New Insights into the Cleaning of Paintings

98 • smithsonian contributions to museum conservation
The cavitation energy, which is a physical property of liquids,
has often been ignored. Looking at the solubilization in energetic
terms, the following equation must be taken into account
(Reichardt, 1990):
DG m
= DH m
– TDS m
FIGURE 1. Model of a segment of an alkyd resin showing the
phthalic acid polyester backbone and the fatty acid branches.
vary significantly. Solvation describes the intermolecular forces
between solvent and solute, whereas selective solvation arises
from a greater affinity of one component of the solvent mixture
to the macromolecules or other components of the paint film
(Marcus, 2002). Of particular interest in a practical context is
cosolvation, where each solvent exhibits a selective affinity to
one type of structural element. This may lead to the increased
solubility of a bistructural material such as the alkyd paints,
which contain a phthalic acid polyester backbone and fatty acid
substituents (Figure 1).
In a dissolution process the free energy of mixing DG m
must
be lowered upon solubilization. The enthalpy of mixing DH m
,
which corresponds to the commonly known rule of “like dissolves
like,” requires similar intermolecular solvent- solvent and
solvent- solute forces for successful action and is mostly positive
and small (Reichardt, 1990). Therefore, the entropy of mixing
DS m
at a given temperature T is of relevance. It can be calculated
using the Flory- Huggins model of thermodynamics of polymer
solubility (Flory, 1942; Huggins, 1942). The change in entropy
is largely dependent on the strength of the intermolecular interaction
within the liquid because the liquid cohesion has to be
overcome to form a cavity in the liquid to incorporate the solute
(Chipperfield, 1999). Cavity formation can be described by the
cohesive energy of the liquid and can be qualified by the Hildebrand
parameter d H2
, a parameter that controls the entropy of
the dissolution process. This process of dissolution comprising
both endo- and exothermic steps is schematized in (Figure 2).
The exothermic step, an enthalpic process, can be described by
the intermolecular interaction between solute and solvent. These
can be dispersive, aprotic, or protic interactions.
Thus, the weaker the cohesive forces within a liquid are,
the better the material solubilization is. The so- called cavitation
energy is of direct relevance since the energy of cohesion is constant
in pure solvents but varies strongly in solvent mixtures
(Marcus, 2002).
FIGURE 2. Schematic representation of the two- step dissolution process. The first endoenergetic step represents the cavity formation. The
exoenergetic step corresponds to the intermolecular interactions between solute and solvent, i.e., SSP, dispersive; SB, aprotic; SA, protic
interactions.