This work reports thermodynamic and electrostatic parameters for fused silica/water interfaces containing cm2-sized graphene ranging from a single layer of pristine graphene to defected graphene. Second harmonic generation (SHG) measurements carried out at pH 7 indicate that the surface charge density of the fused silica/water interface containing the defected graphene (-0.009(3) to -0.010(3) C/m2) is between that of defect-free single layer graphene (-0.0049(8) C/m2) and bare fused silica (-0.013(6) C/m2). The interfacial free energy of the fused silica/water interface calculated from the Lippmann equation is reduced by a factor of 7 in the presence of single-layer pristine graphene, while defected graphene reduces it only by a factor of at most 2. Subsequent SHG adsorption isotherm studies probing the Mg2+ adsorption at the fused silica/water interface result in fully reversible metal ion interactions and observed binding constants, Kads, of 4(1) � 5(1) � 103 M�1 for pristine graphene and 3(1) � 4(1) � 103 M�1 for defected graphene, corresponding to adsorption free energies, ?Gads, referenced to the 55.5 molarity of water, of -30(1) to -31.1(7) kJ/mol for both interfaces, comparable to Mg2+ adsorption at the bare fused silica/water interface. Maximum Mg2+ ion densities are obtained from Gouy�Chapman model fits to the Langmuir adsorption isotherms and found to range from 1.1(5) � 1.5(4) � 1012 ions adsorbed per cm2 for pristine graphene and 2(1) � 3.1(5) � 1012 ions adsorbed per cm2 for defected graphene, slightly smaller than those of for Mg2+ adsorption at the bare fused silica/water interface ((2�4) � 1012 ions adsorbed per cm2), assuming the magnesium ions are bound as divalent species. We conclude that the presence of defects in the graphene sheet, which we estimate here to be around 1.3 � 1011 cm2, imparts only subtle changes in the thermodynamic and electrostatic parameters quantified here. Results for binary mixtures of hexanol and cyclohexane are shown as well and discussed in the context of a general view emerging for these interfaces in which single layer graphene is merely a spectator as opposed to an active participant in interfacial processes at fluid/solid interfaces.

Molecular and ion adsorption to single layer graphene sheets studied by vibrational sum frequency and second harmonic generation

248th National Meeting (2014)

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