Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

Phase of the second-order susceptibility in vibrational sum frequency generation spectroscopy: Origins, utility, and measurement techniques

Vibrational sum frequency generation can provide valuable structural information at surfaces and buried interfaces. Relating the measured spectra to the complex-valued second-order susceptibility χ(2) is at the heart of the technique and a requisite step in nearly all subsequent analyses. The magnitude and phase of χ(2) as a function of frequency reveal important information about molecules and materials in regions where centrosymmetry is broken. In this tutorial-style perspective, the origins of the χ(2) phase are first described, followed by the utility of phase determination. Finally, some practical methods of phase extraction are discussed.

Ultra-broadband detection of coherent infrared pulses by sum-frequency generation spectroscopy in reflection geometry

We have newly developed, to the best of our knowledge, a detection method for broadband infrared pulses based on sum-frequency generation spectroscopy in reflection geometry, which can avoid a restriction of the detection bandwidth originating from the phase mismatch that is inevitable for the upconversion in transmission geometry. Using a GaAs crystal, we successfully demonstrated the ultra-broadband detection of the infrared pulses generated from a two-color laser-induced air plasma filament in a region from 300 to 3300 cm-1. With the advantage of ultra-short infrared pulses, the present detection method holds promise for application to time-resolved, ultra-broadband vibrational spectroscopy.

Lamellar orientation at the surface of isotactic polystyrene thin films analyzed by sum frequency generation spectroscopy

Analyzing the orientation of polymeric crystalline lamella at the surface of thin films can be challenging. Even though atomic force microscopy (AFM) is often sufficient for this analysis, there are cases when imaging is not sufficient to confidently determine lamellar orientation. Here, we used sum frequency generation (SFG) spectroscopy to analyze the lamellar orientation at the surface of semi-crystalline isotactic polystyrene (iPS) thin films. The SFG orientation analysis indicated that the iPS chains are oriented perpendicular to the substrate (flat-on lamellar orientation), which was confirmed by AFM. By analyzing the evolution of the SFG spectral features with the progress of crystallization, we demonstrated that the ratios of the SFG intensities of the phenyl ring resonances are a good indication of the surface crystallinity. Furthermore, we explored the challenges associated with SFG measurements of heterogeneous surfaces, which is commonly present in many semi-crystalline polymeric films. To the best of our knowledge, this is the first time that the surface lamellar orientation of semi-crystalline polymeric thin films was determined by SFG. Also, this work pioneers in reporting the surface conformation of semi-crystalline and amorphous iPS thin films by SFG and in linking the SFG intensity ratios to the progress of the crystallization and the surface crystallinity. This study demonstrates the potential applicability of SFG spectroscopy in the conformational analysis of polymeric crystalline structures at interfaces and opens the way to the investigation of more complex polymeric structures and crystalline arrangements, especially for the case of buried interfaces, where AFM imaging is not an option.

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

Dynamic Wetting of Photoresponsive Arylazopyrazole Monolayers is Controlled by the Molecular Kinetics of the Monolayer

Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. AAP monolayers were deposited on α-Al₂O₃(0001) and showed reversible E/Z photoswitching that can trigger contact angle changes of up to ∼10°. We monitored these changes on the macroscopic level by recording the contact angle while the monolayer was switched in situ. On the molecular level, time-dependent vibrational sum-frequency generation (SFG) spectroscopy provided information on the kinetic changes within the AAP monolayer and the characteristic times for E/Z switching. In addition, vibrational SFG at different relative humidity indicates that the thermal stability of the Z configuration is largely influenced by the presence of water which can stabilize the Z state and hinder E → Z switching of the AAP monolayer when it is wetted with H₂O. Having established the switching times on the molecular scale, we additionally measured the dynamic contact angle and show that the time scales of the substrate and droplet dynamics can be extracted individually. For that, we report on a relaxation model that is solved analytically and is verified via a comparison with simulations of a Lennard-Jones system and with experimental data. The slower E to Z switching in the presence of the droplet as compared to the vapor phase is rationalized in terms of specific interactions of water with the exposed AAP moieties.

Quantitative and qualitative studies for real monitoring of interfacial molecular water

HYPOTHESIS

Interfacial water plays an essential role in natural phenomena and scientific applications despite causing many economic losses. Therefore, its real monitoring is no question mandatory; however, suitable techniques are quite rare and/or with limitations.

