Nanoconfined Ionic Liquids
2.3 Confinement Effects
2.3.2 Impact of Confinement on Physicochemical Properties of Ionic Liquids
Considering the potential applicability of confined ionic liquids, special attention is being paid to the effect of confinement on such thermochemical properties as melting temperatures [116, 117] and glass transitions [25, 118] as well as dynamic properties,
Nanoconfined Ionic Liquids 37
namely, diffusivity [119–122], viscosity [108, 110, 123], and conductivity [124–126].
Data on several exemplary systems characterized by the changes in IL properties caused by their confinement in spatial systems are presented in Table 2.3. This data refer to the most intensively investigated systems of porous silica and CNT materials. The factors
Real image (a)
(b)
Schematic representation
D= 2.0 nm 5 nm (i)
(ii)
(iii)
(iv)
D= 2.5 nm
D= 3.7 nm D= 3.0 nm
Simulated image
Single-chain
Double-helix
Zigzag tube
Random tube
Figure 2.6 (A) Packing arrangement of [Me3NC2H4OH][ZnCl3] inside SWNTs of different nanotube diameters. The observed (left‐hand side) and simulated (right‐hand side) HRTEM images of four typical morphologies of [Me3NC2H4OH][ZnCl3] ((i)single‐chain, (ii) double‐helix, (iii) zigzag tubes, and (iv) random tubes) are shown. On the basis of the TEM images, structural models (center) were constructed and the diameter of the SWNTs suitable for each configuration were determined (center right). The calculated tube diameters for the single‐chain, double‐helix, zigzag tubes, and random sizes are 1.2, 1.4, 1.8, and 2.1 nm, respectively. Source: Reproduced with permission from Reference [115]. Copyright 2009 American Chemical Society. (B) Representative simulation snapshots of [C4C1IM][PF6] confined inside MWCNTs of different diameters. Source: Reproduced from Reference [112]. Copyright 2010 American Chemical Society. Reproduced with permission of the American Chemical Society.
in various matrices.
IL Confining conditions
Structure and properties of confined IL Change of
Tm, Tg, and Tca Reference
[C2C1IM][Br] Mesoporous silica (3.7 and 7.1 nm), 31.1 and 29.7 wt% IL, post‐impregnation (under high vacuum)
Tm increases (ΔTm = 5 or
22 K) [116]
[C2C1IM][DCA] Porous silica monolith (6.2–19.3 nm, 496–512 m2 g−1), post‐impregnation
Tc increases (ΔTc = 4 K),
Tm decreases (ΔTm = 14 K) [127]
[C2C1IM][SCN] Porous silica monolith (6.2–19.3 nm, 496–512 m2 g−1), post‐impregnation
Tg slightly increases (ΔTg = 1 K), Tc and Tm
disappear
[127]
Silica gels (3.7–7.5 nm, 431–546 m2 g−1, 0.63–
1.19 cm3 g−1), in situ sol–gel method
Tg increases by about
5–8 K [128]
[C2C1IM][BF4] Silica gels (7.4–7.8 nm, 125–202 m2 g−1, 0.46–
0.60 cm3 g−1), in situ sol–gel method
Tc and Tg increase, change
in Tm is complex, [129]
MCM‐41 (3.4 nm, 764 m2 g−1, 0.84 cm3 g−1), 23–34 wt% IL, post‐impregnation
Tg decreases (ΔTg = 14 K),
Tc and Tm disappear [23]
BP2000 carbon black (1374 m2 g−1, 2 cm3 g−1), post‐impregnation
Tc and Tm disappear [130]
[C2C1IM][TfO] Porous silica monolith (6.2–19.3 nm, 496–512 cm3 g−1), post‐impregnation
Tg disappear, Tc and Tm
