Structure-Property Relationships in Ionic Liquids - American Chemical

Chapter 32

Downloaded by NORTH CAROLINA STATE UNIV on September 14, 2012 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch032

Structure-Property Relationships in Ionic Liquids James D. Martin Department of Chemistry, North Carolina State University, Raleigh, N C 27695-8204

Abstract: The structure of an ionic liquid directly impacts its properties. A perspective on the structure of ionic liquids is given with a consideration of the impact of size, shape and charge distribution on the coulombic interactions between the constituent ions. Dramatic changes in structure of molten salts, described as metallotropism, can be affected by the relative AX/MX composition, as well as by the choice of templating cations. Solid-state structures of low melting salts provide the foundation to understand structural principles governing the organization of ions in the liquid phase. m

Introduction It is well known that the characteristic properties of ionic liquids can be significantly varied by the choice of anions and cations. To exploit this tunablilty of ionic liquids as solvents it is necessary to understand those forces that give rise to specific properties such as the melting point, polarity and solubility. Undoubtedly these properties are strongly influenced by the structure of the respective ionic (and/or molecular) species in the liquid. In their recent

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

413


414 review Wasserscheid and Keim summarized the influence of the local structural properties of anions and cations on properties such as melting point, vapor pressure, thermal stability, density, viscosity, solvation strength and acidity ( i ) . The size, charge, and distribution of charge of the respective ions are the origin of many of these properties in an ionic liquid. To further understand the structure property relationships in ionic liquids it is important to look beyond the local structure of the respective ions to the structural organizations in the bulk liquid. Significant randomness in organization is necessary to describe the structure of a liquid. However, a majority of materials exhibit only a 10-15% volume expansion on going from the crystalline to the liquid state. Since volume increases with the cube of distance the relatively small volume increase upon melting requires that atom-atom or ion-ion distances in the liquid state are not significantly different from those in the solid state. This is born out in the analysis of numerous X-ray and neutron scattering experiments in liquids as diverse as N a C l (2) and S i 0 (3). Thus, it is possible to gain insight into structure property relationships in ionic liquids from a consideration of structural organizations in the solid and liquid state. A unique class of liquids are ambient temperature ionic liquids, whose properties present a variety of curious features that can begin to be understood from a consideration of the liquid structure. Based on a freshman chemistry understanding of ionic interactions, salts, the combination of anions and cations are expected to have high melting points. Yet as is now apparent from the field of ionic liquids, numerous salts have melting points well below room temperature. While it might be expected that molten salts should dissolve ionic species, Rogers et. al. found that a variety of ionic dyes were partitioned into an aqueous layer whereas when neutral they partitioned into the ionic liquid (4). This raises the question as to what detennines solubility in a molten salt? A n d while polarity is an important concept for the synthetic chemist, the measurement of polarity and its impact on the solvent characteristics of molten salts remains a matter of significant discussion. These curious properties are directly related to the structural organization in the melt. 2

O n the Melting Point of Salts The dominant force of attraction between ions that must be overcome upon melting a salt to an ionic liquid is the coulombic attraction between oppositely charged particles (Coulomb's law). F= 6Γ

2



415 However two features distinguish those salts that exhibit relatively low melting points from those with quite high melting points. In low temperature melting salts, the charge, q, on the ions is normally ± 1 ; and the size of the ions is large, thus the distance, r, separating them must also be relatively large. In ambient temperature ionic liquids the small charge and large distance reduces the coulombic attractions to a magnitude similar to that of intermolecular interactions in molecular liquids. The size of the respective cations not only decreases the Coulombic attraction by separating the centers of charge, but often the charge in a polyatomic ion will be delocalized over the surface (or volume) of the anion resulting in a dramatically reduced charge density. Comparison of the melting points of the sodium salts in Table 1 serves to demonstrate this effect. On increasing the thermochemical anionic radius from CI" < [BF ]" < [PF ]~ < [AICI4]" the melting point of the sodium salts is dramatically reduced from 801 °C to 185 °C. 4

6

Table 1: Relative Size of Ions and Melting Points of Salts

+

Similarly increasing the size of the cation from N a to l-ethyl-3methylimidazolium results in a further decrease in the melting temperature of the respective salts (Table 1). The charge distribution within the E M I M cation


