International Journal of Molecular Sciences Review Review Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis Relationship: A Critical Analysis Wagner Silva 1 , Marcileia Zanatta 2 , Ana Sofia Ferreira 1 , Marta C. Corvo 2 and EuricoWagner J. Silva Cabrita 1, Marcileia1,* Zanatta 2, Ana Sofia Ferreira 1, Marta C. Corvo 2 and Eurico J. Cabrita 1,* 1 UCIBIO,UCIBIO, Chemistry Chemistry Department, Department, School School of of Science Science and and Technology, Technology, NOVA NOVA University Lisbon, 2829-5162829-516 Caparica, Caparica, Portugal; Portugal; [email protected] [email protected] (W.S.); (W.S.); asd.ferreira@ [email protected] (A.S.F.) (A.S.F.) 2 2 i3N|Cenimat,i3N|Cenimat, Materials Materials Science Science Department, Department, School School of of Science Science and and Technology, Technology, NOVA UniversityUniversity Lisbon,Lisbon, 2829-5162829-516 Caparica, Caparica, Portugal; Portugal; [email protected] [email protected] (M.Z.); (M.Z.); [email protected] [email protected] (M.C.C.) (M.C.C.) ** Correspondence:Correspondence: [email protected] [email protected] Received: 2 October 2020; Accepted: 16 October 2020; Published: 19 19 October October 2020 2020 Abstract: In the the last last few few years, years, ionic ionic liquids liquids (ILs) (ILs) have have been been the the focus focus of of extensive extensive studies studies concerning concerning the relationship between structure structure and and properties properties and and how how this this impacts impacts their their application. application. Despite Despite a alarge large number number of of studies, studies, several several topics topics remain remain controversial controversial or or not not fully answered, such as:as: the existence of ion ion pairs, pairs, the the concept concept of of free free volume volume and and the the effect effect of ofwater water and and its implications its implications in the in themodulation modulation of ILs of physicochemical ILs physicochemical properties. properties. In this Inpaper, this paper,we present wepresent a critical a review critical of review state-of- of state-of-the-artthe-art literature literature regarding regarding structure–property structure–property relationship relationship of ILs, we of re-examine ILs, we re-examine analytical analytical theories theorieson the structure–property on the structure–property correlations correlations and present and new present perspectives new perspectives based on basedthe existing on the data. existing The data.interrelation The interrelation between betweentransport transport properties properties (viscosity, (viscosity, diffusion, diff usion,conductivi conductivity)ty) of IL ofstructure IL structure and andfree freevolume volume are analysed are analysed and anddiscussed discussed at a atmolecu a molecularlar level. level. In addition, In addition, we demonstrate we demonstrate how how the theanalysis analysis of microscopic of microscopic features features (particularly (particularly using using NMR-derived NMR-derived da data)ta) can can be be used used to to explain explain and predict macroscopic properties, reachingreaching newnew perspectivesperspectives onon thethe propertiesproperties andand application of ILs. Keywords: ionicionic liquid; liquid; supramolecular supramolecular organization; organization; free volume; water; ion pair; physicochemical physicochemical properties; transporttransport propertiesproperties 1. Introduction Ionic LiquidsLiquids (ILs) (ILs) are are organic organic salts salts that meltthat commonlymelt commonly below 100below◦C, therefore100 °C, theytherefore are constituted they are entirelyconstituted by charged entirely species,by charged usually species, an organic usually cation an organic and an cation organic and or inorganican organic anion. or inorganic The extensive anion. 6 18 amountThe extensive of possible amount combinations of possible ofcombinat knownions cations of known and anions cations (10 and–10 anions)[1], (10 results6–1018 in) di[1],ff erentresults and in uniquedifferent physicochemical and unique physicochemical properties such properties as high thermalsuch as high stability, thermal large stabilit electrochemicaly, large electrochemical window and lowwindow vapor and pressure. low vapor Because pressure. of these Because attractive of these properties, attractive ILs properties, have garnered ILs have industrial garnered and industrial scientific interest,and scientific see Figure interest,1[2, 3see]. Figure 1 [2,3]. Figure 1. Typical ions in ionic liquids (ILs). Int. J. Mol. Sci. Sci. 20202020,, 2121,, x; 7745; doi: doi: FOR10.3390 PEER/ ijms21207745REVIEW www.mdpi.com/journal/ijmswww.