EXPERIMENTS

Moisture Sensor (MS) was used to detect the galvanic response current arising from the stacked interfacial water molecules between two dissimilar electrodes under controlled relative humidity (RH). Simultaneously, the frequency response was detected using QCM sensor as a quantitative tool. Bare and Hydrophilic (HP) sensor surfaces were used to examine the surface wettability. Moreover, sum frequency generation (SFG) was used to investigate the qualitative formation and the nature of stacked interfacial water molecules on bare and HP modified surfaces of quartz prism.

FINDINGS

Results revealed that, response current and frequency change were increased as the number of stacked water molecules increased. Correlating response current and frequency gave a clear quantitative estimation of stacked water molecules on the sensors' surfaces. Interfacial water molecules possessed strong H-bonding nature at the bare prism surface whereas, strong and weak H-bonding were existing at the HP/prism surface. Such findings provide feasible evaluation for the galvanic current source due to stacked interfacial water molecules at different levels of RH.

Probing the Bovine Hemoglobin Adsorption Process and its Influence on Interfacial Water Structure at the Air-Water Interface

*These authors contributed equally to this work.The molecular-level insight of protein adsorption and its kinetics at interfaces is crucial because of its multifold role in diverse fundamental biological processes and applications. In the present study, the sum frequency generation (SFG) vibrational spectroscopy has been employed to demonstrate the adsorption process of bovine hemoglobin (BHb) protein molecules at the air-water interface at interfacial isoelectric point of the protein. It has been observed that surface coverage of BHb molecules significantly influences the arrangement of the protein molecules at the interface. The time-dependent SFG studies at two different frequencies in the fingerprint region elucidate the kinetics of protein denaturation process and its influence on the hydrogen-bonding network of interfacial water molecules at the air-water interface. The initial growth kinetics suggests the synchronized behavior of protein adsorption process with the structural changes in the interfacial water molecules. Interestingly, both the events carry similar characteristic time constants. However, the conformational changes in the protein structure due to the denaturation process stay for a long time, whereas the changes in water structure reconcile quickly. It is revealed that the protein denaturation process is followed by the advent of strongly hydrogen-bonded water molecules at the interface. In addition, we have also carried out the surface tension kinetics measurements to complement the findings of our SFG spectroscopic results.

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

In cosmetic, pharmaceutical, and food applications, many active ingredients have limited bioavailability in an aqueous environment, and in that context, nanoemulsions provide a mechanism for encapsulation, protection, and transport. These dispersed oil droplets are on the order of 100s of nanometers in diameter and owe their long-term stability to emulsifiers that are commonly charged. More recently, applications have been utilizing nonionic species as stabilizing agents due to their enhanced biosafety. DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) theory has been central in the description of colloid stability, which emphasizes repulsive electrostatic forces, while extended DLVO theory also accounts for steric effects. Past studies of nanoemulsions have largely employed charged surfactants and polyelectrolytes, making it difficult to decouple electrostatic and steric effects as they relate to droplet stability. To better understand steric and molecular factors contributing to the stability of "uncharged" droplets, we have created nanoemulsions with sodium dodecyl sulfate (SDS) and poly(N-vinylacetamide) (PNVA). Though SDS is anionic, with PNVA coating the droplet surfaces, the ζ-potentials of these nanoemulsions are ∼0 mV. Despite minimizing electrostatic contributions, these nanoemulsions are stable for upward of a month with interesting dynamics. By employing dynamic light scattering, vibrational sum frequency scattering spectroscopy, and calculating interaction pair potentials using extended DLVO theory, we learn that the thickness of the PNVA layer plays a critical role in stabilizing these "uncharged" nanoemulsions. Beyond the sterics, the molecular conformation of the PNVA strands also contributes to the droplet stability. The adsorbed PNVA strands are shown to form stratified, rigid polymer networks that prevent the nanoemulsions from rapid destabilization.

