“They did a good job,” agrees Eric Hoek, an environmental engineer at UCLA who trained under Elimelech 20 years ago but was not involved in the study. “Finally, someone has hit the nail in the coffin.”
The roots of the new idea of solution friction is actually old. The molecular math behind it dates back to the 1950 And 1960when Israeli researchers Ora Kedem and Aharon Katzir-Kachalsky, and UC Berkeley researcher Kurt Samuel Spiegler, derived desalination equations that accounted for friction — meaning how water, salt and pores in the plastic membrane interact.
Friction is resistance. In this case, it tells you how hard it is for something to get across the membrane. If you construct a membrane that is less resistant to water, and more resistance to salt or whatever you want to remove will give you a cleaner product with potentially less work.
But that model was shelved in 1965, when another group introduced a simpler model fashion model. It assumed that the plastic polymer of the membrane was dense and had no pores through which water could flow. Nor did friction appear to play a role. Instead, it hypothesized that water molecules in a saltwater solution would dissolve in the plastic and diffuse out the other side. For that reason it is called the “solution-diffusion” model.
Diffusion is the flow of a chemical from where it is more concentrated to where it is less concentrated. Think of a drop of dye spreading through a glass of water, or the smell of garlic coming from a kitchen. It keeps moving toward equilibrium until the concentration is the same everywhere, and it doesn’t depend on a pressure difference, like the suction that pulls water through a straw.
The model stuck, but Elimelech always suspected it was wrong. To him, accepting that water diffuses through the membrane implied something strange: that the water dispersed into individual molecules as it passed through. “How can it be?” asks Elimelech. Breaking up clusters of water molecules requires a tons of energy. “You have to almost evaporate the water to get it into the membrane.”
Still, says Hoek, “20 years ago it was anathema to suggest it was false.” Hoek didn’t even dare to use the word “pores” when talking about reverse osmosis membranes because the dominant model didn’t recognize them. “For many, many years,” he says wryly, “I’ve called them ‘interconnected free volume elements’.”
Over the past 20 years, pictures taken with advanced microscopes have reinforced Hoek and Elimelech’s doubts. Researchers discovers that the plastic polymers used in desalination membranes are not so dense and porous after all. They actually contain interconnected tunnels – though they are absolutely miniscule, peaking at about 5 angstroms in diameter, or half a nanometer. Yet one water molecule is about 1.5 angstroms long, so that’s plenty of room for small clusters 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 abandon the solution diffusion model. He worked with a team: Li Wang, a postdoc in Elimelech’s lab, studied fluid flow through tiny membranes to make real measurements. Jinlong He, at the University of Wisconsin-Madison, tinkered with a computer model that simulated what happens on a molecular scale when salt water is forced through a pressurized membrane.
