pH dependent isotropic to nematic phase transitions in graphene oxide dispersions reveal droplet liquid crystalline phases

Abstract

a Size fractionation, amplified by the surface charge density of graphene oxide (GO) sheets, broadens the pH dependent isotropic (I) to nematic (N) phase transition in aqueous dispersions of graphene oxide (GO). In this biphasic region, a highly organized droplet nematic phase of uniform size (20 ± 2.8 μm diameter) with an isotropic interior is observed.Suspensions of 2D sheet-like materials exhibit transition from a disordered isotropic (I) phase to ordered nematic (N) phase. As noted by Onsager, 1 despite their loss of orientational entropy, the N phase, is stabilized by a gain in excluded volume (or configurational) entropy. Graphene -a single atom thick layer of carbon atoms is receiving immense attention owing to the combination of properties such as large intrinsic mobility of electrons, massive surface area, immense mechanical strength and large thermal conductivity. Although significant progress has been made in the solid-state synthesis of graphene, liquid-phase graphene possess opportunities for large-scale synthesis and novel mesophases that may lead to ordered macroscopic structures such as thin films and fibers with enhanced electrical, mechanical and optical properties.2, 3 Graphene solvated in chlorosulfonic acid 2 and stabilized by surfactant 4 exhibit liquid crystalline behavior, as well as their composites with discotic molecules.5 Chemically oxidized graphene sheets or graphene-oxide (GO) suspensions in water, the most accessible precursor to graphene-based materials, not surprisingly demonstrate I-N phase transitions, too.6-10 These studies have shown that the nematic phases of graphene exhibit 'brush-like' texture in massively organized continuous domains that may reach length-scales of millimeters. These length-scales are relevant to achieving macroscopcially ordered materials and fluid-phase alignment. However, graphene sheets are flexible and although this flexibility is likely to yield isolated droplet liquid crystalline phases, yet such confined phases have not been reported so far. The ability to form highly ordered N phases from GO arises because GO can be stabilized in aqueous solvent by electrostatic repulsive forces between the particles as demonstrated by large (-20 to -40 mV) zeta potential measurements.11-13 The charged functional groups are primarily carboxylic groups on the edges and hydroxyls on the basal planes, which can be protonated/deprotonated by change in pH, thus surface charging of these particles depend on the pH. pKa of GO is ~ 4 for the carboxylic groups and ~9 for the phenolic groups, 14 however GO suspensions can be stable at lower pH through hydrogen bonding between the oxidized groups and water molecules. 15 In addition, the basal plane contains hydrophobic domains that make GO sheets amphiphilic, and the ionisable groups render this amphiphilicity a dependence on pH and size of the GO sheets with smaller GO sheets being more hydrophilic than larger GO sheets. 13,[16][17][18] It can thus be expected that the I-N phase transitions are dependent upon factors suc...

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“…This behavior is compensated by an increase in the translation entropy, whereby a colloidal phase transition occurs from the isotropic to the nematic liquid crystalline phase. 82 In the liquid crystalline state, the GO sheets become anisotropic. However, they can still flow and react to shear stresses.…”

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“…24. Dependence of the mass fraction on the NaCl concentration for the dispersion of graphene oxide: S -is the solid phase, I -is the isotropic phase, I+N -is the two-phase region of the nematic -isotropic, N -is the nematic phase [41] Интересно влияние рН [48]. Были приготовлены дисперсии при разных концентрациях (от 1 до 20 мг/мл) с пятью значениями рН (1, 2, 6, 9 и 14).…”

“…26. Микрофотографии тактоидов окиси графена при вращении поляризатора и анализатора, скрещенных под углом 45º: а -схема, показывающая расположение частиц окиси графена в тактоиде (вверху), b -схема поля директора в тактоиде (внизу) [48] Fig. 26.…”

“…26. Microphotographs of the graphene oxide tactoids during rotation of the polarizer and analyzer crossed at an angle 45º: a -a scheme showing the arrangement of particles of graphene oxide in the tactoid (top), b -the scheme of the director field in the tactoid (bottom) [48] ≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡ Кроме этого, было найдено, что рН влияет на размеры частиц окиси графена. Так, при рН 2 ши-рина частиц в изотропной и нематической фазах была, соответственно, 390 нм ± 59 % и 430 нм ± 71 %, а при рН 6  330 нм ± 51 % и 515 нм ± 76 %.…”

“…25. Dependence of the volume fraction of the nematic phase on the concentration of graphene oxide in the dispersion [48] Рис. 26.…”