Surface-sensitive Sum Frequency Generation Research Articles

Lead (II) ions enable the ion-specific effects of monovalent anions on the molecular structure and interactions at silica/aqueous interfaces

HypothesisThe adsorption of heavy metal ions such as Pb(II) onto negatively charged minerals such as silica is expected to alter the structure and the interactions at the silica/aqueous interfaces. Besides the solution pH, the inner-sphere sorption of Pb(II) is expected to regulate the surface charge/potential, hypothesized to control the actions of monovalent anions in the aqueous environment. These complex pictures can be probed directly using surface-sensitive sum-frequency generation (SFG) spectroscopy. ExperimentsThe pH-dependent water structure within the double layer at silica/aqueous interfaces under the influence of different ions was examined using SFG. The recorded SFG spectra were deconvoluted into the Stern layer (SL) and diffuse layer (DL) using the maximum entropy method in conjunction with the electrical double-layer theory. FindingsStandalone monovalent sodium salts do not exhibit ion-specific effects on the silica/aqueous interfaces. However, the mixture of Pb(II) species and each of these salts display profound ion-specific effects on the structure of silica/aqueous interfaces, indicating the role of Pb(II) as an enabler of the ion-specificity of the investigated monovalent anions. The interesting effect arises from a complex interplay between the physical processes (i.e., electrostatic interactions, screening effects, etc.) and chemical processes such as the hydrolysis of Pb(II) ions, ion complexation, protonation and deprotonation of the surface silanol group.

Diffusely Charged Polymeric Zwitterions as Loosely Hydrated Marine Antifouling Coatings.

Polymeric zwitterions exhibit exceptional fouling resistance through the formation of a strongly hydrated surface of immobilized water molecules. While being extensively tested for their performance in biomedical, membrane, and, to a lesser extent, marine environments, few studies have investigated how the molecular design of the zwitterion may enhance its performance. Furthermore, while theories of zwitterion antifouling mechanisms exist for molecular-scale foulant species (e.g., proteins and small molecules), it remains unclear how molecular-scale mechanisms influence the micro- and macroscopic interactions of relevance for marine applications. The present study addresses these gaps through the use of a modular zwitterion chemistry platform, which is characterized by a combination of surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and marine assays. Zwitterions with increasingly delocalized cations demonstrate improved fouling resistance against the green alga Ulva linza. SFG spectra correlate well with the assay results, suggesting that the more diffuse charges exhibit greater surface hydration with more bound water molecules. Hence, the number of bound interfacial water molecules appears to be more influential in determining the marine antifouling activities of zwitterionic polymers than the binding strength of individual water molecules at the interface.

Methanol Adsorption and Reaction on TiO2(110) at Near Ambient Pressure

Although methanol on rutile TiO2(110) has been widely investigated under ultrahigh vacuum (UHV) conditions as a prototypical model, the adsorption structure and reaction of this system under or near real reaction conditions are still unclear. Surface-sensitive sum frequency generation vibrational spectroscopy (SFG-VS) has been utilized to in situ study the adsorption and reaction of methanol on TiO2(110) at near ambient pressure and 400 K. The SFG results indicate that the bridge-bonded methoxy (CH3Ob) is the dominant adsorbed species on TiO2(110) at an elevated pressure of methanol, and its density rises with increasing the methanol pressure. By evacuating the methanol gas, only CH3Ob species leave behind on the TiO2(110) surface, which is also confirmed by ultraviolet (UV) irradiation experiments. Furthermore, by a comparative SFG study of methanol adsorption on TiO2(110) as a function of methanol pressure at 250 K, we concluded that the recombinative desorption of bridging hydroxyl (OHb) groups in the form of water can facilitate the continuous dissociative adsorption of methanol on bridging oxygen vacancy (Ov) sites and the continuous generation of CH3Ob. Our results not only shed light on the adsorption and reaction of methanol on TiO2(110) at near ambient pressure but also demonstrate the studies of catalytic reaction under or near real reaction conditions at the molecular level.

Strong Surface Hydration and Salt Resistant Mechanism of a New Nonfouling Zwitterionic Polymer Based on Protein Stabilizer TMAO.

