04 Jan 2015 Learning from lung surfactants
I've been writing about Dynamic Surface Tension, DST and my Practical Surfactants website has two apps that provide useful insights into the phenomenon. You start with some fresh surface with no surfactant and the surface tension is ~70 mN/m because it's essentially pure water. As the surfactant comes to the surface the surface tension falls, eventually reaching the equilibrium value of, say, 25 mN/m
All I had to do was make a few tweaks to what I'd written and that was another chunk of the book sorted. But I came across a paper on the DST behaviour of lung surfactants, a subject about which I knew precisely nothing. I suddenly realised that I'd been totally unaware of a whole important area of DST. The starting point is the equilibrium value that I had always taken as the end point. What happens next is that the surface is compressed and expanded. On compression the surfactant packs more tightly and the surface tension decreases and on expansion the opposite happens and the surface tension increases.
Or not. If the surfactant moves much faster than the contraction/expansion process then the surface tension is mostly unchanged because any excess surfactant is rejected and any lack is quickly filled with fresh molecules.
For a lung to function it is vital that the surface tension should be very low when the lung is fully compressed. This is because a low surface tension stops the compressed lung surface from sticking to itself and it requires very little energy to start expanding - so the vital oxygen molecules can start to arrive as quickly as possible. And it's important that the surface tension should be high when expanded so that any part of the lung that has expanded too much is held back so that other parts can catch up.
Because breathing isn't a very fast process, and yet very large decreases (25 down to 5 mN/m) are required, the surfactant molecules must be big and slow so they respond at timescales of seconds rather than milliseconds. They are large protein molecules so the meet that criterion. Just as importantly, they must be resistant to being pushed out into the aqueous environment under compression. As it happens, they are rather insoluble in water, so they are good at resisting the compression.
This is all fascinating. But it turns out that these principles are important in many other areas of surfactant science. One area where I was supposed to know something, Ostwald Ripening, can use some of these tricks. A "slow", insoluble surfactant will not get crushed out of the surface of an emulsion drop that is starting to shrink (and larger drops are starting to grow) so its surface tension, γ, will fall. But it's the larger pressure (proportional to γ/r) which causes the small drop to shrink and if γ decreases then the pressure decreases and the tendency to shrink decreases. So to avoid Ostwald Ripening, one strategy is to use a big, "bad", insoluble surfactant. Where can you readily find these? In food science. Food scientists, through luck or brilliance, have known about such tricks for decades. It took the chance reading of a paper on lung surfactants to help me understand some key phenomena in food emulsions. That's the joy of writing books. In trying to tell others what you know, you discover your own ignorance and learn much, much, more.
To try out the new app for yourself, go to DST around equilibrium.