Absolute surface coverage measurement using a vibrational overtone

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm(-1) line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm(-1), while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO(2) surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection.     

Evanescent-wave cavity ring-down investigation of polymer/solvent interactions

Evanescent-wave cavity ring-down spectroscopy (EW-CRDS) is used to measure interfacial phenomena when methanol or water is placed in contact with a film of poly(dimethylsiloxane) (PDMS), which is attached to the face of a fused-silica prism that constitutes part of a ring cavity. In the first few minutes after contact, the uptake of methanol is slower than that of water, but after this initial period the methanol diffuses more rapidly in the film than water does. Bulk weight-gain measurements confirm this result and yield diffusion coefficients of (25.1 +/- 0.7) x 10(-7) cm(2)/s for methanol in PDMS and (7 +/- 2) x 10(-7) cm(2)/s for water in PDMS. The interfacial optical losses found in the EW-CRDS measurements result primarily from scatter. In particular, we find that delamination of the film from the fused-silica substrate dominates the optical losses in the case of methanol. This conclusion is confirmed by separate surface plasmon resonance experiments.     

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) as a probe of macromolecule adsorption kinetics at functionalized interfaces

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been employed to study the interfacial adsorption kinetics of coumarin-tagged macromolecules onto a range of functionalized planar surfaces. Such studies are valuable in designing polymers for complex systems where the degree of interaction between the polymer and surface needs to be tailored. Three tagged synthetic polymers with different functionalities are examined: poly(acrylic acid) (PAA), poly(3-sulfopropyl methacrylate, potassium salt) (PSPMA), and a mannose-modified glycopolymer. Adsorption transients at the silica/water interface are found to be characteristic for each polymer, and kinetics are deduced from the initial rates. The chemistry of the adsorption interfaces has been varied by, first, manipulation of silica surface chemistry via the bulk pH, followed by surfaces modified by poly(L-glutamic acid) (PGA) and cellulose, giving five chemically different surfaces. Complementary atomic force microscopy (AFM) imaging has been used for additional surface characterization of adsorbed layers and functionalized interfaces to allow adsorption rates to be interpreted more fully. Adsorption rates for PSPMA and the glycopolymer are seen to be highly surface sensitive, with significantly higher rates on cellulose-modified surfaces, whereas PAA shows a much smaller rate dependence on the nature of the adsorption surface.     

Total internal reflection Raman spectroscopy

Total internal reflection (TIR) Raman spectroscopy is an experimentally straightforward, surface-sensitive technique for obtaining chemically specific spectroscopic information from a region within approximately 100-200 nm of a surface. While TIR Raman spectroscopy has long been overshadowed by surface-enhanced Raman scattering, with modern instrumentation TIR Raman spectra can be acquired from sub-nm thick films in only a few seconds. In this review, we describe the physical basis of TIR Raman spectroscopy and illustrate the performance of the technique in the diverse fields of surfactant adsorption, liquid crystals, lubrication, polymer films and biological interfaces, including both macroscopic structures such as the surfaces of leaves, and microscopic structures such as lipid bilayers. Progress, and challenges, in using TIR Raman to obtain depth profiles with sub-diffraction resolution are described.     

隐藏高分子界面及生物界面分子结构的和频振动光谱研究

界面的分子结构决定界面的性质.为了以优化界面的结构来改进材料的性质,原位实时地研究界面的分子结构是很重要的.近年来和频振动光谱已发展成为一个很有效及独特的手段来研究隐藏界面的分子结构,例如液/液界面、固/液界面及固/固界面等.这篇综述讨论了和频振动光谱在研究高分子界面及生物界面等复杂界面的分子结构上的应用.具体说来,本文论述了高分子表面在水里的分子结构变化,高分子及模型粘合促进剂硅烷在界面相互作用的分子机理和隐藏的高分子/高分子及高分子/金属界面的结构.另外,此文还将介绍不同二级结构的多肽及几个有代表性的蛋白分子在界面的结构.界面在诸如化学、生物、物理、材料科学及工程和纳米技术等许多领域都很重要.发展一个独特的能原位研究隐藏界面的分子结构的技术会有力地促进这些领域的研究及跨学科研究的发展.     

