SDS solutions behaving, um, oddly.....

Gys de Jongh GysdeJongh at
Thu Jul 20 14:41:50 EST 2000

----- Original Message -----
From: <sam_ross at>
Newsgroups: bionet.molbio.methds-reagnts
Sent: Wednesday, 19 July, 2000 01:37 AM
Subject: SDS solutions behaving, um, oddly.....

> Dear Netter Nutters,
> A couple of SDS solutions in our lab have been behaving, well, oddly.
> This isn't actually a 'problem', as such,  but it's driving my nuts
> wondering why.
> We've had a bit of cold weather Down Here in Oz, and I've noticed that a
> 1% SDS solution has a tendency to pop out of solution overnight - while
> the 10% solution next to it stays dissolved.  Both were prepared from
> the same SDS stock (1% diluted from the 10%), with water from the same
> MilliQ system, stored in bottles of the same type, in fact from the same
> wash-up batch... "Ah", intoned a wise lab mate, "But as soon as you so
> much as touch the bottle of 10%, the whole thing will solidify".  Nope,
> it has so far stayed happily dissolved.  Tea room enquiries so far
> suggest two possible reasons for this strangeness:
> 1) last weekend's lunar eclipse
> 2) aliens abducted my reagents and returned them after experimenting on
> them
> Has anybody else ever had a dilute reagent solution precipitate out,
> whilst a more concentrated one stays dissolved?  Anybody have any ideas
> about why this might be happening?

Sodium Dodecyl Sulfate  , SDS , is a surfactant. There is a polar and a a-polar
group in the same molecule. The 1% solution consists of normal micelles. The
a-polar groups are inside and polar groups are outside. The polar , water ,
phase is continuous. If the concentration SDS is increased from 1% to 10% than
at some point there will be more apolar phase than polar , water , phase. The
10% solution consists of inverse or inverted micelles where the polar , water
phase is no longer continuous. Inverse micelles are a topic in membrane
structure , liposomes and studies in transfection. The situation where there are
two extremes with no intermediates is not unkommon in physics. So there is : "a
lot phenol in equilibrium with a little bit TE" and "a lot TE in equilibrium
with a bit phenol" . The lower and upper layer of "phenol" which we use for
"phenol" extractions.

See also :
The Surfactants Virtual Library
( the background of this page is a composition of normal and inverted micelles)

Schematic representation of a reverse micelle :


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