A novel process is developed to synthesize graphene oxide sheets with an ultralarge size based on a solution-phase method involving pre-exfoliation of graphite fl akes. Spontaneous formation of lyotropic nematic liquid crystals is identifi ed upon the addition of the ultralarge graphene oxide sheets in water above a critical concentration of about 0.1 wt%. It is the lowest fi ller content ever reported for the formation of liquid crystals from any colloid, arising mainly from the ultrahigh aspect ratio of the graphene oxide sheets of over 30 000. It is proposed that the self-assembled brick-like graphene oxide nanostructure can be applied in many areas, such as energy-storage devices and nanocomposites with a high degree of orientation.

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Colloidal stability of graphene oxide (GO) is studied in aqueous and organic media accompanied by an improved aggregation model based on Derjaguin-Landau-Verwey- Overbeek (DLVO) theory for ultrathin colloidal flakes. It is found that both magnitude and scaling laws for the van der Waals forces are affected significantly by the two-dimensional (2D) nature of GO. Experimental critical coagulation concentrations (CCC) of GO in monovalent salt solutions concur with DLVO theory prediction. The surface charge density of GO is largely affected by pH. However, theoretical calculations and experimental observations show that the colloidal stability of the 2D colloids is less sensitive to the changes in the surface charge density compared to the classical picture of 3D colloids. The DLVO theory also quantitatively predicts the colloidal stability of reduced GO (rGO). The origin of lower stability of rGO compared to GO is rooted in the higher van der Waals forces among rGO sheets, and particularly, in the removal of negatively charged groups, and possibly formation of some cationic groups during reduction. GO also exfoliates in the polar organic solvents and results in stable dispersions. However, addition of nonpolar solvents perturbs the colloidal stability at a critical volume fraction. Analyzing the aggregation of GO in mixtures of different nonpolar solvents and N-methyl-2-pyrrolidone proposed that the solvents with dielectric constants of less than 24 are not able to host stable colloids of GO. However, dispersions of GO in very polar solvents shows unexpected stability at high concentration (>1 M) of salts and acids. The origin of this stability is most probably solvation forces. A crucial parameter affecting the ability of polar solvents to impart high stability to GO is their molecular size: the bigger they are, the higher the chance for stabilization.

Dispersion of graphene in a polymer matrix as mono layers is an important step towards fabricating high performance polymer-graphene nanocomposites. In this paper, a novel method based on Pickering emulsion polymerization has been introduced that assures fine dispersion and enhances loading. The major idea is to use a high affinity of graphene oxide (GO) for assembly at the liquidliquid interface for Pickering emulsion polymerization. A guideline for ensuring stable hybrid colloids of polymer-graphene oxide with an appropriate polymer particle size has been introduced. Then a system of poly (methyl methacrylate) (PMMA)-GO has been selected and the nanocomposites have been made by Pickering emulsion polymerization to examine the theory. TEM studies of the products show various interesting arrangements of PMMA and GO for a different size ratio of nanolayers to polymer particles. The new method paves the way for an environmentally benign process for the production of high quality polymer graphene nanocomposites as it is water-based (no organic solvent is employed) and soap free. Furthermore, resulting hybrid particles were melt mixed with PMMA as a master batch. The resulting nanocomposites with 0.3 wt% graphene showed improved thermal stability and stiffness.

The colloidal probe technique was used to accurately measure forces between water–solid interfaces of negatively charged latex particles in aqueous solutions of linear, cationic oligoamines of different valence up to roughly +4. These measurements were realized between pairs of particles with the atomic force microscope. Monovalent and divalent amines behave as simple electrolytes, and the forces are dominated by double layer repulsion at low concentrations and van der Waals attraction at high concentrations, as suggested by the classical theory by Derjaguin, Landau, Verwey, and Overbeek (DLVO). Additional attractive non-DLVO force of a short range can be evidenced, and its origin is attributed to hydrophobic interaction between the surfaces. Trivalent and tetravalent oligoamines induce a charge reversal and equally an additional attractive non-DLVO force. The charge reversal originates from the adsorption of these oligoamines to the particle surface. The additional non-DLVO force is more long-ranged than the ones observed in the presence of amines of low valence. This additional attraction is probably related to ion–ion correlations, existing surface heterogeneities, and the chainlike nature of the amines investigated

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