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Water, the Original Solvent

‘green chemistry’ is trying to increase the scope of water & other non-hydrocarbons as solvents.

Water is the solvent of life, yet despite this, most of us don’t immediately think of water when we think of solvents. By definition, a solvent is a substance able to dissolve other substances. This is important in chemistry because the solution phase allows molecules to adopt the orientations necessary to react with each other– a process of no greater importance than in our cells. It is also important to engineers as dissolution allows different molecules to be mixed and transferred onto substrates in coatings.

Entire academic careers have been devoted to the study of water. Being by far the most prevalent liquid in our world, it is striking that its behaviour is about as non-ideal and complex as the world of chemistry can conjure; rather like the verb ‘to be’ being irregular. Its gas and solid phases are no better behaved. We are taught the water cycle at a young age but never stop to consider that no other compound is regularly observed on this planet as a solid, a liquid and as a gas and that each phase manifests strange properties.

Given the miraculous nature of water, why then do organic and industrial chemists often tend to use organic solvents instead? For the former, it is because water is rewater dropletactive in numerous ways, to the point that it would scupper many reactions by reacting/participating non-selectively. For the latter, it is simply because many organic molecules are soluble in organic solvents but not water. Methanol is arguably the closest solvent in nature to water in terms of its polarity and solubility profile, yet it possesses many starkly different characteristics e.g. flammability, toxicity and a much lower surface tension, and is also a poor solvent for many organic compounds.

Although the use of water as a solvent in organic chemistry has been relatively uncommon, it is becoming more prevalent today as ‘green chemistry’ seeks to increase the scope of water and other non-hydrocarbons as solvents. Likewise, in industries such as coatings, everything’s going green. Water-based coatings have always been around, but the world is now moving away from organic solvents (VOCs) towards water. While there are some disadvantages to water such as solubility of organics and wetting, the benefits to health, safety and sustainability are too great. Additionally, solvents are a major contributor to cost in chemical processes and coatings; the overall material efficacy of a chemical process (E factor) is largely governed by the amount of waste materials a process entails, of which solvents make up a large part.

One way around insolubility in water is through dispersions/suspensions of partially soluble/insoluble materials. Many adhesives, paints and primers are dispersions. A curious facet of water-based coatings is that the very solvent one uses to transfer the coating to a substrate is the one which we are most worried about removing the coating through weathering. if the coating was in water before why wouldn’t it go into water again? How can we ensure that a water-based coating isn’t water-susceptible? A chemical curing/cross-linking step can change the nature of a coating, so that it may be water-soluble before reaction but not after, which is where OAS’s Onto™ technology comes in. Onto™ technology employs very thin coatings of adhesion promoters which can be applied without the need for flammable, hazardous solvents and which also enables the use of water-based coatings thereafter – all in all, a greener option. A perfect use for such technology would be in vehicle interior leather wrapping;  by using Onto™ adhesion promoters and removing the use of VOCs in car interiors altogether, we envisage the potentially harmful ‘new car smell’ becoming a thing of the past.

Key Solvent Properties of Water

  • High polarity: one of the most polar covalent substances, strong intermolecular forces in the form of hydrogen bonding result, and are responsible for, many aspects of its behaviour and macro properties
  • High surface tension: as evidenced by visible menisci and pond skaters. Pure water gives poor wetting of low surface energy substrates in coatings: this necessitates surfactant chemistry
  • Capillarity: what makes a paint brush or cloth absorb liquids, and allows plants to transport liquids easily
  • Simultaneously slippery and sticky: take two sheets of glass with water in between. They are easy to slide over each other yet very hard to pull apart
  • Amphoteric: as well as being acidic, water is basic, which has important consequences in biological systems
  • Low vapour pressure and high heat capacity: due to its strong intermolecular forces, water requires a lot of energy to evaporate thus having a low vapour pressure. Similarly, a lot of energy is required to heat water compared to other substances
  • Electrolytic: a predominantly covalent compound but conductive when containing dissolved ions
  • Hydrophobic effect: Immiscibility with many organic solvents, which allows liquid/liquid separations
  • not a good solvent for everything but a uniquely good solvent for many important things: gases (especially at lower temperatures), salts (sea water solution ~0.5N HCl), acids and bases, sugars, proteins