Fluorochromes in solvent gels

Sophie Haake <sophie.haake [at] iname__com> writes

>For a paper that I write at the University of Applied Art and
>Science in Hildesheim, I am testing the effect of solvent gels on
>stone. I would like to mix a fluorochrome into my solvent gel to
>visualize under ultraviolet light the amount of gel residue left in
>the pores of the stone. I tried to add Calcofluor-White M2R (by
>Sigma) in the gel, but then no gel formed.

This is a rather complex question and there is no simple answer but
some of the following hints may aid in properly preparing solvent
gels and finding an appropriate fluorochrome. I almost doubt, that
the addition of a fluorochrome, which should be added in the order
of 0.01-0-2% in proportion to the (small) amount of water, would
affect the gel formation process. However, excess amounts of
fluorochrome could be critical in that respect, so you may want to
reevaluate the quantity.

In terms of gel preparation, it is very important to first achieve a
smooth, homogeneous paste of the Carbopol with the neutralizing
base, such as Ethomeen, avoiding the formation of any lumps. You
should then add the solvent and blend it with the paste, again to a
homogeneous suspension. Only now and under constant stirring should
water be added dropwise utilizing a pipette or syringe which will
initiate the neutralization and thickening of the Carbopol.

It takes some practice, but if the proportions of your ingredients
are correct it will always gel. You may encounter problems with
solvents that are not perfectly "dry" i.e. have absorbed water from
the air etc. One has to work with clean and dry chemicals.

Now, the choice of a fluorochrome is much more complicated and will
certainly depend on the preparation of the gel/s (such as the base
used for neutralization) and other parameters such as pH, ionic
strength etc. First of all, Carbopol is a polyacrylic acid polymer
that upon neutralization with a base will create negative charges
all along the backbone due to ionization of the carboxylic acid
groups. Since your objective seems to be marking the Carbopol
polymer, it would make most sense to look for a cationic (basic)
fluorochrome that would have the capacity to form a salt with some
of the free carboxylic acid groups of the Carbopol. However, the
cationic dye molecules will have to compete with the base (such as
Ethomeen used for neutralization) for those free carboxylic acid
groups to form salts. In that respect it may be helpful to reduce
the amount of the neutralizing base to a minimum by keeping in mind
that effective staining, i.e. salt formation will only occur at
basic pH's. Finally, it would also depend on the amount of water and
the polarity of the solvent as the dye has to be completely
dissolved in order to "stain".

My recommendation would be to dissolve the dye in the water that is
used to form the gel. Most of the possibly suitable cationic dyes
are soluble in water and ethanol. As mentioned in one of the
previous replies, there is still a possibility of dye molecules
partitioning from the gel and getting trapped in porous materials
such as stone. Therefore, staining after gel application may be
another possibility although dry Carbopol residues seem to be quite
insoluble and consequently it may be difficult to insert the dye
solution in a way that salt formation i.e. chemical staining can
occur. It would certainly take some testing with different cationic
dyes and the proportions of the other gel ingredients to find out
whether the dye actually binds to the free carboxylic acid groups of
the Carbopol.

A widely available and inexpensive cationic fluorochrome is Acridine
Orange, but there are of course a great number of other choices.

If I can be of further help, please contact me personally via
e-mail. It would e.g. be very interesting to know, why
Calcofluor-White M2R was chosen as the only fluorochrome and how the
results will be examined. Macroscopically, microscopically, in
cross-sections? There is a wealth of information on fluorescent
labeling of polymers in the fields of materials science and polymer
chemistry.

Stephan Schaefer
University of Sao Paulo
Institute of Organic Chemistry
ArtCare & Consulting
Sao Paulo, Brasil


                                  ***
                  Conservation DistList Instance 14:33
                 Distributed: Monday, December 18, 2000
                       Message Id: cdl-14-33-001
                                  ***