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Characterization and Solution Properties of QuaternaryAmmonium-Salt-Type Amphiphilic Gemini Ionic Liquids Risa Kawai, Shiho Yada, and Tomokazu Yoshimura* Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan
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ABSTRACT: Quaternary-ammonium-salt-type amphiphilic gemini compounds (Cm-2-Cn X, where m and n represent the alkyl chain lengths; m = 4, 6, 8, 10; n = 2, 4, 6, 8, 10; m ≥ n; and X indicates the counterion BF4, PF6, OTf, FSA, or NTf2) were synthesized by the quaternization of N,N,N′,N′tetramethylethylenediamine and n-alkyl bromide and a subsequent ion-exchange reaction with five different counterions. For comparison, the corresponding monomeric compounds (Cn X, n = 2, 4, 6, 8, and 10) were also synthesized. The melting points of the compounds were evaluated using differential scanning calorimetry, and those with melting points lower than 100 °C were treated as ionic liquids during the subsequent measurements. The amphiphilic gemini compounds exhibited the lowest melting points (44−49 °C) when bulky NTf2− was the counterion and the degree of dissymmetry between the two alkyl chains was 0.4 < n/m < 0.75. However, their melting points were not similar to those of the monomeric compounds with NTf2− and n = 4−10 ( OTf− > FSA− > NTf2− (Figure S1). Cn NTf2 compounds with n = 8 and 10 were liquid at temperatures below 0 °C, and their melting points could not be obtained. An increase in the length of the alkyl chain from 2 to 4 resulted in a significant decrease in the melting point; this was true for all counterions. The attraction between the cation and the anion is greater for shorter alkyl chains, while the van der Waals interaction between the alkyl chains is stronger for longer alkyl chains, resulting in a high melting point.29 The melting point increased with an increase in the alkyl chain length from 6 to 8 because the van der Waals interactions between the alkyl chains became stronger than the electrostatic interaction between the cations and anions. A similar behavior has been reported in the literature.30,31 Figure 2 shows the relationship between the melting point and degree
Figure 1. Chemical structures of (a) quaternary-ammonium-salt-type amphiphilic compounds gemini-type Cm-2-Cn X, (b) monomeric-type Cn X, and (c) counterion X.
Figure 2. Relationship between melting point and degree of dissymmetry of two alkyl chains, n/m, for Cm-2-Cn NTf2: orange triangle, down, solid, m = 4; blue triangle, up, solid, m = 6; pink circle, solid, m = 8; and purple square, solid, m = 10.
ing monomeric compounds (Cn X, n = 2, 4, 6, 8, 10; see Figure 1b). This was done with the aim of developing novel amphiphilic ionic liquids that show the properties of both ionic liquids and surfactants as well as those of gemini surfactants. We investigated their properties as ionic liquids as well as their surface-active properties in aqueous solutions. Further, we discuss the effects of the alkyl chain length, number of alkyl chains, degree of dissymmetry of the chains, and structure of the counterion of the ionic liquids on their properties.
of dissymmetry of the two alkyl chains, n/m (m ≥ n), for the amphiphilic gemini compounds Cm-2-Cn NTf2 (the values of the melting points of the amphiphilic gemini compounds containing the other counterions, namely, BF4−, PF6−, OTf−, and FSA−, are listed in Table S1). The error in the melting points of the amphiphilic gemini ionic liquids Cm-2-Cn NTf2 was ±0.6 °C. The melting points of Cm-2-Cn NTf2 were high for n/m = 1 (alkyl chains of the same length) but were low for 0.2 ≤ n/m < 1 (except for m = 4). This can be ascribed to the fact that the corresponding ionic liquids are packed tightly because of the strong van der Waals interactions between the alkyl chains owing to their high symmetry. It is known that a dissymmetric molecular structure or the presence of flexible alkyl chains leads to a decrease in the melting point because the lattice energy in this case is low, resulting in poor crystallinity.27 Thus, it can be concluded that the amphiphilic gemini compounds that exhibited low melting points did so because of the presence of dissymmetric alkyl chains. It is worth noting that the melting point of amphiphilic gemini compounds can be lowered to approximately 40 °C without causing an increase in the van der Waals force by varying the length (degree of dissymmetry) between the two alkyl chains. For example, despite their asymmetric structure, amphiphilic gemini compounds C4-2-C2 BF4 (n/m = 0.5), C6-2-C2 BF4 (n/ m = 0.33), and C6-2-C2 FSA (n/m = 0.33) showed higher melting points than those of C4-2-C4 BF4, C6-2-C6 BF4, and C6-2-C6 FSA, respectively, which contained the same alkyl chains. It can be surmised that the former gemini compounds exhibit high ionicity owing to their short chain (n = 2) and are
