Monolayer Surface Coverage Research Articles

The effect of varying levels of surfactant on the reactive uptake of N<sub>2</sub>O<sub>5</sub> to aqueous aerosol

Abstract. Recent observations have detected surface active organics in atmospheric aerosols. We have studied the reaction of N2O5 on aqueous natural seawater and NaCl aerosols as a function of sodium dodecyl sulfate (SDS) concentration to test the effect of varying levels of surfactant on gas-aerosol reaction rates. SDS was chosen as a proxy for naturally occurring long chain monocarboxylic acid molecules, such as palmitic or stearic acid, because of its solubility in water and well-characterized surface properties. Experiments were performed using a newly constructed aerosol flow tube coupled to a chemical ionization mass spectrometer for monitoring the gas phase, and a differential mobility analyzer/condensation particle counter for determining aerosol surface area. We find that the presence of ~3.5wt% SDS in the aerosol, which corresponds to a monolayer surface coverage of ~2×1014 molecules cm-2, suppresses the N2O5 reaction probability, γN2O5, by approximately a factor of ten, independent of relative humidity. Consistent with this observation is a similar reduction in the rate of ClNO2 product generation measured simultaneously. However, the product yield remains nearly constant under all conditions. The degree of suppression is strongly dependent on SDS content in the aerosol, with no discernable effect at 0.1wt% SDS, but significant suppression at what we predict to be submonolayer coverages with 0.3–0.6wt% SDS on NaCl and natural seawater aerosols, respectively.

Quantitative Determination of the Speciation of Surface Vanadium Oxides and Their Catalytic Activity

A quantitative method based on UV-vis diffuse reflectance spectroscopy (DRS) was developed that allows determination of the fraction of monomeric and polymeric VO(x) species that are present in vanadate materials. This new quantitative method allows determination of the distribution of monomeric and polymeric surface VO(x) species present in dehydrated supported V(2)O(5)/SiO(2), V(2)O(5)/Al(2)O(3), and V(2)O(5)/ZrO(2) catalysts below monolayer surface coverage when V(2)O(5) nanoparticles are not present. Isolated surface VO(x) species are exclusively present at low surface vanadia coverage on all the dehydrated oxide supports. However, polymeric surface VO(x) species are also present on the dehydrated Al(2)O(3) and ZrO(2) supports at intermediate surface coverage and the polymeric chains are the dominant surface vanadia species at monolayer surface coverage. The propane oxidative dehydrogenation (ODH) turnover frequency (TOF) values are essentially indistinguishable for the isolated and polymeric surface VO(x) species on the same oxide support, and are also not affected by the Brønsted acidity or reducibility of the surface VO(x) species. The propane ODH TOF, however, varies by more than an order of magnitude with the specific oxide support (ZrO(2) > Al(2)O(3) >> SiO(2)) for both the isolated and polymeric surface VO(x) species. These new findings reveal that the support cation is a potent ligand that directly influences the reactivity of the bridging V-O-support bond, the catalytic active site, by controlling its basic character with the support electronegativity. These new fundamental insights about polymerization extent of surface vanadia species on SiO(2), Al(2)O(3), and ZrO(2) are also applicable to other supported vanadia catalysts (e.g., CeO(2), TiO(2), Nb(2)O(5)) as well as other supported metal oxide (e.g., CrO(3), MoO(3), WO(3)) catalyst systems.

Water Exclusion at the Nanometer Scale Provides Long-Term Passivation of Silicon (111) Grafted with Alkyl Monolayers

This work is a quantitative study of the conditions required for a long-term passivation of the interface silicon-alkyl monolayers prepared by thermal hydrosilyation of neat 1-alkenes on well-defined H-Si(111) surfaces. We present electrochemical capacitance measurements (C-U) in combination with ex situ atomic force microscopy (AFM) observations and X-ray photoelectron spectroscopy (XPS) measurements. Capacitance measurements as a function of the reaction time and XPS data reveal close correlations between the chemical composition at the interface and its electronic properties. A very low density of states is found if suboxide formation is carefully prevented. The monitoring of C-U plots and AFM imaging upon exposure of the sample in diverse conditions indicate that the initial electronic properties and structure of the interface are long-lasting only when the monolayer surface coverage is theta > 0.42. A model demonstrates that this threshold value corresponds to a monolayer with intermolecular channels narrower than approximately 2.82 A, which is equal to the diameter of a water molecule. Water exclusion from the monolayer promotes long-term passivation of the silicon surface against oxidation in air and water as well as perfect corrosion inhibition in 20% NH(4)F. We provide two criteria to assess when a sample is optimized: The first one is an effective dielectric constant <2.5, and the second one is a very characteristic energy diagram at open circuit potential.

