Interaction of Magnesium Ions with Pristine Single-Layer and Defected Graphene/Water Interfaces Studied by Second Harmonic Generation

Abstract

This work reports thermodynamic and electrostatic parameters for fused silica/water interfaces containing cm(2)-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/m(2)) is between that of defect-free single layer graphene (-0.0049(8) C/m(2)) and bare fused silica (-0.013(6) C/m(2)). 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 Mg(2+) adsorption at the fused silica/water interface result in fully reversible metal ion interactions and observed binding constants, Kads, of 4(1) - 5(1) × 10(3) M(-1) for pristine graphene and 3(1) - 4(1) × 10(3) M(-1) for defected graphene, corresponding to adsorption free energies, ΔGads, referenced to the 55.5 molarity of water, of -30(1) to -31.1(7) kJ/mol for both interfaces, comparable to Mg(2+) adsorption at the bare fused silica/water interface. Maximum Mg(2+) 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) × 10(12) ions adsorbed per cm(2) for pristine graphene and 2(1) - 3.1(5) × 10(12) ions adsorbed per cm(2) for defected graphene, slightly smaller than those of for Mg(2+) adsorption at the bare fused silica/water interface ((2-4) × 10(12) ions adsorbed per cm(2)), 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 × 10(11) cm(2), imparts only subtle changes in the thermodynamic and electrostatic parameters quantified here.