I published this on my website a few months back and thought I’d share here.
An Introduction from a Chemist’s Perspective
What is a solvent? In the simplest of terms, a solvent is a liquid that dissolves another substance (liquid, solid or gas). The best way to understand solvents is to think of them in terms of ‘water-like’ or ‘oil-like’. You could also apply the old ‘like dissolves like’ saying. This simply means that greasy solvents prefer to dissolve greasy substances (solutes). There is even a scale to describe if a particular solvent or chemical (in its purest form) will be oil (nonpolar) or water-like (polar). This scale is known as the ‘Partition Coefficient‘ or ‘P’ and is usually presented as its log form (logP). The greasier the substance (more oil-like) the larger the logP. Because some solvents are actually mixtures of many different solvents, a logP can not be assigned. But, we can still get a feel for their ‘greasy’ or ‘water-like’ nature of the solvent from experience (you could obtain a logP for each component).
For the chemistry and math daring:
log P = log [octanol]/[water]
In simple terms it is the log of the ratio of its concentration in octanol or water after shaking the substance in a flask filled with equal parts of octanol and water.
Probably the most common woodworking solvents are mineral spirits, naptha, toluene, turpentine, ethanol, methanol, isopropylalcohol, lacquer thinner, and the ketones acetone and MEK (methylethylketone). All of these solvents could be ranked according to their water or oil-like nature and the pure-form (non-mixture solvents) solvents could be assigned a logP. When choosing a solvent it is important to estimate where the solvent falls on the scale with regard to its hydrophobic (literally ‘water fearing’) nature.
Hexanes → (Naptha – Mineral Spirits)->MEK->Acetone->Methanol->Water
–>Decreasing hydrophobicity (Increasing hydrophilicity)–>
At the extreme, ‘greasy’ (hydrophobic) end of the spectrum are solvents like mineral spirits (composed of a mixture of greasy hydrocarbons like hexanes), turpentine, and toluene. While at the other extreme end are solvents like water, methanol, ethanol and isopropanol (listed in order of increasing LogP or increasing hydrophobicity). Because the alcohols are more hydrophilic than mineral spirits (the other end of the spectrum) they are better suited for dissolving more hydrophilic substances such as analine dyes (like-dissolves-like).
Solvents that are somewhere in the middle of the scale are the ketones such as MEK and acetone. Acetone has a logP of -0.25 so it prefers water slightly more than oil (around 2-fold more).
Aside from considering a solvents water-like or oil-like nature, the boiling point is also important. For example isopropanol (82 C) has a higher boiling point than methanol (65 C) or ethanol (78 C) and will evaporate slower. If this is important to you then choose accordingly.
I’m not suggesting that when choosing a solvent for a woodworking purpose that we get too analytical when thinking about their properties; this would just bog you down. But, I think we should at least try and keep their “rough estimate” properties in mind when choosing a solvent for a particular task.
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