Surface activity of lysozyme and dipalmitoyl phosphatidylcholine vesicles at compressed and supercritical fluid interfaces

The surface activities of lysozyme and dipalmitoyl phosphatidylcholine (DPPC) vesicles at aqueous/compressed fluid interfaces are examined via high-pressure interfacial tension measurements using the pendant drop technique. The density and interfacial tension in compressible fluid systems vary significantly with pressure, providing a versatile medium for elucidating interactions between biomolecules and fluid interfaces and a method to elicit pressure-dependent interfacial morphological responses. The effects of lysozyme concentration (0.0008, 0.01, and 1 mg/mL) and pressure (> or = 7 MPa) on the dynamic surface response in the presence of ethane, propane, N2, and CO2 at 298 K were examined. Interfacial lysozyme adsorption reduced the induction phase and quickly led to interfacial tensions consistent with protein conformational changes and monolayer saturation at the compressed fluid interfaces. Protein adsorption, as indicated by surface pressure, correlated with calculated Hamaker constants for the compressed gases, denoting the importance of dispersion interactions. For DPPC at aqueous/compressed or aqueous/supercritical CO2 interfaces (1.8-20.7 MPa, 308 K), 2-3-fold reductions in interfacial tension were observed relative to the pure binary fluid system. The resulting surface pressures infer pressure-dependent morphological changes within the DPPC monolayer.     

Molecular-level investigation on electrochemical interfaces by Raman spectroscopy

The structure and dynamics of electrode/liquid interfaces play an increasingly important role in electrochemistry. Raman spectroscopy is capable of providing detailed structural information at molecular level and new insight into the interfacial structure, adsorption, reaction, electrocatalysis and corro-sion. In this account we will summarize some progresses of surface Raman spectroscopy in the study of electrochemical interfaces, mainly based on our group's work, laying emphasis on the detection sensitivity, spectral resolution, time resolution and spatial resolution as well as the hyphenated technique.     

pH dependence of the crystal violet adsorption isotherm at the silica-water interface

The pH-dependent adsorption isotherms for the charged chromophore crystal violet, CV(+), have been measured with three different bases by a free-running cavity implementation of evanescent wave cavity ring-down spectroscopy. The ratio of the maximal absorbance measurements at pH 5.10 and 9.05 is consistent with a Q2:Q3 silanol site ratio of 72.8:27.2. The adsorption isotherms have been interpreted in terms a cooperative binding adsorption allowing more than one ionic species to bind to each silanol group. The surface concentration is consistent with a silanol charge density of 1.92 +/- 0.55 nm(-2) and a total neutralized interface layer structure extending 9 nm from the surface. Binding constants and stoichiometric coefficients are derived for CV(+) to both the Q2 and Q3 sites. A variation of the adsorption isotherm with base is observed so that the isotherm at pH 9.05 adjusted with ammonium hydroxide sets up a competitive acid-base equilibrium with the SiOH groups with only 49% of the surface silanol sites dissociated. The implications for functionalized surfaces in chromatography are discussed.     

Excited-state dynamics of organic dyes at liquid/liquid interfaces

Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (～1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.     

Dynamic surface and interfacial tensions of surfactant and polymer solutions

Dynamic surface and interfacial tensions are the most frequently measured non-equilibrium properties of adsorption layers at liquid interfaces. The review presents the theoretical basis of adsorption kinetics, taking into consideration different adsorption mechanisms, and specific experimental conditions, such as liquid flow and interfacial area changes. Analytical solutions, if available, approximations as well as numerical procedures for direct solution of the physical models are presented.Several experimental techniques are discussed frequently used in studies of the dynamic adsorption behaviour of surfactants and polymers at liquid interfaces: drop volume, maximum bubble pressure, and pendent drop technique, drop pressure tensiometry, pulsating bubble and elastic ring method. Experimental results, most of all obtained with different technique on one and the same surfactant system, are then discussed on the basis of current theories.Finally, the role of dynamic interfacial properties in several practical applications is discussed: foam and emulsion film formation and stabilisation, rising of bubbles and drops in a surfactant solution.     

Molecular orientation study of methylene blue at an air/fused-silica interface using evanescent-wave cavity ring-down spectroscopy

Using evanescent-wave cavity ring-down spectroscopy (EW-CRDS), we monitored the change in the absorbance of a thin film of methylene blue (MB) at an air/fused-silica interface while varying the polarization of the incident light (600 nm). We derived the average orientation angle of the planar MB molecules with respect to the surface normal and observed that the average orientation angle decreases as the surface concentration increases. At low surface concentrations, the MB molecules lie almost flat on the surface, whereas at higher surface concentrations the molecules become vertically oriented.     

