Colloidal stability of graphene in aqueous medium: a theoretical approach through molecular dynamics

Graphene has been used as reinforcement of polymeric nanocomposites to increase mechanical and electrical properties. Recently, graphene suspensions have been used for the development of nanofluids in automotive applications, where improvements in convection heat transfer coefficients and pressure drops have been reported. However, dispersions of graphene sheets in a polymeric matrix as well as in a solvent medium are difficult to achieve; that is because Van der Waals, [Formula: see text] and Coulombic interactions cause agglomerations. Surface chemical modifications have been considered as viable options to improve the graphene integration. In this work, we studied the colloidal stability of aqueous solutions of graphene sheets functionalized with (i) carboxylic groups, (ii) 3-amino-propyl tri-ethoxy silane (amphiphilic behavior), (iii) graphene oxide, and (iv) pristine graphene. Results show that the lower sedimentation velocity corresponds to the graphene functionalized with carboxylic groups, which presents the higher colloidal stability. However, the amphiphilic group enhances the interaction energy between graphene and the solvent; we believe that there is a threshold percentage of functionalization that improves the colloidal stability of graphene.

Transport properties of graphene solutions were estimated by using Non-Equilibrium Molecular Dynamics simulations to generate Poiseuille flow in an NVT ensemble. Simulations were developed in the LAMMPS code. The COMPASS Force Field was used for the graphene systems and the TIP3P for the water molecules. Bonds and angles of hydrogen atoms were kept rigid by using the shake algorithm. The molecular models were built through MedeA and visualized with the Ovito software.