Sedimentation Equilibrium of Mixtures of Charged Colloids

1.2 Sedimentation equilibrium of mixtures ofcharged colloids 2I know the word ’of’. . .What is a colloid?!I’m glad you ask that question so soon. It is very important to know what acolloid is before reading about suspensions of them.Suspensions?That is what I mean. Wait, first the colloid. . . The physicist uses the wordcolloid for an object of a specific size. Size is the only property that makessomething a colloid and another object not. To become a colloid an objectmust be small. Readers, fingers, letters, books are all too big to be calledcolloids. On the other hand, objects may not be too small either. At leastnot smaller than 10 nm, that is 0.00000001 meter. Their maximum size isabout 1µm, or 0.000001m.What’s so special about this size? Why is the word ’colloid’ invented?Colloids are very special. They live between two size-ordering worlds, themicroscopic and the macroscopic one.Size-ordering worlds?Physicists like to make orderings in mind. They make a difference betweenbig things and very small things. This imaginary collection I call ’world’. Letme show you around. The macroscopic world categorizes all things biggerthan 1µm. Since it it very common to speak of these sizes in everyday life,the objects with these sizes have long been observed and described already.Like running boars, the sun, the moon, clouds, apples and many, many otherthings. The surviving physical theories that describe the laws of this worldare therefore called ’classical’ mechanics. In the other world, the microscopicworld, these laws proved unsatisfactory for many physicists. All objects thatare described in this world are not measurable by eye nor ruler, ever heardabout ’atoms’ or ’electrons’? They have a size smaller than 10 nm. Nota typical size one experiences during childhood. These objects are ratherdescribed by ’quantum’ mechanics, and may behave magically for classicalpeople. And since 1905 many of these objects have been moving freely andrandomly in an ever growing chaos called ’entropy’.Sounds scary.Big (=macroscopic) groups of these objects however, can be described withclassical statistical mechanics, or thermodynamics.And the colloids?Well, they live in between. That is why the physicist often uses the word2 Chapters in italic script can be skipped by physicists2