Amphiphilic derivatives of dextran: adsorption at air/water and oil/water interfaces

Ionic amphiphilic dextran derivatives were synthesized by the attachment of sodium sulfopropyl and phenoxy groups on the native polysaccharide. A family of dextran derivatives was thus obtained with varying hydrophobic content and charge density in the polymer chains. The surface-active properties of polymers were studied at the air-water and dodecane-water interfaces using dynamic surface/interfacial tension measurements. The adsorption was shown to begin in a diffusion-limited regime at low polymer concentrations, that is to say, with the diffusion of macromolecules in the bulk solution. In contrast, at long times the interfacial adsorption is limited by interfacial phenomena: adsorption kinetics or transfer into the adsorbed layer. A semiempirical equation developed by Filippov was shown to correctly fit the experimental curves over the whole time range. The presence of ionic groups in the chains strongly lowers the adsorption kinetics. This effect can be interpreted by electrostatic interactions between the free molecules and the already adsorbed ones. The adsorption kinetics at air-water and oil-water interfaces are compared.     

Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation

The orientation angle is an important parameter that reflects the structure of molecules at interfaces. In order to obtain this parameter, second order nonlinear spectroscopic techniques including second harmonic generation (SHG) and sum frequency generation-vibrational spectroscopy (SFG-VS) have been successfully applied through analysis of the nonlinear signal from various polarizations. In some SHG and SFG-VS experiments, total internal reflection (TIR) configuration has been adopted to get enhanced signals. However, the reports on the detailed procedure of the polarization analysis and the calculation of the orientation angle of interfacial molecules under TIR configuration are still very few. In this paper, we measured the orientation angles of two molecules at the hexadecane-water interface under TIR and Non-TIR experimental configurations. The results measured from polarization analysis in TIR configuration consist with those obtained from Non-TIR configuration. This work demonstrates the feasibility and accuracy of polarization analysis in the determination of the orientation angle of molecules at the interfaces under TIR-SHG configuration.     

Surfactant adsorption at the salt/water interface: comparing the conformation and interfacial water structure for selected surfactants

We report in situ spectroscopic measurements monitoring the adsorption of a series of carboxylate surfactants onto the surface of the semisoluble, ionic solid fluorite (CaF2). We employ the surface-specific technique, vibrational sum-frequency spectroscopy (VSFS), to examine the effect that surfactant adsorption has on the bonding interactions and orientation of interfacial water molecules through the alteration of the electric properties in the interfacial region. In addition, we report on the chain length and headgroup dependence of the formation of hydrophobic self-assembled monolayers on the surface of the solid phase. Differences in chain length and headgroup functionality lead to large changes in the adsorption behavior and structuring of the monolayers formed and the interactions of interfacial water molecules with these monolayers. Fundamental studies such as these are essential for understanding the mechanisms involved in the surfactant adsorption process, information that is important for industrially relevant processes such as mineral ore flotation, waste processing, and petroleum recovery.     

Dynamic behaviour at a nematic liquid crystal-rubbednylon interface using evanescent wave photon correlation spectroscopy

Photon correlation spectroscopy of light scattered by director fluctuations from an evanescent optical wave propagating in the nematic liquid crystal 5CB is used to study the interfacial dynamic behaviour of the liquid crystal. The intensity correlation function of light scattered by interfacial orientation fluctuations is measured by illuminating to give a short optical penetration depth within the nematic. These surface scattering correlation functions strongly differ from the bulk correlation function and are interpreted in terms of a nematic surface orientation mode arising from the coupling between the director field and the fluid velocity. It is shown that the analysis of the surface mode gives a method for measuring anchoring energies in liquid crystals. The anchoring energy obtained for rotation of the director away from the rubbing direction about an axis normal to the surface for 5CB at a rubbed nylon surface is 7.14±0.7 × 10-² ergcm-².     

Probing polymer surfaces and interfaces using sum frequency generation vibrational spectroscopy - a powerful nonlinear optical technique

Sum frequency generation (SFG) vibrational spectroscopy has been proved to be a powerful technique which substantially impacts on many research areas in surface and interfacial sciences. This paper reviews the recent progress of applying this nonlinear optical technique in the studies of polymer surfaces and interfaces. The theoretical background of SFG is introduced first. Current applications of SFG in polymer science are then described in more detail to demonstrate the significance of this technique. Finally, a short summary is presented on this relatively new but widely applicable spectroscopic technique.     

Measurement of the surface dilational viscoelasticity of adsorbed layers with a capillary pressure tensiometer

A capillary pressure tensiometer has been set up to measure the dilational surface viscoelasticity in liquid-air and liquid-liquid surfactant systems, according to the oscillating drop/bubble technique. A specific model which allows the dilational surface viscoelasticity to be inferred from the acquired pressure data is proposed and the critical points concerning the experimental procedure and the data interpretation are discussed. In order to optimize the method, side measurements utilizing the same tensiometer to evaluate equilibrium interfacial tensions and the system compressibility are coupled to this technique. Some nonionic surfactants, polyoxyethylene glycol ethers (C(i)EO(j)) and alkyl dimethyl phosphine oxide (C(12)DMPO), at water-air and water-hexane interfaces have been investigated by this technique. The measured dynamic dilational viscoelasticities are compared with the predictions of theoretical models which consider different adsorption mechanisms.     

Adsorption of Brij 35-dodecylpyridinium bromide mixtures at air-aqueous solution and Teflon-aqueous solution interfaces

Tensiometry, spectrophotometry, the radioactive-tracer technique, and contact angle measurements are employed to study the adsorption of mixtures of a nonionic surfactant Brij 35 and a cationic surfactant dodecylpyridinium bromide (DDPB) at interfaces between their solutions and air or Teflon. It is established that adsorption layers at both of the interfaces are enriched with Brij 35. Brij 35 adsorption on Teflon is nearly independent of the presence of DDPB. On the contrary, DDPB adsorption is enhanced in the presence of Brij 35. Negative deviations from the ideal behavior are revealed for mixtures with small fractions of Brij 35 with respect to a decrease in the interfacial tension in air-solution and Teflon-solution systems.     

Ellipsometric approach for the real-time detection of label-free protein adsorption by second harmonic generation

Second harmonic generation (SHG) was performed using a novel ellipsometric detection approach to selectively probe the real-time surface binding kinetics of an unlabeled protein. The coherence of nonlinear optical processes introduces new possibilities for exploiting polarization that are unavailable with incoherent methods, such as absorbance and fluorescence. Adsorption of bovine serum albumin (BSA) at silica/aqueous solution interfaces resulted in changes in the polarization state of the frequency-doubled light through weak, dynamic interactions with a coadsorbed nonlinear optical probe molecule (rhodamine 6G). Using a remarkably simple instrumental approach, signals arising exclusively from surface interactions with BSA were spatially isolated and selectively detected with high signal-to-noise. The relative intensities acquired during the kinetics experiments using both circularly and linearly polarized incident beams were in excellent agreement with the responses predicted from SHG ellipsometry polarization measurements. Analysis of the polarization-dependent SHG generated during BSA adsorption at glass/aqueous solution interfaces provided direct evidence for slow conformational changes within the protein layer after adsorption, consistent with protein denaturation. This polarization selection approach is sufficiently general to be easily extended to virtually all coherent nonlinear optical processes and a variety of different surface interactions and architectures.     

Measuring the optical chirality of molecular aggregates at liquid–liquid interfaces

Some new experimental methods for measuring the optical chirality of molecular aggregates formed at liquid–liquid interfaces have been reviewed. Chirality measurements of interfacial aggregates are highly important not only in analytical spectroscopy but also in biochemistry and surface nanochemistry. Among these methods, a centrifugal liquid membrane method was shown to be a highly versatile method for measuring the optical chirality of the liquid–liquid interface when used in combination with a commercially available circular dichroism (CD) spectropolarimeter, provided that the interfacial aggregate exhibited a large molar absorptivity. Therefore, porphyrin and phthalocyanine were used as chromophoric probes of the chirality of itself or guest molecules at the interface. A microscopic CD method was also demonstrated for the measurement of a small region of a film or a sheet sample. In addition, second-harmonic generation and Raman scattering methods were reviewed as promising methods for detecting interfacial optical molecules and measuring bond distortions of chiral molecules, respectively.