A "chemical dispersant" is a clever name for a surfactant, which is nothing more than a soap! Well, sort of. Surfactants are molecules which are defined by having two parts (think of soap on a rope): a water-loving head group (the soap), and an oil-loving tail group (the rope). Now, the chemicals deployed in the Gulf are a little more complex than common house soap, but their mechanism is the same. These dispersants work their way to the oil-water interface, and push their tails into the oil while keeping their heads in the water.
I mentioned the idea of "minimum energy" in a previous post. By this, I mean that most of these chemical systems are trying to be as comfortable as possible (who can blame them?). Well, the same logic applies here. By keeping their heads in water and tails in oil, these dispersants are able to minimize the energy of the water, dispersant, and oil system! A byproduct of this minimum energy is that the droplets can afford to be smaller in size. At the end of the day, the logic is that adding a good dispersant allows you to have smaller droplets.
If your objective is to keep oil from rising all the way to the top of the ocean, then why are small droplets good? As the droplets get smaller, they have a higher and higher pressure inside (think of squeezing a sealed balloon to 10% of its original size). When the inside pressure is high enough, then the forces of buoyancy and gravity become very small, and the motion of the droplets is primarily governed by the motion of the ocean (meaning that they go with the flow … literally).
Another byproduct of dispersants is that the resulting droplets have these water-loving heads (the soap part) sticking out all over them! If the droplets are small enough (something less than the width of a hair), then these head groups will act as spikes to stop any one droplet from interacting with any other droplet. Remember back to when you used to blow bubbles as a kid? When the bubbles collided, they burst and the air inside of them combined. For the sake of analogy, imagine those bubbles were clad in metal spikes; it would be pretty darn difficult to burst any of them since you couldn't get past the spikes! This illustrates the second effect of dispersants; they prevent the oil droplets from getting back together in water, which means they can stay suspended in water indefinitely.
I hope that you found this humorous little exploration of dispersants useful; please post any questions!