Unveiling the mechanism of hydrotropy: towards a sustainable future

• Autor(a) principal: Abranches, João Dinis Oliveira
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/28936
• topic: Hydrotropy; Sustainable chemistryc; Hydrophobic effect; Glycerol ethers; Cyrene; Phenolic acids

Hydrotropes, with their ability to increase the solubility of hydrophobic substances in water, can expand the applicability of the greenest and most abundant of all solvents. However, and even though broadening the repertoire of safer solvents is in line with the principles of green chemistry and is essential for a sustainable future, hydrotropy is often overlooked as a promising tool for green chemistry. This is due to a lack of fundamental understanding on its mechanism, which hampers the design of novel hydrotropic systems and limits its applications to a few well-known examples. This work starts by using glycerol ethers as a case-study of hydrotropy by investigating their ability to enhance the solubility of gallic and syringic acids in water. The results obtained suggest that the solubility enhancement ability of a hydrotrope, and by extension its hydrotropic capability, depends on its concentration in water. Furthermore, using the concept of the Setschenow constant, it is shown that the hydrophobicities of both solute and hydrotrope play an important role in the solubility enhancement by hydrotropy. Building on the preliminary results obtained with glycerol ethers, experimental evidence for the cooperative theory of hydrotropy, which holds that hydrotropy occurs due to water-mediated aggregation of hydrotropes around the solute, is obtained here for the first time, using 1H-NMR. Moreover, a new computational approach to quantify apolarity is introduced, and is used to clarify the role of the apolarity of both solute and hydrotrope. In fact, it is shown that the number of hydrotrope molecules aggregated around the solute is maximum when there is a match between the apolarity of the two species. Using these newly-found fundamental concepts of hydrotropy, the solubility of hydrophobic solutes in Cyrene, an emerging bio-based green solvent, and its mixtures with water is herein explored. It is shown that hydrotropy is the solubilization mechanism of hydrophobic solutes in the water-Cyrene system, in most of its concentration range. Furthermore, the ketone form of Cyrene is shown to be the principal hydrotrope of the system, with the diol form acting as a hydrotrope only at low Cyrene concentration. The parameters of the cooperative model are shown to be correlated with the hydrophobicity of the solutes, which is explored to successfully predict the solubility curves of phthalic acid, aspirin, gallic acid and vanillin in water-Cyrene mixtures. Finally, it is shown that water, when added to Cyrene in a small amount, acts as a cosolvent by establishing strong hydrogen bonding with the solute. This shows that a system may solubilize hydrophobic solutes through very different mechanisms, depending on the concentration of each species.