This has been a long-standing pre-occupation that has seen compounds used as solvents, such as benzene, Footnote 4 carbon disulfide and carbon tetrachloride, first used and then substituted as new substances became available which were more efficacious or less hazardous. The environmental and other consequences that arise from the large-scale use of solvents highlight the need to reconcile their usefulness with the minimisation of their impact. Would these criteria be the same or different if one were to consider different types of solvent, such as one used as a paint stripper or nail-varnish remover or a shampoo rather than a floor cleaner? And, would the priority given to one criterion over another also be different, depending on how it was used?, Footnote 2, Footnote 3 Would one be prepared to pay more for an ‘excellent’ cleaning outcome compared with an ‘acceptable’ outcome? Or, should it be based on the environmental impact of the product, and would one be prepared to accept an inferior cleaning outcome if the product was (however judged) more environmentally benign? How should one address the overall acceptability of such diverse products? Should they primarily be judged on the basis of the efficacy of the cleaning process (and how would one judge that?) or the cost-effectiveness of the product in bringing about the desired effect. Is the ‘solvent’ in this context the entire composition, or one (or all) of the liquid components? It is not surprising, therefore, that products designed for this purpose are formulations containing several components (some liquid, some solid) each designed to fulfil a particular additional function, such as wetting, penetration or dispersion (or even making the product smell nice), and not simply dissolution. Recent work, also discussed in “ Solvent selection and design: empirical database and computational methods” section, has sought to incorporate toxicological and environmental characteristics into such screening.Įveryday use of solvents, such as in washing dishes or in cleaning the floor, involves the dissolution (or removal in some other way) of a complex mixture of materials, some adhering to the surface to be cleaned with varying degrees of tenacity. Tables of such data are now available to guide the screening of the large number of possible solvents for those most likely (singly or in combination) to dissolve a solute of choice. This empirical idea has been refined and extended (Barton 1991) (and will be discussed further in “ Solvent selection and design: empirical database and computational methods” section) so that certain molecule-specific properties (such as polarisability, dipolarity, hydrogen-bond donor and acceptor ability) can be put into numerical form as ‘solubility parameters’ (Hansen 1969) or, latterly, as ‘solvatochromic parameters’ (Marcus 1993 Kamlet et al. A polar material, like common salt, sodium chloride, is more likely to dissolve in a polar solvent, like water, than in a non-polar solvent, like hexane. The simplest and oldest rule of thumb is that ‘ like dissolves like’. Precisely which solvent will dissolve which solute and which solute will dissolve in which solvent (and why) are determined by a delicate balance of a range of factors that include the differing nature and relative strengths of inter-molecular forces in the pure solute and solvent and interactions between molecules of the solute and solvent (coupled with entropic factors governing the overall process of solute + solvent → solution). Solvents used as media for chemical reactions have a profound effect on both the thermodynamics and kinetics of physical and chemical processes (Buncel et al. Solvation encompasses all types of inter-molecular interaction, such as van der Waals bonding, ionic/dipolar interactions, hydrogen bonding and charge transfer. The chemistry of solutions and solvents, therefore, is of interest in its own right (Reichardt and Welton 2010), being of wide chemical relevance, encompassing phase behaviour and the nature of solids, liquids and gases and the properties of mixtures of mainly liquids and solids. But what is a solution? What is a solvent or a solute? What is the difference between a solution and a suspension, or a dispersion, a gel or a slurry? Solvency, solvents and solutions are part of everyday life, from making a cup of coffee (and putting sweetener in it and washing the cup afterwards) to washing one’s hair in the shower, from applying and removing nail varnish to rinsing an apple under the tap before eating it, from applying a coat of paint to taking a soluble pain killer for a headache.
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