Cholesteric colloidal liquid crystals from phytosterol rod-like particles

Abstract

We report the first observation of chiral colloidal liquid crystals of rod-like particles from a low molecular weight organic compoundphytosterols. Based on the particles shape and crystal structure, we attribute this phenomenon to chiral distribution of surface charge on the surface of neighbouring rods.Shape anisotropic (disc-like or rod-like) colloidal particles have been drawing the attention of scientists for many centuries, starting with the use of natural clay particles.1 The particular interest in their phase behaviour started already with the work of Onsager who theoretically predicted the occurrence of entropically driven phase transition in suspensions of disc-like particles that leads to the formation of liquid crystal (LC) phases.2 Nowadays, available synthetic methods readily provide a vast amount of (organic and inorganic) shape anisotropic colloidal particles, 3,4 which indeed in many cases give rise to the formation of various LC phases.5-7 Interestingly, shape anisotropic colloidal particles can also form a cholesteric (or chiral nematic) LC phase,8-11 initially thought to be an exclusive property of molecular LC phases which are typically composed of molecules containing a chiral centre. The latter produces intermolecular forces that favour alignment between molecules at a slight angle to one another in different layers resulting in a long-range chiral order.12 Surprisingly, chiral colloidal LC phases have been observed only in the case of shape-anisotropic particles from self-assemblies of large biopolymers (e.g. cellulose, 13 chitin 14) and macromolecules (e.g. viruses 15) and not in low molecular weight organic compounds. Although a lot of research has been done in relation to cholesteric colloidal LCs, the connection between molecular and macroscopic chirality is still not understood. 8-11,16Here we report the first observation of a cholesteric LC phase in a colloidal suspension of rod-like particles from phytosterols (PSs): a low molecular weight organic molecule. To our knowledge, this is the first observation of cholesteric LC from colloidal particles made of chiral low molecular weight organic molecules. Phytosterols are a group of plant-derived sterols that have structures similar to that of cholesterol (Fig. 1).17 Due to their different biochemistry, phytosterols are used as an efficient cholesterol lowering agent in various food products. 17,18The procedure developed for the synthesis of phytosterol colloidal particles is based on anti-solvent precipitation.19 Transmission electron microscopy (TEM) pictures (Fig. 2) show that the colloidal particles have a rod-like shape with aspect ratios L/D varying between 3 and 20, with D the particle width and length L typically in between 2 mm and 3 mm.20 Phytosterols are known to form rod-and plate-like crystals of hemi-and monohydrates when crystallized from oil solutions 21 or organic solvents in the presence of water.22 Recent studies have shown that cholesterol, which also forms hemi-and monohydrates, forms plate-like particles.23 Our rod...

Density functional theory for chiral nematic liquid crystals

“…The wavelength associated to a full rotation of the nematic director around the chiral directorχ is known as the cholesteric pitch P . Cholesteric phases are commonly found in both thermotropic molecular compounds (e.g., derivatives of cholesterol [2][3][4][5]) and in lyotropic colloidal suspensions of, e.g., DNA [6,7] and filamentous viruses [8][9][10][11][12]. Their widespread occurrence explains why cholesterics were the first liquid-crystal phase experimentally observed [2].…”

Phase behaviour of polarizable colloidal hard rods in an external electric field: A simulation study

“…1,2 Additionally, a body-centered orthogonal crystal phase is observed for intermediate field strength and density in the case of soft repulsive spheres. [1][2][3] The phase behaviour of anisotropic colloidal particles, such as rods, [4][5][6] dumbbells, 7-9 snowman particles, 10,11 and bowls, 12,13 has received significantly less attention, even though great progress has been made in the synthesis of such particles in recent times (see, e.g., Ref. 14).…”

Twisting with a twist: supramolecular helix fluctuations in chiral nematics

“…A rough estimate forε c can be produced by assuming helical rods with some chiral charge pattern [10,58] with an effective total charge Q eff residing on the particle sur-face, so that chiral forces are mediated via some screened Coulomb potential Q 2 eff λ B exp(−κr)/r with λ B the Bjerrum length and κ the Debye screening constant related chiefly to the ionic strength of the solvent. Using this in Eq.…”

“…Examples of chiral biopolymers capable of forming (chiral) liquid crystals include DNA [4], chitin [5], collagen [6,7], cellulose [8,9], phytosterol [10], and filamentous fd virus particles [11,12]. Some of these systems currently witness an active field of experimental research in which the role of biomolecular chirality on the mesoscopic material properties in relation to possible applications as functional materials is being extensively explored [13][14][15][16][17].…”