Measurement of the Osmotic Coefficients in Aqueous Solutions of Imidazolium Ionic Liquids ([C_nmim]Cl or [C_nmim]Br, n = 4, 6, 8) Over a Wide Salinity Range at 298.15 K

Abstract

The goal of this work is to provide accurate and detailed experimental data on the osmotic coefficients and the activity of water for the binary aqueous solutions of ionic liquids [C n mim]Cl and [C n mim]Br (n = 4, 6, and 8), covering the broad range of salinity up to 10 mol•kg −1 . Such data form the basis for the thermodynamic description of the system, and are needed, in particular, for the estimation of parameters of thermodynamic models. Our data supplement and extend to a broader concentration range the existing literature data on the studied systems at 298.15 K. The systematic presentation of the data includes the method used, the conditions, the concentration range, and the estimates of the experimental errors. To help in the assessment of the data quality, we compare all of the available data sets. Using vapor pressure osmometry and isopiestic methods to perform experiments, we discuss possible sources of error and explain in every detail the construction of the apparatus designed in our lab to reduce the experimental errors to a minimum. For [C 8 mim]Cl and [C 8 mim]Br, known to form micelles in aqueous solutions, we estimate the critical micelle concentration from our experimental data on the osmotic coefficients and the activity of water.

Modeling Osmotic Coefficients in Aqueous Solutions of 1-Alkyl-3-methylimidazolim Halides: A Theory That Reflects the Electrical Structure of Ions and ePC-SAFT

“…For solutions of C 8 mimCl and C 8 mimBr, the osmotic coefficient data show rather strong attraction that may not be modeled by the DH theory of a 1:1 electrolyte at constant dielectric permittivity (see Figure ). The longer-chain C n mim + ( n ≥ 8) are known to form micelles in solution: the CMC is 0.28 mol·kg –1 for C 8 mimCl and 0.21 mol·kg –1 for C 8 mimBr . Formation of charged micelles is followed by strong downturn of the experimental ϕ − m curves, as Figure demonstrates for these binaries.…”

“…Osmotic coefficients in aqueous solution of ILs (298.15 K, 101 kPa) as a function of salinity calculated with the aid of two versions of ePC-SAFT: “ePC-SAFT Revised” (dashes) and “ePC-SAFT Advanced” (solid curves). Symbols: experiment . Numbers shown at different curves are the ARD values for the osmotic coefficients and the activities of water (in parentheses); see Table S3.…”

“…The modeling protocol, including parameter estimation, is described in our previous works. , Here, we re-estimate parameters for ILs from new data on osmotic coefficients of aqueous IL solutions over a wide concentration range . To improve the results of parameter estimation, we measured experimentally the densities of aqueous solutions [C n mim]Cl and [C n mim]Br ( n = 4, 6, 8) at T = 298.15 K and p = 101 kPa (see Tables S1 and S2).…”

“…Applying this version of the model, we introduced association between the cations and anions of ILs to take into account ion pairing, as suggested in ref . The model parameters for water are taken from ref and for Cl – and Br – from ref ; parameters for systems with [C n mim] + are regressed from the data on densities (refs − and data of this work given in Table S2) and osmotic coefficients of [C n mim]X ( n = 4, 6, 8, X = Cl, Br) aqueous solutions. All data sets are limited to 298.15 K.…”

“…The association term a assoc has not been included in our applications of this version of the model. The model parameters for water are taken from ref ; parameters for Cl – and Br – are regressed from data on densities, − mean ionic activity coefficients (MIAC), and osmotic coefficients of KCl and KBr aqueous solutions; and parameters for [C n mim] + are from data on densities (refs − and data of this work given in Table S2) and osmotic coefficients of [C n mim]X ( n = 4, 6, 8, X = Cl, Br) aqueous solutions.…”