416 ( E M I M = l-ethyl,3-methyl-imidazolium), however is not spherical (5). The acidity of the hydrogen bound to the C2 of the imidazolium cation is commonly noted. Though less acidic, a partial positive charge, δ , also resides on the hydrogens bound to C4 and C5, as is common to all aromatic rings. In contrast to the normally observed δ" charge on the face of an aromatic ring, some of the positive charge of the imidazolium cation is also distributed over the face of the ring, consistent with resonance structures that assign a positive formal charge to the nitrogens. (Calculations which describe a negative charge for nitrogen, a result of its greater electronegativity, do not differentiate between polar bonds in the σ- and π-systems. It is the removal of one π-electron that gives the immidizolium ring aromaticity and the positive charge.) Examination of the crystal structure of [EMIM][PF ] (6) finds the close contacts between the anions and cations to be associated with the two nitrogens and the three aromatic hydrogens of the imidazolium ring (see below). Thus from a simple consideration, the positive charge of the imidazolium cation can be considered to be distributed over a cylinder defined by the imidizolium core. The alkyl groups bound to the ring increase some cation/anion distances, but these do not significantly participate in charge derealization. However, in addition to the large size the asymmetrical shape of the E M I M cation disfavors crystallization such that salts with melting points well below room temperature are frequently observed (Table 1). Interestingly, the reported melting point for the [EMIM][PF ] salt is higher than might be predicted from a simple trend based on anion and cation sizes. This may in part be due to a r+/r_ radius ratio that favors crystal formation, and hence crystals suitable for a structure determination were obtained.


+

6

6

The composition of the ionic liquid also has a major impact on the observed melting temperature. In a binary mixture, A + B , a maximum in the liquidus is observed at the compositions of specific compounds; for example compound A B as described in the qualitative binary phase diagram in Figure 1.

L

2> 2 ω A +L CL

\ / ™ \ / +L

Ε ι2

AB\ / +L γ / B+ L

A +AB A

AB + B AB Composition

Β

Figure 1: Schematic of a binary phase diagram with a single compound


417 Deviation from the A B composition results in a lowering of the melting temperature until the minimum observed at the eutectic composition. Such compositional variation is anticipated for most metal-halides that are observed to form low temperature ionic liquids. For example, in the cMoroaluminate system [AICI4]", [AI2CI7]", [AI3CI10]' [AI4CI13]' have been described, although only the 50% composition results in the formation of a compound [EMIMJfAlCU] according to the phase diagram (7). In this [EMIM][C1]/A1C1 binary system eutectics at approximately 38% and 66% result in liquids with melting points (or glass transitions) below -70 °C. A more complex phase diagram has been reported in the [HPy]/ZnCl system with compounds reported at 20, 33, 50 and 66 mole percent (8). We have also demonstrated this compositional variation in the new class of metallotropic liquid crystals of Z n C l with cationic surfactants, which exhibit at least two distinct crystalline phases and a variety of liquid crystalline phases over the composition range of 0 to 85 mole % Z n C l (P). Similar eutectic behavior can be expected for all salts [ M X ] ~ anions (e.g. [F"] [PF ]-, [PF ]-, [PF ] or [CI]", [CuCU] ", [ C u C l f ... [ C u C l ] - , CuCl ). Furthermore, this eutectic behavior accounts for the fact that small deviations in preparations of ionic liquids can result in liquids with significantly different melting temperatures.


3

2

2

2

x

n

m

2

6

5

3

2

2

6

m

2m+2

2

Polarity of Salts The coulombic interactions, described in the previous section, are responsible for the attractive forces that hold anions and cations into a crystalline lattice. Unlike covalent bonds however, these ionic interactions are not directional. Rather in the salt, anions and cations are arranged in space so as to cancel the build-up of charge throughout the bulk material. For example in the well known NaCl structure, Figure 2, no N a C l molecules exist, rather each sodium cation is surrounded by six nearest neighbor chloride anions, and each

Figure 2. Ion packing in the structure of NaCl


418 chloride anion is surrounded by six sodium cations. Numerous investigators have probed the structure of liquid NaCl, and it is generally concluded that a similar structural organization exists in the liquid state (2). Thus, while an ionic liquid is a polarizable medium it is not anticipated to be a polar liquid unless either the anion or cation itself is polar. O f course a strong dipole in either the anion or cation is likely to increase the melting temperature of a salt. The crystal structures of [EMIM][PF ] and [C MIM][PF ] ( C M I M = l-dodecyl-3-methylimidazolium) have recently been reported (6,10), and they demonstrate that the organization in these complex salts is analogous to that observed in NaCl. The non-standard unit cell of the [EMIM][PF ] salt, shown in Figure 3 viewed perpendicular to the (1 0 -2) plane, demonstrates the NaCl-type packing arrangement that is observed in this salt. Five reasonably short contacts are observed between the imidazolium cation and neighboring P F anions (C-P or N - P contacts of less than 4.5Â) with a sixth contact (of about 6Â) that has been lengthened by the ethyl group bound to the imidazole ring. Similarly six imidazolium cations surround each P F anion in a distorted octahedral fashion. The close points of contact between the imidazolium cation and the P F anion are the aromatic protons and the nitrogen atoms consistent with the charge distribution described in the previous section (although the alkyl C H protons are also in close proximity to the anions). The methyl and ethyl groups are oriented in alternating directions so as to give the most efficient packing while beginning to separate the charged and neutral portions of the imidazolium cation. When the neutral portion of the imidazolium cation is increased to the size of a C H ( C H ) n chain more dramatic phase separation between the neutral and charged portions of the lattice is observed resulting in a lamellar bi-layer type


