33 - 2 molecular aggregation - Review of surfactants

So last time we were finishing up about micelles and surfactants. So surfactants are like SDS here. They've got what are called amphiphiles. They have sort of a polar water soluble functionality on one end of the molecule. And the other end of the molecule doesn't like water. Like in this case, it's just a straight chain hydrocarbon. So it doesn't like to dissolve in water. And these are surface active agents. So when you put SDS or a detergent in water, it segregates to the surface and has an impact on things like surface tension. So here you go. Here's water surface tension with the amount of-- well this is a mixture of SDS and this other surfactant. So the open circles there are the pure SDS. And this one here, the squares, are AOT. And that's down here. But you can see is basically you start off around 72 dynes per centimeter. And it drops as you add the surfactant. And that's a general rule that surfactant tends to lower the surface tension. And what's happening there is this part's sticking in the water. And this part's sticking out into the air because the air is, if you think about it, is hydrophobic. It's not water. But then what we noticed is something very strange happens here is once you get to a certain point, it turns out the surface tension doesn't change anymore. And what's happening there is a very unusual thing. These molecules have decided to join one another, aggregate spontaneously to create these things called micelles. And I'll go to the next slide. So there's one here. Once you get above this critical micelle concentration that we talked about, you start to see these nanometer scale particles that scatter light. And it gets a little bluish tinge because of the way light scatters at different wavelengths. And these can form aggregates, all kinds of complexity these surfactants. So two that we'll talk about more today are these-- the phospholipid bilayer and the liposome. But we can also get things that are tubular. So in this case, it's an aqueous solution. If you have an oil, you'll actually get the inverse micelle, where the polar groups stick together in the middle and the aliphatic groups stick out into the oil. Same thing happens. So, for instance, if you want to disperse water and oil, you use surfactants to do exactly the opposite. Right? There'll be water in the side here, inside this little droplet. And so instead of being-- phase separating into oil and water or oil and vinegar, you can add a surfactant and get that vinegar to disperse within the oil. So there's lots of food grade surfactants to help you do that kind of thing.