Study of the Gemini Surfactants’ Self-Assembly on Graphene Nanosheets: Insights from Molecular Dynamic Simulation

Abstract

Understanding the mechanism of adsorption and self-assembly of surfactants on graphene is highly important to perform better optimization of the graphene dispersion process. Because of Gemini surfactants' special structure, they have a high charge capacity, high hydrophobicity, and unique self-assembly properties compared to single-chain surfactants. Therefore, Gemini surfactants with their small concentrations are expected to disperse and stabilize graphene nanosheets in aqueous solutions more effectively. We conducted molecular dynamics simulations to study adsorption and self-assembly of single-chain cationic surfactant dodecyltrimethylammonium bromide (CTAB) and its same family Gemini surfactant dimethylene-α,β-bis(dodecyldimethylammonium bromide) ([12-2-12]Br) on graphene nanosheets. The results showed that assemblies morphology formed on graphene is affected by surfactant structure. We observed that increasing surface coverage, especially for [12-2-12]Br, leads to a transmission in adsorption mechanism and most [12-2-12]Br head groups tend toward the aqueous phase and prevent water molecules from accessing graphene surface. It can be concluded from morphological assessments that [12-2-12]Br is more effective than CTAB in stabilizing graphene aqueous suspensions. Moreover, we investigated the effect of graphene sheet size and Gemini surfactant spacer length on the structure of surfactant assemblies on graphene. The present study results can expand our comprehension of dispersion mechanism of graphene nanosheets by Gemini surfactants.

Effect of Surfactant Choice and Concentration on the Dimensions and Yield of Liquid-Phase-Exfoliated Nanosheets

“…Authors have noted that there is an increase in the hydrophobic interactions between surfaces using SDC for graphene dispersions leading to a higher overall nanosheet yield. 21,26 On the contrary, we did not see an appreciable difference in concentration or nanosheet dimension when comparing WS2 dispersions stabilised with sodium cholate and sodium deoxycholate (SI.9 for detailed comparison).…”

“…Although binding of particular surfactants such as sodium dodecyl sulfate to graphite has been examined in detail, 18 studies of surfactant-stabilized nanosheets are limited and have been performed mainly for graphene. [19][20][21][22] Ionic surfactants are the most commonly studied with sodium cholate (SC), sodium dodecyl sulfate (SDS) and other long alkyl chain surfactants at the forefront. 17,[22][23][24][25] In broad terms, stabilization of nanosheets (and colloids in general) using ionic surfactants is relatively simple, though the surfactant conformation on the nanosheet surface in solution requires further exploration, with various binding orientations and mechanisms suggested.…”

“…Although binding of particular surfactants such as sodium dodecyl sulfate to graphite has been examined in detail, studies of surfactant-stabilized nanosheets are limited and have been performed mainly for graphene. − Ionic surfactants are the most commonly studied with sodium cholate (SC), sodium dodecyl sulfate (SDS), and other long alkyl chain surfactants at the forefront. ,− In broad terms, stabilization of nanosheets (and colloids in general) using ionic surfactants is relatively simple, though the surfactant conformation on the nanosheet surface in solution requires further exploration, with various binding orientations and mechanisms suggested. ,− The surfactant tail groups (or nonpolar part of the molecule in general) tend to adsorb on the nanosheet surface yielding a layer of bound charge which is compensated by a diffuse, semimobile cloud of counterions associated with the head groups. The resultant double layer acts as an electrical multipole and, from a distance, appears to carry a net charge.…”

Molecular Dynamics Simulations and Density Functional Theory on Unraveling Photoresponsive Behavior of Wormlike Micelles Constructed by 12-2-12·2Br– and trans-ortho-Methoxy Cinnamate

“…However, a great challenge about explaining the different properties between trans / cis -OMCA and microscopic mechanisms of micelle transformation after UV irradiation still exists. Atomistic molecular dynamics (MD) simulations and quantitative calculations provide us with a new solution to obtain specific self-assembly process of micelles, the details of packing patterns of certain molecule on the micelle surface, and energy changes on atom and molecular scales, respectively. , In the past few years, the self-assembly process of the surfactant has been well investigated using the simulation method. − For example, Heerdt et al employed time-dependent DFT calculations and atomistic MD simulation to study photoisomerization of trans / cis -OMCA isomers and the self-assembly of CTAB micelles with trans / cis -OMCA in water solution . In order to save simulation time, preassembled rodlike micelles in water solution were chosen as the initial configuration in their simulation.…”

“…Atomistic molecular dynamics (MD) simulations and quantitative calculations provide us with a new solution to obtain specific self-assembly process of micelles, the details of packing patterns of certain molecule on the micelle surface, and energy changes on atom and molecular scales, respectively. 33,34 In the past few years, the self-assembly process of the surfactant has been well investigated using the simulation method. 35−38 For example, Heerdt et al employed time-dependent DFT calculations and atomistic MD simulation to study photoisomerization of trans/cis-OMCA isomers and the self-assembly of CTAB micelles with trans/cis-OMCA in water solution.…”