The
use of nanostructured fluids (NSFs), that is, micellar solutions
and microemulsions, in art conservation is often associated with cleaning
purposes as the removal of polymeric coatings and/or soil from artistic
surfaces. In both cases, the use of NSFs grants significant improvements
over the use of traditional cleaning techniques that employ neat unconfined
organic solvents, water, or aqueous solutions. The study of the nature
and properties of surfactants present in NSF formulations is important
to boost the effectiveness of these systems in applicative contexts
and in the search of innovative and highly performing amphiphiles.
This work reports on the methoxy-pentadeca(oxyethylene) dodecanoate
(MPD) surfactant in two different NSFs, whose utilization in conservation
of cultural heritage is new. Its effectiveness is compared to the
conventional nonionic amphiphiles used in conservation practice, as
pentadeca(oxyethylene) dodecyl ether, for the cleaning of poly(ethyl
methacrylate/methyl acrylate) 70:30, p(EMA/MA), and artificially soiled
surfaces. The mechanism, through which NSFs interact with polymeric
coatings or soiled surfaces, was investigated by confocal laser scanning
microscopy, fluorescence correlation spectroscopy, photographic observation,
contact angle, surface tension measurements, and small-angle X-ray
scattering. The results highlighted the superior MPD’s performance,
both in inducing polymer removal and in detaching the soil from coated
surfaces. At the microscale, the cleaning involves dewetting-like
processes, where the polymer or the soil oily phase is detached from
the surface and coalesce into separated droplets. This can be accounted
by considering the different surface tensions and the different adsorption
mechanisms of MPD with respect to ordinary nonionic surfactants (likely
due to the methyl capping of the polar head chain and to the presence
of the ester group between the hydrophilic and hydrophobic parts of
the MPD surfactant molecule), showing how a tiny change in the surfactant
architecture can lead to important differences in the cleaning capacity.
Overall, this paper provides a detailed description of the mechanism
and the kinetics involved in the NSFs cleaning process, opening new
perspectives on simple formulations that are able to target at a specific
substance to be removed. This is of utmost importance in the conservation
of irreplaceable works of art.
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