Probing Ionic Liquid Aqueous Solutions Using Temperature of Maximum Density Isotope Effects

Mohammad Tariq,José Esperança,José Lopes,Luís Rebelo

doi:10.3390/molecules18043703

Abstract

This work is a new development of an extensive research program that is investigating for the first time shifts in the temperature of maximum density (TMD) of aqueous solutions caused by ionic liquid solutes. In the present case we have compared the shifts caused by three ionic liquid solutes with a common cation—1-ethyl-3-methylimidazolium coupled with acetate, ethylsulfate and tetracyanoborate anions—in normal and deuterated water solutions. The observed differences are discussed in terms of the nature of the corresponding anion-water interactions.

Highlights

Water is a complex substance whose unique properties derive from a balance between its ability to perform multiple hydrogen bonds (HBs) and its rather small molar volume and symmetric nature
Even in the liquid state such open hydrogen-bonded structure is only progressively lost: pure water continues to contract until a temperature of maximum density (TMD) is reached around 4 °C [1]
One way to probe the nature of the hydrogen-bonded structure of aqueous solutions is to measure the shifts in the TMD, = TMD-TMD, of water solutions: solutes that promote more stable HB/deuterium bonds (DBs) networks should yield solutions with higher TMDs, whereas those that break the HB/DB network should decrease the TMD value

Summary

Introduction

Water is a complex substance whose unique properties derive from a balance between its ability to perform multiple hydrogen bonds (HBs) and its rather small molar volume and symmetric nature. One of the most well known consequences of such subtle balance is the open structure of ice and the associated density increase upon melting (+9%). Even in the liquid state such open hydrogen-bonded structure is only progressively lost: pure water continues to contract until a temperature of maximum density (TMD) is reached around 4 °C (the density increases +0.013% between 0 and 4 °C) [1]. Given the different nature of the deuterium bonds (DBs), the melting point temperature and the TMD of D2O only occur at 3.82 and. One way to probe the nature of the hydrogen-bonded structure of aqueous solutions is to measure the shifts in the TMD, = TMD (aqueous solution)-TMD (pure water), of (normal or heavy) water solutions: solutes that promote more stable HB/DB networks should yield solutions with higher TMDs, whereas those that break the HB/DB network should decrease the TMD value

Methods

Results

Discussion

Conclusion