Phase Control of Lyotropic Liquid Crystals Using Amino Acid Ionic Liquids As Sovlents

Liquid crystals have attracted considerable attention as functional soft materials because they form well-ordered nanostructures. They have been applied in a variety of fields. In particular, they are used for the development of nanochannel materials transporting substances, such as ion, electron, and small molecules. Previously, we have developed ion-transporting liquid crystals by using ionic liquid. For example, columnar (Col) liquid-crystalline (LC) phosphonium salts give one-dimensional ion conductive pathways while smectic (Sm) LC imidazolium salts form two-dimensional ion conductive layers. In the course of these studies, we have focused on the use of bicontinuous cubic (Cub_bi) phases having three-dimentionally interconnected nanochannel networks. For the construction of Cub_bi LC assembles, we have used lyotropic LC systems containing ionic liquids (ILs) as solvents.

Previously, we reported that lyotropic LC properties of amphiphiles can be controlled by the selection of anion of ILs. In these systems, induction of Cub_bi phases has been successfully achieved. However, for the construction of LC system forming Cub_bi phases in a wide temperature region, further insight how IL design influence self-organization of amphiphiles is still required.

In the present study, we newly synthesized some ionic liquids. As cations, 1-(2-methoxyethyl)-3-methylimidazolium ([Im]), and cholinium ([Ch]) were used. As anions, we selected amino acids ([AA]). L-alanine ([Ala]), L-phenylalanine ([Phe]), and 3-(2-naphthyl)-L-alanine ([Nap]) were employed. We mixed these ILs with amphiphiles1 in various ratios and examined the LC properties of the obtained mixtures (1/IL) by varying temperature and component ratio and then constructed lyotropic LC phase diagrams. Comparing these phase diagrams, we examined the effect of physicochemical properties of ILs on the lyotropic LC behavior of the amphiphile.

1/ILs mixtures exhibit lyotropic LC behavior. Comparing the lyotropic LC phase diagrams, it was found that the LC region in the phase diagrams consists of Col, Cub_bi, Sm, and so on. Induction of Cub_bi phases having 3D ionic nanochannels is observed for the phase diagrams of 1/[Im][AA] and 1/[Ch][AA] while there are differences in their appearance. When examining the effect of anion species, it was found that the use of [Ala] is more effective for the expansion of Cub_bi LC area than [Phe]. When examining the effect of cation species, 1/[Ch][AA] keeps LC phases in a wider temperature range than 1/[Im][AA].

For understanding factors of ILs that govern lyotropic LC behavior, we examined physicochemical properties of ILs. The thermal properties of the ILs were examined by differential scanning calorimetry measurements. All ILs showed glass transition temperature (T_g). T_g increased as the increase of bulkiness of the side chain group in the anion; -73 °C ([Im][Ala]), -50 °C ([Im][Phe]), and -32°C ([Im][Nap]). It is probably attributed to the increase of the van der Waals force and some other interactions between the component ions. To examine the hydrophilicity of ILs, contact angle measurements were performed. We used two substrates, one has a surface bearing hydroxyl groups and the other has a surface bearing alkyl chain groups. On the hydrophilic substrate, the contact angle of [Im][Ala] is smaller than that of [Im][Phe]. In contrast, contact angle of [Im][Ala] is larger than that of [Im][Phe] on the hydrophobic substrate. It is well known that contact angle becomes smaller as the increase of affinity between liquids and substrates. It was strongly suggested that the hydrophilicity of [Im][Ala] is higher than that of [Im][Phe]. Considering these physicochemical properties of ILs, we found that bulkiness of anions and hydrophilicity of ILs are significant factors that govern the self-organization behavior of amphiphiles.

One of interesting applications of these LC phases is for ion conductive pathways. For the ion conduction measurements for LC thin film samples, we employed two types of cells; one equips comb-shaped gold electrodes and the other equips indium tin oxide electrodes. The former cell provides ionic conductivity parallel to the substrate (σ_//). The latter one provides ionic conductivity perpendicular to the substrate (σ_⊥). By comparingσ_// and σ_⊥, we examined the relationships between the macroscopic continuity of ionic nanochannels and dimensional-order of LC phases. The ionic conductivity of the present materials would also be discussed in the presentation.