Zwitterionic polymers exhibit excellent nonfouling performance due to their strong surface hydrations. However, salt molecules may severely reduce the surface hydrations of typical zwitterionic polymers, making the application of these polymers in real biological and marine environments challenging. Recently, a new zwitterionic polymer brush based on the protein stabilizer trimethylamine N-oxide (TMAO) was developed as an outstanding nonfouling material. Using surface-sensitive sum frequency generation (SFG) vibrational spectroscopy, we investigated the surface hydration of TMAO polymer brushes (pTMAO) and the effects of salts and proteins on such surface hydration. It was discovered that exposure to highly concentrated salt solutions such as seawater only moderately reduced surface hydration. This superior resistance to salt effects compared to other zwitterionic polymers is due to the shorter distance between the positively and negatively charged groups, thus a smaller dipole in pTMAO and strong hydration around TMAO zwitterion. This results in strong bonding interactions between the O- in pTMAO and water, and weaker interaction between O- and metal cations due to the strong repulsion from the N+ and hydration water. Computer simulations at quantum and atomistic scales were performed to support SFG analyses. In addition to the salt effect, it was discovered that exposure to proteins in seawater exerted minimal influence on the pTMAO surface hydration, indicating complete exclusion of protein attachment. The excellent nonfouling performance of pTMAO originates from its extremely strong surface hydration that exhibits effective resistance to disruptions induced by salts and proteins.

Interfacial Structure and Interfacial Tension in Model Carbon Fiber-Reinforced Polymers.

Carbon fiber-reinforced plastics (CFRPs) are widely used materials with outstanding mechanical properties. The wettability between the polymer matrix and carbon fiber in the interphase region significantly influences the strength of the composite. Sizing agents consisting of multiple components are therefore frequently applied to improve wetting and interfacial adhesion between polymers and carbon fiber in CFRPs. However, the complex compositions of sizing solutions make detailed interpretations of their impacts on interfacial wetting difficult. In this work, surface-sensitive sum frequency generation (SFG) spectroscopy was utilized to characterize the sizing/polymer and sizing/carbon fiber interfacial structures to gain molecular-level understandings of the wetting improvements afforded by sizing. A mixture sizing solution containing polyethylenimine (PEI, adhesion promoter) and Lutensol (surfactant) was investigated when contacting nylon (model plastics), polypropylene (model plastics), and graphite (model carbon fiber). Our results demonstrated that although the addition of the surfactant led to an interfacial tension decrease (in comparison to pure PEI solution) on nylon and polypropylene, the interfacial tension was surprisingly increased on graphite, contrasting with the commonly accepted function of surfactants. SFG characterizations revealed the multilayer molecular structures at these buried interfaces. The peculiar interfacial tension increase at the graphite/sizing interface was then correlated to the strong amine-π interactions between PEI and graphite. PEI was therefore demonstrated to be an effective adhesion promoter for carbon fiber. This article reports the first investigation of (polymer + surfactant) complex structures at solid-liquid interfaces. The valuable structural insights obtained by SFG analysis enable more accurate understandings of the composition-wettability (structure-function) relationship. These detailed understandings of interactions between sizing and the substrates promote more informed and optimized selections of sizing formulae.

Why Are Water Droplets Highly Mobile on Nanostructured Oil-Impregnated Surfaces?

Porous lubricated surfaces (aka slippery liquid-infused porous surfaces, SLIPS) have been demonstrated to repel various liquids. The origin of this repellency, however, is not fully understood. By using surface-sensitive sum frequency generation vibrational spectroscopy, we characterized the water/oil interface of a water droplet residing on (a) an oil-impregnated nanostructured surface (SLIPS) and (b) the same oil layer without the underlying nanostructures. Different from water molecules in contact with bulk oil without nanostructures, droplets on SLIPS adopt a molecular orientation that is predominantly parallel to the water/oil interface, leading to weaker hydrogen bonding interactions between water droplets and the lubrication film, giving SLIPS their water repellency. Our results demonstrate that the molecular interactions between two contacting liquids can be manipulated by the implementation of nanostructured substrates. The results also offer the molecular principles for controlling nanostructure to reduce oil depletion-one of the limitations and major concerns of SLIPS.

Spectroscopic Identification of Peptide Chemistry in the Caulobacter crescentus Holdfast.

