How does salt melt snails?

In light of our recent snail eradication project:

Why does salt "melt" snails and slugs? (And how do people manage to prepare escargot without ending up with a big pot of goo?)

To answer this question, let us consider the snail as seen by the chemist:

The snail is an animal whose sliding-along-the-plants part (the foot) is made up of cells. Animal cells are, roughly, bags of aqueous solution and organelles wrapped in phospholipid bilayers (the cell membranes). For what we're looking at here, the important thing to know is that cell membranes are semipermeable membranes: some stuff travels easily across the membrane, owing to its small size, while other stuff is too big to pass through the membrane.

There's a particular property of semipermeable membranes with aqueous solutions on either side called osmotic pressure. Roughly, this is the pressure on the membrane due to the solutions on either side.

Let's detour for a moment to solutions, and then we'll return to what's going on with the semipermeable membrane.

A solution is a mixture of two or more substances that is homogeneous. The components of this mixture will associate with each other by way of intermolecular forces.

For many solutions, there will be a relatively large amount of one of the substances (the solvent) and a smaller amount of the other substance or substances (the solute(s)). So, for example, in a solution of sodium chloride (NaCl, also known as table salt), the NaCl is the solute and the water is the solvent.

You'll recall that solid NaCl is a lattice of Na⁺ and Cl^- held together by ionic bonds, strong electrostatic attractions between the positively and negatively charged ions. You'll also recall that the H₂O molecules in liquid water associate with each other by way of hydrogen bonding (strong dipole-dipole attractions between the partial-positive hydrogen ends and the partial-negative oxygen ends of the neighboring water molecules). When you dissolve NaCl in water, the ionic bonds between Na⁺ and Cl^- are replaced with ion-dipole interactions between these ions and water. Here's the chemist's cartoon:

Notice that water molecules are arranged around Cl^- with their partial-positive H-ends pointing in, while the water molecules around Na⁺ have their partial-negative O-ends point in. And, because water is the solvent, you can assume there are plenty of water molecules in the solution interacting with other water molecules by way of hydrogen bonding. Another thing worth noting: Na⁺ and Cl^- ions are significantly larger than water molecules. (One way to talk about the size of ions, and the magnitude of their positive or negative charge, is to compare the average number of water molecules in their solvation shells.)

What happens when you put aqueous solutions on either side of a semi-permeable membrane? Those solutions will tend to shift whatever stuff can be shifted across the membrane until the concentration (roughly, how much solute is dissolved in the solution relative to the amount of solvent) is the same on both sides of the membrane. We could tell a detailed story about this in terms of energies and entropies and that kind of thing, but we're not going to go into those details today. Here, we do fine noting that the water molecules are small enough to pass through the membrane, and that the solute (here, Na⁺ and Cl^- ions) are too big to pass through the membrane. So the water will flow across the membrane, tending to flow from the side that has less solute to the side that has more solute. The net flow of water across the membrane stops when the solutions on both sides of the membrane have the same concentration. (At this point, water molecules are still passing through the membrane -- they're little enough that the membrane won't restrict their passage. However, once equilibrium has been achieved, the water molecules flow in both directions at the same rate.)

As you might guess, you'll see the biggest osmotic pressures in response to extreme concentration differences. For example, if you put a saline solution on one side of the membrane and pure water on the other side:

what will result is a rush of water from the "pure" side to the "salt solution" side of the membrane. (Here, I haven't drawn the Na⁺ and Cl^- ions separately, since as far as the water is concerned, both are just stuff dissolved in the solution that can't get across the membrane.) In effect, that rush of water is trying to dilute the salt solution so much that it's indistinguishable from pure water. This usually results in a swelling of the membrane, and might even lead to it popping if it cannot contain all the extra water molecules that have rushed in.

Now let's return to the snail.

The snail is made up of a bunch of cells which have aqueous solutions wrapped in semipermeable membranes. The content of the snail cells is not pure water -- there're likely even some Na⁺ and Cl^- ions kicking around in there. So what happens if you place, on the other side of that membrane, a very concentrated solution of NaCl?

Here, the water tends to cross the membrane in the direction that will dilute the NaCl to the same concentration as what's inside the snail cells. If there's enough salt on the salty side, this means pretty much all the water inside the snail cells will have to cross the membrane.

This kind of water-loss is not consistent with the continued biological functioning (or structural integrity) of the snail.

At this point, you might point out that the bucket-of-salt method of dispatching snails doesn't actually put a salt solution on the exterior of the snail cell membranes. However, snails excrete mucus to help them slide by reducing friction; the mucus contains water. Also, the snails I'm picking tend to be out when the plants they're sliding on are wet with dew. Between the mucus and the dew, there's enough water clinging to the outside of the snail to dissolve some salt and put the membrane in contact with a very salty solution. That's enough to get the water flowing out of the snail, which dissolves more of the salt and keeps the concentration high enough to "melt" the snail.

So, how on earth can you cook these critters -- including seasoning to taste with salt -- without ending up with a big puddle of slime?

Cooking involves raising the temperature of the snails and their cells enough to denature the proteins in the snails. This increased temperature also changes the cell membranes -- after the cooking, the membranes are no longer semipermeable, but rather allow diffusion of water and solute in both directions. This (plus the denaturing of the proteins inside the snail cells) helps keep the snail insides in, rather than drawing all the water out and deflating the snail that was holding the water.