Sedimentation Equilibrium of Mixtures of Charged Colloids

Chapter 1Introduction1.1 Sedimentation Equilibrium of Mixturesof Charged ColloidsThis thesis is not an attempt to gain a Platonic idea by careful inductionand comparisons between diversities. Instead of unifying and generalisingexisting theories, we will go the other way. Some good old ideas, known bythe names Thermodynamics, classical statistical mechanics, classical electrostaticsand Newton’s gravity 1 , will be taken as solid laws, with which we willtry to understand complex systems. Systems that contain groups of different,interacting objects, in equilibrium.The objects we consider are colloids and salt-ions. Colloids are particleswith a size between 10nm and 1µm. These ’big’ particles will be suspendedin a solvent, where they can act like a gas. They interact with gravity andother particles. By the influence of gravity, the colloids sediment, while themuch lighter salt-ions do not. By the influence of their charge, the colloidsrepel each other and interact with the salt-ions. Our main goal will be todetermine the equilibrium state of the system, by looking at the distributionof the particles in space, and their ’influence’, the electric field. We will seethat the balance will lead to structures, very different from ’ordinary’ idealgases.Still, the feeling of understanding often leads to wild analogies, and generalisations,although it was not initially intended. Something shown by manyspecialists. Care was taken to keep these ideas from the thesis.1 a 0 th order approximation1