The bacterium Caulobacter crescentus is known to attach irreversibly to underwater surfaces by utilizing an adhesive structure called the holdfast, which exhibits the greatest known adhesive strength of any organism. The very small size of the holdfast (∼400 nm wide and ∼40 nm high) has made direct chemical analysis difficult, and its structure remains poorly understood. In this study, we employ spectroscopic techniques, including attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy, to probe holdfast chemistry. The data indicate the presence of a peptide signal within the holdfast polymer. By comparing the ATR-IR spectrum of the holdfast to peptidoglycan spectra from other bacterial species, we demonstrate the similarity of the holdfast chemistry to that of peptidoglycan, suggesting peptide cross-linking may play a role in holdfast architecture. To probe the molecular groups at the interface, surface-sensitive sum frequency generation spectroscopy was used to show that aromatic and hydroxyl groups related to this protein content at the adhesive interface could be playing a crucial role in adhesion. On the basis of these results, we propose a model of the holdfast architecture with similarities to the peptide cross-linking observed in the peptidoglycan polymer of the bacterial cell wall. These results not only provide information about the development of adhesives that could be based on holdfast chemical architecture but also reveal a potentially yet unexplored biosynthetic pathway in holdfast synthesis that has not yet been revealed by genetic approaches, thereby opening up a potentially new avenue of research in holdfast synthesis.

Transition in the Acid–Base Component of Surface Free Energy of Ice upon the Premelting of Its Second Molecular Bilayer

Molecular disordering of the ice surface occurs below the bulk melting temperature of 273 K, termed surface premelting. The top-most molecular layer begins gradually premelting at 200 K, and has been linked to its low coefficient of friction through an increase in molecular mobility. The second molecular bilayer premelts around 257 K, but no study has linked this transition to a change in any macroscopic phenomena. Here, we show that the thermodynamic work of adhesion between polydimethylsiloxane (PDMS) and ice changes abruptly at 257.0 $\pm$ 0.1 K. Surface-sensitive sum frequency generation spectroscopy shows that there are no molecular level changes at the PDMS surface or the ice-PDMS interface near the transition in adhesion, indicating that the transition arises from changes of the ice surface. Using existing contact angle data in the literature, we show that this transition is due to a decrease in the acid-base component of the surface free energy of ice by 17 $\pm$ 2 mJ/m$^2$ at 257.0 $\pm$ 0.1 K. The change in surface energy provides a possible explanation for a variety of unexplained phenomena seen across the literature including ice adhesion, friction, and the morphology of snowflakes.

The Role of Specific Ion Effects in Ion Transport: The Case of Nitrate and Thiocyanate

The selective transport of trivalent rare earth metals from aqueous to organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the coextracted background anions are not only necessary for charge balance but also have a large impact on extraction efficiency and selectivity. In particular, the opposite selectivity trends observed throughout the lanthanide series in the presence of nitrate and thiocyanate ions have not been explained. To understand the role of background anions in the phase transfer of lanthanide cations, we use a positively charged long-chain aliphatic molecule, modeling a common extractant, and gain molecular level insight into interfacial headgroup-anion interactions. By combining surface sensitive sum frequency generation spectroscopy with X-ray reflectivity and grazing incidence X-ray diffraction, we observed qualitative differences in the orientational and overall interfacial structure of nitrate and thiocyanate solutions at a positively charged Langmuir monolayer. Though nitrate adsorbs without dramatic changes to the solvation structure at the interface or the monolayer ordering, thiocyanate significantly alters the water structure and reduces monolayer ordering. We suggest that these qualitatively different adsorption trends help explain a reversal in system selectivity toward lighter or heavier lanthanides in solvent extraction systems in the presence of nitrate or thiocyanate anions.

Reorganization of an Amphiphilic Glassy Polymer Surface in Contact with Water Probed by Contact Angle and Sum Frequency Generation Spectroscopy

We address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hysteresis measurements are also affected by surface roughness and chemical heterogeneity, we have used surface-sensitive sum frequency generation spectroscopy (SFG) in conjunction with water contact angles to answer this question. To increase the magnitude of the surface reorganization, we have designed an amphiphilic polymer, poly(α-hydroxymethyl-n-butyl acrylate) (PHNB), to study the changes in the structure of polar hydroxy groups and nonpolar (methyl and methylene) groups at the interface. The SFG and the water contact angles show that reorganization does occur for PHNB below Tg. However, complete reorganization requires heating the sample above the bulk Tg. These heating experiments were conducted by first heating the sample in the presence of water and then followed by cooling the sample to room temperature in the presence of water to lock the changes in the surface str...

