Thermotropic Liquid Crystal Phases

A thermotropic compound is a liquid that has some form of anisotropy in its liquid

state over some temperature range; that is, it forms a liquid crystal. This leads to many properties, such as refractive index, electrical permitivity and some mechan-

ical properties, being dependent on the direction in which they are measured. All liquid crystals have an upper temperature limit, beyond which structuring is lost

and the liquid becomes isotropic; this effect is entropically driven. In lyotropic sys- tems (that is, multi-component systems) the ordered component is typically referred

to as an amphiphile; in thermotropic systems (that is, single component systems) it is called a mesogen. While many molecules that are amphiphiles will also be

mesogens, the most common moieties of each class of compound are different. Ly- otropic amphiphiles most commonly consist of a charged head group with an alkyl

tail. Mesogens share the alkyl tail, although the head group is typically a rigid

∗_{In this paper the exact nature of the aggregate was not confirmed; only DLS measurements of}

aggregate sizes and distributions were collected; hence the use of critical aggregate concentration rather than a more specific term, like CMC

substituted biaryl structure, for example N-(4-methoxybenzylidene)-4-butylaniline, shown in Fig. 4.1.

Figure 4.1: N-(4-methoxybenzylidene)-4-butylaniline

Liquid crystals are generally conceptualised as pipes or as being pill shaped, however they can also be disks and there are a few interesting examples of banana shaped

molecules exhibiting liquid crystallinity.114–117

Thermotropic liquid crystals exist in one of a number of phases which have varying degrees of local and long range order. Since most ILs are rod-like, discussion here

will be limited to the liquid crystal phases of rod-like molecules. There are two broad categories of liquid crystalline phases for rod-like molecules, these are the nematic

phase and the smectic phase, sketches of which are shown in Figs. 4.2 & 4.3.118

Figure 4.2: Sketch of the nematic phase118

The nematic phase is the least ordered of the liquid crystal phases. In the nematic

phase the mesogens are aligned along the long axis of the molecule, which is called the director. The nematic phase is fluid in all directions, however the rates of

diffusion parallel and perpendicular to the director are not necessarily equal. In an isotropic liquid diffusion is equal in any direction.

The smectic phases have an additional degree of ordering. The simplest of which

Figure 4.3: Sketch of the smectic A and smectic C phases118

are organised into layers in the plane perpendicular to the director, as shown in

Fig. 4.3. Smectic phases are completely fluid in the plane, however the exchange of mesogens between layers is slow (although not zero). There are a wide variety of

smectic phases, and both nematic and smectic phases can exist in chiral forms.

The use of ionic components in liquid crystals is not unusual; their use is particularly relevant in areas where anisotropic electrical properties are required, for example

liquid crystalline displays.

The use of ionic compounds as liquid crystals has been reviewed comprehensively up until 2005 by Binnemans.119 _{What follows is an overview of the literature in which}

ILs or similar compounds are the subject; while there are fascinating examples of biphenyl substituted imidazolium compounds showing thermotropic behaviour,120

they do not inform our discussion of the structure in RTILs.

4.2.2.2 Thermotropic Behaviour of long chain Ammonium compounds

Alkylammonium chlorides have been investigated with alkyl chains from hexyl to

octadecyl121 and upon heating all undergo a plastic transition (where the alkyl chains melt but the head group remains fixed), then transition into the liquid crys-

talline smectic A phase, followed by a ‘clearing’ transition when the liquid becomes isotropic.

Primary amines have also been combined with various acids,122,123 while mesophases were seen in decylammonium phenylsulfonate, the mesophases were sta-

bilised by using pyridine-3-sulfonate and various napthalene sulfonates, instead of the simple phenylsulfonate. In this context stabilised means the temperature range

over which the liquid crystal phase is observed is increased.

There are also some examples of alkylammonium salts with more complex anions

that exhibit mesomorphic behaviour, including carboxylic acid derivatives of guada- nine and cytosine,124 _{and tetrachlorometalates}125–129 _{which exhibit some unusual}

anion structures.

Tetraalkylammonium compounds are more widely studied than N-alkylammonium compounds, however alkyltrimethylammonium halides are thermally unstable, which

limits their thermal range and longevity. Alkyldimethylpropylammonium halides have improved thermal stability (the addition of a single propyl group hinders the

approach of a nucleophile to σ* of the alkyl group, which prevents dealkylation).

Mesophases have been observed for dodecyldimethylpropylammonium bromide and various similar compounds substituted at the 3-propyl position.130–133 _{The ther-}

motropic behaviour of alkyldimethylpropylammonium alkane- and benzene-sulfonates have been investigated and all samples exhibited liquid crystalline behaviour.122,134

A new type of smectic phase, the smectic T phase, was discovered following inves-

tigation into dialkyldimethylammonium bromides,135 where the T phase has Bragg peaks at integer ratios (in reciprocal space) in the small angle X-ray diffraction

(SAXS) region, which indicate a lamellar phase, and peaks in the wide angle X- ray diffraction (WAXS) region with relative spacings of √2:√4:√5, which indicate a tetragonal lattice. A smectic T phase was also found in dialkyldi-(2-hydroxy)- ethylammonium bromide. Some more complex systems incorporating diazobicy-

clooctane also exhibit a smectic T phase.136

Similar to the alkylammonium tetrachlorometallates, tetraalkylammonium tetra- chlorometallates show thermotropic behaviour;137 _{the addition of the metal centre}

in these examples induces mesomorphic behaviour in tetraalkylammonium halides that otherwise don’t exhibit liquid crystallinity. As both alkylammonium halides

and tetraalkylammonium halides can have mesomorphism induced by the addition of metal halides, it is clear that the formation of hydrogen bonds between the cations

an anions is not the driving force for the mesophase formation. The ability of the anions to form extended stable structures with themselves is what induces the me-

somorphism.

A vast range of variations of tetraalkylammonium compounds have been synthe-

sised and categorised, such as using cholestanyl groups (very similar to cholesterol) on the ammonium centre.138 _{There are also examples of bis-(2-hydroxyethyl)-ω-}

(biphenyl)alkylammonium chlorides,139_{α−ω−diquaternary ammonium salts}140_and

complexes of benzenehexacarbolxylic acid and (6 equivalents of) didodecyldimethy-

lammonium bromide, all of which have exhibited thermotropic behaviour, although

the last example surprisingly existed in a lamellar phase rather than a discotic phase.141

Phosphonium based mesogens have been examined in detail elsewhere119_{and gener-}

ally have a wider mesophase range and higher clearing point than the corresponding

ammonium salts. They also tend to spontaneously form a non-centrosymmetric bi- layer system from which arises spontaneous polarisation caused by displacement of

the smectic layers.142,143 _{This is different to ammonium based systems, which may}

not exhibit spontaneous birefringence under a polarising microscope because they

form extended homeotropic domains; they often only exhibit birefringence under mechanical strain.