🚀 go-pugleaf

In article <richard.657537509@fafnir.la.locus.com> richard@locus.com (Richard M. Mathews) writes:
>eddy@boulder.Colorado.EDU (Sean Eddy) writes:
>
>>Osmosis is a colligative property. That is, osmotic pressure is
>>dependent on the *number* of particles in solution -- *not*
>>their size, mass, etc. 
>
> Think of a large particle in
>the solution as a perfectly rigid, lumpy blob.  Think of the membrane
>as a perfectly rigid, lumpy surface.  In general when you bring 2 lumpy
>surfaces together they will touch at no more than 3 points.  When the
>particle touches at just one point, that might be enough to start
>bouncing it away.  If the gaps between the particle and the membrane
>are large enough for water molecules to fit (which isn't very big) then
>we find that each particle is only capable of blocking water in 1 to 3
>spots.  At most we have to account for a factor of 3 difference between
>a monatomic particle and a huge molecule rather than a factor based on the
>ratio of surface areas.  If the assumption of perfect rigidity is wrong,
>the factor could be a bit larger; but in the time scale of a collision,
>I would expect rigidity to be a halfway decent approximation.

In fact deviations from the ideal law of a factor of two or three are
relatively easy to find with large polymers.  This is the point I was
attempting to make in a previous post: the truth isn't as simple as the
textbooks make it out to be.

				Marc R. Roussel
                                mroussel@alchemy.chem.utoronto.ca