Study of the molecular orientation of steroidal side chains at polyimide surfaces using sum frequency generation vibrational spectroscopy

The molecular orientation of steroidal side chains at rubbed polyimide (PI) surfaces is studied by sum frequency generation (SFG) vibrational spectroscopy. The main objective is to find a correlation between the molecular structure of the PI film and the liquid crystal alignment on the polymer. Analysis of the SFG spectra shows that rubbing of the polymer film appears to cause conformational changes in the methyl group of the polymer side chain near the steroidal structure. However, rubbing does not significantly influence the orientation of the isopropyl group at the end of the polymer side chain. This shows that the liquid crystal alignment is not correlated with the orientation of the isopropyl group.

Influence of Polymer Molecular Weight on Chain Conformation at the Polystyrene/Silver Interface

The dependence between the conformation of polystyrene (PS) and its molecular weight (Mw) in the vicinity of a metal interface was investigated by sum frequency generation (SFG) spectroscopy. Tilt angles θ ≥ 50^° (the angle between the C₂ axis of the pendant phenyl ring and the surface normal) were observed for all samples because of the interaction between the aromatic rings and the metal surface. Furthermore, it was found that θ decreases with increasing Mw for PS samples ranging from 20 × 10³ g/mol to 400 × 10³ g/mol. The intensity of the backbone SFG signal was higher for high Mw PS, compared to low Mw PS, indicating a greater number of backbone interactions with the silver substrate surface for the high Mw sample. These structural differences are driven by different entropic and enthalpic contributions to the free energy of adsorption for different polymer molecular weights. Differences in the polymer free volume and in the relative amount of chain ends with higher mobility may also influence the chain conformation. These results suggest that important interfacial properties of polymeric thin films, such as adhesion and wettability, could be tailored by modifying the polymer Mw to achieve the desired interfacial conformation.

Sensitivity of sum frequency generation experimental conditions to thin film interference effects

Sum-frequency generation (SFG) spectroscopy has furthered our understanding of the chemical interfaces that guide key processes in biology, catalysis, environmental science, and energy conversion. However, interpreting SFG spectra of systems containing several internal interfaces, such as thin film electronics, electrochemical cells, and biofilms, is challenging as different interfaces within these structures can produce interfering SFG signals. One potential way to address this issue is to carefully select experimental conditions that amplify the SFG signal of an interface of interest over all others. In this report, we investigate a model two-interface system to assess our ability to isolate the SFG signal from each interface. For SFG experiments performed in a reflective geometry, we find that there are few experimental conditions under which the SFG signal originating from either interface can be amplified and isolated from the other. However, by performing several measurements under conditions that alter their interference, we find that we can reconstruct each signal even in cases where the SFG signal from one interface is more than an order of magnitude smaller than its counterpart. The number of spectra needed for this reconstruction varies depending on the signal-to-noise level of the SFG dataset and the degree to which different experiments in a dataset vary in their sensitivity to each interface. Taken together, our work provides general guidelines for designing experimental protocols that can isolate SFG signals stemming from a particular region of interest within complex samples.

Local Orientation of Polystyrene at the Interface with Poly(methyl methacrylate) in Block Copolymer

The local conformation of polystyrene (PS) at the phase-separated lamellar interface with poly(methyl methacrylate) (PMMA) in their diblock copolymer (BCP) was examined by sum-frequency generation spectroscopy in conjunction with a full-atomistic molecular dynamics simulation. While PS phenyl groups of BCP were oriented in the interfacial region, they were random in the bulk. Such an interfacial orientation of phenyl groups was not clear for the corresponding blend of PS and PMMA. The PS backbone of BCP was in-plane oriented and folded near to the chemical junction point located in the interfacial region and the orientation became random at several nanometers distant. No evidence for the chain folding at the interface was found for the blend system.

Development of quadrupole susceptibility automatic calculator in sum frequency generation spectroscopy and application to methyl C-H vibrations

Sum frequency generation (SFG) spectroscopy has been established as a powerful interface probe technique based on the electric dipole approximation, while possible signals of quadrupole and bulk origin have also been known for a long time. In this work, we developed a computational tool, namely, Qsac (quadrupole susceptibility automatic calculator), to evaluate the comprehensive contributions of the dipole/quadrupole and interface/bulk in the arbitrary vibrational bands of SFG spectra. The calculations of relevant susceptibility terms are performed on the basis of the theory of energy representation using quantum chemical calculation and molecular dynamics simulation, which allows for semi-quantitative comparison among these terms on the same footing. We applied the Qsac to the methyl C-H stretching bands of organic molecules and found a general trend that the weak asymmetric bands are more sensitive to the bulk contribution than the symmetric ones. The phases of interface and bulk terms tend to cancel in the asymmetric band, which results in the reduced band intensity in the SFG spectra.

