Colloidal Stability of Graphene Oxide: Aggregation in Two Dimensions

Abstract

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.

New insights into the solubility of graphene oxide in water and alcohols

“…Recently, Gudarzi discussed the stability of GO solutions with respect to the DLVO theory, 8 and found a good correlation between experimental data and theory. However, in our opinion, the application of this theory to truly two-dimensional GO is not straightforward due to the well-known non-additivity principle for nanomaterials.…”

Aggregation and sedimentation of shattered graphene oxide nanoparticles in dynamic environments: a solid-body rotational approach

“…1 Among various nanomaterials, the use of graphene nanomaterials is rapidly growing due to their broad constructional, industrial, environmental, and medical applications and arising from their use with other nanomaterials as nanohybrids. [2][3][4][5][6][7] Graphene nanosheets may end up in the environment in various forms of particulate matter such as crumpled graphene, [8][9][10] multilayer graphene, 4,5,11 shattered nanosheets, 12 and fractal aggregates/hetero-aggregates. 13,14 Once they have entered aquatic environments, aggregation of such particles can significantly affect their functionality and transport behaviour, particularly in aqueous and porous media.…”

Self-powered therapeutic release from conducting polymer/graphene oxide films on magnesium

“…25,29 It was found that salt Dex induced aggregation of the GO sheets due to shrinking of the electric double layer the keeps the GO sheets suspended. 42 To prevent aggregation, polymerization of the PEDOT/GO/Dex films was accomplished using a non-salt form of Dex in ethanol based PEDOT/GO solution.…”