Investigation of a family of structurally-related guanidinium ionic liquids through XPS and thermal analysis Aldo Moscardini a , Andrea Mezzetta a , Nicola Calisi b,c , Stefano Caporali b,c , Christian Silvio Pomelli a , Lorenzo Guazzelli a, ⁎, Cinzia Chiappe a a Department of Pharmacy, University of Pisa, Via Bonanno 33, 56126 Pisa, Italy b Department of Industrial engineering (DIEF), University of Firenze, 50139 Firenze, Italy c Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), 50123 Firenze, Italy abstract article info Article history: Received 16 October 2018 Received in revised form 11 December 2018 Accepted 14 December 2018 Available online 15 December 2018 A family of structurally-related guanidinium bistriflimide ionic liquids has been prepared and characterized. TGA analyses showed a high thermal stability for all the proposed ionic liquids while DSC and XPS analyses divided them into distinct subsets depending on whether one or more constraining cycles were present. The results obtained highlighted the influence of the cation structure on some of the physico-chemical properties and thus the possibility to tune them by selecting proper substituents. The solvatochromic parameters of a selected guanidinium IL have also been studied. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids (ILs) are salts which are, by definition, liquid at a tem- perature lower than 100 °C. They are composed by an organic cation and an organic or inorganic anion. During the last 20 years they attracted a great deal of interest as neoteric solvents characterized by remarkable and sometimes unique physico-chemical properties [1]. In particular, the high thermal stability, negligible vapor pressure, low flammability and wide electrochemical window suggested from the be- ginning their potential use as alternatives to traditional volatile organic solvents (VOCs). During the years, the idea of ILs as intrinsically green species has been mitigated by investigating their toxicity [2,3] and deg- radation [4], and looking at the synthetic strategies to prepare them in more detail [5]. Recently, natural (e.g. amino acids, terpenes, fatty acids) [6,7] or biobased ions [8] have been employed in the develop- ment of new ILs as a way to reduce the use of fossil derived materials and enhance their biodegradability. Nowadays, ILs are employed as sol- vents or additives in a wide and disparate range of applications which span for instance from organic synthesis [9], to biopolymers dissolution and modification [10,11], from innovative separation techniques [12] to biosensor development [13]. Another aspect of ILs which is often praised is the almost limitless number of structures that can be prepared from known ions. This means that their properties can be finely tuned to address a specific problem, hence their reputation as designer solvents. Although this is true, a recent work by Bara et al [14], focused on the frequency of use of selected ILs deriving from the combination of the most studied 1,3 dialkyl imidazolium cation and 16 anions, demonstrated that only 5% of the possible total structures under consideration are actually pres- ent in the literature, even for this simple cation. Guanidinium ILs have been far less investigated than the imidazolium one, although they showed interesting potential applications as well. In fact, they were used as electrolytes for lithium batteries [15,16], and as possible media for CO 2 capture [17], organic reactions [18,19], cellulose dissolution [20], and electrochemical deposition of elemental titanium [21]. Furthermore, a series of tetramethyl guanidinium ILs resulted as biodegradable and non-toxic for normal HEK 293 and cancerous DLD-1 cell cultures [22]. A peculiar trait of guanidinium ILs is the charge delocalization of the cation core where the positive charge is spread over three nitrogen atoms. Instead, in pyrrolidinium and imidazolium ILs the positive charge is localized on the sole nitrogen atom present in the first case or distributed across the π-ring system in the latter case. As a conse- quence of the different positive charge density, the interaction between the guanidinium cation and its anion is weaker when compared to pyrrolidinium and imidazolium ILs, as shown by X-ray photoelectron spectroscopy (XPS) analysis [23]. XPS is a surface-sensitive spectro- scopic technique which works under high vacuum and is particularly suitable for studying ILs thanks to their negligible vapor pressure [24]. Several features of ILs have been investigated by means of XPS. For in- stance, anion-cation interactions [25], in situ reaction monitoring [26], surface composition [27], and even the different orientations of chiral enantiopure ionic liquids at the IL/vacuum interface [28]. To gain further insight into the guanidinium ILs family, and in partic- ular into the correlation between cation structure and physico-chemical Journal of Molecular Liquids 277 (2019) 280–289 ⁎ Corresponding author. E-mail address: lorenzo.guazzelli@unipi.it (L. Guazzelli). https://doi.org/10.1016/j.molliq.2018.12.083 0167-7322/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq