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The adsorption of 1-hexanol from cyclohexane-d12 at single-layer graphene/α-Al2O3 interfaces was probed at mole percent values as low as 0.05 in the C–H stretching region using vibrational sum frequency generation (SFG). The SFG spectra are indiscernible from those obtained in the absence of graphene, and from those obtained in the presence of graphene oxide films prepared via oxygen plasma treatment of pristine single-layer graphene. A Langmuir adsorption model yields observed free adsorption energies of −19.9(5) to −20.9(3) kJ/mol for the three interfaces. The results indicate that the molecular structure of the hexanol alkyl chain is subject to the same orientation distribution when graphene, oxidized or not, is present or absent at the α-Al2O3/cyclohexane-d12 interface. Moreover, it appears that the adsorption of 1-hexanol in this binary mixture is driven by hexanol interactions with the underlying oxide support, and that a single layer of graphene does not influence the extent of this interaction, ev...

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.

This Article reports measurements of the intra- and intermolecular ordering of tight-binding octylphosphonate ligands on the surface of colloidal CdSe quantum dots (QDs) within solid state films, and the dependence of this order on the size of the QDs. The order of the organic ligands, as probed by vibrational sum frequency generation (SFG) spectroscopy, decreases as the radius of the QDs decreases; this decrease is correlated with a decrease in the order of underlying Cd(2+), as detected by X-ray photoelectron spectroscopy (XPS) line width measurements, for radii of the QDs, R2.4 nm, and is independent of the disorder of the Cd(2+) for R2.4 nm. We believe that, for R2.4, the decreasing order of the ligands with decreasing size is due to an increase in the curvature of the QD surfaces. Disorder in the Cd(2+) results from the presence of a shell of Cd(2+)-surfactant complexes that form during synthesis, so this work demonstrates the possibility for chemical control over molecular order within films of colloidal QDs by changing the surfactant mixture.

Proton transfer across single layer graphene is associated with large computed energy barriers and is therefore thought to be unfavorable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here, we subject single layer graphene supported on fused silica to cycles of high and low pH and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid-base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.68 to 0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavorable energy barriers to helium and hydrogen transfer indicate the transfer process is selective for aqueous protons., Comment: 80 pages, including Supporting Information, 3 Figures, 1 Table, final pre-edited version

The ability to study the interactions of hydrocarbons on carbon surfaces is an integral step toward gaining a molecular level understanding of the chemical reactions and physical properties occurring on them. Here, we apply vibrational sum frequency generation (SFG) to determine the tilt angle of toluene, a common organic solvent, on millimeter-thick highly oriented pyrolytic graphite (HOPG). The combination of a time-delay technique, which results in the successful suppression of the nonresonant SFG response, and a null angle method is shown to overcome the "strong optical absorber" problem posed by macroscopically thick carbon samples and yields a molecular tilt angle of toluene in the range of 37° to 42° from the surface normal. The implications of this approach for determining the orientation of organic species adsorbed on carbon interfaces, which are important for energy-relevant processes, are discussed.

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.

SOA particle formation ranks among the least understood processes in the atmosphere, rooted in part in (a) the limited knowledge about SOA chemical composition; (b) the availability of only little concrete evidence for chemical structures; and (c) little availability of reference compounds needed for benchmarking and chemical identification in pure and homogenous form. Here, we address these challenges by synthesizing and subjecting to physical and chemical analysis putative isoprene-derived SOA particle constituents. Our surface-selective spectroscopic analysis of these compounds is followed by comparison to synthetic SOA particles prepared at the Harvard Environmental Chamber (HEC) and to authentic SOA particles collected in a tropical forest environment, namely the Amazon Basin, where isoprene oxidation by OH radicals has been reported to dominate SOA particle formation (Martin et al., 2010b; Sun et al., 2003; Hudson et al., 2008; Yasmeen et al., 2010). We focus on the epoxides and tetraols that have been proposed to be present in the SOA particles. We characterize the compounds prepared here by a variety of physical measurements and polarization-resolved vibrational sum frequency generation (SFG), paying particular attention to the phase state (condensed vs. vapor) of four epoxides and two tetraols in contact with a fused silica window. We compare the spectral responses from the tetraol and epoxide model compounds with those obtained from the natural and synthetic SOA particle samples that were collected on filter substrates and pressed against a fused silica window and discuss a possible match for the SFG response of one of the epoxides with that of the synthetic SOA particle material. We conclude our work by discussing how the approach described here will allow for the study of the SOA particle formation pathways from first- and second-generation oxidation products by effectively "fast-forwarding" through the initial reaction steps of particle nucleation via a chemically resolved approach aimed at testing the underlying chemical mechanisms of SOA particle formation.

Electrostatics and counterions play important roles in many supramolecular processes, including surfactant adsorption and aggregation at interfaces. Here, we assess their influence on how the common surfactant cetyltrimethylammonium (CTA) interacts with fused silica/aqueous interfaces by determining thermodynamic, kinetic, and electrostatic parameters for CTA adsorption across a range of NaCl concentrations (10-500 mM NaCl) using second harmonic generation (SHG). Using vibrational sum frequency generation (SFG), we demonstrate that vibrationally resonant contributions and nonresonant background contributions to the SFG signal intensity that depend on the interfacial potential can be quantified simultaneously during the adsorption process, which provides insight into the nonequilibrium dynamics of CTA adsorption. By analyzing the adsorption free energies as a function of interfacial potential at these four salt concentrations, the charge density per adsorbate is determined, indicating that CTA coadsorbs with counterions at a ratio of approximately 4 to 3 (i.e., 4 CTA(+) ions for every 3 Cl(-) ion). The chemical (i.e., non-Coulombic) portion of the free energy is found to dominate the overall free energy of adsorption, indicating that CTA adsorption at these ionic strengths is primarily driven by the favorable hydrophobic interactions between interdigitated surfactant hydrocarbon chains in the adsorbed aggregate. By applying Gouy-Chapman-Stern theory to our data, an average charge density of 2.8(3) x 10(13) charges/cm(2), which corresponds to 0.7 to 1.7 molecules/nm(2), was obtained for the four NaCl concentrations.

Nonlinear optical imaging is a powerful method for observing bulk and interfacial phenomena in time and space. Here, we present a step-by-step description of how to carry out second harmonic generation imaging with a kHz amplifier laser system and demonstrate its applicability for SHG microscopy studies of highly size-resolved colloidal CdSe quantum dots having radii of 1-2 nm deposited on glass slides. It is found that not all quantum dots are SHG active, which suggests that environmental effects and particle distributions are important for SHG activity of quantum dots.