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Nanoscience for the Conservation of Works of Art

The Royal Society of Chemistry

Cultural Heritage Artefacts and Conservation: Surfaces and Interfaces

G. CAMINATI

University of Florence, Italy

Email: gabriella.caminati@unifi.it

1.1Thermodynamics of Interfaces: Surface Energy and Surface Tension

Our appreciation of any work of art belonging to our Cultural Heritage is definitely intertwined with the observation and interpretation of the surface of the object itself. As a matter of fact, whether we are observing a Renaissance fresco, a Maya painting, or the surface of a grotto painted by an unknown artist of our prehistory (see Figure 1.1), our attention is drawn entirely to the surface of the artefact.

An art object is devised so as to observe, read and experience its surface: the surface is the locus where the artist transferred their message and emotions, but it is also the place where different materials, with their own specific chemical composition and mechanical properties, coexist. Surface also plays a leading role in another unfortunately unavoidable process that affects our Cultural Heritage: all works of art deteriorate over time upon exposure to light, temperature stresses and relative humidity cycles, insects, or microorganisms, depending on the particular location and exposure to the environmental factors. The effects of deterioration may proceed deeper into the artefact but the first screen and the first point of attack is undoubtedly what the object exposes to the external surroundings: its surface, or better the interface with the world outside the art object. No matter whether the causes of deterioration are physical, chemical or biological, the surface will be the first frontier to be modified both in structure and composition. Exposure of the work of art to the atmosphere will result sooner or later in the growth of nano- or microlayers of different chemical composition readily adsorbed on the surface, as depicted in Figure 1.2. The mechanism and the kinetics of interactions of these compounds as well as their correct removal can be understood by proper application of the principles of physical chemistry of surfaces.

In fact, a proper conservative intervention should proceed only if the properties of the chemical media used are known and the mechanism with which these systems interact with the artefact's surface is understood.

It has long been recognized and described in several textbooks and reference books that surface science extends its branches in many realms of science and technology, but its importance in art restoration and conservation has often been undervalued and only in recent times has a schematic scientific framework been attempted.

Art conservators should therefore acquire a sound competence not only in the nature and behaviour of materials but they should also master the science of surface phenomena. In addition, surface phenomena play a leading role in dictating the behaviour of many of the nanosystems conceived for art restoration that will be described in this book. Nanosystems, including nanoparticle dispersions, micelles, micro- and nanoemulsions, and polymer gels, will be described and their applications to art conservation illustrated.

The subject of this chapter is the thermodynamics of surface and interfacial phenomena involved in many aspects of art restoration and conservation, including their interpretation in terms of basic physics and chemistry. Both the general thermodynamic principles and the theoretical approach for the determination of molecular properties are widely discussed in literature, and will not be reported here.

The thermodynamic treatment will be presented in the case of a planar surface (as far as microscopic scale domains are concerned, curved surfaces found in statues or bas-reliefs are still considered planar). Extension to the important case of fluid curved surfaces will be described in section 1.4.2.

Interfacial properties are modified by changing the adjoining phases, so that the thermodynamic properties of these bulk phases must be mastered and understood in the first place. A complete description of classical thermodynamics is beyond the scope of this book but the interested reader may refer to several textbooks on this subject reported in the "Further Suggested Reading" section. This chapter will present the thermodynamic basis of surface and interface science, underlining those aspects that can be readily employed in art restoration and conservation.

This chapter is directed to restorers searching for the scientific basis of their own work, as well as to students in applied chemistry and restoration techniques.

Table 1.1 reports a summary of some of the thermodynamic phenomena and variables that will be dealt with in the remainder of this chapter, indicating how they are correlated with the specific application in art conservation.

Readers unfamiliar with the physical chemistry of interfaces are certainly aware of many everyday life phenomena that are strictly correlated with the surface properties reported in Table 1.1: plants receive water and nutrients from the ground and many common insects such as water spiders may actually walk on the surface of a pond. The above phenomena share common roots in the physical chemistry of interfaces; in particular, they are examples of the larger domain of wetting phenomena, a domain that will be proven to be of relevance when conservation and restoration are involved.

1.1.1Definition of Surfaces and Interfaces

This section deals with the discussion of a physico-chemical approach to surfaces and interfaces. The discussion on surfaces will lead to the definition of a very important physical entity, surface tension, which will be found repeatedly through this entire book. What we provide in this section is therefore a general and fundamental approach that underlies the following sections.

