Water as Green Solvent: Methods of Solubilisation and Extraction of Natural Products - Past, Present and Future Solutions

Water is considered the greenest solvent. Nonetheless, the water solubility of natural products is still an incredibly challenging issue. Indeed, it is nearly impossible to solubilize or to extract many natural products properly using solely water due to their low solubility in this solvent. To address this issue, researchers have tried for decades to tune water properties to enhance its solvent potential in order to be able to solubilise or extract low-water solubility compounds. A few methods involving the use of solubilisers were described in the early 2000s. Since then, and particularly in recent years, additional methods have been described as useful to ensure the effective green extraction but also solubilisation of natural products using water as a solvent. Notably, combinations of these green methods unlock even higher extraction performances. This review aims to present, compare and analyse all promising methods and their relevant combinations to extract natural products from bioresources with water as solvent enhanced by green solubilisers and/or processes.

Introduction

Water is seen as the solvent of life. Indeed, it is essential for every known living organism, and it may even be necessary for every unknown life form in the universe [1]. Within organisms, water acts as a useful solvent, supporting many vital physiological functions. Amongst other things, this solvent is able to solubilise numerous different molecules, it is part of various fundamental metabolic pathways and enables acid-base neutrality and enzyme function. All these advantages can also be utilised in the laboratory by chemists. Besides being a useful solvent, many researchers consider water as the greenest solvent in chemistry both from an experimental and an industrial point of view [2,3,4,5]. In addition, it is clear that there has been a continuous growth in interest for water in solubilisation and extraction since the 1980s, as shown in Figure 1.

Such an interest in using water as a solvent may be attributed to its easy accessibility and low cost, along with its green properties (non-toxicity, renewability, safety and ease of handling, ease of treatment and degradation, etc.). Nevertheless, when water is used to solubilise or to extract natural products (NPs) from actual biological resources, this solvent appears to be relatively inefficient. For instance, the flavonoid rutin—which is theoretically quite polar according to its partition coefficient (Kow ≈ −0.47) [6]—is only sparingly soluble in water (S ≈ 130–150 mg/L) [6,7]. To overcome such a low efficiency in solubilising or extracting NPs, researchers have developed different methods to enhance the water solvent potential while taking advantage of its green qualities. By the turn of the millennium, Yalkowsky [8] had summed up the main methods to enhance the water solvent potential primarily in order to solubilise drugs more efficiently. These methods consist of the following: pH range and salts, cosolvents, surfactants, complexing ligands, inclusion complexes, stacking complexes and hydrotropes.

Once these initial seven methods to solubilise NPs in water were described at the beginning of the 2000s, additional ways to enhance the solubility of such compounds in water were introduced. Namely, switchable solvents were discovered in 2010 [9] followed by the Natural Deep Eutectic Solvents (NADES) first described in 2011 [10]. These two relevant solubilisation methods can easily be applied to modify water’s solvent properties. In fact, switchable solvents include switchable water and NADES can be dissolved in water so that the system remains an aqueous solvent. These techniques have continued to capture scientists’ attention to the extent that still today numerous articles are published with a view to extending their use and discovering and patenting new ingredients.

At this point, a total of nine methods have been established and are largely described as solubilisation techniques. All these methods may also be applied to the extraction of NPs from biological material. Whilst a relevant solubilisation technique does not necessarily constitute an efficient extraction technique—as was notably shown with the rosemary case study [11]—with the appropriate adjustments implemented, such techniques may be used for the purpose of both solubilisation and the extraction of NPs. If we now consider water-based extraction methods, we can enrich this set of methods with four other innovative techniques, namely the use of enzymes [12], reactive extraction, in situ plant water extraction (ISPWE) [13] and subcritical water extraction (SWE) [14], which gives a total of thirteen water-based extraction methods.

This review introduces the basic principle of thirteen methods to enhance the solvent power of water in the green extraction of NPs. These methods are then analysed, compared and evaluated, with a score reflecting their green extraction global efficiency. Moreover, the future of water-based green extraction is expected to consist of relevant combinations of these methods to achieve higher extraction yields and greater extraction profiles. The interest of some of these combined methods will be discussed. Finally, the potential impact of these methods as a prospect is assessed through the prism of green extraction and the principles of green chemistry, then linked to the corresponding Sustainable Development Goals (SDGs) as defined by the United Nations.

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Lajoie, L.; Fabiano-Tixier, A.-S.; Chemat, F. Water as Green Solvent: Methods of Solubilisation and Extraction of Natural Products—Past, Present and Future Solutions. Pharmaceuticals 2022, 15, 1507. https://doi.org/10.3390/ph15121507