Thursday, January 7, 2016


Occasionally, I go off on a tangent; last night was one of those times. I searched the internet for a description of the forces of diffusion. The standard explanation was that the concentration gradient is often called the driving force in diffusion but goes on to explain that it is a not a force but rather mechanistic sense moving atoms or what ever down a concentration gradient. I found this explanation weak, incomplete, and therefore, unsettling. In simplistic terms, the concept is that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient. Scientists label this as Fick’s law. The question of why is not answered. Something called random walk theory enters the conversation. Under the right parameters when we look at particles under a microscope, we see what scientists describe as Brownian motion, which is the random movement away from high concentrations to low or zero concentration in a zigzag pattern resulting from collision of one molecule with other dissimilar molecules in the environment. The concept is simple and well known, which is why I expected to find an explanation of the force or forces involved. What my question relates to is why particles, atoms, molecules, compounds, or what ever, move from high concentration to low concentration. In addition, the movement is specifically in relation to other molecules their own kind. For example, amino acid and sugar molecules diffuse in an aqueous solution in respect to their own concentration gradients. Why, what forces or force is at play? The random walk explanation, which is often given,is especially perplexing in this regard. If a particles, atom or whatever moves down a concentration gradient in a random manner, that means it moves north the same distance as it move south or it moves east the same distance as it move west or moves up or down. I fail to see how this results in the described movement from high to low concentration, in fact how the random walk results in any net movement at all. Even a drunk on a random walk is motivated such as going home from the pub. The only thing I can think as an explanation of the force involved with the specificity it seems to have is that likes repel. In chemistry, a mole of a specific amino acid, or mole of any other atom or compound, has a specific number of atoms, which is Avogadro’s number, which is 6.022 x 1023. Each atom is identical to each other in that context. Atoms and compounds have different internal structure, which creates electrostatic forces in molecules apparently act like magnets; like repel and opposites attract. That repelling force seems to be the force of diffusion, which answers my question. Just as drunk’s motivation to go home may be the driving force that give direction to a drunk’s random walk, the force of like molecules repelling like molecules supply the direction motivation to random molecular movement down a concentration gradient. I am sure this answer is on the internet somewhere, I just could not find it. URL: Comments Invited and not moderated

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