How to define and locate the surface of an object exactly is not a trivial question. The surface of a painted table or canvas may be approximated by a geometric plane, but when a section of the pictorial layer is enlarged we can easily prove that the surface of separation between the two phases is somewhat more extended and heterogeneous (see Figure 1.3).

Interfaces are boundaries between different phases, but the physical and chemical properties of the interface differ from those of the adjacent bulk phases. It is customary to use the symbols S, L and G to denote a solid, a liquid and gaseous phase, respectively. Using this terminology we can summarize different interfaces as SS, SL, SG, LL and LG interfaces as schematically described in Figure 1.4.

Examples of such interfaces are ubiquitous in the real world. Specific cases where art conservation is involved will be dealt with separately in this book, including solid–liquid interfaces (colloidal particles for wall painting consolidation), liquid–liquid interfaces (oil–water: micro- and miniemulsion for the cleaning of paintings), solid–solid interfaces (glue–cement, glue–canvas: adhesives).

The notion of "interface" is indeed the most general one, whereas "surface" is more restrictive, for example it is a boundary between a gas phase and a condensed phasewhen gas–liquid or gas–solid boundaries are considered.

The term "surface" is often also used when referring to boundaries of a particle, no matter what its dimensions are, i.e. from macro to nanoparticles, and independently of what is around the particle. A surface has a different physicochemical nature with respect to the associated bulk phases, but as we approach smaller dimensions surface effects become much more dominant. This concept is easily visualized in Figure 1.5: assuming that they have the same overall volume, a single large particle exhibits a total surface area smaller than the total surface area of a collection of smaller objects filling the same total volume. In other words, the higher the surface-to-volume ratio the larger the predominance of surface forces with respect to bulk forces.

The smaller the particles the greater the specific area defined as the surface area per unit weight. The same concept applies also to a sequence of crystalline solid phases: the more finely the material is divided, the larger the surface area. It is therefore apparent that the surface rules the behaviour of the entire system as far as nano-objects are concerned, e.g. Ca(OH2) (calcium hydroxide) nanoparticles for the treatment of wood acidity or for the consolidation of wall paintings. Definition and modelling of interfaces are fundamental in order to describe the parameters that rule all the phenomena occurring between two phases (e.g. degradation reactions of artefacts).

The three-dimensional region of contact between two generic phases, α and β, is called the interfacial region or interfacial layer. In two-phase systems where one of the phases is crystalline it may be tempting to identify the division surface (DS) with the geometrical plane that intersects the centres of the atoms forming the first surface layer, as depicted in Figure 1.6. This simple case may be extended to any solid surface, as in a typical case of a marble, wood or bronze surface in contact with air.

When planar liquid phases are concerned, the location of the dividing surface is much more controversial: at the liquid–vapour and at some liquid–liquid interfaces the boundary layers extend over the dimension of few molecules, and more rarely several molecular layers can be involved. Although this boundary region may appear static, in real systems the interface is in a very turbulent state: for a liquid–vapour interface, the liquid is in equilibrium with its vapour, meaning that molecules from the vapour phase hit and condense on the surface while molecules from the liquid bulk phase escape from the surface and evaporate.

In many cases of interest for the treatment of works of art, liquid–liquid and solid–liquid interfaces will be involved. The interfacial region between two condensed phases is shown schematically in Figure 1.7. It is generally assumed that, in the absence of electrolytes, this region is a few molecules in thickness (approximately 1–2 nm) and only an extremely small fraction of the molecules in the system are present in the interfacial region owing to geometrical constraints.

If a system (for instance an artefact's surface) is in equilibrium with its surroundings, its macroscopic properties are fixed, and the system can be defined as a given thermodynamic state. Practically, a system is in equilibrium if no further spontaneous changes take place at constant surroundings. Out of equilibrium, a system is under stress, and tends to equilibrate to a fully relaxed state. Many degradation reactions occur at the interfaces of artefacts (metal oxidation, tarnishing, etc.) and such systems evolve to a stable state that may hinder the readability of the surface.

In order to inquire further into surface thermodynamics it is necessary to recall a fundamental state function, i.e. the Gibbs free energy of the system, which is the maximum amount of work a system can do at constant pressure (isobaric changes). The importance of this function for the description of phenomena related to the chemistry of art conservation is immediately apparent once we notice that in conservation studies all the systems considered are usually at constant pressure.

For closed systems at equilibrium with a fixed temperature (T) and pressure (P), the Gibbs free energy reaches a minimum. All spontaneous, irreversible processes (e.g. the degradation of monuments) occurring at constant T and P proceed in a direction such that the total Gibbs energy of the system still decreases:

dGT,P ≤ 0 (1.1)

Thus the equilibrium state of a heterogeneous closed system is the state with the minimum total Gibbs energy attainable at the given T and P.