Interstitial Water Enhances Sliding Friction.

This study examines how surfaces with different water contact angles (wettability) affect dry and underwater adhesion and friction. These studies were conducted by bringing a deformable hydrophobic poly(dimethylsiloxane) lens in contact with surfaces of gradient wettability. On the basis of our adhesion and friction results, we divide the results in three regions. In region I (water contact angles greater than 80°), the dry adhesion is lower than underwater adhesion. In contrast, in region III, (water contact angles less than 50°), the dry adhesion is higher than underwater adhesion. For surfaces with water contact angles between 50 and 80° (region II), the dry and wet adhesion values are comparable. Interestingly, in this region II, the underwater coefficient of friction (COF) values are higher than those in regions I and III. We have used surface-sensitive sum frequency generation (SFG) spectroscopy to probe whether the contact interface in static conditions and during dynamic sliding is dry or wet. The SFG results reveal that the contact is dry in region I. If this dry contact is maintained, the underwater COF follows the trend of adhesion hysteresis in dry conditions (adhesion hysteresis decreases with an increase in water contact angles). In region III, the contact is wet and the underwater COF follows the trend for adhesion hysteresis in wet conditions (adhesion hysteresis increases with an increase in water contact angles). By knowing whether the contact interfaces are dry or wet, we can relate the trends in COF with the trends in adhesion hysteresis. For conditions where the contact interfaces have both dry and wet patches (region II), the COF values are higher than those in completely dry conditions, suggesting that a partially lubricated system can exhibit a higher COF.

Effect of Surface Crowding and Surface Hydrophilicity on the Activity, Stability and Molecular Orientation of a Covalently Tethered Enzyme.

We have investigated two surface properties that are generally thought to have an important influence of enzyme activity and stability: surface hydrophobicity and surface crowding. Here two variants of an engineered bacterial nitro-reductase were covalently tethered to orient the protein's pseudo-2-fold symmetry axis either parallel or perpendicular to the surface. The surface hydrophobicity was systematically varied by changing the ratio of methyl- to hydroxyl-groups displayed on the SAM surface, and the effects on enzyme activity, thermal stability, and structure investigated. Increasing surface hydrophobicity progressively decreased enzyme activity, but had no effect on thermal stability. Surface-sensitive sum frequency generation and attenuated total reflectance Fourier transform IR spectroscopies indicated that the enzyme is not denatured by the more hydrophobic surface, but is more likely trapped in less active conformations by transient hydrophobic interactions. In contrast, increasing enzyme surface concentration increased the specific activity of the parallel oriented enzyme, but had no effect on the activity of the perpendicularly oriented enzyme, suggesting that crowding effects are highly context dependent.

Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy.

The lipopeptide surfactin produced by certain strains of Bacillus subtilis is a powerful biosurfactant possessing potentially useful antimicrobial properties. In order to better understand its surface behavior, we have used surface sensitive sum frequency generation (SFG) vibrational spectroscopy in the C-H and C═O stretching regions to determine its structure at the air/water interface. Using surfactin with the leucine groups of the peptide ring perdeuterated, we have shown that a majority of the SFG signals arise from the 4 leucine residues. We find that surfactin forms a robust film, and that its structure is not affected by the number density at the interface or by pH variation of the subphase. The spectra show that the ring of the molecule lies in the plane of the surface rather than perpendicular to it, with the tail lying above this, also in the plane of the interface.

Probing Site-Specific Structural Information of Peptides at Model Membrane Interface In Situ.

Isotope labeling is a powerful technique to probe detailed structures of biological molecules with a variety of analytical methods such as NMR and vibrational spectroscopies. It is important to obtain molecular structural information on biological molecules at interfaces such as cell membranes, but it is challenging to use the isotope labeling method to study interfacial biomolecules. Here, by individually (13)C═(16)O labeling ten residues of a peptide, Ovispirin-1, we have demonstrated for the first time that a site-specific environment of membrane associated peptide can be probed by the submonolayer surface sensitive sum frequency generation (SFG) vibrational spectroscopy in situ. With the peptide associated with a single lipid bilayer, the sinusoidal trend of the SFG line width and peak-center frequency suggests that the peptide is located at the interface beneath the lipid headgroup region. The constructive interferences between the isotope labeled peaks and the main peptide amide I peak contributed by the unlabeled components were used to determine the membrane orientation of the peptide. From the SFG spectral peak-center frequency, line width, and polarization dependence of the isotope labeled units, we deduced structural information on individual units of the peptide associated with a model cell membrane. We also performed molecular dynamics (MD) simulations to understand peptide-membrane interactions. The physical pictures described by simulation agree well with the SFG experimental result. This research demonstrates the feasibility and power of using isotope labeling SFG to probe molecular structures of interfacial biological molecules in situ in real time.

