Article pubs.acs.org/jced
Density, Viscosity, and Conductivity of Binary Mixtures of the Ionic Liquid N‑(2-Hydroxyethyl)piperazinium Propionate with Water, Methanol, or Ethanol Qi Cao, Xiaoxing Lu, Xi Wu, Yongsheng Guo, Li Xu, and Wenjun Fang* Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: The densities, viscosities, and electrical conductivities of the binary systems of the ionic liquid (IL), N-(2-hydroxyethyl)piperazinium propionate ([(OH) 2 C 2 pi][C 2 CO 2 ]), with three protic solvents (water, methanol, or ethanol) were measured in the IL concentration range from (0.05 to 3.00) mol·kg −1 at different temperatures T = (293.15 to 323.15) K and atmospheric pressure p = 0.1 MPa. The concentration dependences of apparent molar volume, viscosity, and electrical conductivity of the binary mixtures were correlated by the Pitzer equation, the Jones−Dole equation, and the Casteel−Amis equation, respectively. It is observed that both the density and viscosity increase with the elevated IL concentration or the decreased temperature. The electrical conductivity at a given IL concentration increases with the temperature, while the conductivity at a given temperature changes clearly against the IL concentration, and a maximum value can be observed. The measured results and the correlations present help for a better understanding and applications of the piperaziniumbased IL.
1. INTRODUCTION Ionic liquids (ILs) are salts composed of organic cations and various anions and usually have melting points lower than 373 K.1,2 Due to their unique properties, such as low melting points, nonvolatility, low flammability, and high themal stability,3,4 interests in IL have increased extensively in recent years. ILs have been used in many fields as reaction media, mechanical lubricants, and separation solvents.5 The most widely studied ILs are imidazolium- and pyridinium-based ILs that consist of aromatic rings.6−11 By contrast, as a large class of ILs, the ammonium-based ILs have also been studied and applied. Akbari and co-workers12 found that 1,1,3,3-tetramethylguanidinium acetate was an efficient, homogeneous, and recyclable catalyst for Boc protection of amines. Yuan et al.13 presented a series of hydroxyl ammonium carboxylates for the absorption of SO2. The saturated solubilities of SO2 in tri(2-hydroxyethyl)ammonium lactate are as high as 0.5 in mole fraction. Xin et al.14 synthesized a cyclic guanidinium lactate ionic liquid that can catalyze the Knoevenagel condensation of one of the aromatic aldehydes with each of the active methylene compounds at room temperature in a high yield of greater than 90% within (1 to 7) min. Attri et al.15 reported that triethylammonium acetate is a strong stabilizer and more biocompatible for α-chymotrypsin stability. These achievements indicate that the ammonium-based ILs have the potential to be promising materials in many studies and applications. As a type of ammonium-based IL, several piperazinium-based ILs have been prepared and investigated in our larboratory recently, such as N-methylpiperazinium lactate, N-ethylpiperazinium lactate, and N-ethylpiperazinium propionate, and they © XXXX American Chemical Society
have already been used to extract aromatics from hydrocarbonbased fuels.16−18 On the other hand, the determination and understanding of basic physical and transport properties of ILs, such as density and viscosity, are of great importance for process design and equipment options. Electrical conductivity is an indication of the movement of electric charges and further gives information about the interactions between solvent and solute. In some practical processes, an IL is used as one of the components in the mixed solvents, in consideration of the cost and performance. For example, N-methylpiperazinium propionate with methanol as cosolvent could be used effectively to extract aromatics from hydrocarbon fuels.16 Thus, the fundamental data about ILs with molecular liquids may provide a better understanding of these ILs. Densities and viscosities of the N-ethylpiperazinium propionate IL with n-alcohol systems have been determined at different temperatures.19 To obtain more useful information on the piperazinium-based ILs, a novel IL, N-(2-hydroxyethyl)piperazinium propionate ([(OH)2C2pi][C2CO2]), has been synthesized. In this work, the densities, viscosities, and conductivities for binary systems of the IL with three protic solvents (water, methanol, or ethanol) at the molality (m) of the IL from m = (0.05 to 3.00) mol·kg−1 were measured Received: April 29, 2014 Accepted: January 7, 2015
