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On the Nature of Mesoscopic Organization in Protic Ionic Liquid-Alcohol Mixtures Alessandro Triolo, Wolffram H. Schroer, and Olga Russina J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b01422 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016
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The Journal of Physical Chemistry
On the Nature of Mesoscopic Organization in Protic Ionic Liquid-Alcohol Mixtures. Alessandro Triolo‡,*, Wolffram Schroer§,* and Olga Russina†,* ‡ Laboratorio Liquidi Ionici, Istituto Struttura della Materia, Consiglio Nazionale delle Ricerche, Rome, Italy § Universität Bremen Fachbereich 2 Biologie-Chemie, Bremen, Germany † Dipartimento di Chimica, Università di Roma Sapienza, Rome, Italy
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ABSTRACT.
The mesoscopic morphology of mixtures of ethylammonium nitrate, a protic ionic liquid, and npentanol is explored for the first time using Small Angle X-ray Scattering as a function of concentration and temperature. Both compounds are amphiphilic and characterized by an extended hydrogen bonding network; however, though macroscopically homogeneous, their mixtures are highly heterogeneous at the mesoscopic spatial scales. Previous structural studies rationalized similar features in related mixtures proposing the existence of large aggregates or micelle- and/or microemulsion-like structures. Here we show that a detailed analysis of the present concentration and temperature resolved experimental data set supports a structural scenario where the mesoscopic heterogeneities are the due to density fluctuations that are precursors of liquid-liquid phase separation. Accordingly no existence of structurally organized aggregates (such as micellar or microemulsion aggregates) is required to account for the mesoscopic heterogeneities detected in this class of binary mixtures.
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The Journal of Physical Chemistry
Introduction Ionic liquids (IL), being salts that are liquid at ambient temperatures, represent an exciting class of compounds1–9 from the scientific point of view because of the large field of possible applications. They are often built up by asymmetric ions, thus showing an enhanced tendency to remain liquid over extended temperature windows. Increasing efforts are currently being paid to understand the chemical-physical properties of binary mixtures of ILs with molecular compounds, in an attempt to fine tune and modulate the bulk properties of the resultant media10– 21
. ILs’s amphiphilic nature (very often ILs ions bear covalently bound polar and apolar
moieties) leads to a complex morphology as a consequence of the polar vs apolar segregation into separate domains22–35, such a behavior has also been detected in protic ILs (PIL)36–42 such as Ethylammonium Nitrate (EAN)28,43,44. When mixing ILs with molecular compounds, the latter tend to distribute preferentially in the domains with larger philicity45–50; however this is not always the case: when dissolving amphiphilic compounds into ILs, a different phenomenology occurs that cannot be straightforwardly rationalized in terms of the commonly accepted polar-vsapolar dualism51. It is very well known that EAN is characterized by a miscibility gap with a Critical Temperature (Tc) at 315 K when mixed with 1-octanol (C8-OH, hereinafter we will indicate with Cn-OH normal alcohols with alkyl chain as CnH2n+1)52–56. The critical temperature is shifted to higher temperatures for alcohols with longer chain length, while alcohols with shorter chain are fully miscible with EAN at ambient conditions, but the mixtures can phase separate when cooled. This is a common scenario for the thermodynamics of IL-based solutions. 15–18,57–59 Recent Small Angle X-ray Scattering (SAXS) studies on PIL-alcohol mixtures detected low momentum transfer (Q) scattering features prompting for a complex structural behavior for these
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mixtures50,51,60,61; the latter were explained proposing the formation of large aggregates50 or even postulating the existence of “small micelles or microemulsions”60 in order to account for the experimental data. On the other hand in our exploration of binary mixtures of EAN with methanol using a combination of x-ray and neutron scattering together with Reverse Monte Carlo computations, we assessed that the mixing of the two amphiphilic compounds does not lead to mesoscopically homogeneous mixtures, but rather to the unstructured clustering of the ionic species over an enhanced spatial scale that is supposedly the precursor for the phase separation occurring in the case of longer chain alcohols.51 This proposal nicely fits the observation that in selected binary mixtures of alcohols and ILs, such as the mixture EANoctanol where phase separation has been observed, Small Angle Neutron Scattering, Light scattering and dielectric measurements detected concentration fluctuations that constitute mesoscopic inhomogeneities in the near-critical region and were proposed as precursors of the liquid-liquid phase separation.52,53,55 All the above mentioned studies have been conducted at ambient conditions and so far no diffraction pattern is available at lower temperature, for short (n97% (NMR), with EA and nitrate content of 99.8% (ionic conductivity)) with halides content
