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Organothiol monolayers on metal substrates (Au, Ag, Cu) and their use in a wide variety of applications have been extensively studied. Here, the growth of layers of organothiols directly onto muscovite mica is demonstrated using a simple procedure. Atomic force microscopy, surface X‐ray diffraction, and vibrational sum‐frequency generation IR spectroscopy studies revealed that organothiols with various functional endgroups could be self‐assembled into (water) stable and adaptable ultra‐flat organothiol monolayers over homogenous areas as large as 1 cm2. The strength of the mica–organothiol interactions could be tuned by exchanging the potassium surface ions for copper ions. Several of these organothiol monolayers were subsequently used as a template for calcite growth., Organothiols with various functional endgroups could be self‐assembled into (water) stable and adaptable ultra‐flat organothiol monolayers on muscovite mica over homogenous areas as large as 1 cm2. The strength of the mica–organothiol interactions could be tuned by exchanging the potassium surface ions for copper ions.

A combination of surface tension, surface-specific vibrational spectroscopy, and differential scanning calorimetry experiments was performed to examine the ability of lipid films to enrich interfacial organic content by attracting soluble, neutral saccharides from bulk aqueous solution. This "cooperative adsorption" hypothesis has been proposed as a possible source of the high organic fractions found in sea spray aerosols and is believed to be responsible for cryoprotection in some organisms. Experiments described in this work show that the neutral disaccharide trehalose (Tre) is drawn to lipid films composed of dipalmitoylphosphatidylcholine (DPPC), a saturated lipid that is a major component of most eukaryotic cells. The effects of Tre on DPPC monolayer structure and organization were tested with tightly packed monolayers in the two-dimensional solid phase (40 A2/molecule) and more expanded monolayers in the two-dimensional liquid condensed phase (55 A2/molecule). Surface tension data show that DPPC monolayer behavior remains largely unchanged until Tre bulk concentrations are sufficiently high (≥50 mM). In contrast, surface-specific vibrational sum frequency spectra show that when Tre bulk concentrations are ≥10 mM, DPPC monolayers in their liquid condensed state (55 A2/molecule) became more ordered, implying relatively strong noncovalent interactions between the two species. Tre also induces changes in DPPC bilayer behavior as evidenced by a gel-to-liquid crystalline phase transition temperature that increases with increasing Tre concentration. This result suggests that Tre associates with the DPPC headgroups in very specific ways leading to partial dehydration. Together, these results support the cooperative adsorption mechanism under some circumstances, namely, that there is a minimum aqueous phase Tre concentration required to induce observable structural changes in a lipid monolayer and that these effects are most pronounced with DPPC monolayers in their liquid condensed state compared to those of a tightly packed two-dimensional solid.

We examine synthesized standards of structurally related β-caryophyllene ozonolysis products at the air/water and air/ammonium sulfate (aq) interfaces using polarization-resolved standard- and high...

Mineral oxide/water interfaces are important for a wide range of industrial, geochemical, and biological processes. The reactivity of these interfaces is strongly impacted by the presence of ions. ...

Surface tension, surface-specific vibrational spectroscopy and differential scanning calorimetry measurements were all used to test cooperative adsorption of glucuronic acid (GU) to DPPC monolayers adsorbed to the aqueous/vapor interface. Experiments were performed using GU solutions prepared in Millipore water and in carbonate/bicarbonate solutions buffered to a pH of 9.0. The effects of GU on DPPC monolayer structure and organization were carried out with tightly packed monolayers (40 A2/DPPC) and monolayers in their liquid condensed phase (55 A2/molecule). Surface tension data show that GU concentrations of 50 mM lead to expanded DPPC monolayers with diminished surface tensions (or higher surface pressures) at a given DPPC coverage relative to monolayers on pure water. With unbuffered solutions, GU induces significant ordering within liquid condensed monolayers although the effects of GU on tightly packed DPPC monolayers are less pronounced. GU also induces a second, higher melting temperature in DPPC vesicles implying that GU (at sufficiently high concentrations) strengthens lipid-lipid cohesion, possibly by replacing water solvating the DPPC headgroups. Together, these observations all support a cooperative adsorption mechanism. In buffer solutions, the effects of dissolved GU on DPPC structure and organization are muted. Only at sufficiently high GU concentrations (when the solution's buffering capacity has been exceeded) do the data again show evidence of cooperative adsorption. These findings place limits on cooperative adsorption's ability to enrich interfacial organic content in alkaline environmental systems such as oceans.

Muscovite mica (001) is a widely used model surface for controlling molecular assembly and a common substrate for environmental adsorption processes. The mica (001) surface displays near-trigonal symmetry, but many molecular adsorbates-including water-exhibit unequal probabilities of alignment along its three nominally equivalent lattice directions. Buried hydroxyl groups within the muscovite structure are speculated to be responsible, but direct evidence is lacking. Here, we utilize vibrational sum frequency generation spectroscopy (vSFG) to characterize the orientation and hydrogen-bonding environment of near-surface hydroxyls inside mica. Multiple distinct peaks are detected in the O-H stretch region, which we attribute to Si/Al substitution in the SiO4 tetrahedron and K+ ion adsorption above the hydroxyls based on density functional theory simulations. Our findings demonstrate that vSFG can identify the absolute orientation of -OH groups and, hence, the surface termination at a mica surface, providing a means to investigate how -OH groups influence molecular adsorption and better understand mica stacking-sequences and physical behavior.

The synthesis and surface-specific spectroscopic analysis of α-pinene isotopologues and their corresponding secondary organic material is reported., Atmospheric aerosol–cloud interactions remain among the least understood processes within the climate system, leaving large uncertainties in the prediction of future climates. In particular, the nature of the surfaces of aerosol particles formed from biogenic terpenes, such as α-pinene, is poorly understood despite the importance of surface phenomena in their formation, growth, radiative properties, and ultimate fate. Herein we report the coupling of a site-specific deuterium labeling strategy with vibrational sum frequency generation (SFG) spectroscopy to probe the surface C–H oscillators in α-pinene-derived secondary organic aerosol material (SOM) generated in an atmospheric flow tube reactor. Three α-pinene isotopologues with methylene bridge, bridgehead methine, allylic, and vinyl deuteration were synthesized and their vapor phase SFG spectra were compared to that of unlabeled α-pinene. Subsequent analysis of the SFG spectra of their corresponding SOM revealed that deuteration of the bridge methylene C–H oscillators present on the cyclobutane ring in α-pinene leads to a considerable signal intensity decrease (ca. 30–40%), meriting speculation that the cyclobutane moiety remains largely intact within the surface bound species present in the SOM formed upon α-pinene oxidation. These insights provide further clues as to the complexity of aerosol particle surfaces, and establish a framework for future investigations of the heterogeneous interactions between precursor terpenes and particle surfaces that lead to aerosol particle growth under dynamically changing conditions in the atmosphere.

By integrating organic synthesis, secondary organic aerosol synthesis and collection, density functional theory (DFT) calculations, and vibrational sum frequency generation (SFG) spectroscopy, we identify close spectral matches between the surface vibrational spectra of β-caryophyllene-derived secondary organic material (SOM) and those of β-caryophyllene aldehyde and β-caryophyllonic acid at various interfaces. Combined with the record high surface tension depression described previously for these same oxidation products, we discuss possibilities for an intrinsically chemical origin for cloud activation by terpene-derived surfactants. Although the present study does not unequivocally identify the synthesized and analyzed oxidation products on the β-caryophyllene-derived SOM surfaces, these two compounds appear to be the most surface active out of the series and have also been foci of previous β-caryophyllene field and laboratory studies. An orientation analysis by phase-resolved SFG spectroscopy reveals a...

Interfaces between minerals and water play a vital role in many geochemical and industrial processes such as dissolution and growth, sorption of heavy metals and organic contaminants, heterogeneous catalysis, corrosion, and many more. Due in part to its simple geometric structure, MgO (periclase) is a useful model system for understanding how the interaction between specific surface structure and water yield macroscopic observables such as wettability. Here we report the first vibrational sum frequency generation (vSFG) spectroscopy study of MgO (100) and (111) single crystal surfaces, using hydrated conditions both in the ambient atmosphere and in contact with a bulk aqueous fluid. Although most metal oxide surfaces are hydroxylated and strongly interacting with water, for MgO crystal surfaces in air or water vibrational sum frequency generation (vSFG) spectra show the presence of strong non-H-bonded OH stretching vibrations (~ 3700 cm− 1), suggesting an unusual molecularly hydrophobicity.

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.

