Polyelectrolytes

Polyelectrolytes jls164

Polyelectrolytes are a class of polymers that have rather special and useful properties when dissolved in solution. Polyelectrolytes are characterized by having a charged backbone. A few considerations:

  • There are counter ions – how do they affect things?
  • The polymer backbone is charged, what does that do?

What about osmotic pressure? (Remember what it is??)

Recall that osmosis is the movement of molecules through a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. Osmotic pressure is then the minimum pressure that is required to prevent the flow of solvent across the membrane, and in effect is a measure of the tendency of the solution to take up solvent.

PROBLEM

Which of these polymers would be a polyelectrolyte when dissolved in water?

molecular diagrams of A-sodium polyacrylate, B-polyacrylic acid, and C-poly(methyl acrylate)
  1. sodium polyacroylate
  2. polyacrylic acid
  3. poly(methyl acrylate)

ANSWER

A and B

molecular diagrams of A-sodium polyacrylate, B-polyacrylic acid

A is a salt and B is an acid that would be dissociated in water.

Let’s say we start with a polyelectrolyte that is a salt, like sodium polyacrylate. When the polymer not in water, the counter ion is electrostatically attracted and tightly associated with the ion on the polymer backbone. In this picture (figure 10.3), the blue negative charge is on the polymer and the red positive charge is the counter ion. When we add water – pure water, with limited ions in it – we have now created an environment in which there is osmotic pressure. We have lots of ions around the polymer, no ions in the surrounding water. The water diffuses near the polymer and the counter ions diffuse away from the polymer some distance due to the osmotic pressure. As the counter ions drift further away, the charge on the polymer backbone is less shielded and so the charges repel each other and the coil expands. This is called the polyeletrolyte effect.

diagrams showing the polyelectrolyte effect
Figure 10.3: Polyelectrolyte effect
Source: Lauren Zarzar

Polyelectrolytes are very important commercially. Some of them, such as the sodium polyacrylate that is pictured below, can absorb and “trap” hundreds of times their weight in water. This is how diapers work! Other great examples of where superabsorbent polymers like sodium polyacrylate are used include “instant snow” and “moisture control” potting soil.

molecular diagram of sodium polyacrylate
Figure 10.4: sodium polyacroylate
Source: Lauren Zarzar

If the polyelectrolyte is a linear polymer, when you keep adding water to it, it will eventually become completely diluted. Imagine dumping a bowl of spaghetti into the ocean – the noodles will all float away from each other. But if you have a network – where all the polymers are connected – then this network will retain its shape and can’t get infinitely diluted. If you throw a net into the ocean, it remains a net! If the polymer is swollen in water and it is crosslinked, we refer to this as a hydrogel. Some examples of hydrogels are those water beads that you put in candles which are popular at weddings – they look like solid beads when you buy them, but after soaking in water, they swell up into large hydrogel spheres. Even if you left them in water forever, they would never get any larger than their equilibrium swelling volume because the polymer is crosslinked. Another example is contact lenses; obviously, you don’t want your contacts to dissolve into your eye, so it has to be crosslinked!

Schematic of a crosslinked polymer, a hydrogel, swelling in water
Figure 10.5: Schematic of a crosslinked polymer, a hydrogel, swelling in water.
Source: Lauren Zarzar

PROBLEM 2

If you added NaCl to an aqueous solution of sodium polyacrylate, how would the polymer conformation change?

molecular diagram of an aqueous solution of sodium polycarbonate plus NaCL
  1. Nothing would change
  2. The polymer would be more extended (swell)
  3. The polymer would become more coiled (shrink)

ANSWER 2

C. The polymer would become more coiled (shrink)

The added salt ions will reduce the osmotic pressure and will help shield the charges on the polymer backbone from each other, both will drive the polymer to “shrink”.

We can use polyelectrolytes to make pH-responsive polymers. Consider the polyacrylic acid, shown below. Here, we are basically switching between a polyelectrolyte and non-polyelectrolyte; in base, the backbone of the polymer is charged, and this polymer would then swell in water as we expect. But when acid is added, and the polymer is fully protonated and has no charge, then it is not a polyelectrolyte, it is not affected by the osmotic pressure, and shrinks in volume.

molecular diagram showing how the polymer shrinks in low pH and swells in high pH
Figure 10.6: Effect of pH on a crosslinked polyacrylic acid hydrogel.
Source: Lauren Zarzar