DLVO Theory of Colloid Stability

Question

DLVO theory gives the curve of potential energy vs distance of two colloid particles. Potential energy curve is derived for colloids being only electrostatically stabilized and not sterically.

Looking at the image which shows potential energy curve, we can see two local minima and one maximum. Between local maximum and secondary minimum is a domain of repulsive forces since potential energy increases by making distance between particles smaller.

Theory predicts that if colloid particles have enough energy (usually energy of Brownian motion), they can overcome repulsive forces and bond at primary minimum which causes flocculation and colloid instability.

If they don't have enough energy colloid should be stable according to this theory, however this doesn't make sense since particles can still bond at secondary minimum. Bonding at secondary minimum forms much weaker bond which can be seen on the curve, but it should still cause flocculation since particles arrive at local minimum.

If this is so, how can this theory predict colloid stability?

Answer 1

I had read about the DLVO theory and colloids a long time ago, so in case I make a mistake, feel free to correct me.

The secondary minimum is not deep enough to hold the particles for a long enough time to cause flocculation. Basically, in room temperature, colloid particles should have enough energy to get out of that secondary minimum and drift away from each other (i.e. to the right of the curve you have drawn). However, they should not have enough energy to cross the barrier on the left (so it cannot go to the primary minimum). When two particles collide due to thermal motion, they cannot cross the barrier on the left, so they rebound away from each other (i.e. move to the right in the graph).

Only at a very low temperature would you see enough particles arrive at the secondary minimum at the same time so as to cause some kind of coagulation. At higher temperatures, the thermal motion ensures that particles are always traversing that potential energy surface.

Also notice that at the secondary minimum, there is a considerable distance between the two particles. So even if you could get all the particles to stay in that shallow minimum at the same time, I am not sure it can be called coagulation.