular scale when pressure pushes brine across a membrane.
The prediction based on the 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 pressures at the entrance and exit of the membrane were different. This suggests that pressure pushes water through the membrane rather than simple diffusion.
