Phase Transition and Liquid Crystalline Organization of Colloidal Graphene Oxide as a Function of pH

Abstract

(1 of 11) 1600391 This paper discusses the influence of pH on colloidal graphene oxide (GO) liquid crystals. The results indicate that there is a crossing point where suspensions at pH > pK a , the GO sheets have different behavior than suspensions at pH < pK a . This behavior is evidenced by broadening of the biphasic region in the isotropic to nematic phase transition, by observing different shape and internal configurations of the nematic droplets, and in the magnitude of fractionation between large and small sheets in the nematic and isotropic phases. The behavior of GO liquid crystal droplets under the influence of a static magnetic field is studied, and evidence is found in the magnetically induced distortions for the support of our interpretation of the internal configurations of the nematic droplets. This study also demonstrates that magnetic distortions are reversible on a time scale of minutes to hours. This research is based on the LC-PolScope, a polarized light system, which is found to be a powerful tool for characterization of GO liquid crystals due to its fast and easy image acquisition and analysis.Lyotropic liquid crystals are systems that contain at least two components: mesogens and a continuous medium. Mesogens are usually large molecules or colloids and the continuous medium acts as a solvent to the mesogens. Liquid crystallinity is obtained at a certain range of concentration, depending on the size and shape of the mesogens, and the nature of the interaction between the components. There are three main types of lyotropic liquid crystals: nematic and smectic for rod-like (or calamitic) systems, and nematic and columnar for plate-like (or discotic) systems. In a nematic liquid crystal, the mesogens have a long-range orientation order, where their normal vectors are pointing along the same direction called the director, but have no translational order. Smectic liquid crystals have a laminated structure in which the mesogens have translational order in one dimension in addition to orientational order.A convenient way to identify the liquid crystalline phase is by polarized light microscopy, where the sample is located between two cross polarizers. The mesophase is birefringent due to the anisotropic arrangement of its building blocks that produce an orientated and long-range director field. To distinguish between a nematic and columnar phase, X-ray measurements can be used. [3] Liquid Crystals of Plate-Like MaterialsPlate-like liquid crystals of bentonite clay suspensions were first observed by Langmuir in 1938, although these liquid crystals were not stable. [4] He suggested that the phase separation arises from osmotic pressure considerations instead of energy considerations. Plate-like colloidal phase transition has not been studied as much as rod-like colloids; however, in the last three decades there has been major progress in this field. Lekkerkerker and co-workers pioneered research on plate-like liquid crystals. [5][6][7][8] They investigated mineral platelets, mainly gibbsite but also...

Graphene Oxide Liquid Crystal Domains: Quantification and Role in Tailoring Viscoelastic Behavior

“…Onsager’s theories and subsequent improvements by others for plate-like systems provide a reasonable qualitative prediction of LC phase formation, but the experimental nature of these phase transitions are affected by factors contributing to electrostatic interactions such as pH, surface charge density, and ionic strength as well as the inherent polydispersity of the particle dimensions. , The latter has been shown to critically influence the phase transition in various 2D colloidal systems. − It is quite clear that the phase transitions in GO systems are rather complex because the associated surface-charged groups are prominent, polydispersity is usually large, and the morphology of the platelet may change over time …”

Graphene Oxide Liquid Crystal Membranes in Protic Ionic Liquid for Nanofiltration

“…48 Polarized optical microscopy is a useful characterization technique to detect anisotropy, which is indicated by birefringence. 32 Schlieren brushlike birefringence patterns have been previously seen as an indication of nematic liquid crystal phase formation in GO aqueous systems. 14,30 A Schlieren texture was observed with various disclinations, which is a reflection of GO platelets, orienting in a preferential direction.…”

Segmentation, retardation and mass approximation of birefringent particles on a standard light microscope

“…Since its first introduction this so‐called LC‐PolScope system has been widely used (e.g. Schindelman et al ., 2001; Aikawa et al ., 2006; Tkacz et al ., 2017) and it can be considered a standard method of measuring full‐field retardation within a range of 0–273nm. A recent study also used liquid crystal phase retarders to measure the retardation of a silk fibre with values exceeding 1000nm (Honda et al ., 2018).…”