Welcome to the Geiger Laboratories

for Nonlinear Optics at Interfaces

at Northwestern University

in Focus 
Interaction of Magnesium Ions with Pristine Single-Layer and Defected Graphene/Water Interfaces Studied by Second Harmonic Generation
Jennifer L. Achtyl, Ivan V. Vlassiouk, Sumedh P. Surwade, Pasquale F. Fulvio, Sheng Dai, and Franz M. Geiger
J. Phys. Chem. B, Articles ASAP (As Soon As Publishable)
Publication Date (Web): February 11, 2014 (Article)
DOI: 10.1021/jp410298e

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, ΔGads, 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.
Towards the identification of molecular constituents associated with the surfaces of isoprene-derived secondary organic aerosol (SOA) particles
C. J. Ebben, B. F. Strick, M. A. Upshur, H. M. Chase, J. L. Achtyl, R. J. Thomson, and F. M. Geiger
Atmos. Chem. Phys., 14, 2303-2314, 2014

Secondary organic aerosol (SOA) particle formation ranks among the least understood chemical processes in the atmosphere, rooted in part in the lack of knowledge about chemical composition and structure at the particle surface, and little availability of reference compounds needed for benchmarking and chemical identification in pure and homogenous form. Here, we synthesize and characterize SOA particle constituents consisting of the isoprene oxidation products α-, δ-, and cis- and trans-β-IEPOX (isoprene epoxide), as well as syn- and anti-2-methyltetraol. Paying particular attention to their phase state (condensed vs. vapor), we carry out a surface-specific and orientationally selective chemical analysis by vibrational sum frequency generation (SFG) spectroscopy of these compounds in contact with a fused silica window. Comparison to the vibrational SFG spectra of synthetic isoprene-derived SOA particle material prepared at the Harvard Environmental Chamber yields a plausible match with trans-β-IEPOX, suggesting it is an abundant species on their surfaces, while the other species studied here, if present, appear to be SFG inactive and thus likely to be localized in a centrosymmetric environment, e.g., the particle bulk. No match is found for authentic SOA particle material collected at the site of the Amazonian Aerosol Characterization Experiment (AMAZE-08) with the surface SFG spectra of the compounds surveyed here, yet we cannot rule out this mismatch being attributable to differences in molecular orientation. The implications of our findings for SOA formation are discussed in the context of condensational particle growth and reactivity.http://pubs.acs.org/doi/pdf/10.1021/ja3120899

Member of the DOE Fluid Interface Reactions, Structures and Transport (FIRST) EFRC at Oak Ridge and the NSF Center for Chemical Innovation (CCI) for Sustainable Nanotechnology (CSN) at the University of Wisconsin, Madison - check out the CCI CNS blog!

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