ARTICLE pubs.acs.org/JPCB Ionic Parachor and Its Application in Acetic Acid Ionic Liquid Homologue 1-Alkyl-3-methylimidazolium Acetate {[Cnmim][OAc](n = 2,3,4,5,6)} Wei Guan, Xiao-Xue Ma, Long Li, Jing Tong, Da-Wei Fang, and Jia-Zhen Yang* Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China bS Supporting Information ABSTRACT: Five acetic acid ionic liquids (AcAILs) [Cnmim][OAc](n = 2,3,4,5,6) (1-alkyl-3-methylimidazolium acetate) were prepared by the neutra- lization method and characterized by 1HNMR spectroscopy and differential scanning calorimetry (DSC). The values of their density and surface tension were measured at 298.15 ( 0.05 K. Since the AcAILs can strongly form hydrogen bonds with water, the small amounts of water are difficult to remove from the AcAILs by common methods. In order to eliminate the effect of the trace water, the standard addition method (SAM) was applied to these measurements. As a À new concept, ionic parachor was put forward. [OAc] was seen as a reference ion, and its individual value of ionic parachor was determined in terms of two extrathermodynamic assumptions. Then, the values of ionic parachors of a À À À À number of anions, [NTf2] , [Ala] , [AlCl4] , and [GaCl4] , were obtained by using the value of the ionic parachor of the reference ion; the parachor and surface tension of the investigated ionic liquids in literature were estimated. In comparison, the estimated values correlate quite well with their matching experimental values. 1. INTRODUCTION strongly form hydrogen bonds with water, which is problematic Ionic liquids (ILs) have several unique properties including as an impurity, the standard addition method (SAM) was applied in these measurements;14 (3) as a new concept, the ionic parachor minuscule vapor pressure, nonflammability, and dual natural À polarity, making them greatly applicable in many physical and was put forward and was applied to the AcAILs. [OAc] was seen chemical fields.1 Acetic acid ionic liquids (AcAILs) have attracted as a reference ion, and its particular value of ionic parachor was considerable attention from industry and the academic commu- determined in terms of two extrathermodynamic assumptions, then values of ionic parachor for the corresponding cations, nity as a new-generation greener ionic liquid. The AcAILs have + [Cnmim] , in the AcAILs were calculated from the reference strong solubility and good catalytic properties, which are useful À fi for an enzyme-friendly cosolvent for the resolution of amino value of [OAc] ; (4) nally, in terms of the estimation of the 2 parachor and surface tension of the investigated ionic liquids in acids, ultrasonic irradiation toward synthesis of trisubstituted À + imidazoles,3 assisted transdermal delivery of sparingly soluble literature using the ionic parachors for [OAc] and [Cnmim] , 4 5 the credibility of the new concept, ionic parachor, was tested and drugs, and some catalytic reactions. Thus, AcAILs have been fi expected to be applied in various fields, such as industrial veri ed. chemistry and pharmaceutical chemistry. In recent years, there has been a developing trend in the 2. EXPERIMENTAL SECTION literature toward the estimation of the physicochemical proper- ties for compounds by semiempirical methods, in particular, for 2.1. Chemicals. Deionized water was distilled in a quartz still, 6À8 À À ILs. Although the estimated result cannot be regarded as and its conductance was 0.8À1.2 Â 10 4 S 3 m 1. Acetic acid was accurate physicochemical data, it is to be commended because it distilled and dried under reduced pressure. N-Methylimidazole provides valuable insight into the origins of the behavior of the (AR-grade reagent) was vacuum-distilled prior to use. 1-Bro- materials. Among all the empirical methods, parachor is the moethane, 1-bromopropane, 1-bromobutane, 1-bromopentane, 9À11 12,13 simplest. As a continuation of our previous investigation, and 1-bromohexane (AR-grade reagent) were distilled before this article reports the following: (1) five acetic acid ionic liquids use. Ethyl acetate and acetonitrile were distilled and then stored [Cnmim][OAc] (n = 2,3,4,5,6) (1-alkyl-3-methylimidazolium acetate) were prepared by the neutralization method; (2) the Received: August 16, 2011 values of the density and surface tension for [Cnmim][OAc] (n = Revised: September 15, 2011 2,3,4,5,6) were measured at 298.15 ( 0.05 K. Since AcAILs can Published: October 06, 2011 r 2011 American Chemical Society 12915 dx.doi.org/10.1021/jp207882t | J. Phys. Chem. B 2011, 115, 12915–12920 The Journal of Physical Chemistry B ARTICLE Table 1. Density Values of Ionic Liquids Containing Various À a Amounts of Water at 298.15 K (G/g 3 cm 3) F density for [Cnmim][OAc] (n = 2,3,4,5,6) Figure 1. Preparation of AcAILs by the neutralization method. 