Ask A Scientist© General Science Archive

name         Henry E.
status       educator
age          40s

Question -   Please explain the difference between " hydroscopic " and
" hygroscopic ".
Would it be better to refer to (for example)the water absorbing nature of
nylon as a hydroscopic or hydroscopic material?
---------------------------------
Henry,

The answer to your question is, neither.

My dictionary offers the following definitions:

hy.gro.scop.ic Pronunciation: "hI-gr&-'skä-pik  Function: adjective Etymology:
hygroscope, an instrument showing changes in humidity + [^1]-ic; from the use
of such materials in the hygroscope Date: 1790
1 : readily taking up and retaining moisture
2 : taken up and retained under some conditions of humidity and temperature
  - hy.gro.scop.ic.i.ty /-(")sk@-'pi-s&-tE/ noun

"hydroscopic": <=== Dictionary has no entry for hydroscopic

Nylon is not at all good at absorbing water. It is polymeric structure is
hydrophobic (water hating) rather than like cotton which is hydrophilic (water
loving). Thats why nylon clothing is so uncomfortable in hot and humid 
weather.
Under those conditions, I will take cotton every time.

Regards,
ProfHoff
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As far as I am aware and can find in dictionaries, there is no word
"hydroscopic."  "Hygroscopic", on the other hand, is a respectable word,
meaning the tendency of some materials to absorb moisture from humid air.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
=========================================================
up-date: January 12, 2005
-----------------
I agree with the analysis of the experts with regard to word selection.
However, I would point out that in the world of synthetic polymers, nylon -
with a water absorption of 8% - is considered hygroscopic.  compared to most
major polymers. The polymer chains of nylons contain polar areas (where the
monomers are linked together to make the chain), as well as nonpolar areas
that extend the chain as well.  The polar regions give nylon some water
absorption capabilities, but not to the extent of fibers such as cotton.

ProfHoff correctly notes the clammy feeling associated with most synthetics
compared to natural fibers such as cotton.  There is a really good way to
demonstrate this at home.  The next time you take clothes from the washer to
dry (let's hope we all share in this task at home), compare the relative
wetness of the fabrics.  Cotton will be wettest, followed by nylon, then
polyester, then acrylic.  The acrylic clothing will be almost completely dry
after the spin cycle because it absorbs virtually no water.  That is why
cotton, for example, is such a good fiber for socks - it's durable and
absorbs lots of moisture.  Nylon looks dressier than cotton, but doesn't
have quite the moisture absorption, so while I'm wearing nylon socks to work
today, I won't wear them in my Nikes when hitting the treadmill.  I'm old
enough to remember owning a few pairs of acrylic socks (then tradenamed
OrlonR).  Now THAT'S clammy!    I wasn't sorry when the dryer ate them!.

Then again, moisture absorption isn't the only word on comfort.
Polypropylene, which absorbs almost no moisture, is an excellent cold
weather sock because a) it readily wicks moisture away from the skin and b)
it has a positive heat of wicking, which means that the wicking doesn't have
a cooling effect of the skin, or at least has less of one.  The problems
with polypropylene as a major apparel fabric include poor abrasion
resistance and poor "memory" - a woman sitting in polypropylene stockings,
for example, would see the stockings retain the shape of her bent knees when
she stood up.

Synthetic polymers are pretty amazing materials, but in many ways they can't
match their natural counterparts.

Regards,
David M. Stiles
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