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RESULTS AND DISCUSSION Melting Point. The melting points of the gemini and monomeric compounds could be arranged in the following order, based on the counterion present: PF6−, OTf− > BF4− > FSA−, NTf2−. Thus, the compounds containing FSA− and NTf2− had the lowest melting points (Table S1). In general, the radii of the above-mentioned anions can be arranged as follow: BF4− (3.44 Å) < PF6− (3.60 Å) < OTf− (3.79 Å) < NTf2− (4.39 Å).26 The radius of FSA− can be assumed to lie between those of OTf− and NTf2−, based on their structures. The melting points of the compounds containing counterions FSA− and NTf2−, which have a bulky structure, were very low. As stated previously, ionic liquids are salts consisting of cations and anions, which undergo electrostatic interactions.27,28 Ionic liquids show low melting points as compared to inorganic salts because they contain organic ions, which have bulky structures. On the other hand, the melting points of the monomeric compounds, Cn X with n = 2, were higher than 100 °C and could be arranged in the following order based on the B
DOI: 10.1021/acsomega.9b01660 ACS Omega XXXX, XXX, XXX−XXX
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bis(trifluoromethanesulfonyl)amide (50 mPa s) as well as those of the protic ionic liquids such as ethylammonium nitrate (32 mPa s) and propylammonium nitrate (66.6 mPa s).32 The viscosity of ionic liquids depends on the interactions that occur between their molecules, such as collisions, hydrogen-bond formation, van der Waals interactions, and electrostatic interactions: the stronger these interactions are, the higher the viscosity will be.33,34 In general, it is known that ionic liquids show viscosities 100−1000 times higher than those of water and organic solvents because ionic liquids consist of a cation and an anion and the electrostatic interactions between them are strong. This, in turn, determines the viscosity.27,34,35 However, the amphiphilic gemini ionic liquids showed extremely high viscosities, suggesting that their gemini-type structure enhanced the van der Waals interactions between the two alkyl chains. The conductivity of the amphiphilic gemini ionic liquid C62-C4 NTf2 (2.84 mS m−1 at 50 °C) was 1/58 and 1/32 of those of the corresponding monomeric ionic liquids, namely, C4 NTf2 and C6 NTf2, respectively. That is to say, the gemini ionic liquid showed significantly lower conductivity than did the corresponding monomeric ones. The charge mobility of gemini ionic liquids is small because of their structure, in which two cation molecules are connected by a spacer. The conductivity of gemini ionic liquid C8-2-C6 FSA was 5.2 times that of C8-2-C6 NTf2 at 65 °C; thus, the conductivity of the ionic liquid containing FSA− was higher than that of the ionic liquid containing NTf2−, in keeping with the conductivity behavior of monomeric ionic liquids (Table S3). In the counterion NTf2−, the fluorine (−F) of FSA− is replaced with the trifluoromethyl group (−CF3). Further, FSA− has a more flexible structure as compared to that of NTf2−. Therefore, the ionic liquid with FSA− showed higher conductivity and lower viscosity.36 The conductivity of the gemini ionic liquids C8-2Cn X reduced as their structure became more symmetric. This was also the case when the length of the alkyl chains of the monomeric ionic liquids was increased. This can be ascribed to the fact that the van der Waals force becomes stronger and the mobility reduces with these changes. The quaternary-ammonium-salt-type amphiphilic monomeric ionic liquids Cn X exhibited lower conductivities compared to those of the imidazolium-type ionic liquid 1buthyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (4.06 × 102 mS m−1)37 and protic ionic liquids such as methylammonium formate (4.38 × 103 mS m−1),38 ethylammonium nitrate (2.69 × 103 mS m−1),39 and ethylammonium acetate (222 mS m−1).40 In general, ionic liquids with an aromatic part, where