Critical Packing Parameter

Quick Start

Some people suggest that CPP represents a fundamental insight into surfactants. If you are interested in the behaviour of higher concentrations of pure surfactants in water, this is partially true. But for those of us who formulate in the real world, this behaviour is irrelevant.

Play with the Tail Volume and length and head area, but don't believe that it helps to formulate emulsions.

Critical Packing Parameter

Tail Volume V ų
Tail Length l Å
Head Area A Ų
CPP

For a surfactant with a tail volume, V, a head area A and a tail length l a simple dimensionless number, the CPP, can be calculated:

CPP=V/(A.l)

If V ~ A.l then the surfactant is fairly symmetrical (CPP~1) and it's quite easy for it to pack into Cubic or simple Lα phases. If the head is very large (e.g. a bulky ethoxylate) and the tail volume is fairly small then CPP<1/3 and it's easy to pack the surfactant into familiar o/w micelles. In between these two cases (1/3<CPP<1/2) hexagonal packing becomes possible. At the other extreme with a very bulky tail (V is large) and a small head and/or short tail then CPP>1 and any attempt to pack the surfactant molecules together is going to have difficulties and the phases are quite complex - or you have reverse (w/o) micelles.

That's the story, and in many cases it is somewhat informative. But the idea is much abused. First, it is assumed that V, A and l are constants for a given surfactant. This simply isn't true; changing the temperature of a surfactant solution can cause big phase changes which at least in part can be ascribed to changes in the parameters. And the existence of HLD-NAC (and the existence of complementary viewpoints discussed in the Curvature section) shows that different oils and different salinities (as well as different temperatures) change the effective values of V and A.

So, treat this illustration of CPP effects with some skepticism.