Insights on the Molecular Characteristics of Molecularly Imprinted Polymers as Monitored by Sum Frequency Generation Spectroscopy

Sensing in molecularly imprinted polymers (MIPs) requires specific interactions of the imprinted polymer and the approaching template molecule. These interactions are affected by the morphology of the polymer surface, the affinity of the template molecule to the polymer network, and the steric approach. In this particular study, a template molecule, metronidazole, is studied with respect to the typically used methacrylic acid-based imprinted polymer using a combination of bulk and surface techniques. The resulting infrared (IR) spectra exhibited the presence of the template molecule in the polymer matrix as well as their efficient removal after washing. Dipping of the MIP according to what is expected of facile sensing in an aqueous solution of metronidazole did not show any presence of the template molecule in the bulk of the MIP, as observed by IR spectroscopy. However, using sum frequency generation (SFG) spectroscopy, the CH aromatic stretch of the imidazole ring positioned at ∼3100 cm^-1 was observed at the polymer surface, including its inner pores or cavities, and at the buried polymer-fused silica interface after dipping. SFG studies have also shown the vibrational signatures of the polymer matrix, the presence of the template molecule on the surface, and the detection of residual template molecules after washing. Increasing the washing time to 50 min has proven to be less effective than increasing the washing cycles to three. However, after the third cycle, reorganization of the polymer matrix was evident as also the complete removal of the template molecule. The observed changes from the acquired images using scanning electron microscopy and atomic force microscopy show the structural morphologies of MIPs and a good distribution of the pores across the MIP surface. The study demonstrates the importance of combining both bulk and surface characterization in providing insight into the template molecule-polymer network interactions.

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

Polymer-stabilized liquid/liquid interfaces are an important and growing class of bioinspired materials that combine the structural and functional capabilities of advanced synthetic materials with naturally evolved biophysical systems. These platforms have the potential to serve as selective membranes for chemical separations and molecular sequencers and to even mimic neuromorphic computing elements. Despite the diversity in function, basic insight into the assembly of well-defined amphiphilic polymers to form functional structures remains elusive, which hinders the continued development of these technologies. In this work, we provide new mechanistic insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous interface, in which the headgroups consist of positively charged methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes covalently attached to the headgroups. We demonstrate using vibrational sum frequency generation spectroscopy and pendant drop tensiometery that the composition of the bulk aqueous phase, particularly the ionic strength, dictates the kinetics and structures of the amphiphiles in the organic phase as they decorate the interface. These results show that H-bonding and electrostatic interactions taking place in the aqueous phase bias the grafted oligomer conformations that are adopted in the neighboring oil phase. The kinetics of self-assembly were ionic strength dependent and found to be surprisingly slow, being composed of distinct regimes where molecules adsorb and reorient on relatively fast time scales, but where conformational sampling and frustrated packing takes place over longer time scales. These results set the stage for understanding related chemical phenomena of bioinspired materials in diverse technological and fundamental scientific fields and provide a solid physical foundation on which to design new functional interfaces.

Abnormal spectral bands in broadband sum frequency generation induced by bulk absorption and refraction

In this paper, the time-resolved broadband sum frequency generation (BB-SFG) spectra from a bare Au surface with a distorted infrared (introduced with a 10 µm polyethylene film in the IR light path) and principal component generalized projection (PCGP) algorithm were used to investigate the bulk distortion on the measured BB-SFG spectra. Besides the SFG intensity reduction from the bulk absorption, the frequency dependent refraction of the bulk layer led to misleading SFG features at the positive delay times beyond the Au dephasing time. These results suggest that SFG spectroscopy is not entirely 'bulk-free' for the buried interfaces because of the bulk absorption and refraction of the incident pulses.

