DOI: 10.1002/cphc.201100852 Density, Thermal Expansion and Viscosity of Cholinium-Derived Ionic Liquids Anabela J. L. Costa, [a] Mµrio R. C. Soromenho, [a] Karina Shimizu, [b] Isabel M. Marrucho,* [a] JosØ M. S. S. EsperanÅa,* [a] JosØ N. Canongia Lopes,* [a, b] and Luís Paulo N. Rebelo [a] 1. Introduction Choline, N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium chlo- ride, is a water-soluble essential nutrient usually grouped within the B-complex vitamins. It can be found in food ob- tained from vegetables or animals, and supports several essen- tial biological functions. [1] Studies show that this nutrient is es- pecially important during pregnancy and lactation. [1–3] More- over, Meck et al. and Jones et al. have observed that a choline- supplemented diet in rats play a critical role in brain develop- ment and memory function. [2, 3] Choline is used nowadays as a starting material for the de- velopment of environmentally friendly ionic liquids. The goal is to obtain ionic liquids that can unite the most relevant ionic liquid properties (such as a wide liquid range, non-flammabili- ty, [4] and negligible vapour pressure at ambient conditions) [5–7] with biocompatible properties. In this context, the presence of the hydroxyl group of the choline cation increases the biode- gradability of the compound. [8] Unfortunately, it is known that, for a given anion, ionic liq- uids based on the cholinium cation show higher melting points than those based on 1-alkyl-3-methylimidazolium cat- ions. [9] Herein we have chosen bis(trifluoromethanesulfonyl)- imide (bistriflimide) as the anion so that one could have a wider range of compounds in the liquid state at around room temperature. Our objective is to show the effect of in- creasing the alkyl chain length of one of the alkyl groups con- nected to the nitrogen in the cholinium cation in terms of den- sity and viscosity. Choline is thought to be safe for the environment and human health. However, care must be taken regarding the choice of the anion and the size/functionality of the alkyl side chains of the cation in order to preserve the “green” status of the final product. On one hand it is already known that, in combination with distinct cations, bistriflimide anions generally yield ionic liquids with relatively high toxicities. [10] Moreover, the increase of the length of the alkyl side chain also negative- ly affects toxicity, due to the increasing hydrophobicity of the ionic liquid. [11] There are already several studies dealing with the toxicity and biodegradability of cholinium-based ionic liq- uids with different anions. [12–16] However, only one of them [13] presents toxicity data for the bistriflimide anion, showing their higher toxicity when compared to that of their analogues hal- ides. That study also shows the negative effect on toxicity of increasing the alkyl side chain in the cholinium cation. Nevertheless, the scientific interest in cholinium-based ionic liquids is increasing as shown by several studies already pub- lished on possible applications, for example, in catalytic reac- tions, [17–21] as reaction media, [22] in processing of metals and metal oxides, [23] as crosslinking agents [24] or as protein handling media. [25] Only one study on the separation of aliphatic and ar- omatic compounds with [N 11 n 2(OH) ][Ntf 2 ] has been found in the Density and viscosity data of the N-alkyl-N,N-dimethyl-N-(2-hy- droxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ionic liquids homologous series [N 11 n 2(OH) ][Ntf 2 ] with n = 1, 2, 3, 4 and 5 have been measured at atmospheric pressure in the 283 < T/K < 373 temperature range and the corresponding iso- baric thermal expansion coefficients have been calculated. This work studies the effect of increasing the alkyl chain length of the cholinium-based cation on the density, viscosity and relat- ed properties of this family of ionic liquids. A volumetric pre- dictive method based on the effective molar volume of cations and anions is used to estimate the effective molar volume of the different cations present in this study. The results agree with data for other cation families that show a molar volume increment per CH 2 group on the alkyl chain of the cation of about 17.2 cm 3 mol 1 , except for [N 1 1 1 2(OH) ] + , which exhibits an outlier behaviour. Molecular dynamics simulation results are used to explain the volumetric behaviour along the homolo- gous series from a molecular perspective. The predictive power of group contribution methods for density and viscosity is also tested. [a] A. J. L. Costa, M. R. C. Soromenho, Dr. I. M. Marrucho, Dr. J. M. S. S. EsperanÅa, Dr. J. N. C. Lopes, Prof. L. P. N. Rebelo Instituto de Tecnologia Química e Biológica Universidade Nova de Lisboa Av. Repfflblica, 2780-157 Oeiras (Portugal) E-mail : imarrucho@itqb.unl.pt jmesp@itqb.unl.pt jnlopes@ist.utl.pt Homepage: http://www.itqb.unl.pt [b] Dr. K. Shimizu, Dr. J. N. C. Lopes Centro de Química Estrutural Instituto Superior TØcnico 1049-001 Lisboa (Portugal) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201100852. ChemPhysChem 0000, 00, 1 – 9  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