re: cmc of Orvus

[David Walker <david@xxxxxxxxxxxxxxxxxxxxxxx>]

Nicole Rode wrote:
"There has been, however, some discrepency in the literature 
concerning Orvus's critical micelle concentration and I am curious 
how other conservators prepare a washing solution with Orvus. 
Specifically, I was wondering what cmc value you understand Orvus to 
have and what concentration of the detergent do you use when wet 
cleaning (i.e. at how many times it's cmc do you find effective)?"

Orvus WA paste is 28-30% sodium lauryl sulfate - SLS (which would be 
called sodium dodecyl sulfate - SDS - if it were made from 
petrochemicals).  There is also a minor amount of salt and excess 
fatty acid.  The CMC of SLS is generally accepted as .008M (.23% w/v 
or 2.3 g/l) - but there are many variations of this value (see 
"Critical micelle concentrations of aqueous surfactant systems" by 
Pasupati Mukerjee and Karol J. Mysels, Washington, U.S. National 
Bureau of Standards; U.S. Govt. Print. Off., 1971)

In order to get a 2.3 g/l concentration of SLS from a 28% product, 
one would need 8.2 g/l, which (given the specific gravity of Orvus 
paste - 1.04) is approximately 8 cc/l or about 1 liquid ounce per 
gallon.

In practice, it is necessary to use an amount above the CMC, since 
the surfactant adsorbs strongly onto the textile and soil - 
decreasing the bath concentration.  Under "average" bath/textile 
ratios and "average" wash times and "average" mechanical action, 
temperature, etc - in order to achieve the CMC in the bath, one would 
use at least 1.5 times the CMC (12 cc/l or 1 cup of Orvus paste per 5 
gallons of water).  Depending on the textile/bath ratio, Australian 
researchers have shown that wool can adsorb half of the Orvus - and 
this can be pushed higher as the pH drops below the isoelectric point 
(4.5-5.5).

The history of the use of Orvus in textile conservation traces back 
to one conservator's search for a non-formulated product (a 
surfactant with no additives) that was available "over the counter" 
in a small package (something less than a 55 gal. drum!).  Proctor 
and Gamble had such a product.  This has never meant that it was - or 
is - the best choice (or that many situations can be served by one 
product).