The structure and ultrafast dynamics of the electric double layer (EDL) are central to chemical reactivity and physical properties at solid/aqueous interfaces. While the Gouy-Chapman-Stern model is widely used to describe EDLs, it is solely based on the macroscopic electrostatic attraction of electrolytes for the charged surfaces. Structure and dynamics in the Stern layer are, however, more complex because of competing effects due to the localized surface charge distribution, surface-solvent-ion correlations, and the interfacial hydrogen bonding environment. Here, we report combined time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD) to get direct access to the molecular-level understanding of how ions change the structure and dynamics of the EDL. We show that innersphere adsorbed ions tune the hydrophobicity of the silica-aqueous interface by shifting the structural makeup in the Stern layer from dominant water-surface interactions to water-water interactions. This drives an initially inhomogeneous interfacial water coordination landscape observed at the neat interface toward a homogeneous, highly interconnected in-plane 2D hydrogen bonding (2D-HB) network at the ionic interface, reminiscent of the canonical, hydrophobic air-water interface. This ion-induced transformation results in a characteristic decrease of the vibrational lifetime (T1) of excited interfacial O-H stretching modes from T1 ∼ 600 fs to T1 ∼ 250 fs. Hence, we propose that the T1 determined by TR-vSFG in combination with DFT-MD simulations can be widely used for a quantitative spectroscopic probe of the ion kosmotropic/chaotropic effect at aqueous interfaces as well as of the ion-induced surface hydrophobicity.

Vibrational sum frequency generation (VSFG) and surface tension experiments were used to examine the effects of aqueous phase soluble saccharides on the structure and organization of insoluble lipid monolayers adsorbed to aqueous-air interfaces. Changes in dipalmitoylphosphocholine (DPPC) chain structure as a function of aqueous phase saccharide concentration and pH are reported. Complementary differential scanning calorimetry (DSC) measurements performed on solutions containing soluble saccharides and DPPC vesicles measured the effects of the saccharides on the lipid membrane phase behavior. Data show that the saccharides glucosamine and glucuronic acid induce a higher degree of organization in compressed DPPC monolayers regardless of the saccharide’s charge.

While ions are known to perturb hydrogen bonding networks in bulk water, our understanding of such effects is less developed for interfaces. Alumina/water interfaces are highly ordered due to strong hydrogen bonding interactions between interfacial water molecules and adjacent aluminol groups. However, how ions alter this interaction is not yet known. Herein, to address the effect of sodium halide salts on the hydrogen bonding environment of interfacial water, we investigated charged alumina (0001) surfaces using steady-state and time-resolved vibrational sum frequency generation (vSFG) spectroscopy. Our results indicate that the effect of halide anions on the attenuation of the vSFG signal next to positively and negatively charged alumina surfaces followed the sequence F– ≫ Br– > Cl– > I– (slightly varied direct Hofmeister series) and Br– > I– ≈ Cl– > F– (slightly varied indirect Hofmeister series), respectively. Additionally, time-resolved vSFG reveals that only F– perturbs the vibrational lifetime of w...

Steady-state and time-resolved vibrational sum frequency generation (vSFG) were used to investigate the structure and dynamics of water at the α-Al2O3(0001) surface. The vSFG spectra of the O—H stretch of water next to the Al2O3(0001) surface are blue-shifted compared to the Al2O3(1120) surface, indicating its weaker hydrogen bonding network. Consequently, the vibrational dynamics of the O—H stretch of the neutral Al2O3(0001) surface is two times slower than the neutral Al2O3(1120) surface. Furthermore, the vibrational dynamics of the O—H stretch of water next to charged Al2O3 surfaces is observed to be faster than that in bulk water and at charged SiO2 surfaces, which could be due to (a) fast proton transfer dominating the vibrational relaxation and/or, (b) efficient coupling between the O—H stretch and the bend overtone via the presence of low frequency (∼3000 cm–1) O—H stretching modes. Lastly, the addition of excess ions (0.1 M NaCl) seems to have little to no effect on the time scale of vibrational...

Water vapor binding to metal oxide surfaces produces thin water films with properties controlled by interactions with surface hydroxo sites. Hydrogen bonding populations vary across films and induce different molecular orientations than at the surface of liquid water. Identifying these differences can open possibilities for tailoring film-mediated catalytic reactions by choice of the supporting metal oxide substrate.The (0001) face of a single sapphire (α-AlSFG-VS revealed a submonolayer film on sapphire exposed to 43% relative humidity (R.H.), and a multilayer film at 78% R.H. Polarization dependent SFG-VS spectra showed that median tilt angles of free OH bonds on the top of films are at ∼43° from the normal of the (0001) face but at 38° on neat liquid water. These values align with MD simulations, which also show that up to 36% of all OH bonds on films are free. This offers new means for understanding how interfacial reactions on sapphire-supported water films could contrast with those involving liquid water.