decrease (ΔTc = 405 K, ΔTm = 8 K)
[127]
Porous silica monolith (6.2–7.1 nm, 919 m2 g−1), post‐impregnation
Tg increases (ΔTc = 2 K),
Tm decreases (ΔTm = 9 K) [131]
Carbon aerogels (276–
308 m2 g−1, 0.43–0.70 cm3 g−1), post‐impregnation
Tg and Tc disappear for
C‐900 and C‐700 [15]
[C2C1IM][NTf2] Silica gels (2.2–12.1 nm, 576–634 m2 g−1, 1.04–2.23 cm3 g−1), in situ sol–gel method
Change in Tc and Tm,
binding energy [132]
ZIF‐8 (MOF), 25 wt% IL,
post‐impregnation Tc and Tm disappear [133]
ZIF‐8 (MOF, 1.16 nm, 1947 m2 g−1, 0.636 cm3 g−1), post‐impregnation
Tc and Tm disappear
(except for EZ125) [134]
[C4C1IM][Cl] Silica gels (3–12 nm, 300–
700 m2 g−1, 0.6–1.1 cm3 g−1), 26.5 wt% IL, in situ sol–gel method
Tc and Tm disappear [13]
(Continued )
IL Confining conditions
Structure and properties of confined IL Change of
Tm, Tg, and Tca Reference
[C4C1IM][Br] Ordered mesoporous silica (3.7 nm), post‐impregnation (under high vacuum)
Tm increases (ΔTm = 50 K) [135]
[C4C1IM][TfO] Controlled‐pore glasses (7.5–11.5 nm, 120–140 m2 g−1), post‐impregnation
Tm decreases; ΔTm
depends on the pore diameter
[136]
[C4C1IM][BF4] Silica gels (5.9–10.4 nm, 350–390 m2 g−1,
0.8–1.1 cm3 g−1), 5.2–40.2 wt%
IL, in situ sol–gel method
Tg disappears [13]
Graphene multilayers,
post‐impregnation Tg increases from 201.7 K
to 329.2 K [25]
BP2000 carbon black (1374 m2 g−1, 2 cm3 g−1), post‐impregnation
Tc decreases (ΔTc = 12 K) [136]
[C4C1IM][PF6] Silica gels (1.6–5.4 nm, 338–843 m2 g−1), in situ sol–gel method
Tm decreases (ΔTm = 2 °C) [137]
Silica gels (11.4–22.6 nm, 182–299 m2 g−1, 1.3–
1.6 cm3 g−1), in situ sol–gel method
Tg increases [138]
Mesoporous silica (2–6 nm), 35.9 wt% IL, post ‐
impregnation (under high vacuum)
Tm greatly increases to
474.8 K [139]
[C4C1IM][NTf2] Silica gels (12 nm, 780 m2 g−1, 1.5–3.5 cm3 g−1), in situ sol−gel method
Tm and Tc disappear [126]
Silica gels (11–15 nm,
780–1200 m2 g−1, 1.50 cm3 g−1), in situ sol–gel method
Tm and Tc disappear while
Tg kept constant [125]
Controlled‐pore glasses (7.5–11.5 nm, 120–140 m2 g−1), post‐impregnation
Tm decreases or
disappears; ΔTm depends on the pore diameter
[136]
SnO2 monolith ionogels (<2 nm, 257 m2 g−1, 0.13 cm3 g−1), in situ sol–gel method
Tm and Tc disappear; Tg
increases (ΔTg = 5 K) [118]
[C6C1IM][Br] MWCNTs (internal diameters:
10 nm), post‐impregnation (under high vacuum)
Tm increases
(ΔTm = 213.2 K) [117]
[C8C1IM][BF4] Silica gels (11.1 nm, 353 m2 g−1, 1.1 cm3 g−1), 14.9–31.7 wt% IL, in situ sol–gel method
Tg disappears [13]
Table 2.3 (Continued)
that affect the structure of ILs confined in pores are chemical composition of the pores’
surface, size, and homogeneity of pores, type of cation and anion in IL, and degree of loading of porous material. These factors have an impact on their diverse behavior in contrast to bulk liquid. In terms of changes of both thermal and dynamic properties caused by confinement very few tendencies may be observed. However, the number of exceptions makes it almost impossible to define the basis for predicting the behavior of novel IL–support systems.