6

12

6

1 2

6

6

6

6

3

2

Figure 3: Crystal Structure of [EMIM] [PF ] illustrating the NaCl-Type packing 6


419 organization as seen in Figure 4. Nevertheless, examination of the interaction between the cationic heads of the [ d M I M ] and the [PF ]" anions finds an analogous sodium chloride-type organization. Here each [PF ]" anion is surrounded by five imidazolium cations and each imidazolium cation is surrounded with five [PF ]" anions (anions (C-P or N-P contacts of less than 4.5Â), consistent with a two layer slice of the NaCl-type lattice. Thus, the bulk structural organization consists of two alternating non-polar regions, the salt-like packing of anions and cationic heads, and the hydrocarbon-like regions of the alkyl tails. +

2

6

6


6

Figure 4. Structure of [C MIM]PF emphasizing the NaCl-type packing within the cationic head/anion region and the phase separation of the ionic and hydrocarbon regions in the bulk crystal (The alkyl chains are truncated in 4a.) 12

6

Although the electrostatic interaction of the cations and anions require a non-polar structural organization, polar characteristics of an ionic liquid can be realized for materials with polar cations or anions. For example, the observed polarity of several [EMIM][X] ionic liquids, which have been compared with C H C N or low alcohols (77), may originate from the asymmetric charge distribution in the imidazolium cation in which the C2 hydrogen is significantly more acidic than the C4 and C5 protons (5). The polarity of this cation is somewhat unusual in that the distribution of positive charge creates the dipole as 3


420 opposed to a dipole resulting from an increased electron density on an electronegative atom. This contrasting origin of the dipole makes [ E M I M ] a polar, but generally non-coordinating species. From a similar consideration of the geometry of the anion, it would be expected that nitrite salts, [N0 ]", will form more polar solvents than nitrate, [N0 ]~, salts. However, the solubility characteristics of a polar ionic liquid and a polar molecular solvent are expected to be notably different because of the energetic cost of charge separation associated with solvation. In an ionic liquid, ions must be separated from the network of charge compensating counter ions to form a solvation shell around the solute. The electrostatic competition for a polar ion to interact with its counter ion or a solute will tend to reduce solubility in ionic liquids as compared to neutral polar solvents in which the respective dipole moments of the polar ions and molecules are of a similar magnitude. +

2


3

Crystalline vs. L i q u i d Structures What then are the relationships between the crystalline structures described above and their structures in the liquid phase? Understanding the structure of liquids at an atomic or molecular level of organization is quite challenging and has been the subject of considerable investigation and controversy; with most significant early advances coincident with the development of X-ray and Neutron scattering techniques (12). Watching children "swim" through a pool of plastic spheres in a playground "ball-pen" leads one to believe that this may be a quite reasonable macroscopic model of the structural organization in liquids. Similar plastic spheres when ordered into a periodic lattice provide a macroscopic model of many crystalline structures as well. The difference between the organization of balls in the playground pool or in a model of a crystalline lattice of NaCl is minimal. Both exhibit an approximately close-packed arrangement of spheres. These structural observations led us to think about the structure of liquids from a starting point of a crystalline model, albeit with the break down of long range order. The neutron scattering pattern for liquid N a C l at 820 °C is shown in Figure 5 (2b). Such measurements have been made and analyzed by a number of authors, and the general conclusion is that in liquid the Na-Cl distance is about 2.78Â and the C l - C l distance is about 3.91Â. These values are quite similar to those of crystalline NaCl at high temperature just prior to melting (2). It has further been noted that the strong scattering features in the S (the charge density structure factor, which describes the charge ordering of the anions and cations) (Figure 5a) occur at values of Q (the reciprocal space wave vector, Q = (4π sin θ)/λ)) which reasonably correspond to the position of the first Bragg reflections of the crystalline material (Figure 5d) (2b). When we ignore the Q Q



421 periodic lattice conditions for crystalline diffraction and calculate the expected scattering for an NaCl-type structure in which the scattering originates from atom atom contacts within a 5Â radius, the pattern shown in Figure 5c is obtained (13) which shows definite similarity to the measured SNN (the number density structure factor) of Figure 5b. These data suggest that the charge ordering may have a longer coherence length than the atom ordering. A n d while long range order is lost on going from the crystal to the liquid, similarities in structure remain as a consequence of the coulombic forces between anions and cations.