Quantifying Surface Coverage of Colloidal Silica by a Cationic Peptide Using a Combined Centrifugation/Time-Resolved Fluorescence Anisotropy Approach

Recent experimental studies have shown that time-resolved fluorescence anisotropy (TRFA) is a promising methodology for in situ characterization of the surface modification of aqueous silica nanocolloids. Here we provide a more fundamental insight into the principle of this approach and discuss how the adsorption parameters for a cationic peptide, Lys-Trp-Lys (denoted using the standard shortform KWK), onto Ludox nanoparticles (NPs) are linked to the rotational dynamics of rhodamine 6G (R6G) dispersed in the KWK/Ludox mixture. First, the adsorption isotherm of KWK on hydrophilic controlled pore glass (CPG-3000) was obtained using the traditional centrifugation method, which provides the total molar amount of KWK per unit surface area of the silica. Assuming that both CPG and Ludox particles possess identical surface properties when suspended in the same aqueous buffer, both materials should also have identical adsorption properties. Thus, the adsorbed amount of KWK per unit area at a given total KWK concentration, as determined by the centrifugation method, can be plotted against the fractions of R6G anisotropy decay components at the same KWK concentration to relate the anisotropy components to the absolute surface coverage. Using this approach, it was determined that the concentration of KWK at which the CPG surface was saturated corresponded to the condition g = 0 in the R6G decay, where g is the fraction of the nondecaying anisotropy component. This condition means that there is no R6G bound to the fraction of Ludox NPs with a radius R > 2.5 nm at maximum KWK coverage, consistent with the adsorbed peptide forming a continuous layer on the Ludox surface. Hence, the g value obtained from TRFA analysis can be used to assess the absolute surface coverage of monolayer coatings on colloidal nanoparticles.

A Kinetic Model of the Formation of Organic Monolayers on Hydrogen-Terminated Silicon by Hydrosilation of Alkenes

We have analyzed a kinetic model for the formation of organic monolayers based on a previously suggested free radical chain mechanism for the reaction of unsaturated molecules with hydrogen-terminated silicon surfaces (Linford, M. R.; Fenter, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc 1995, 117, 3145). A direct consequence of this mechanism is the nonexponential growth of the monolayer, and this has been observed spectroscopically. In the model, the initiation of silyl radicals on the surface is pseudo first order with rate constant, ki, and the rate of propagation is determined by the concentration of radicals and unreacted Si-H nearest neighbor sites with a rate constant, kp. This propagation step determines the rate at which the monolayer forms by addition of alkene molecules to form a track of molecules that constitute a self-avoiding random walk on the surface. The initiation step describes how frequently new random walks commence. A termination step by which the radicals are destroyed is also included. The solution of the kinetic equations yields the fraction of alkylated surface sites and the mean length of the random walks as a function of time. In mean-field approximation we show that (1) the average length of the random walk is proportional to (kp/ki)1/2, (2) the monolayer surface coverage grows exponentially only after an induction period, (3) the effective first-order rate constant describing the growth of the monolayer and the induction period (kt) is k = (2ki kp)1/2, (4) at long times the effective first-order rate constant drops to ki, and (5) the overall activation energy for the growth kinetics is the mean of the activation energies for the initiation and propagation steps. Monte Carlo simulations of the mechanism produce qualitatively similar kinetic plots, but the mean random walk length (and effective rate constant) is overestimated by the mean field approximation and when kp >> ki, we find k approximately ki0.7kp0.3 and Ea = (0.7Ei+ 0.3Ep). However the most striking prediction of the Monte Carlo simulations is that at long times, t >> 1/k, the effective first-order rate constant decreases to ki even in the absence of a chemical termination step. Experimental kinetic data for the reaction of undec-1-ene with hydrogen-terminated porous silicon under thermal reflux in toluene and ethylbenzene gave a value of k = 0.06 min(-1) and an activation energy of 107 kJ mol(-1). The activation energy is in reasonable agreement with density functional calculations of the transition state energies for the initiation and propagation steps.