Polymer Solar Cells-Interfacial Processes Related to Performance Issues

Harnessing solar energy with solar cells based on organic materials (in particular polymeric solar cells) is an attractive alternative to silicon-based solar cells due to the advantages of lower weight, flexibility, lower manufacturing costs, easier integration with other products, low environmental impact during manufacturing and operations and short energy payback times. However, even with the latest efficiencies reported up to 17%, the reproducibility of these efficiencies is not up to par, with a significant variation in the efficiencies reported across the literature. Since these devices are based on ultrathin multilayer organic films, interfaces play a major role in their operation and performance. This review gives a concise account of the major interfacial issues that are responsible for influencing the device performance, with emphasis on their physical mechanisms. After an introduction to the basic principles of polymeric solar cells, it briefly discusses charge generation and recombination occurring at the donor-acceptor bulk heterojunction interface. It then discusses interfacial morphology for the active layer and how it affects the performance and stability of these devices. Next, the formation of injection and extraction barriers and their role in the device performance is discussed. Finally, it addresses the most common approaches to change these barriers for improving the solar cell efficiency, including the use of interface dipoles. These issues are interrelated to each other and give a clear and concise understanding of the problem of the underperformance due to interfacial phenomena occurring within the device. This review not only discusses some of the implemented approaches that have been adopted in order to address these problems, but also highlights interfacial issues that are yet to be fully understood in organic solar cells.

Evidence of chemical-bond formation at the interface between an epoxy polymer and an isocyanate primer

Sum frequency generation spectroscopy was applied to confirm the presence of a chemical bond at the joining interfaces, suggesting that MDI primers can serve as a chemical bridge for urethane adhesives.

Effects of Ca2+ Ion Condensation on the Molecular Structure of Polystyrene Sulfonate at Air-Water Interfaces

The structure of poly(sodium 4-styrenesulfonate) (NaPSS) polyelectrolytes at air-water interfaces was investigated with tensiometry, ellipsometry, and vibrational sum-frequency generation (SFG) in the presence of low and high CaCl₂ concentrations. In addition, we have studied the foaming behavior of 20 mM NaPSS solutions to relate the PSS molecular structure at air-water interfaces to foam properties. PSS polyelectrolytes without additional salt exhibited significant surface activity, which can be tuned further by additions of CaCl₂. The hydrophobicity of the backbone due to incomplete sulfonation during synthesis is one origin, whereas the effective charge of the polyelectrolyte chain is shown to play another major role. At low salt concentrations, we propose that the polyelectrolyte is forming a layered structure. The hydrophobic parts are likely to be located directly at the interface in loops, whereas the hydrophilic parts are at low concentrations stretched out into near-interface regions in tails. Increasing the Ca²⁺ concentration leads to ion condensation, a collapse of the tails, and likely to Ca²⁺ intra- and intermolecular bridges between polyelectrolytes at the interface. The increase in both surface excess and foam stability originates from changes in the polyelectrolyte's hydrophobicity due to Ca²⁺ condensation onto the PSS polyanions. Consequently, charge screening at the interface is enhanced and repulsive electrostatic interactions are reduced. Furthermore, SFG spectra of O-H stretching bands reveal a decrease in intensity of the low-frequency branch when c(Ca²⁺) is increased whereas the high-frequency branch of O-H stretching modes persists even for 1 M CaCl₂. This originates from the remaining net charge of the PSS polyanions at the air-water interface that is not fully compensated by condensation of Ca²⁺ ions and leads to electric-field-induced contributions to the SFG spectra of interfacial H₂O. A charge reversal of the PSS net charge at the air-water interface is not observed and is consistent with bulk electrophoretic mobility measurements.

Sum-Frequency Generation Vibrational Spectroscopy Investigations of Phosphonic Acids on Anodic Aluminum Oxide Films

Self-assembled monolayers of alkyl phosphonic acids on anodic aluminum oxide (AlOx) surfaces are important as dielectric layers in thin film electronic devices. Assessing the properties and quality of these monolayers on amorphous AlOx is limited to a few surface-sensitive methods. In this work, we study using nonlinear optical measurements the molecular ordering in n-alkyl phosphonic acids with various alkyl chain lengths (6 to 18 carbons) deposited on AlOx and show the influence of temperature on stability and conformational order. The results demonstrate that the octadecylphosphonic acid has fewest defects in the chain orientation. A detailed comparison of the longest and the shortest alkyl chain revealed different behavior in conformational ordering upon annealing.