Table 2.3 (Continued)
IL Confining conditions
Structure and properties of confined IL Change of
Tm, Tg, and Tca Reference
Decomposition temperature [C2C1IM][DCA] Carbon aerogels
(276–308 m2 g−1, 0.43–0.70 cm3 g−1), post‐impregnation
Thermal stability slightly
improves [15]
[C2C1IM][SCN] Silica gels (3.7–7.5 nm, 431–546 m2 g−1, 0.63–1.19 cm3 g−1), in situ sol–gel method
Thermal stability
improves [128]
[C2C1IM][BF4] Silica gels (7.4–7.8 nm, 125–202 m2 g−1, 0.46–0.60 cm3 g−1), in situ sol–gel method
Thermal stability
decreases [129]
MCM‐41 (3.4 nm, 764 m2 g−1, 0.84 cm3 g−1), 23–34 wt% IL, post‐impregnation
Thermal stability
decreases [23]
[C2C1IM][TfO] Carbon aerogels (276–308 m2 g−1, 0.43–0.70 cm3 g−1), post‐impregnation
Thermal stability slightly
improves [15]
[C4C1IM][Br] MIL‐101 (MOF, 2956 m2 g−1,
1.63 cm3 g−1), ship‐in‐bottle Slightly improved thermal
stability [140]
NaY (zeolite), 11.4–22.5 wt%
IL, ship‐in‐bottle Improved thermal
stability [103]
[C4C1IM][PF6] Silica gels (14.8–23.2 nm), 26.9–65.0 wt% IL, in situ sol–gel method
Thermal stability
decreases [141]
Silica gels (11.4–22.6 nm, 182–299 m2 g−1,
1.3–1.6 cm3 g−1), in situ sol–gel method
Thermal stability
decreases [138]
[C4C1IM][NTf2] Silica gels (12 nm, 780 m2 g−1, 1.5–3.5 cm3 g−1), in situ sol−gel method
High thermal stability (Td = 630 K in air) close to bulk ILs
[126]
a Tm, Tg, and Tc denote melting, glass‐transition and crystallization temperatures, respectively.
Nanoconfined Ionic Liquids 41
In general, for silica porous materials a decrease in melting temperature is observed [23, 136, 137, 142]. For example, for the ionic liquid [C4C1IM][Ntf2] confined in mesopores the mentioned decrease was around 10 K [96] whereas for [C2C1IM][DCA]
it is around 13 K. The results of studies on the second IL also show that the degree of melting temperature decrease is affected by the size of the anion. Exchanging [DCA]
anion with octyl sulfate resulted in a decrease in melting temperature of up to 52 K.
The results of studies on the impact of IL loading on melting temperature indicate another important factor that may determine the direction of Tm change. It is the loca- tion of IL in the pores [108, 139, 143]. As shown in Figure 2.7, for complete filling of mesopores with [C4C1IM][PF6] under high vacuum an increase in melting temperature value up to 475 K may be observed.
However, if the same IL is located close to the pore entrance, it melts at a slightly lower temperature than bulk liquid does. The same behavior was observed for other ILs based on imidazole core, i.e. 1‐propyl‐3‐methyl‐imidazolium and 1‐allyl‐3‐methyl‐
imidazolium. However, the results of studies on IL based on tributylhexadecylphospho- nium cation and bromide anion show a lowered melting temperature regardless of the IL location [135]. The only difference is the value of Tm decrease, which was equal to 8 K for IL loaded under vacuum whereas for surface‐bound IL it was 14 K.
