Self-assembly of amphiphiles was first reported in ionic media in series of papers by Reinsborough and Bloom in the late 1960s. They investigated surfactants in molten
(ethylene glycol) also induces self-assembly of amphiphiles. These two discoveries are believed to be the first reports of self-assembly in non-aqueous media.
The first report of an IL as a medium for self-assembly was ethylammonium nitrate (EAN).98,99 This puts ILs on a short list of solvents that can promote self-assembly.
This list include various glycols, formamides and hydrazine. The inclusion of ILs in this select group of solvents opens up an interesting avenue for exploring the
behaviour of lyotropic systems, because normally only the surfactant can be sys- tematically modified (with the exception of the small range of formamides which
have been systematically studied in some limited sense100). Up until 2005, other
than EAN, the only ILs investigated were [C4C1im][PF6] and [C4C1im]Cl.101 There
are also two short reviews on the use of ILs in the synthesis of inorganic materi-
als, which are of interest because of the way in which amphipilic substances can be used to template nanoparticle growth.102,103This application has driven much of the
recent interest in ILs as self-assembly media.
Ethylammonium nitrate is the most widely studied of all ILs in this area. Thermo-
dynamic measurements showed that the free energy of transfer of a non-polar gas into EAN is similar, but slightly less, than the free energy of transfer of a non-polar
gas into water. The behaviour is dominated by entropic contributions at room tem- perature.104 In contrast to this, the transfer of alcohols into EAN was found to be
dissimilar to the transfer of alcohols into water but similar to transfer of alcohols into polar aprotic solvents such as DMF, DMSO and ethylene glycol.105 These data
suggest that the alkyl chains of the alcohols are more stable in EAN than in water. This means there is less of an entropic force driving self-assembly in EAN systems
than in aqueous systems. This is because self-assembly of the alcohols in EAN re- quires less structure breaking of the EAN solvation sphere around the alcohol alkyl
chains. This may seem counter-intuitive, however alkyl chains in water induce ex- tremely organised structuring in the solvation shell, which must be broken before
self-assembly occurs. That the alkyl chains are more stable in EAN means the sol- vation shell need not be so strictly organised in order to minimise the free energy
of the system. This data has been corroborated by measurements on phosphlipids
aqueous systems.106,107
Another feature of self-assembly in EAN that is different than in similar aqueous
systems is the partial molar volumes of each component in the system. In aqueous systems the partial molar volume of water increases upon micelle formation, whereas
in EAN there is no significant change in its partial molar volume.108 Again this can
be resolved by the need for water to tightly structure around the alkyl chains of the
solute molecule. When the water is tightly bound around the freely solvated alkyl chains of the solute, its partial molar volume decreases. Above the CMC (critical
micelle concentration) there is no water-alkyl chain interaction, so the partial molar volume increases. In EAN there is no significant rearrangement around the freely
solvated alkyl chains, thus there is no increase of partial molar volume above the
CMC.
One of the distinguishing features of self-assembly behaviour is the presence of a
CMC. Below the critical micelle concentration the amphiphiles are almost entirely at the solution interfaces (typically the liquid gas interface, as water interacts favorably
with glass). This effectively minimises the free energy by minimising the number of unfavourable interactions between the water and the solute alkyl chains. However
above a certain concentration of amphiphile the most effective way to minimise the free energy is to form micelles in the solution with the head-groups towards the
solvent and the non-polar chains clustered in the interior. This sudden switch from surface activity to micelle formation is characteristic of self-assembly and when it
occurs sharp deviations in properties such as surface tension and conductivity are observed.
It has been suggested that in polar aprotic solvents micelle formation is via freely
solvated monomers, dimers, trimers and small aggregates combining, which explains the lack of a sharp changes in the properties of these systems,109 in comparison to
the sharp changes in the properties of similar aqueous systems. At present this seems to be a hypothetical explanation based only on the absence of a sharp CMC;
dynamic light scattering (DLS) experiments would provide valuable data on the polydispersity of aggregates in these systems, as micelles in aqueous media are quite
While there is only a limited data set available thus far, the ILs studied all have a sharp CMC. So far there are data on EAN, [C2C1im][NTf2], [C4C1im]Cl and
[C4C1im][PF6] with a wide range of surfactants.42 There is also a more extensive
list of Gordon parameters of ILs. The Gordon parameter (G) is the surface tension
(γ) divided by molar volume (V) to the power of one third, and is shown in Eqn. 4.1. The Gordon parameter is a predictor of a solvent’s ability to induce self-assembly in
amphiphiles, with higher values indicating an greater ability to induce self-assembly in amphiphiles. The Gordon parameters of ILs predict that many of them will be
capable of inducing self-assembly behaviour.
G = γ
V 13
(4.1)
While most studies of lyotropic behaviour focus on relatively dilute solutions, there are a few investigations that discuss the whole concentration range of an am-
phiphile in an IL. Hexadecyltrimethylammonium bromide (CTAB) and 3,7,11,15- tetramethylhexadecane-1,2,3-triol (phytantriol) were investigated as the amphiphiles
in a number of ILs.110,111 Two amphiphiles were needed in order to examine all of the lyotropic phases, because of the effect of the surfactant shape on the liquid
crystal phases which can be formed. This is quantified by the critical packing pa- rameter. The critical packing parameter (CPP) is shown in Eqn. 4.2 is the ratio
of the amphiphile volume (v) and the product of the head group cross section area and the amphiphile length. The CPP is a predictor of the type of lyotropic phase
an amphiphile will form.
CCP = v
a.l (4.2)
CTAB has a low critical packing parameter and tends to form normally curved surfaces (micelles and hexagonal phases). Phytantriol has a higher critical packing
parameter and tends to form negatively curved surfaces (inverse micelles and inverse hexagonal phases). Using these two amphiphiles it was found that all of the liquid
crystalline phases can be formed in ILs.110,111
[C16C1im][BF4] and [C16C1im]Cl in EAN,112 which found that critical aggregate
concentrations∗ were similar to other amphiphiles in EAN and about a factor of 100 greater than the CMC in water.
These data all show that ILs act as highly polar environments, in which aliphatic
molecules are unstable and tend towards segregation. Many ILs have an aliphatic segment that is destabilised and will tend to segregate or separate where possible.
This effect is discussed in the next section, which is divided into ‘long chain’ and ‘short chain’ sections. For the purposes of this thesis ‘long chain’ refers to systems
where the longest alkyl chain (or chains) is dodecyl or longer. This cut-off has been chosen because it is the chain length of the shortest birefringent ILs reported to date
in the literature; [C12C1im]Cl113 and [C12C1im][PF6]53 are both birefringent.