Size distributions and stability of toluene diluted heavy oil emulsions

AbstractThe sizes and stability of oil droplets created from various concentrations of heavy oil‐in‐toluene at a fixed oil:water ratio were investigated during turbulent flow in model process water. The Reynolds number (Re) ranged from 17,000 to 34,500 and was obtained by stirred tank mixing with a Rushton turbine. The droplet sizes were monitored using laser light scattering. Results showed that at high Re and low oil concentrations (that is, low drop‐surface coverage), breakage of the droplets was the dominant process, but as Re was reduced, coalescence was dominant. Droplets were less prone to breakage as the oil concentrations in toluene increased, and droplet sizes approached a steady state quickly during mixing. Their size distributions broadened and stability increased as heavy oil in toluene increased. Stability was attributed to a surface coverage by asphaltenes and the consequent interfacial elasticity that provided resilience to breakage. Equilibrium interfacial tension σE was determined by fitting a diffusion‐limited kinetic mathematical model to the data. The Gibbs adsorption model gave a monolayer surface coverage of 3 nm2/mol asphaltenes, consistent with other published results. High zeta potential of the droplets also hindered coalescence. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Characterization of alkanethiol-self-assembled monolayers-modified gold electrodes by electrochemical impedance spectroscopy

The structure and electrochemical properties of pure and mixed monolayers of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) self-assembled on a gold disk electrode (AuE) have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes containing a one-electron redox couple. These results are compared with those obtained by cyclic voltammetry (CV). The monolayer surface coverage was determined using impedance measurements through the charge-transfer resistance, and by the pore size model. Cyclic voltammetry considering gold oxide formation/removal, as well as the oxidation/reduction process of a redox probe, was also employed to calculate the surface coverage. Taking into account the stability of the studied monolayers during the electrochemical experiment, the comparison of the barrier to charge transfer at both, the modified and the unmodified electrodes, resulted to be an appropriate method for estimating the surface coverage percentage given by each of the considered monolayers. The obtained results demonstrated that the MPA-monolayer-coated electrode behaves like a microelectrode array, with pinholes acting as the microelectrodes. The average size and the separation of the pinholes, estimated by the pore size model, are 1.9 and 21 μm, respectively. However, MUA forms a closely packed monolayer that acts as an ionic insulator and is virtually defect-free. The adsorption kinetics of MPA and MUA SAMs on the gold electrode (AuE) was evaluated. As expected, mixed monolayers of MPA and MUA show characteristics of both, long and short-chain alkanethiol monolayers.

Reaction pathways on NiMo/Al 2O 3 catalysts for hydrodesulfurization of diesel fuel

The ranking of catalytic activities of several NiMo catalysts was performed using various model compounds: thiophene (TH), tetrahydrothiophene (ThTH), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). So-called type I and type II NiMo catalysts were prepared by varying Mo loading from below to above monolayer coverage. Type I and II differ most at high loading. Below monolayer surface coverage, type II active phase shows higher HG activity than type I. It is likely that type II active phase consists of higher stacking active phases, favoring the HG pathway. Consequently, type II catalyst shows higher HDS activity for TH and 4,6-DMDBT, as these sulfur compounds react mainly via HG pathway. Above monolayer surface coverage, type I active phase forms aggregated crystals on the support alumina, resulting in a low dispersion, whereas type II active phase can maintain high dispersion. Higher loading type I catalyst shows lower DDS activity and only slightly increased HG activity. Higher loading type II phase shows higher DDS and extremely high hydrogenation (HG) activity and, as a consequence, high HDS activity for TH and 4,6-DMDBT. In order to be able to generalize the results, we included a CoMo catalyst in the study. The results at first sight are not fully consistent. Although this catalyst shows a low HG activity in HDS of DBT, it shows a relatively high activity in HDS of 4,6-DMDBT. Two reaction schemes were examined, one of them taking into account a dehydrogenation step. Although a dehydrogenation step could occur under our experimental conditions, the main cause of ranking differences is the differences of susceptibility for NiMo and CoMo catalysts to H 2S.