With silica materials, the quantitative effect of melting temperature decrease is usually described by the Gibbs–Thomson equation. This equation, however, refers to energetic changes of the system caused by the curvature of the interphase surface.
mp of IL: increased
mp of IL: decreased Filling in vacuum10–5 Pa
(70 °C)
Filling at atmospheric pressure
(70 °C) Ionic liquid (IL)
Nanoporous SiO2
Figure 2.7 Schematic diagram of the change in melting point of an IL entrapped by mesoporous silica oxide particles under vacuum conditions versus that at atmospheric pressure. Reproduced from Reference [139]. Copyright 2012 American Chemical Society. Reproduced with permission of the American Chemical Society.
Therefore, its applicability depends on the dominance of forces related to either liquid rearrangement caused by its interaction with pore surface or those related to the capillary effect. Nevertheless, the results of studies on ILs with an imidazole cation and different anions ([TfO], [PF6], [NTf2]) are in accordance with the above‐mentioned relationship, which means that the decrease in melting temperature is inversely proportional to the pore diameter of the silica material (Figure 2.8) [136]. The above‐
mentioned equation is not applicable in case of IL confinement in CNTs. It is caused by far‐reaching structural changes. Moreover, the melting temperature of confined IL usu- ally increases, which is a consequence of a highly arranged IL structure. The melting point shift in the case of CNT confinement is much more significant than in silica mate- rials and it seems to be independent of IL hydrophobicity. For example, the melting temperature after confinement of hydrophobic [C4C1IM][PF6] and hydrophilic [C6C1IM][Br] ionic liquids increased from 279 to 539 K and 215 to 428 K, respectively [97, 117]. Similarly to silica materials, the diameter of the nanotubes also affects the degree of melting temperature change. However, taking into account previous consid- erations it is presumed to be the consequence of decreasing the radial and axial IL structuring with increasing IL distance from the inner surface of the CNT [144].
The effect of IL confinement on dynamic properties such as diffusivity and conduc- tivity is also of interest. Similarly to thermal properties, the dynamics of confined ILs depend specifically on the IL–support system. For ILs confined in silica materials, a decrease of dynamic properties is usually observed [119, 121, 145, 146]; however, oppo- site effects are also described in the literature [107, 124]. A noticeable effect is that the degree of loading of porous material with IL impacts the dynamic properties. It may be stated that when interactions between IL and silica surface are strong, ion mobility is limited and, as a consequence, dynamic properties are decreased. The increased degree of pore loading results in higher average ion mobility as the contribution of IL located closer to the center increases. Taking into consideration the degree of their structure disturbance, some other dependencies for mobility of ILs confined in CNTs may be
40
30
20
10
ΔT/K
0.0000 0.005 0.010
H2O [EMIM][PF6] [BMIM][(CF3SO2)2N]
[BMIM][CF3SO3] [BMIM][PF6]
d–1/(10–10 m)–1
0.015 0.020
Figure 2.8 Melting point depression of ILs as a function of the inverse of the pore diameter; [C4C1IM][TfO], [C4C1IM][PF6], [C2C1IM][PF6], [C4C1IM][NTf2]. The dotted line represents the melting point
depression of H2O. The arrows indicate the estimated errors of Tm(pore) within 1.5 K.
Source: Reproduced from Reference [136].
Copyright 2006 the Royal Society of Chemistry. Reproduced with permission of the Royal Society of Chemistry.
Nanoconfined Ionic Liquids 43
indicated. According to the results of theoretical calculations it has been noted that ionic liquids that have a highly organized structure in the bulk phase caused by hydro- gen and electrostatic bonding (e.g. [C2C1IM][Cl]) are characterized by a significant increase of self‐diffusion coefficient while confined in CNTs of 1.36–3 nm diameter range [21, 147]. This effect may be explained by disturbance of the original IL structure.
An analogous consideration may be made in the case of ILs characterized by relatively loose structuring in the bulk phase. In such cases, IL confinement in CNTs causes their arrangement, which leads to the decrease of dynamic properties, as has been observed for [C4C1IM][PF6] or [C4C1IM][NTf2] ionic liquids [112, 119].