Solute solubility in any aqueous solution is controlled by the amount of salt added; either a decrease or an increase in solubility. Generally, these phenomena, relating to the former and the latter, are termed: salting out and salting in, respectively. In addition, the extent of the effects depends on the salt concentration, valence and nature of the cation and the anion, and their complex interactions with water and dissolved solutes. This topic is of much interest and has even seen several theories put forward in a bid to rationalize the underlying mechanisms and predict quantitative and qualitative effects of added salts. Specifically, in recent chemical and bio-technological advances, separating small, hydrophilic molecules from aqueous solutions on one side and the dissolution of hydrophobic organic molecules in water on the other has become a daunting challenge. Therefore, it is imperative that an in-depth study be undertaken in order to understand the underlying molecular-level mechanism of the ions’ action in solutions.
Recently, Queen’s University Belfast researchers led by Dr. Marijana Blesic from the School of Chemistry and Chemical Engineering investigated the salting in/out propensity of ions and assess the importance of both in salt-solute interaction and their concentration. To be precise, they evaluated the effects of addition of zwitterionic salts, (ZWSs) on miscibility and alteration of critical solution temperature in binary systems. They also utilized zwitterionic salts: a novel group of hybrid materials, incorporating characteristics of inorganic salts, zwitterions, and ionic liquids. Their work is currently published in the research journals, Green Chemistry and ChemPhysChem.
To begin with, the researchers commenced with the synthesis of four zwitterionic salts differing in the structure and the length of the alkyl chain attached. They the selected acetone and butanol for use in their work owing to the industrial significance of the two. Next, the critical solution temperature for the ternary mixtures prepared were determined by cloud-point measurements, while nuclear magnetic resonance and UV absorption spectroscopies were used to determine the phase compositions. Lastly, the effects of zwitterionic salts additives on the solubility of a range of organic compounds were analyzed.
The authors observed that, in aqueous solutions, several mechanisms simultaneously took place resulting to the effects which could not be disentangled. Additionally, they noted that contrary to classical inorganic salts that affect the solubility of nonpolar or low-polar solutes to very different extents, but generally exhibit similar salt orders, zwitterionic salts behaved differently and did not follow the same salt order.
In conclusion, Queen’s University Belfast scientists successfully demonstrated the solubility-modifying power of zwitterionic salts. Despite the fact that separation of small, hydrophilic molecules from aqueous solutions on one side and the dissolution of hydrophobic organic molecules in water on the other are seemingly of opposite nature, their work demonstrated that both processes could be facilitated by addition of zwitterionic salts. Altogether, zwitterionic salts offer possibility of modulation and functionalization, good thermal stability, non-toxicity and could even be used as salting out or salting in agents to polymer, amino acid,or ionic liquids solutions for extraction, recovery or purification purposes among other uses.
“Zwitterionic salts are molecules introduced only in 2017 by my group, so they are in a very early stage of investigation. We have recently received a funding grant from EPSRC for development of these novel molecular structures. It will help us to get a deeper knowledge of ZWSs’ properties, behaviour, and diverse design, with aim to explore and, hopefully, bring some technological advances.” said Dr. Marijana Blesic.
Marijana Blesic, Brendan F. Gilmore, John D. Holbrey, Johan Jacquemin, Gaelle Level, Peter Nockemann, Lorenzo Stella, An Introduction to Zwitterionic Salts, Green Chemistry 2017 (Hot Article 2017), volume 19, issue 17, page 4007-4011.Go To Green Chemistry
Gaelle Level, Mariana Vieira Fadul, and Marijana Blesic. Solubility-Modifying Power of Zwitterionic Salts. ChemPhysChem 2018, volume 19, page 575 – 580.Go To ChemPhysChem