Consequences of water between two hydrophobic surfaces on adhesion and wetting.

The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the water-free contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a self-assembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS-PDMS contact. The Young-Dupré equation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting.

Surface orientation control of site-specifically immobilized nitro-reductase (NfsB).

We demonstrate the control of enzyme orientation for enzymes chemically immobilized on surfaces. Nitro-reductase (NfsB) has the ability to reduce a broad range of nitro-containing compounds and has potential applications in a broad range of areas including the detection and decomposition of explosives. The enzyme was tethered through unique surface cysteine residues to a self-assembled monolayer (SAM) terminated with maleimide groups. One cysteine was introduced close to the active site (V424C), and the other, at a remote site (H360C). The surface-tethered NfsB variants were interrogated by a combination of surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) to determine how the mode of attachment altered the enzyme's orientation. The activities of the two immobilized NfsB variants were measured and can be well correlated to the deduced orientations. The relationships among enzyme engineering, surface immobilization, enzyme orientation, and enzyme activity were revealed.

Liquid/air interfacial structures of various molecular weights PBMA end-capped with 2-perfluorooctylethyl methacrylate units in toluene solutions

Various molecular weight poly(butyl methacrylates) (PBMA) end-capped with 2-perfluorooctylethyl methacrylate (FMA) units (PBMAm-ec-FMAn) have been synthesized by atom transfer radical polymerization (ATRP).The liquid/air interfacial structure of PBMAm-ec-PFMAn in toluene solutions was investigated by surface tension measurement and surface-sensitive sum frequency generation vibrational spectroscopy (SFG).The results indicate that the surface activity of PBMAm-ec-PFMAn toluene solution is related to the degree of polymerization of PBMA and the number of FMA units at the chain end.When the number of FMA units about 4 and 6,the surface tension of the polymer increases from 21 to 25 mN/m with increasing of PBMA chain length,approaching to the surface tension of pure PBMA solution.SFG results reveal that there is no fluorinated component absorbed at the solution surface for this polymer.When the PBMA chain is short,fluorinated component is absorbed at the solution surface and forms the order of the packing orientation of the perfluoroalkyl side chains,which results in great decrease of the surface tension compared to that of PBMA homopolymer.As to PBMAm-ec-PFMA1,the surface tension of toluene solution increases slightly (from 22 to 23 mN/m) with increasing of the degree of polymerization of PBMA.SFG results reveal that the decrease in surface tension for PBMAm-ec-PFMA1 with short PBMA chain may be attributed to the segregation of fluorinated component with the disorder of the packing orientation of the perfluoroalkyl side chain,that for polymer with longer PBMA chain may be attributed to the segregation of both fluorinated component and ester butyl group in PBMA chain at the solution surface.

Infrared Signatures of HNO3 and NO3− at a Model Aqueous Surface. A Theoretical Study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3(-) at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+* level of theory on the HNO3 x (H2O)3 --> NO3(-) x H3O(+) x (H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species' intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts.

Order To Disorder Transition at The Polymer-Air Interface for Hydrophobic Octadecyl Side Chain Copolymers

ABSTRACTMolecular orientation of octadecyl alkyl side chains at the poly (vinyl octadecyl carbamates-co-vinyl acetate) polymer-air interface has been studied using surface sensitive sum frequency generation (SFG) spectroscopy. At 280C, below the side chain crystalline transition temperature, the SFG spectra show strong methyl vibrations indicating ordered alkyl side chains at the polymer-air interface. In the liquid state (980C), the SFG spectra show higher contributions from the methylene vibrations indicating higher gauche defects at the interface. This surface order to disorder transition is gradual and spans over a broad temperature range (50-900C), where the bulk is in the smectic liquid crystalline state.