Negatively charged graphene layers from a graphite intercalation compound spontaneously dissolve in N-methylpyrrolidone, without the need for any sonication, yielding stable, air-sensitive, solutions of laterally extended atom-thick graphene sheets and ribbons with dimensions over tens of micrometers. These can be deposited on a variety of substrates. Height measurements showing single-atom thickness were performed by STM, AFM, multiple beam interferometry, and optical imaging on Sarfus wafers, demonstrating deposits of graphene flakes and ribbons. AFM height measurements on mica give the actual height of graphene (ca. 0.4 nm).
In degassed water graphene re-aggregation is drastically slowed down due to the small intergraphene attractive dispersive forces (a consequence of graphene two-dimensional character) and the stabilizing electrostatic repulsion. As has been reported before for many hydrophobic objects, (i.e. hydrocarbon droplets 11 , 12 or air bubbles 13 ) graphene becomes electrically charged in water as a consequence of the spontaneous adsorption on its surface of OH -ions coming from graphenide oxidation and water dissociation. As two graphene flakes come together, they experience a repulsive force due to the overlap of their associated counterion clouds.Accordingly, graphene can be efficiently dispersed in water at a concentration of 0.16 g/L with a shelf life of a few months.The pH values after graphene transfer to water is very revealing. While the system resulting from the mixture with non-degassed water (left vial of Fig. 1b) has a pH close to 11, stable graphene suspensions have a pH close to neutrality (pH between 7 and 8; right vial of Fig. 1b). As the same amount of OH -is produced in both cases after graphenide oxidation, the remarkable difference in pH is attributed to the adsorption of OH -on the suspended graphene flakes. This hypothesis is supported by the electrophoretic mobility and zeta potential ζ of the graphene flakes. Negative ζ values (ζ = -45 ± 5) were observed at neutral pH conditions; on the contrary, charge reversal was observed in acidic pH environment (ζ = +4 ± 2 at pH 4). It could be argued that this ζ variation is due to the reduction of pH below the pK a of functional groups dissociated at basic pH. To discard this hypothesis, we measured ζ of water-dispersed graphene in presence of tetraphenylarsonium chloride, Ph 4 AsCl which contains a hydrophobic cation known to readily 3 adsorbs on hydrophobic surfaces 14 . As reported in Table 1, we observed a progressive increase in ζ with increasing concentration of the hydrophobic cation, with charge reversal at sufficiently large cation concentrations. Stability of SLG iw is determined by the interaction between the individual graphene plates. In regular laboratory conditions, gases dissolved in water (about 1 mM) adsorb on the graphene surface, inducing long-range attractive interaction between the dispersed objects and promoting aggregation (a, bottom left, gas bubbles and ions are not at scale). On the contrary, if water is degassed (removing dissolved gases) water-ions readily adsorb on the graphene surface, conferring a certain charge to the dispersed objects. The repulsive electrostatic interaction favors the stability of the dispersed material (b) Left vial: mixture of graphene in THF after addition to water which was not degassed. The aqueous dispersion is not stable and black aggregates visible to the eye begin to form a few minutes after mixing. Right vial: stable dispersion of graphene in degassed water after THF evaporation. No evidence of aggregation is observed after several months of storage at room temperature (c) UV-visible absorption...
The sliding of adhesive surfactant-bearing surfaces was studied with a surface forces apparatus nanotribometer. When the surfaces are fully immersed in an aqueous solution, the dynamic behavior is drastically different and more varied than under dry conditions. In solution, the shear stress exhibits at least five different velocity regimes. In particular, the sliding may proceed by an "inverted" stick-slip over a large range of driving velocities, this regime being bounded by smooth (kinetic) sliding at both lower and higher driving velocities. The general behavior of the system was studied in detail, i.e., over a large range of experimental conditions, and theoretically accounted for in terms of a general model based on the kinetics of formation and rupture of adhesive links (bonds) between the two shearing surfaces with an additional viscous term.
We have measured the friction forces between molecularly smooth mica surfaces confining
thin films of different branched hydrocarbons, using the surface forces apparatus (SFA). The evolution
of the systems to steady-state sliding from rest or after a change in sliding velocity was thoroughly studied,
and the presence of different length and time scales was observed. Using a new “extended bimorph slider”
which allows continuous shearing for distances well beyond the contact diameter, we show that the
evolution to steady-state sliding in these films is governed by the distance the surfaces are sheared rather
than the time. From these results it is clear that both time and distance of sliding have to be considered
in order to fully describe the dynamic response of lubricants and complex fluids under shear.
A surface force apparatus was used to measure the transient and steady-state friction forces between molecularly smooth mica surfaces confining thin films of squalane, C30H62, a saturated, branched hydrocarbon liquid. The dynamic friction "phase diagram" was determined under different shearing conditions, especially the transitions between stick-slip and smooth sliding "states" that exhibited a chaotic stick-slip regime. The apparently very different friction traces exhibited by simple spherical, linear, and branched hydrocarbon films under shear are shown to be due to the much longer relaxation times and characteristic length scales associated with transitions from rest to steady-state sliding, and vice versa, in the case of branched liquids. The physical reasons and tribological implications for the different types of transitions observed with spherical, linear, and branched fluids are discussed.
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