Using the word "all" is dangerous, blochee. Since they are manmade,
semi-permeable membranes can be designed however and for whatever
function is necessary.
The baic idea in a semi-permeable membrane is that there are small
holes in the membrane which only allow atoms/molecules/particles of a
certain size to pass through. These holes can be anywhere from the
atomic scale to the whatever size scale. They're effectively a
glorified filter.
Technically, a collander/strainer is a semi-permeable membrane,
blocking the vegetables from going through, but allowing water to pass
through.
Imagine you had a hypothetical wall with 4 inch diameter circular
holes in it. If you had a group of humans and squirrels on one side,
then bam, you have yourself a semi-permeable membrane. Squirrels can
pass through; humans can't.
Typically, though, for a number of scientific applications, we're
talking water. H20. I have seen some of your previous threads, and
know you want these for an experiment, but you need to keep in mind
that a semi-permeable membrane can be anything from dialysis tubing to
my hepothetical wall.
Anything which is advertised as being semi-permable will tell you the
hole size, typically given in microns, (10^-6 meters). From there,
you can look at the typical size of your particles and ask yourself -
are these going to be bigger than my hole size or not.
I have no clue what you are trying to do (it may help if you tell us),
but from your http://www.answers.google.com/answers/threadview?id=350915
thread, I recommend you go with something like perfluorooctane, a
colorless liquid which is insoluble in water, denser than water (SG =
1.73), and other than potentially irritating the eyes/skin, I don't
believe this is toxic.
Finding a semi-permeable membrane should be pretty easy at this point.
I invite you to check out this thread, which may help you:
http://answers.google.com/answers/main?cmd=threadview&id=197449 |
