Wednesday, August 25, 2010

Nanoemulsions for Dissolving Polymer Films

When you see this as the opening of an abstract, you know you're in for a fun read:
"Conservation of works of art often involves the inappropriate application of synthetic polymers."
The article certainly doesn't disappoint (open access). In this case, the researchers were trying to selectively remove polymeric films that earlier conservationists had applied to wall paintings in an effort to isolate the paintings from the environment. To the conservationists' credit, the paintings were isolated from the outer environment. Unfortunately, this led to crystallization of salts in the pores of the wall, thereby putting pressure on the coatings which then peel off the wall, often taking the paint with. The coatings also degraded in such a manner that their removal with solvents is either impossible or undesirable as the solvents would pass into the pores taking the coating with them. The only option was to use emulsions of the solvents in water.

The authors spend quite some time preparing and characterizing their nanoemulsions - I won't go into the details here. What I really was excited about was the effort they put into studying the mechanism for the polymer removal which involved light scattering experiments to characterize the emulsion particle size, and AFM to look at the removal of the coating over time. The results are worth is as they are able to propose the following mechanism:

The emulsion particle, consisting of sodium dodecyl sulfate (SDS) and 1-pentanol (1-PeOH) as (co)emulsifiers for ethyl acetate (EA) and propylene carbonate (PC), moves to the surface and releases the solvents into the polymer. At an optimal polymer/solvent ratio, the polymer is able to disperse itself into the water largely without the aid of the surfactants. The surfactants then reform into micelles, albeit smaller ones as they are now lacking in their solvent core.

I love seeing new mechanisms such as this, particularly when they are well documented. If you've studied emulsion polymerization chemistry, then you know that micelles are dynamic entities, not the static objects that they are diagrammed as.

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