Solubility Parameters: Theory and Application

John Burke
The Oakland Museum of California
August 1984
Appeared in the AIC Book and Paper Group Annual, Volume 3, 1984, Craig Jensen, Editor, pg. 13-58

Part 1 - Solutions and Molecules (this page)
Part 2 - The Hildebrand Solubility Parameter
Part 3 - Other Practical Solubility Scales
Part 4 - Component Polarities
Part 5 - Two Component Parameters
Part 6 - Three Component Parameters
Part 7 - Fractional Parameters
References


Solvents are ubiquitous: we depend on them when we apply pastes and coatings, remove stains or old adhesives, and consolidate flaking media. The solubility behavior of an unknown substance often gives us a clue to its identification, and the change in solubility of a known material can provide essential information about its ageing characteristics.

Our choice of solvent in a particular situation involves many factors, including evaporation rate, solution viscosity, or environmental and health concerns, and often the effectiveness of a solvent depends on its ability to adequately dissolve one material while leaving other materials unaffected. The selection of solvents or solvent blends to satisfy such criterion is a fine art, based on experience, trial and error, and intuition guided by such rules of thumb as "like dissolves like" and various definitions of solvent "strength". While seat-of-the-pants methods are suitable in many situations, any dependence on experiential reasoning at the expense of scientific method has practical limitations. Although it may not be necessary to understand quantum mechanics to remove masking tape, an organized system is often needed that can facilitate the accurate prediction of complex solubility behavior.

SOLUBILITY SCALES

Product literature and technical reports present a bewildering assortment of such systems: Kauri-Butanol number, solubility grade, aromatic character, aniline cloud point, wax number, heptane number, and Hildebrand solubility parameter, among others. In addition, the Hildebrand solubility parameter, perhaps the most widely applicable of all the systems, includes such variations as the Hildebrand number, hydrogen bonding value, Hansen parameter, and fractional parameter, to name a few. Sometimes only numerical values for these terms are encountered, while at other times values are presented in the form of two or three dimensional graphs, and a triangular graph called a Teas graph has found increasing use because of its accuracy and clarity.

Understandably, all this can be slightly confusing to the uninitiated. Graphic plots of solvent-polymer interactions allow the fairly precise prediction of solubility behavior, enabling the control of numerous properties in practical applications that would be very difficult without such an organizing system. Yet the underlying theories are often extremely complex, and an understanding of the "why" of a particular system can be very difficult, enough to discourage the use of such systems. Many of the systems mentioned, however, are actually quite simple (this is especially true of the Teas graph) and can be used to advantage with little understanding of the chemical principles at work.

This paper will attempt to bridge these two realities by briefly introducing solubility theory as well as its application so that the conservator will be both better able to understand and profitably apply the concepts involved. The discussion will center on Hildebrand solubility parameters and, after laying a theoretical foundation, will concentrate on graphic plots of solubility behavior. It should be remembered that these systems relate to non-ionic liquid interactions that are extended to polymer interactions; water based systems and those systems involving acid-base reactions cannot be evaluated by simple solubility parameter systems alone.

Solutions and Molecules

A solvent, usually thought of as a liquid, is a substance that is capable of dissolving other substances and forming a uniform mixture called a solution. The substance dissolved is called the solute and is usually considered to be the component present in the smallest amount. According to this definition, an almost-dry or slightly swollen resin film comprises a solution of a liquid (the solute) in a resin (the solvent), even though conventionally the liquid is usually referred to as the solvent, and the resin as the solute.

MOLECULAR ATTRACTIONS

Liquids (and solids) differ from gases in that the molecules of the liquid (or solid) are held together by a certain amount of intermolecular stickiness. For a solution to occur, the solvent molecules must overcome this intermolecular stickiness in the solute and find their way between and around the solute molecules. At the same time, the solvent molecules themselves must be separated from each other by the molecules of the solute. This is accomplished best when the attractions between the molecules of both components are similar. If the attractions are sufficiently different, the strongly attracted molecules will cling together, excluding the weakly attracted molecules, and immiscibility (not able to be mixed) will result. Oil and water do not mix because the water molecules, strongly attracted to each other, will not allow the weakly attracted oil molecules between them.

VAN DER WAALS FORCES

These sticky forces between molecules are called van der Waals forces (after Johannes van der Waals who first described them in 1873). Originally thought to be small gravitational attractions, Van der Waals forces are actually due to electromagnetic interactions between molecules.

The outer shell of a neutral atom or molecule is composed entirely of negatively charged electrons, completely enclosing the positively charged nucleus within. Deviations in the electron shell density, however, will result in a minute magnetic imbalance, so that the molecule as a whole becomes a small magnet, or dipole. These electron density deviations depend on the physical architecture of the molecule: certain molecular geometries will be strongly polar, while other configurations will result in only a weak polarity. These differences in polarity are directly responsible for the different degrees of intermolecular stickiness from one substance to another. Substances that have similar polarities will be soluble in each other but increasing deviations in polarity will make solubility increasingly difficult.

Van der Waals forces, then, are the result of intermolecular polarities. As we shall see, accurate predictions of solubility behavior will depend not only on determining the degree of intermolecular attractions between molecules, but in discriminating between different types of polarities as well. A single molecule, because of its structure, may exhibit van der Waals forces that are the additive result of two or three different kinds of polar contributions. Substances will dissolve in each other not only if their intermolecular forces are similar, but particularly if their composite forces are made up in the same way. (Such types of component interactions include hydrogen bonds, induction and orientation effects, and dispersion forces, which will be discussed later.)

Next: Part 2 - The Hildebrand Solubility Parameter

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