[Matt R.]

English is my first language. Given the context of the question, I believe it is reasonable to assume that the OP was questioning "What force (if any) is responsible for molecules diffusing into open space." If you theoretically take away the electronmagnetic force, it will take MUCH MUCH longer for a gas to expand into the space. There would be nothing pushing them apart. Thus, given the speed at which a gas under high pressure expands into open space, I think it is reasonable to assume the OP is wondering what force makes the process of diffusion faster/easier.

[ QUOTE ]
When they collide with other molecules there is no net force acting on them on average.

[/ QUOTE ]

I think this may be where we disagree. Take a molecule inside a pressurized canister. It has no NET force acting on it, because it is being bombared equally in every direction by other molecules or the walls of the container. Now, if the container busts, the molecules on the outside of the "system" are only being hit by molecules that were further inside the container. Thus they have a net force AWAY from the other molecules. The ones further in are still being hit from all directions and they tend to stay put. But over time these distances spread out as the molecules diffuse, and they reach equilibrium again in the larger space.

Basically, I think as the molecules initially diffuse when they are released from a pressurized container there is definitely a net force acting on some of the molecules causing them to spread out.