In the derivation of the previous equation, we ignored any special change at the dividing interface (boundary), or the effect of the variation of the interfacial area.

Gibbs treated this thin layer as a quasi-two-dimensional phase having no volume (see Figure 1.7). The Gibbs dividing plane concept is a departure from the physical reality but it is consistent and allows us to apply thermodynamics to surface processes. Extensive thermodynamic quantities can thus be written as a contribution to the Gibbs free energy from the system bulk phases plus a surface term.

In the case of a system with surface area A:

dGTOT = dGBulk + GSdA (1.2)

where Gs is the extra Gibbs free energy per unit area. Although the composition varies in the neighbourhood on the surface, according to Gibbs we consider the system as uniform up to this interface.

For reversible processes at a completely planar interface, the differential Gibbs energy per unit area, γ, can thus be considered a surface energy at constant temperature, pressure, and composition:

γ = ([partial derivative]G/[partial derivative]A)T,P,N (1.3)

The surface excess free energy term is correlated with the work done in generating an interfacial area increment (dA), which can be expressed as ITLγITLdA. In other words, the surface free energy, γ, is the work that should be supplied to bring the molecules from the interior bulk phase to its surface to create a new surface of unit area (1 m2).

The variable γ is of utmost importance in interfacial science and is called the interfacial, or surface, tension.

The dimension of γis energy per unit area, J m-2 in the SI system. However, these units are used exclusively for the case solid surfaces whereas for liquid interface the equivalent unit N m-1 is adopted (force per unit length). In practical applications surface tension is reported in mN m-1, equivalent to the obsolete dyn cm-1 units.

From the previous discussion it can be stated that in every interfacial region at constant pressure and temperature, there is a tendency for mobile surfaces to decrease spontaneously in area, in order to decrease the Gibbs free energy of the system, since interfaces are the seats of excess Gibbs energy.

If the interface is fluid, i.e. liquid–liquid or liquid–gas boundaries, the action of the interfacial tension determines thus the final shape of the interface. The smallest surface area for a given liquid volume is geometrically a sphere, without any external force acting on it. For example, as far as gravity is concerned large spherical drops tend to flatten, as in raindrops.

The formation of a new surface includes two separate steps: the first is the formation of two new surfaces leaving unaltered the arrangement of atoms and molecules in space. In the second step, atoms and molecules rearrange at the surface until a new equilibrium state with minimum energy is achieved. Molecular rearrangements in liquids are very fast and surface tension can generally be considered an equilibrium value. Theoretically, the surface tensions of real liquids should be strictly measured in liquid–vacuum conditions. However, since liquids will continually evaporate in a high (or complete) vacuum condition, it is physically impossible to measure their real surface tension. In practice, we can only measure the liquid–air interfacial tension instead, under room conditions.

The situation differs for solids, where the greatly reduced molecular mobility slows down the molecular rearrangement at the surface. Therefore, surface energy for solids strongly depends on the local crystalline structure and, owing to the slow kinetics of rearrangement, also on the specific history of surface formation. In the case of crystalline or polycrystalline solid phases the surface energy correlates with the atomic density and number of nearest neighbours on surface plane, therefore γ is a function of orientation of the surface plane and of the specific crystalline structure, i.e., γ is not homogeneous.

Consequently, surface energies of solid surfaces are not as easily determined, and for solid–liquid and solid–gas interfaces the presence of the interfacial tension can only be established indirectly (see also Section 1.4.1).

A rough estimate of the surface energy of a solid surface can be obtained from the example reported in Figure 1.8.

If we separate a rectangular crystalline material into two pieces, two new surfaces will be created. This process requires the breaking of bonds between two layers of molecules or atoms. Depending on the orientation of the slicing plane, a different atomic structure will be exposed at the newly created surface. In Figure 1.8, two typical examples of surfaces created from the rupture of a face-centred cubic crystal are reported: the surfaces were obtained by cutting the same crystal with planes of different orientation. This translates into the fact that each atom is located in an asymmetrical environment where the inward forces are not balanced by the bonds that have been broken to create the surface. The surface energy that we defined in eqn (1.3) can then be equated to the energy involved in bond breaking for unit area:

[ILLUSTRATION OMITTED]

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Excerpted from Nanoscience for the Conservation of Works of Art by Piero Baglioni, David Chelazzi. Copyright © 2013 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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