Re: Orvus/Acetic Acid

[David Walker <cnstnce@xxxxxxxxxx>]

>If one were to use acetic acid  during the cleaning process with Orvus
>wouldn't you be pushing the dissociation constant in a different
>direction? Can someone enlighten me? Mary Ballard

Although, I am not sure how the dissociation of ionic surfactants are
affected by pH - there are a few things I can say about the use of acetic
acid (a weakly dissociated acid) and its effect on the rinsability of Orvus:

The rationale for the use of acetic acid in wetcleaning is primarily to
control the migration of acid dyes.  Although it is an aid in removing
calcium soaps formed by previous washings with true soaps - it would also
neutralize alkalinity if builders (phosphates or citrates) were used in the
cleaning formulation (thus the long term practice of using acidic laundry
sours in commercial laundries).  However, thorough rinsing with de-i water
would be sufficient to eliminate residual alkalinity.  Residual acetic acid
will alter the properties of silk and will degrade cellulose (however, new
cotton textiles are not significantly affected by weak acetic acid
solutions).  Wool is resistent to degradative effects - especially in the
isoelectric range of pH 4.5 - 5.5.  Even though acetic acid will eventually
volatilize and leave (practically) no residue - historic wool should be
rinsed with deionized water as a final step.

Orvus (sodium lauryl sulfate) is anionic, and thus the hydrophilic part is
negatively charged.  Textiles and particulate soils generally also develop
negative surface charges when immersed in water.  The degree of adsorption
of Orvus on the textile surface depends on several factors (concentration,
temperature, flow, interactions with other surfactants, interactions with
finishes, time, strength of charge on the textile, prescence of neutral
electrolytes and pH of the bath).  When acetic acid is used in the bath or
rinse, negatively charged sites on the textile become protonated and the
net negative charge on the surface is depressed and shifted towards neutral
or positive (depending on the concentration of acetic acid).  The reduced
potential at the fiber surface favors greater physical and chemical
adsorption of Orvus and other anionic surfactants.  In the case of protein
fibers, the hydrophilic head group can bond  at the amine sites, with the
hydrocarbon tail oriented into the bath - rendering the surface
increasingly hydrophobic.  For wool, when the pH is below 4.5 a large
amount of Orvus can be chemically adsorbed, which is irreversible under the
conditions of conservation treatments.

Colder temperatures and more acid conditions (i.e. the use of acetic acid)
favor adsorption of Orvus onto the textile.  Warmer temperatures and
alkaline conditions favor desorption.  This general rule is true for most
anionic surfactants - and is not true for most nonionic surfactants.  When
acetic acid is used, nonionic surfactants are indicated.

Normally, the odor of residual acetic acid disappears fairly quickly as the
textile dries.  If the final rinses are deionized water, there should be no
odor  of acetic acid even while the textile is still wet.  If acetic acid
is left in (not recommended for historic textiles), the odor will normally
dissipate after one or two days of open air drying for a thinner textile
(tapestries) - and after three or four days for a denser textile (Oriental
rugs).  The odor should not reappear with subsequent increases in
environmental relative humidity.

As an additional note:  Hardness ions do affect sodium lauryl sulfate.  In
the prescence of calcium: the sodium salt is less soluble in the bath than
the magnesium or ammonia salts; detergency is significantly less than
sodium lauryl (ethoxy) sulfate; the head group of SLS is bound to negative
sites on the fiber by multivalent Ca ion "bridges".


David Walker
Talisman