Adsorption of Asphaltenes on Metals

The adsorption of Athabasca and Cold Lake C7-asphaltenes on stainless steel (304L), iron, and aluminum powders was measured using UV−vis spectrophotometry. The effects of resins, temperature, and n-heptane-to-toluene ratio were also investigated. In all cases, Langmuir (type I) isotherms were observed, indicating that asphaltenes saturated the available surface area for adsorption. The saturation adsorptions of the asphaltenes on metals (0.25−2.7 mg/m2) were of the same order of magnitude as adsorption of asphaltenes on minerals. The saturation adsorptions were less than the monolayer surface coverage observed on water-in-hydrocarbon emulsion interfaces, indicating that there are a limited number of adsorption sites on the metals. Higher molar saturation adsorptions were observed for resins and low molar mass asphaltenes, suggesting that adsorption was limited by the morphology of the metal surface. In general, higher mass saturation adsorptions were observed when asphaltenes self-associated to greater ex...

Electrochemical studies of 6-mercaptonicotinic acid monolayer on Au electrode

The electrochemical characteristics of self-assembled monolayers (SAMs) of 6-mercaptonicotinic acid (MNA) on gold electrode have been studied. The ionization of carboxyl groups of MNA monolayer has been investigated by voltammetric and Faradic impedance analysis using Fe ( CN ) 6 3 - / 4 - redox marker. The surface p K a of MNA was determined to be 5.5 and it does not vary with the ionic strength of supporting electrolyte and surface coverage of monolayer. The carboxyl groups of the monolayer are far apart and the in-plane interaction does not exist between the carboxyl groups. The surface p K a of MNA shifts to positive side when it is co-assembled with MPY on the Au surface due to the increase in the hydrophobicity of the monolayer. The negatively charged MNA monolayer favors the electrode reaction of positively charged hydrophilic Ru(NH 3) 2+ whereas it shows a sluggish voltammetric response for monopositive hydrophobic dopamine (DA). The electrostatic and hydrophobic effects control the selectivity of MNA monolayer towards DA and dihydroxyphenylacetic acid (DOPAC).

Alkyl Chain Conformation and the Electronic Structure of Octyl Heavy Chalcogenolate Monolayers Adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).

Surface Self-Diffusion of Organic Molecules Adsorbed in Porous Silicon

The pulsed field gradient nuclear magnetic resonance method has been employed to probe self-diffusion of organic guest molecules adsorbed in porous silicon with a 3.6 nm pore size. The molecular self-diffusion coefficient and intrapore adsorption were simultaneously measured as a function of the external vapor pressure. The latter was varied in a broad range to provide pore loading from less than monolayer surface coverage to full pore saturation. The measured diffusivities are found to be well-correlated with the adsorption isotherms. At low molecular concentrations in the pores, corresponding to surface coverages of less than one monolayer, the self-diffusion coefficient strongly increases with increasing concentration. This observation is attributed to the occurrence of activated diffusion on a heterogeneous surface. Additional experiments in a broad temperature range and using binary mixtures confirm this hypothesis.

High-density self-assembled GaN nanoislands on SiC (0001) by molecular-beam epitaxy

We report the fabrication of self-assembled coherent GaN nanoislands on Ga-adsorbed √3×√3R30° 6H–SiC(0001) surfaces via Stranski–Krastanov (SK) growth mode by molecular-beam epitaxy at 760°C. The initial GaN deposition reveals a stable two-dimensional growth (2D) mode under Ga-rich growth conditions (Ga∕N⪢1), as evidenced by the variation of in-plane misfit strain (Δa∕a0). We show that above ∼2.2 monolayer (ML) surface coverage, the 2D layer spontaneously transformed into SK islands by the accommodation of elastic strain in the epitaxial lattice misfit under a vacuum. The SK islands density is found to be ∼4×1011cm−2 and the mean height and diameter is 2±0.5nm and 20±5nm for 3.2 ML surface coverage, respectively.

