Lamellar Tetragonal Symmetry of Amphiphilic Thermotropic Ionic Liquid Crystals in the Framework of Other Closely Related Highly Ordered Structures

Michael Arkas,Theodora Katsika,Marilina Douloudi,Michail Vardavoulias

doi:10.3390/sym14020394

Michael Arkas, Theodora Katsika + Show 2 more

Open Access

https://doi.org/10.3390/sym14020394

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Abstract

An overview of the chemical compounds forming the rare smectic T phases is presented with references to the historical context. Thermodynamics (transition temperatures, enthalpies) along with the factors (stereochemical constraints, electrostatic interactions, aliphatic chain stacking, intermolecular forces) contributing to the adoption of tetragonal scaffolds are also discussed. Characteristic optical microscopy textures and X-ray diffraction patterns are presented. In parallel, a comparison of the geometrical parameters such as distances between atoms, molecular areas, volumes, and lattice parameters with the closest two-dimensional and three-dimensional organizations, is performed.

Highlights

Thermotropic liquid crystals [1] are an anisotropic liquid state of matter [2,3]
The molecules of a solid retain their orientation through the increase in temperature, to meet an average orientation, the director ñ, whereas their position according to a three-dimensional lattice is compromised [4,5,6]
Since molecular areas are the same, assuming the 2D tetragonal symmetry is similar to that observed for the smectic T phase of the N, alkyl N, alkyl N, N, dimethyl quaternary ammonium bromides we conclude to an almost cubic three-dimensional sub-cell (a = b = 6.2 Å c = 6.1 Å)

Summary

Introduction

Thermotropic liquid crystals [1] are an anisotropic liquid state of matter [2,3]. The molecules of a solid retain their orientation through the increase in temperature, to meet an average orientation, the director ñ, (orientational order) whereas their position according to a three-dimensional lattice (positional order) is compromised [4,5,6]. Symmetric parts of similar size or moieties that cover similar spaces due to their thermal movement preferentially form hexagonal phases (Smectic B [25], Smectic F [26], Smectic I [27], columnar hexagonal [28]) in proximity with six first neighboring unities. This behavior complies with identical principles prevailing in the formation of solid organic crystals [29]. Distortion of the hexagonal symmetry produces rectangular two-dimensional lattices (Smectic E [30], columnar rectangular [31]) and respective 3D orthogonal (smectic K [32]). AAppaarrttfrfroommththeesseeaabboovvee-d-deessccrribibeeddccaateteggoorrieiessooffrirgigidid““mmooleleccuulalarr””mmeessooggeennss, ,ioionnicic lilqiquuididccrryysstatalsls; ;mmoosstltylyaammpphhipiphhiliilcicflfelexxibibleleeenntittiiteiessrreessuultlitninggfrfroommththeeinintrtroodduucctitoionnooffioionnicic ppaarrtststotohhyyddroropphhoobbicicmmooieietiteiess[4[455––4488],],aalslsoofoformrmtetetrtaraggoonnaal lpphhaasseessccoommmmoonnlylyteterrmmeeddaass ssmmeecctticicTT. .TThheeoobbjejeccttooffththeeppreresesennttwwoorkrkisisththeecocommppaararatitviveeppreresseenntatatitoionnooffththeesseelalatteterr lalammeellalarroorrggaannizizaattioionnssththaattaarreeaassseemmbbleleddfrforommaammpphhipiphhiliilcicmmooleleccuulelessbbeeaarrininggccaatitoionnicic hheeaaddggrroouuppssccoonntataininininggnnitirtrooggeenn(q(quuaateterrnnaarryyaammmmoonniuiumm,,ppipipeerraazzininiiuumm,,ppyyrrrroolliiddiinniiuumm,, ppipipeerrididininiuiumm,,iimmiiddaazzoolium, pyriiddiinniiuumm))..TTwwoo-d-dimimenensisoionnalaol ro3rD3Dionioicniscqusqaurearpelapnleasnaerse afroermfoermd bedy tbhyetphoelaprosluarblsauybelrasywerhsewrehtehreepthoesitpivoseiltyivcehlayrgchedarigoends tiognestthoegrewthiethr twhietihr cthoeuinrctoeurinotnesriaornespalraecepdlapceerdpepnedrpiceunldarictuoltahretosmtheectsicmlaeycteircsl.aIynetrhse. sIenhtyhdersoephhyodbriocpphaortbsi,cflpeaxribtsl,e aliphatic chains have a characteristic positional short-range order, orientational longflexible aliphatic chains have a characteristic positional short-range order, orientational long-range order along the average direction of the long molecular axes, the director, and a 1D-translational long-range order along with the normal layer [49]

Quaternary Ammonium Salts

Aromatic Polar Heads

Conclusions