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 7745 2 of 37 Int.Int. J.J. Mol.Mol. Sci.Sci. 2020,, 21,, xx FORFOR PEERPEER REVIEWREVIEW 22 ofof 3737 The tunability and and versatility versatility of of ILs ILs have have given given rise rise to toseveral several applications applications such such as solvents as solvents for for synthesis and catalysis [4,5] CO capture and storage [6–8] energy generation and storage [9], synthesis and catalysis [4,5] CO22 capturecapture2 andand storagestorage [6–8][6–8] energyenergy generationgeneration andand storagestorage [9],[9], extractionextraction/dissolution/dissolution ofof biomassbiomass [[10,11],10,11], andand activeactive pharmaceuticalpharmaceutical ingredientsingredients [[12,13].12,13]. ILsILs areare normallynormally divideddivided intointo twotwo classes:classes:classes: aprotic aprotic ionic ionic liquids liquids (APILs) (APILs) and and proticprotic ionicionic liquidsliquids (PILs).(PILs). PILsPILs areare formed formed by by the the transfer transfer of of protons protons from from BrønstedBrønsted acid acid toto BrønstedBrønsted base,base, thisthis protonproton transfertransfer doesdoesdoes not notnot take taketake place placeplace in AILSinin AILSAILS due duedue to the toto nature thethe nana oftureture the of ionsof thethe constituting ionsions constituconstitu thetingting salt. the PILsthe salt.salt. are generallyPILsPILs areare preparedgenerally throughprepared a neutralization through a neutralization reaction and APILsreaction by aand quaternization APILs by reactiona quaternization followed byreaction anion exchange,followedfollowed byby Figure anionanion2[ 14exchange,exchange,,15]. FigureFigure 22 [14,15].[14,15]. Figure 2. TypicalTypical synthetic route of a protic ionic liquid (PIL) ( A)) ethylammoniumethylammonium nitrate nitrate and and an an APIL APIL ((BB)) 1-ethyl-3-methylimidazolium1-ethyl-3-methylimidazoli1-ethyl-3-methylimidazolium tetrafluoroborate.tetrafluoroborate. The first first IL reported in the literature is a contro controversialversial matter. Most authors state that the history starts withwith Paul Paul Walden’s Walden’s discovery discovery in 1914 in with 1914 the reportwith ofthe ethylammonium report of ethylammonium nitrate ([EtNH 3][NOnitrate3]) synthesis,([EtNH([EtNH33][NO][NO see33])]) Figure synthesis,synthesis,3[ 16 see];see however, FigureFigure 33 some [16];[16]; however,however, authors attribute somesome authorsauthors the first attributeattribute IL to Gabriel thethe firstfirst and ILIL to Weinerto GabrielGabriel in 1888and Weiner [17], with in 1888 the synthesis [17], with of the ethanolammonium synthesis of ethanolammonium nitrate. This confusion nitrate. This arises confusionconfusion from the arisesarises reported fromfrom meltingthethe reportedreported point, meltingmelting while Walden’s point,point, whilewhile compound Walden’sWalden’s was compoundcompound characterized waswaswith characterizedcharacterized a melting with pointwith aa of meltingmelting 13–14 ◦ pointpointC, Gabriel’s ofof 13–13– compound14 °C, Gabriel’s exhibited compound a melting exhibited point of a 50 melting◦C, substantially point of 50 higher °C, substantially than room temperature. higher than Theseroom workstemperature.temperature. marked TheseThese the beginning worksworks markedmarked of the thethe First beginningbeginning Generation ofof ofththe ILs. First Cations Generation of the of first ILs. generation Cations of of the ILs first are characterizedgeneration of byILs large are volumes,characterized such asby 1,3-dialkyl-imidazolium large volumes, such as or 1,3-dialkyl-imidazolium 1-alkylpyridinium and anionsor 1- +3 basedalkylpyridinium mostly on halogen and anions aluminate based (Al mostly)[1 ,18on]. halogen The disadvantage aluminate of (Al this+3+3)) generation [1,18].[1,18]. TheThe was disadvantagedisadvantage the instability ofof inthisthis relation generationgeneration to air waswas and thethe water. instabilityinstability inin relationrelation toto airair andand water.water. Figure 3. Generations of ILs through time. The introduction of the Second Generation ofof ILsILs comescomes withwith thethe preparationpreparation ofof airair andand waterwater stable ILs, reported in 1992 by Wilkes and Zaworotko [19], based on the 1-ethyl-3-methylimidazolium − − − cation and alternative anions, [CH33CO22]]−,, [NO[NO33]]− andand [BF[BF44]]−,, seesee FigureFigure 3.3. ILsILs fromfrom thethe secondsecond Int. J. Mol. Sci. 2020, 21, 7745 3 of 37 The introduction of the Second Generation of ILs comes with the preparation of air and water stable ILs, reported in 1992 by Wilkes and Zaworotko [19], based on the 1-ethyl-3-methylimidazolium cation and alternative anions, [CH3CO2]−, [NO3]− and [BF4]−, see Figure3. ILs from the second generation are much easier to handle than those