Attachment of gold nanoparticles to glassy carbon electrode and its application for the direct electrochemistry and electrocatalytic behavior of hemoglobin

Gold nanoparticles have been attached onto glassy carbon electrode surface through sulfhydryl-terminated monolayer and characterized by X-ray photoelectron spectroscopy, atomic force microscopy, electrochemical impedance spectroscopy and cyclic voltammetry. The gold nanoparticles-attached glassy carbon electrodes have been applied to the immobilization/adsorption of hemoglobin, with a monolayer surface coverage of about 2.1 x 10(-10) mol cm(-2), and consequently obtained the direct electrochemistry of hemoglobin. Gold nanoparticles, acting as a bridge of electron transfer, can greatly promote the direct electron transfer between hemoglobin and the modified glassy carbon electrode without the aid of any electron mediator. In phosphate buffer solution with pH 6.8, hemoglobin shows a pair of well-defined redox waves with formal potential (E0') of about -0.085 V (versus Ag/AgCl/saturated KCl). The immobilized hemoglobin maintained its biological activity, showing a surface controlled electrode process with the apparent heterogeneous electron transfer rate constant (ks) of 1.05 s(-1) and charge-transfer coefficient (a) of 0.46, and displays the features of a peroxidase in the electrocatalytic reduction of hydrogen peroxide. A potential application of the hemoglobin-immobilized gold nanoparticles modified glassy carbon electrode as a biosensor to monitor hydrogen peroxide has been investigated. The steady-state current response increases linearly with hydrogen peroxide concentration from 2.0 x 10(-6) to 2.4 x 10(-4) M. The detection limit (3sigma) for hydrogen peroxide is 9.1 x 10(-7) M.

Grazing angle infrared microspectroscopy of micropatterned self-assembled monolayers

We report for the first time an infrared spectrum of an alkanethiolate coated microsized metal pattern. Sub-femtomole detection limit of octadecanethiolate is demonstrated using grazing angle Fourier Transform Infrared (GA-FTIR) microspectroscopy. The conformational ordering and surface coverage of alkanethiolate self-assembled monolayer on micropatterned gold substrates obtained using two different approaches are analyzed. Conventional solution incubation of pre-patterned gold on a silicon wafer provides infrared features similar to those reported in the literature on macrosized gold substrate. Infrared spectra showed that microcontact printing gives raise to lower density alkanethiolate monolayer.

Nanoparticles of varying hydrophobicity at the emulsion droplet-water interface: adsorption and coalescence stability.

The coalescence stability of poly(dimethylsiloxane) emulsion droplets in the presence of silica nanoparticles ( approximately 50 nm) of varying contact angles has been investigated. Nanoparticle adsorption isotherms were determined by depletion from solution. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Fully hydrated silica nanoparticles adsorb with low affinity, reaching a maximum surface coverage that corresponds to a close packed monolayer, based on the effective particle radius and controlled by the salt concentration. Adsorbed layers of hydrophilic nanoparticles introduce a barrier to coalescence of approximately 1 kT, only slightly reduce the coalescence kinetics, and form kinetically unstable networks at high salt concentrations. Chemically hydrophobized silica nanoparticles, over a wide range of contact angles (25 to >90 degrees ), adsorb at the droplet interface with high affinity and to coverages equivalent to close-packed multilayers. Adsorption isotherms are independent of the contact angle, suggesting that hydrophobic attraction overcomes electrostatic repulsion in all cases. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics: at or above monolayer surface coverage, stable flocculated networks of droplets form and, regardless of their wettability, particles are not detached from the interface during coalescence. At sub-monolayer nanoparticle coverages, limited coalescence is observed and interfacial saturation restricts the droplet size increase. When the nanoparticle interfacial coverage is >0.7 and <1.0, mesophase-like microstructures have been noted, the physical form and stability of which depends on the contact angle. Adsorbed nanoparticle layers at monolayer coverage and composed of a mixture of nanoparticles with different hydrophobisation levels form stable networks of droplets, whereas mixtures of hydrophobized and hydrophilic nanoparticles do not effectively stabilize emulsion droplets.

Growth and Evolution of Perylene Thin Films on Cu(110)

The growth of ordered perylene structures on a Cu(110) surface has been studied by low-energy electron diffraction and scanning tunneling microscopy. Submonolayer STM images show a centered structure with the molecular plane parallel to the substrate. A sequence of STM images indicates molecular diffusion even at room temperature. The monolayer-covered surface shows a c(8 × 4) periodicity, which transfers into a ( ) structure on annealing at 450 K. On further increasing the surface coverage at room temperature, on top of the c(8 × 4) structure, an ordered multilayer structure grows with molecular rows aligned along the 〈110〉 azimuth.