Understanding mechanistic pathways to radiolytic hydrogen generation by metal oxyhydroxide nanomaterials is challenging because of the difficulties of distinguishing key locations of OH bond scission, from structural interiors to hydroxylated surfaces to physi-sorbed water molecules. Here we exploited the interface-selectivity of vibrational sum frequency generation (VSFG) to isolate surface versus bulk hydroxyl groups for gibbsite and boehmite nanoplatelets before and after 60Co irradiation at dose levels of approximately 7.0 and 29.6 Mrad. While high-resolution microscopy revealed no effect on particle bulk and surface structures, VSFG results clearly indicated up to 83% and 94% radiation-induced surface OH bond scission for gibbsite and boehmite, respectively, a substantially higher proportion than observed for interior OH groups by IR and Raman spectroscopy. Electron paramagnetic spectroscopy revealed that the major radiolysis products bound in the mineral structures are trapped electrons, O , O2− and possibly F-centers in gibbsite, and H , O and O3− in boehmite, which persist on the time frame of several months. The entrapped radiolysis products appear to be highly stable, enduring re-hydration of particle surfaces, and likely reflect a permanent adjustment in the thermodynamic stabilities of these nanomaterials.

Frequency and time-resolved vibrational sum frequency generation (vSFG) are used to investigate the behavior of water at the α-Al2O3(1120) surface. In addition to the typical water OH peaks (∼3200 and ∼3400 cm–1), the α-Al2O3(1120)/H2O interface shows an additional red-shifted feature at ∼3000 cm–1. Addition of ions (0.1 M NaCl) largely attenuates the water OH peaks but has little effect on the 3000 cm–1 peak. The 3000 cm–1 feature is assigned to the O–H stretch of surface aluminol groups and/or interfacial water molecules that are strongly hydrogen bonded to the alumina surface. Density functional theory calculations were performed to test this assignment, revealing the presence of both associated and dissociated H2O configurations (chemisorbed surface OH group) with frequencies at 3155 and 3190 cm–1, respectively, at a hydrated α-Al2O3(1120) surface. IR pump–vSFG probe measurements reveal that the interfacial OH species show very fast (

By integrating organic synthesis, secondary organic aerosol synthesis and collection, DFT calculations, and vibrational sum frequency generation spectroscopy, we identify close spectral matches between the surface vibrational spectra of β-caryophyllene-derived secondary organic material and those of β-caryophyllene aldehyde and β-caryophyllonic acid at various interfaces. Combined with the record high surface tension depression described previously for these same oxidation products, we discuss possibilities for an intrinsically chemical origin for cloud activation by terpene-derived surfactants. Although the present study does not unequivocally identify the synthesized and analyzed oxidation products on the β-caryophyllenederived SOM surfaces, these two compounds appear to be the most surface active out of the series, and have also been foci of previous β-caryophyllene field and laboratory studies.An orientation analysis by phase-resolved SFG spectroscopy reveals a “pincer-like” configuration of the β-caryophyllene oxidation products, albeit on a model quartz surface, that somewhat resembles the orientation of inverse double-tailed surfactants at the surfaces biological systems. The structural information suggests that the less polar moiety of a surface-localized oxidation product, such as those studied here, may be the first site-of-contact for a gas-phase molecule approaching an SOA particle containing surface-active β-caryophyllene oxidation products.

We report the generation of ultrabroadband mid-infrared (mid-IR) pulses by noncollinear difference frequency mixing. The signal and the idler output beams of an optical parametric amplifier are combined in a silver thiogallate crystal (AgGaS2) to generate mid-infrared radiation. We show that a noncollinear geometry facilitates broadband phase matching. Spectral bandwidths up to 1750 cm−1 were obtained at an external noncollinear angle of 4.2 deg, which is more than three times broader than in a collinear geometry. The broadband spectrum is tunable in the range of 1500–4500 cm−1. Pulse energies up to 1 μJ were achieved. The broadband pulses were used in sum frequency generation in ZnSe and in vibrational absorption spectroscopy experiments of liquid samples.

We describe temporal compression of ultrabroadband, few microjoule mid-infrared (mid-IR) pulses from a noncollinear optical parametric amplifier (NOPA) employed in a sum-frequency generation (SFG) vibrational spectroscopic system, operating in total-internal-reflection geometry. The propagation of the mid-IR beam through optical materials results in a significant temporal chirp at the probed interface, which is analyzed and corrected by properly managing the total dispersion of materials introduced into the mid-IR beam path. By employing the simultaneous spatial and temporal focusing of the broadband infrared pulses at the probed interface, we achieve a sub-50-fs full width at half-maximum (FWHM) for the instrument response function, measured via SFG cross correlation of the ultrashort mid-IR pulses with an ultrashort (∼30 fs) near-IR pulse from a synchronized, independently tunable NOPA. From the SFG cross-correlation FWHM, we extract a sub-30-fs mid-IR pulse duration, making it a suitable SFG spectroscopic system to investigate vibrational dynamics in hydrogen-bonded systems at interfaces.