[Gosseyn]

Can the human body by itself be considered a colloid (that is, in the colloidal phase)?  Or would we say that it is a collection of colloids of different types (or not)?  How many possible types of colloids are found in the body, e.g., liquids in liquids, solids in liquids, gases in gases, etc., or are all of them found there?

When something coagulates in the colloidal phase, is the process ever reversable? 

These are not from questions on a test.  I am writing a book, and am including a chapter on the history of colloids.

[Arkcon]

gases in gases

I seem to remember that one as one type that can't exist.  All other combos are possible, just not in the human body.  For example, colloidal gold, dispersed in glass, can be considered a solid-solid colloidal dispersion.  Dunno if people think that's cheating, since there's no way anything dispersed in a solid can settle out, colloid or not.  And lI don't want to get involved in the glass as super-cooled liquid meme again.

Many of the larger proteins in the human body can be considered colloids in water.  But what their state really is, is hard to say.  They may be solids, if rigorously purified away from everything else.  But their structure changes so much in the process, you might say they're not the same thing after that.  So, what, liquid-liquid, maybe?

[mark-sev]

If aproximating colloids as speheres, one could look at the potential diagram, where the sum of the potential of repulsion and attraction is plotted. So in this function there is one small minimum, which corresponds to floculation, which means agregation which can be redisperesed by the use of some little force (like shaking a bottle of some suspension) in a dispersion and is there afterwards also a deep minimum which is a condition of total agregation (the substance forms a cake) and cannot be easily redispersed...I strongly agree with Arckon that proteins require quite constant conditions to preserve their native and working structure, if the denaturate/unwind they simply aren't the same.

[rajan]

colloids are defined as a dispersion in which the dispersed particles have at least 1 dimension measuring between 1nm and 1micron. human bodies don't exactly meet this requirement. there are tons of colloids in the human body though. as previously noted, proteins are definitely on that list. lots of other things also fit.

as for irreversible flocculation/coagulation, that is the crap that keeps me up at night. when my nanoparticles produce a cloudy dispersion, they are coagulated (you shouldn't be able to see nanoparticles) and that's usually bad. various techniques can redisperse them, but in practice, it's really hard to get them back into individual particles floating around in solution. you can try sonication, or changing the solvent or temperature.