Abstract

To explore the effect of ionic liquid hydrophobicity over the volumetric properties, density has been determined for six of these compounds in aqueous solution in the temperature interval (293.15–308.15) K, covering the diluted region. For three of these binary systems, density at 298.15 K in the pressure interval (0.1–95) MPa has been also measured. The atmospheric pressure density is used together with high pressure data to calculate excess volumes as a function of pressure with high accuracy through an integration scheme based on the excess volume derivative against pressure. Since hydrophobic phenomena is known to have a greater effect over second and third order derivatives of the thermodynamic potential, partial molar volume as well as the its derivative against mole number of the ionic liquid are calculated. Clear anomalies are detected in these magnitudes for all binary systems, very similar to those found for low molecular weight amphiphiles. Dependencies of this anomalous behavior against temperature and pressure are also analyzed.

Highlights

  • The study of aqueous solutions of small molecules, has revealed that, for some cases, these systems can present some degree of nanoheterogeneity, i. e. domains of few nanometers rich in water or solute molecules appear for appropriate concentration ranges [1,2,3]

  • Two paramount examples of this kind of molecules are 2butoxyethanol and tert-butanol, both extensively studied using a wide variety of techniques [4,5,6,7,8,9,10,11,12,13,14,15,16]. This behavior has been explained as a consequence of the dual, amphiphilic nature of these compounds. [1,2,3,7,17,18,19,20,21,22,23] At very low composition, amphiphiles are fully surrounded by water molecules, i.e. they are fully hydrated

  • Inorganic salts show regular behavior, without any detectable anomaly in this strongly points towards the volumetric behavior of diluted ionic liquid aqueous solutions is dominated by processes that have their origin in hydrophobic phenomena, and ionic interactions would play a secondary role

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Summary

Introduction

The study of aqueous solutions of small molecules, has revealed that, for some cases, these systems can present some degree of nanoheterogeneity, i. e. domains of few nanometers rich in water or solute molecules appear for appropriate concentration ranges [1,2,3]. Amphiphiles start to interact, and, at enough large concentration, their hydrophobic moieties tend to come together, avoiding contact with water, and they form an aggregate of solute molecules, responsible for the observed nanoheterogeinity. These aggregates could be thought as imperfect micelles, or pseudomicelles; these systems share with micelleforming compounds its amphiphilic character, but, since their hydrophobic parts are quite shorter, they cannot form wellordered structures and do not show proper micellar behavior. The temperature and pressure dependencies of such anomalous behavior are analyzed

Experimental
Results
Effect of pressure
Conclusion
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