Nanometer-Scale Organization of Ethylene Oxide Surfactants on Graphite, Hydrophilic Silica, and Hydrophobic Silica

Abstract

To better understand the role of the interactions between surfactant, solvent, and a solid substrate on surface aggregation, we have studied the adsorption of a series of alkylpoly(ethylene oxide) C n E m surfactants on three different substrates: graphite, hydrophilic silica, and hydrophobic silica. Using atomic force microscopy (AFM), we find that adsorption to hydrophilic silica, with two exceptions, results in the formation of globular structures that are similar to bulk micelles. On silica that has been made hydrophobic by reaction with organosilane, adsorption results in a surface layer that is laterally homogeneous and is probably a monolayer with ethylene oxide groups in contact with the solution. A number of surfactants with ionic and zwitterionic headgroups were also observed to form monolayers on hydrophobic silica. This large perturbation from the solution-aggregate structure on the hydrophobic surface is driven by a minimization of the area of contact between water and the hydrophobic silica substrate. On graphite, the surface layer is either long, thin aggregates (consistent with a hemicylindrical structure) or a laterally homogeneous layer (consistent with a monolayer with the headgroups facing the solution). The nonionic C 12 and C 14 surfactants form hemicylinders, and the C 10 surfactants with the same headgroups form a laterally homogeneous layer. This suggests that above a critical alkyl chain length the interaction between the graphite and the surfactant tail is sufficient to orient a layer of alkyl tail groups parallel to the graphite surface, which then templates further adsorption. Below the critical alkyl length, the arrangement on graphite is similar to that on hydrophobic silica and is probably driven by a minimization of the water-graphite interfacial area. The critical alkyl length for (zwitterionic) sulfobeteine surfactants is not the same as for the nonionic poly(ethylene oxide) surfactants. This shows that the headgroup also plays an important role in determining the adsorbed structure. All measurements were performed in equilibrium with bulk micelles at or above the critical micelle concentration and at approximately 25 °C.

Graphene-philic surfactants for nanocomposites in latex technology

“…First, is the affinity of the alkyl tails to the surface which is driven by the minimisation of graphene/graphite-water interfacial energy as a result of hydrophobic interactions. The second is a preferred orientation of the alkyl chains with the surface lattice because the carbon position in the alkyl chains is closely matched with the graphene surfaces [105,108,116,117,138,139]. The π-π interactions infer face-to-face stacking (see Fig.5A) of the surfactant containing aromatic ring(s) and the graphene basal planes involved in the non-covalent interactions [151], although the nature of this remains controversial [152,153].…”

Modeling self-assembly of surfactants at solid–liquid interfaces. II. hydrophilic surfaces

“…Considerably fewer experiments have been performed to explore the aggregation behavior of nonionic surfactants on hydrophilic surfaces. Nonionic ethylene oxide surfactants have been observed to form either globular aggregates or continuous bilayer structures on hydrophilic silica [7].…”

Supramolecular Structure of Surfactants Confined to Interfaces

“…54 A strictly nonionic system that has been widely studied in the past is the adsorption of polyethyleneoxide (PEO)-based surfactants on silica. A recent AFM study 55 has shown that many of these surfactants make globular (roughly spherical) aggregates on the silica surface, with the spheres randomly arranged on the silica surface at nearestneighbor distances somewhat greater than twice the surfactant length. As with ionic surfactants on silica, these interfacial aggregates reflect bulk micelle morphology.…”

“…56 PEO-based surfactants with relatively longer tailgroups or shorter headgroups, which have a tendency towards bilayer structures in solution, showed similar tendencies on silica. 55 …”

“…This central role of the lattice has since been confirmed for a variety of surfactants with wide differences in headgroup chemistry and molecular geometry. 30,31,45,47,53,55,57,58 Unlike hydrophilic surfaces, graphite interacts primarily with surfactant tailgroups via hydrophobic and van der Waals interactions. It has long been known that the graphite surface adsorbs and orients alkyl chains along its three equivalent symmetry axes, with the alkyl chain following the zigzag line connecting nearest-neighbor carbon atoms on graphite.…”