charge is delocalized, such as imidazolium-type and pyridinium-type ionic liquids, show high conductivities and low viscosities. However, quaternaryammonium-salt-type gemini ionic liquids show low conductivities and high viscosities. This is because of the effects of the localized charge of the ammonium ion and the van der Waals force between the alkyl chains. The conductivity of the ionic liquids also increased with an increase in the temperature (Table S3), in keeping with the behavior of conventional ionic liquids. Ionic liquids show high densities (more than 1.0 g cm−3) because their structure consists only of ions.29,41 The density of gemini ionic liquid C6-2-C4 NTf2 was 1.45 g cm−3 and higher than those of the corresponding monomeric ionic liquids C4 NTf2 (1.40 g cm−3) and C6 NTf2 (1.33 g cm−3) (Table S3), indicating that the gemini ionic liquid was more densely
crystalized readily, even though the van der Waals force between their chains is smaller. This is in spite of the fact that the difference between the lengths of the two alkyl chains is large. That is to say, there is a high degree of asymmetry between the structures of the alkyl chains. On the other hand, the melting-point behavior of Cm-2-Cn OTf was different from those of the compounds with the other counterions, whose melting point increased as the chains became more symmetric. This is because the van der Waals force increases with an increase in the alkyl chain length. Thus, it was found that the melting point of amphiphilic gemini compounds is significantly influenced by the degree of asymmetry between the two alkyl chains as well as the structure of the counterion present. Characterization of Amphiphilic Gemini Ionic Liquids. The amphiphilic gemini compounds Cm-2-Cn X (with the exception of C4-2-C4 FSA, C6-2-C2 FSA, C4-2-C2 NTf2, and C4-2-C4 NTf2) and the monomeric ones Cn X with FSA− or NTf2− (with the exception of C2 FSA and C2 NTf2) were ionic liquids because their melting points were lower than 100 °C. The water contents of the amphiphilic gemini and monomeric compounds were determined to be less than 500 ppm (Table S2); this suggested that investigation of the properties of these ionic liquids would not be problematic. Figure 3 shows the plots of the zero-shear viscosity and
Figure 3. Relationships between (a) zero-shear viscosity and (b) conductivity and degree of dissymmetry of two alkyl chains, n/m (gemini type), and alkyl chain length, n (monomeric type): black triangle, up, solid, C6-2-C4 FSA (50 °C); red triangle, up, solid, C6-2C6 FSA (50 °C); black triangle, open, down, C8-2-C6 FSA (60 °C); orange titled square, solid, C10-2-C4 FSA (50 °C); sky blue square, solid, C6-2-C2 NTf2 (50 °C); purple square, solid, C6-2-C4 NTf2 (50 °C); green square, solid, C8-2-C6 NTf2 (60 °C); red circle, solid, Cn FSA; and blue circle solid, Cn NTf2 (25 °C).
conductivity as functions of n/m (C10-2-C4 FSA and C6-2-C4 NTf2) and n (Cn FSA and Cn NTf2). Here, the zero-shear viscosity was obtained from the relationship between the viscosity and shear rate (Figures S2 and S3). The viscosities of the amphiphilic ionic liquids did not change with increasing shear rate, indicating that they are Newtonian fluids (Figures S2 and S3). The zero-shear viscosities differed according to the structure of the ionic liquids. The viscosity of gemini ionic liquid C6-2-C4 NTf2 was 3.35 × 103 mPa s and 30−40 times higher than that of the corresponding monomeric ionic liquid. This can be attributed to the fact that the van der Waals force between the two alkyl chains enhances their degree of entanglement, resulting in high viscosity. The viscosities of the quaternary-ammonium-salt-type ionic liquids (both gemini and monomeric types) were higher than that of the imidazolium-based ionic liquid 1-butyl-3-methylimidazolium C
DOI: 10.1021/acsomega.9b01660 ACS Omega XXXX, XXX, XXX−XXX
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Table 1. Melting Point (Tm), Krafft Temperature (TK) at 0.20 wt %, Concentration for Ion-Pair Formation (CI.P.), α, Surface Excess Concentration (Γ), Occupied Area per Molecule (A), and Adsorption Efficiency (pC20) Values of Amphiphilic Ionic Liquids Cm-2-Cn FSA and Cn FSA at 25 °C ionic liquid
Tm/°C
TK/°C
CI.P./mmol dm−3
α
C4-2-C2 FSA C6-2-C4 FSA C6-2-C6 FSA C8-2-C2 FSA C8-2-C4 FSA C8-2-C6 FSA C8-2-C8 FSAa C8-2-C8 FSAa (in NaBr solution) C10-2-C4 FSA C4 FSA C6 FSA C8 FSA C10 FSA
61.7 44.1 45.4 44.1 65.5 51.5 66.7