1, [Cnmim][Br]; 2, [Cnmim][OH]; and 3, [Cnmim][OAc]. [C2mim][OAc] 3 2 Â 5 10 w2 6.79 8.03 9.58 10.34 11.87 0 r s 10 À F (g 3 cm 3) 1.1471 1.1478 1.1485 1.149 1.1497 1.1437 0.99 5.5 over molecular sieves in tightly sealed glass bottles. Anion-ex- change resin (type 717) was purchased from Shanghai Chemical [C3mim][OAc] 3 2 Â 5 Reagent Co. Ltd. and activated by the regular method before use. 10 w2 6.31 7.84 9.51 10.31 11.87 0 r s 10 F À3 2.2. Preparation of the AcAILs. The AcAILs [Cnmim][OAc] (g 3 cm ) 1.1219 1.1226 1.1232 1.1238 1.1244 1.1190 0.99 9.9 (n = 2,3,4,5,6) were prepared by a neutralization method 15 [C4mim][OAc] according to Fukumoto et al. Figure 1 is a schematic of this 3 2 Â 5 synthetic route. First, [C mim]Br (n = 2,3,4,5,6) was synthesized 10 w2 5.88 7.18 8.43 9.72 11.06 0 r s 10 n F À3 according to the literature.16,17 Then, aqueous 1-alkyl-3- methy- (g 3 cm ) 1.0995 1.1001 1.1006 1.1012 1.1019 1.0968 0.99 4.8 limidazolium hydroxide ([Cnmim][OH]) (n = 2,3,4,5,6) was [C5mim][OAc] 3 2 Â 5 prepared from [Cnmim]Br (n = 2,3,4,5,6) by use of the activated 10 w2 6.13 7.35 8.69 9.95 11.23 0 r s 10 À anion-exchange resin over a 100 cm column. However, F (g 3 cm 3) 1.0803 1.0809 1.0814 1.0821 1.0828 1.0773 0.99 7.9 [Cnmim][OH] (n = 2,3,4,5,6) is not particularly stable and should be used immediately after preparation. The onium hydro- [C6mim][OAc] 3 2 Â 5 xide aqueous solution was added dropwise to a slight excess of 10 w2 5.74 7.02 8.28 9.52 10.74 0 r s 10 À3 acetic acid aqueous solution. The mixture was stirred under F (g 3 cm ) 1.0633 1.0638 1.0643 1.0649 1.0656 1.0606 0.99 8.9 a 2 ffi cooling for 24 h. Then, water was evaporated under reduced w2 is the water content; r is the squared correlation coe cient; and s is pressure at 40À50 °C. To this reaction mixture was added the the standard deviation. mixed solvent acetonitrile/methanol (volumetric ratio = 9/1), and it was stirred vigorously. The mixture was then filtered to remove Table 2. Surface Tension Values of Ionic Liquids Containing the slight excess of acetic acid. The filtrate was evaporated to re- À a Various Amounts of Water at 298.15 K(γ/mJ 3 m 2) move the solvents. The product of [Cnmim][OAc] (n = 2,3,4,5,6) was dried in vacuo for 2 days at 85 °C. Structures of the resulting γ surface tension for [Cnmim][OAc] (n = 2,3,4,5,6) acetic acid ionic liquids were confirmed by 1HNMR spectroscopy (see Figure A of the Supporting Information). ff [C2mim][OAc] Di erential scanning calorimetric (DSC) measurements showed 3 2 10 w2 6.65 8.05 9.71 11.15 12.74 0 r s that ILs [Cnmim][OAc] (n = 2,3,4,5,6) had no obvious melting À γ (mJ 3 m 2) 39.6 39.9 40.3 40.6 41.0 38.1 0.99 0.018 point, but their glass transition temperatures Tg(n=2,3,4,5,6) = À ° À ° À ° À ° À ° 75.5 C, 73.4 C, 70.5 C, 68.5 C, and 63.2 C, [C3mim][OAc] 3 2 respectively. All traces of DSC are listed in Figure B of the Sup- 10 w2 5.66 7.37 9.13 10.97 12.84 0 r s À porting Information. The water content, w2 (in mass fraction), in γ (mJ 3 m 2) 38.2 38.7 39.1 39.6 40.0 36.8 0.99 0.041 the AcAILs was determined by the use of a Karl Fischer moisture titrator (ZSD-2 type). [C4mim][OAc] 3 2 2.3. Determination of Density and Surface Tension of the 10 w2 7.00 8.26 9.88 11.97 14.72 0 r s À AcAILs. Since the AcAILs can strongly form hydrogen bonds γ (mJ 3 m 2) 37.4 37.7 38.2 38.9 39.4 35.2 0.99 0.106 with water, the small amount of water in the AcAILs is difficult to [C5mim][OAc] remove by common methods so that trace water becomes the 3 2 10 w2 6.43 8.10 9.81 11.39 12.95 0 r s most problematic impurity.