J

1 1

i

c

d 0

5

7

6

3|

/

;

2

4

1

ill

1 Ii., 1î

6

1

k(A- )

Figure 5. (a, b) Bhatia-Thornton structure factors S and S for molten NaCl (reproduced with permission from refrence 2b, © 1986, Institute of Physics Publishing), (c) Simulated neutron scattering for liquid NaCl using a 5Â coherence length, (d) Calculated neutron diffraction for crystalline NaCl. QQ

m

Liquid crystals represent the class of liquids for which the most dramatic structural organization is observed in the liquid phase. When the coherence length of the molecular organization is greater than the wavelengths of


422 visible light, birefringence patterns are observed. Control over the direction of structural organization is the basis for the widely used liquid crystal display technology. We and others have noted that liquid crystalline ionic liquids can frequently be formed by attaching an alkyl tail with a chain length greater than 10 carbons to either the anion or cation (9,10,14). The variable temperature synchrotron X-ray diffraction of Ci TA-ZnCl-33 ([cetyltrimethylammonium]2 [ZnCl ]), shown in Figure 6, clearly demonstrates the similarity between the crystalline and liquid crystalline structures (9). The crystalline structure consists of an A X salt-like organization of cationic trimethylammonium head groups and [ZnCl ] " anions, which are separated by interdigitated surfactant tails resulting in.the overall lamellar structure. The relative charge density of the anion and trimethyammonium head require that the surfactant tails be canted with respect to the salt-type layer in the crystalline phase in order to maintain van der Waals contacts between the surfactant tails. This results in lamellar d-spacing significantly less than the length of the surfactant. (The structure of C T A ZnCl-33 is similar to the structure of [C MIM][PF ] shown in Figure 4.) Upon melting to the first lamellar liquid crystalline phase at about 70 °C, an increase in the lamellar spacing of about 1Â is observed, seen by the shift to lower angle diffraction, as the van der Waals interactions between hydrocarbon chains are relaxed upon chain 'melting.' Nevertheless, the diffraction indicates that 6

4


2

2

4

1 6

12

6

Figure 6. Synchrotron XRD of C TA-ZnCl-33. (a) Crystalline material at room temperature, (b) Lj liquid crystal at 90 °C. (c) L liquid crystal at 200 °C. ]6

2


423 significant lateral organization remains in addition to the lamellar organization in this Sc-type liquid crystal. A liquid-liquid phase transition occurs upon further heating to about 160 °C at which point the long-range lateral organization in the liquid is lost, but sharp diffraction resulting from the lamellar ordering, Q ~ 0 . 2 Â , 0.4Â" , and 0.6Â, remains. Nevertheless, while the sharp diffraction originating from long-range lateral order in the crystal is lost, it is replaced by broad features in the scattering pattern in this S -type liquid crystal. These broad features are indicative of the retention of intermediate range order, which had its origins from the long-range order of the crystalline organization. 1

1

Metallotropism and the Structure of L i q u i d s Finally in our consideration of structure in ionic liquids it is important to understand how composition and cation charge density impact the observed structure of the anions; a concept we describe as metallotropism. The relationship between composition and anion structure to properties such as the melting point was noted above in the complex phase diagrams. For example, metal halide materials can exhibit compositions ranging from the isolated anion [X] " to the neutral composition of the M X parent. The structural preferences of the metal, M , (i.e. oxidation state, coordination geometry and etc.) provide the ground rules for anion oligomerization. But the nature of the oligomers formed reflect a balance of the charge density constraints of the inorganic anion and the templating cation, as represented in Scheme I. Without a readily avalible means to determine the structures of the liquid phase anions, we turn to a consideration of crystalline structures to understand these charge density influences on anion structure, which, as discussed above, provide a reasonable starting model for understanding the structure of liquids. n

n

Inorganic Charge Density Hieh

Hig

'harg e Dens


A

u

Structure-Property Relationships in Ionic Liquids - American Chemical

Recommend Documents