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Nanoscale Density Fluctuations in Ionic Liquid Binary Mixtures With Non-Amphiphilic Compounds: First Experimental Evidence Alessandro Mariani, Rajeev Dattani, Ruggero Caminiti, and Lorenzo Gontrani J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b07295 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016
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The Journal of Physical Chemistry
Nanoscale Density Fluctuations in Ionic Liquid Binary Mixtures with non-Amphiphilic Compounds: First Experimental Evidence Alessandro Mariania, Rajeev Dattanib, Ruggero Caminitia,c, Lorenzo Gontrani*a a
Department of Chemistry, “La Sapienza” University of Rome. Piazzale Aldo Moro 5, 00185
Rome – Italy b
c
Beamline ID02, ESRF-The European Synchrotron. 71, Avenue des Martyrs, Grenoble – France
Centro di Ricerca per le Nanotecnologie Applicate all’Ingegneria, Laboratorio per le
Nanotecnologie e le Nanoscienze, “La Sapienza” University of Rome. Piazzale Aldo Moro 5, 00185 Rome – Italy
[email protected], phone: +390649694273
Abstract
A complex mesoscopic organization is observed in systems containing Ethylammonium Nitrate (EAN) and two non-amphiphilic compounds, using Wide and Small Angle X-ray Scattering and Molecular Dynamics Simulations. The macroscopically homogenous mixtures exhibit a separation where an Ionic Liquid-rich region is percolating a Molecular Liquid-rich one, but no
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unmixing is observed. This effect was already reported in EAN-alcohol mixtures, but the models proposed so far cannot explain this behavior for a non-amphiphilic compound.
Introduction
Ionic Liquids1–4 are low melting point salts often containing an organic, apolar moiety. When the latter is hydrophobic enough, nano-segregation takes place5–11 and the well known spongelike structure of Ionic Liquids (ILs) can be observed in Small Angle X-ray Scattering (SAXS) experiments. The characteristic Low q Peak (LqP) is a fingerprint of this organization, and it is attributed to the repeating distance between two anions12 which is therefore mediated by the hydrophobic part of the molecule (i.e. the apolar domain). Dissolving a co-solvent into an IL may have a variety of effects depending on the nature of the salt and on the mixed compound13– 25
. Taking Ethylammonium Nitrate (EAN)26 as a prototype, there is a wide range of literature
concerning the effects that mixing has on its structure9,24,25,27–36 EAN is the first ever reported room temperature Ionic Liquid and it is capable of building an extended 3D Hydrogen Bond (HB) network37 through its three donors and three acceptors. This overall organization is reminiscent of water and, in fact, EAN and water share a number of similarities, although EAN is without any doubt an amphiphilic compound. Recently it has been reported that the addition of linear alcohols to some ILs leads to an unexpected feature in the extreme low q region of the SAXS pattern23,38,39, which from now on will be termed “Low q Excess” (LqE). Greaves et al.40 observed that in some EAN-alcohol mixtures, micellar-like structures could be found. A confirmation of this behavior came from Jiang et al.38 stating that when an alcohol is longer than
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twice the alkyl chain of the IL, then it is too big to be accommodated into the apolar domain thus forming a series of self-assembled structures where the cation acts as a co-surfactant. This interpretation cannot explain why the same effect is found in EAN-methanol mixtures, where one may expect a total mutual miscibility. Triolo et al.39 have stated that the origin of the LqE is due to density and concentration fluctuations within the bulk. This approach is in line with the proposed paradigm of “nanostructured solvation” described by Méndez-Morales et al.41. In this work, for the first time the LqE is reported for EAN binary mixtures with nonAmphiphilic compounds, namely 1,2-Dimetoxy Ethane (DME) and 1,4-diaminobutane (DAB). Those two molecules are both highly symmetrical (C2V symmetry group), do not posses distinct polar/apolar domains such as alcohols, i.e. the extremities of the molecule are identical and polar (DAB) or alkylic (DME), and have similar sizes compared to each other. Although DME is miscible with water in any proportion at room conditions, we observed that while a χEAN=0.4 (being χ the molar fraction) mixture is macroscopically homogenous, the χEAN=0.3 mixture is clearly separated into two liquid phases, so here we take into account mixtures with χEAN down to 0.4 and not lower . A similar behavior is observed in EAN:n-Octanol mixtures where critical unmixing happens at high alcohol concentrations27,42. DAB is a low melting point solid (2527°C) with an unpleasant odor (it is also called putrescine). It is completely miscible in water and EAN. Figure 1 shows a schematic representation of the chemicals used in this work.
Figure 1. Schematic representation of the molecules in this work. Ethylammonium Nitrate (left); 1,2-Dimetoxy Ethane (upper right) 1,4-Butanediamine (lower right).
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Experimental Section 1,4-diaminobutane and 1,2-dimetoxyethane were both purchased at Sigma Aldrich at the highest purity level available (≥98% and 99.9% respectively) and used without further treatments. Ethylammonium Nitrate was purchased at IoLiTec at nominal purity grade of ≥98%. Before the use, it was pumped in high vacuum under slight warming (45 °C) for 48h, to remove moisture. The final content of water was checked by 1H-NMR and it was undetectable (
