Critical Role of the Spacer Length of Gemini Surfactants on the Formation of Ionic Liquid Crystals and Thermotropic Behavior

Abstract

Numerous reports have shown that the self-assembling properties of 12-s-12 bis(quaternary ammonium) gemini surfactants in aqueous solution are significantly influenced by s, the number of methylene groups in the covalent spacer. However, the role played by s on the phase behavior of the single compounds has not been investigated in a similarly systematic way. Here, we report on the thermotropic phase behavior of the anhydrous compounds with s = 2-6, 8, 10, and 12, resorting to differential scanning calorimetry (DSC), polarized light microscopy (PLM), and X-ray diffraction (XRD). All of the compounds show a stepwise melting behavior, decomposing at 200 °C. As the spacer length increases, nonmonotonic trends are observed for the thermodynamic parameters of the thermotropic phase transitions, mesophase formation, and solid-state d spacings. In particular, the number and type of mesophases (ordered smectic phases and/or fluid smectic liquid crystals) depend critically on s. Further, upon heating molecules with s < 8 decompose before the liquid phase, while those with long spacers, s = 8-12, reach the isotropization (clearing) temperature, hence forming both ionic liquid crystals and ionic liquid phases. We demonstrate that the melting behavior and type of ionic mesophases formed by gemini molecules can be usefully manipulated by a simple structural parameter like the length of the covalent linker.

Ionic liquid crystals based on amino acids and gemini surfactants: tunable phase structure, circularly polarized luminescence and emission color

“…Actually, the mesogenic property of gemini surfactants is largely dependent on the structural degree of freedom originating from the flexibility and length of their spacer group. 50,51 A longer alkyl spacer may boost the flexibility of the complex, thus generating a disturbance in well-packed ILC nanostructures.…”

“…48–50 Their double chains provide sufficient molecular flexibility to suppress crystallization, and their spacer group as a unique component segment affects the phase structure and transition temperature of ILC. 51 Consequently, gemini surfactants are deemed to be favorable candidates to make up the flexible parts in CPL-active ILCs.…”

Bifunctional Double-Salt Ionic Liquids Containing both 4-Chloro-2-Methylphenoxyacetate and l-Tryptophanate Anions with Herbicidal and Antimicrobial Activity

“…Most likely, the most ordered system occurs in the same cation with two bromide anions, which was previously described in the literature. 45 For example, in the case of bis( N -dodecyl- N , N -dimethylammonium) dibromides described in the literature with spacers ranging from 2 to 10 carbon atoms, thermal phase transitions, such crystallization and melting points, were observed in all cases. These dibromides also showed correlations between the length of the spacer and the number and type of mesophases (ordered smectic phases and/or sematic liquid crystals).…”

“… 44 − 46 Described systems containing two identical cations were mainly influenced by the incorporation of two different cation cores into one compound, which resulted in the formation of liquid crystals. 45 − 47 Most bis-cationic compounds described in the literature include a single popular type of anion, e.g., major halides such as bromide, chloride, or NTf 2 and BF 4 in the case of ILs. On the other hand, the effect of the incorporation of two different anions and one cation was described in a monocationic system containing fractional anions at a molar ratio of glyphosate/MCPA equal to 8.48:0.152, and the thermal behavior of the product underwent only slight changes.…”

Effects of Molecular Structure and Temperature on Micellization of Cationic Ammonium Gemini Surfactants in Aqueous Solution

“…Spacer groups can boost hydrophobic interaction between two hydrophobic tails and diminish the electrostatic repulsion between two charged headgroups, therefore, the aggregation behavior of gemini surfactants can be efficiently adjusted by regulating the spacer groups. Extensive studies have revealed that the nature of spacer including chemical composition, flexibility, hydrophobicity, and chain length, and so on, strongly influences the surface activity and aggregation behavior of gemini surfactants (Alami et al, 1993;Bai et al, 2001a;Chauhan et al, 2015;Danino et al, 1995;Fernandes et al, 2017;Frindi et al, 1994;Grosmaire et al, 2002;Hirata et al, 1995;Li et al, 2017;Lin et al, 2013;Pisár cik et al, 2016;Sun et al, 2018;Wang et al, 2004;Xiang et al, 2017;Zana et al, 1991;Zhang et al, 2013;Zhu et al, 2012).…”