A
DOI: 10.1021/je500380x J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
2.2. Synthesis of Ionic Liquid. The ionic liquid, N-(2hydroxyethyl)piperazinium propionate ([(OH) 2 C 2 pi][C2CO2]) was synthesized with a procedure similar to that in our previous work19 as follows (Scheme 1): N-(2hydroxyethyl)piperazine (0.20 mol) and ethyl acetate (100 mL) were loaded into a 500 mL three-neck flask and stirred to obtain a dispersed solution. Propionic acid (0.2 mol) in ethyl acetate (50 mL) was added dropwise to the solution that was cooled in an ice−water bath under the protection of nitrogen gas. The mixture was vigorously stirred for 6 h. The solvent was then removed under vacuum, and the residual white solid was washed with ethyl acetate and further dried by freeze-drying (Peking Sihuan Scientific Instrument, LGJ-10, China). The obtained product was stored in a dry nitrogen atmosphere. The water content in [(OH)2C2pi][C2CO2] was determined by a Karl Fischer titration instrument (Mettler Toledo, C20, Switzerland) and was found to be less than 0.05% of the mass fraction. 1 H NMR spectrum analysis of the synthesized IL was carried out on a Bruker DMX III 500 MHz NMR spectrometer using DMSO-d6 as the solvent with tetramethylsilane as the internal standard. The chemical shifts (δH) of [(OH)2C2pi][C2CO2] are 3.49 (t, 2H, N−C−CH2−O), 2.80 (m, 4H, N−C−CH2−N+), 2.43 (m, 4H, N−CH2−C−N+), 2.37 (t, 2H, N−CH2−C−O), 2.08 (q, 2H, C−CH2−COO), and 0.96 (t, 3H, CH3−C−COO). The 1H NMR spectrum can be found in the Supporting Information. Elemental analysis (Carlo Erba 1110, Italy) of the prepared ILs was found (calcd) in mass fraction of C, 52.2 (52.9); H, 9.9 (9.8); N, 13.4 (13.7). 2.3. Apparatus and Procedure. The binary mixtures were prepared in a 20 mL glass vial sealed with a polytetrafluoroethylene cap by mass using an analytical balance (Mettler Toledo, AL204, Switzerland) with a precision of 1 × 10−4 g. The uncertainty in molality is 1 × 10−4 mol·kg−1. A density meter (DMA 5000M, Anton Paar, Austria) was used to measure the densities of the binary mixtures at T = (293.15 to 323.15) K. The density meter was calibrated with ultrapure water and dry air before the measurements. The uncertainty is 0.01 K for the temperatures and 5 × 10−5 g·cm−3 for the density. Viscosities were measured on a viscometer (Anton Paar, AMVn, Austria) at the same conditions as those used for densities and were determined by measuring the efflux time of the iron ball in the liquid samples. According to the following equations
from T = (293.15 to 323.15) K and at atmospheric pressure p = 0.1 MPa.
2. EXPERIMENTAL SECTION 2.1. Chemicals. N-(2-Hydroxyethyl)piperazine ((OH)2C2pi, mass fraction >0.999, CAS Registry No. 103-76-4) was purchased from Shaoxing Xingxin Chemical Co., Ltd., China. Propionic acid (mass fraction >0.995, CAS Registry No. 79-09-4) and ethyl acetate (mass fraction of >0.995, CAS Registry No. 141-78-6) were purchased from SigmaAldrich. Methanol (mass fraction >0.995, CAS Registry No. 6756-1) and ethanol (mass fraction >0.997, CAS Registry No. 6417-5) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. These reagents were used without further purification. Ultrapure water was produced by a Millipore Q3 system. Detailed information on the chemicals is listed in Table 1. The Table 1. Information on Chemicals in This Work chemical name
source
purity (mass fraction)
water (mass fraction)
>0.999
0.995 >0.995 >0.995