Quartz crystal microbalance: a useful tool for studying thin polymer films and complex biomolecular systems at the solution-surface interface.

The quartz crystal microbalance (QCM) is a simple, cost effective, high-resolution mass sensing technique, based upon the piezoelectric effect. As a methodology, the QCM evolved a solution measurement capability in largely analytical chemistry and electrochemistry applications due to its sensitive solution-surface interface measurement capability. The technique possesses a wide detection range. At the low mass end, it can detect monolayer surface coverage by small molecules or polymer films. At the upper end, it is capable of detecting much larger masses bound to the surface. These can be complex arrays of biopolymers and biomacromolecules, even whole cells. In addition, the QCM can provide information about the energy dissipating properties of the bound surface mass. Another important and unique feature of the technique is the ability to measure mass and energy dissipation properties of films while simultaneously carrying out electrochemistry on solution species or upon film systems bound to the upper electrode on the oscillating quartz crystal surface. These measurements can describe the course of electropolymerization of a film or can reveal ion or solute transport within a film during changes in the film environment or state, including the oxidation state for an electroactive film driven by the underlying surface potential. The past decade has witnessed an explosive growth in the application of the QCM technique to the study of a wide range of molecular systems at the solution-surface interface, in particular, biopolymer and biochemical systems. In this report, we start with a brief historical and technical overview. Then we discuss the application of the QCM technique to measurements involving micellar systems, self-assembling monolayers and their phase transition behavior, molecularly imprinted polymers, chemical sensors, films formed using the layer-by-layer assembly technique, and biopolymer films and point out the utility of the electrochemical capabilities of the technique to characterizing film properties, especially electroactive polymer films. We also describe the wide range of surface chemistries and attachment strategies used by investigators to bring about surface attachment and multi-layer interactions of these thin film systems. Next we review the wide range of recent applications of the technique to: studies of complex biochemical and biomimetic systems, the creation of protein and nucleic acid biosensors, studies of attached living cells and whole cell biosensor applications. Finally, we discuss future technical directions and applications of the QCM technique to areas such as drug discovery.

Gas–solid adsorption of selected volatile organic compounds on titanium dioxide Degussa P25

This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO 2 Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within 60 min of incubation. Adsorption isotherms, determined at a temperature ( T) of 298.2 K and relative humidities (RH) of 0.0% and 57.8% were found to be linear ( R 2>0.993, n=5), indicating that no monolayer surface coverage was reached in the concentration interval studied (0.02 mg l −1⩽C g⩽10.45 mg l −1 ). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression ln K=−( ΔU ads R −1) T −1+a RH+d(R 2>0.94,n=13) for 22% ⩽RH⩽90% and 283 K⩽T⩽313 K . The equilibrium adsorption coefficient K( l g −1) represents the equilibrium concentration ratio C solid ( mg g −1)/C g ( mg l −1) and Δ U ads is the internal energy of adsorption ( J mol −1) . At RH=0.0%, experimental K values were a factor 5–10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO 2 by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO 2 mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models.

Adsorption/desorption behaviour of cysteine and cystine in neutral and basic media: electrochemical evidence for differing thiol and disulfide adsorption to a Au(1 1 1) single crystal electrode

The adsorption/desorption behaviour of the thiol/disulfide redox couple, cysteine/cystine, was monitored at a Au(1 1 1) single crystal electrode. The monolayers were formed electrochemically from 0.1 M KClO 4 and 0.1 M NaOH solutions containing either the thiol or the disulfide. Distinct features in the adsorption potential were noted. An adsorption peak was observed in the cyclic voltammograms (CVs) from Au(1 1 1) in 0.1 M KClO 4 solutions containing cystine at −0.57 V vs. saturated calomel electrode. Under the same conditions, the CVs from solutions containing cysteine showed an adsorption peak at −0.43 V (0.14 V more positive than the corresponding peak from disulfide solutions). This showed that the thiol and disulfide species have different adsorption properties. Similar behaviour was observed in 0.1 M NaOH. Cyclic voltammetric and chronocoulometric data were employed to determine the surface coverage of the different monolayers. Cysteine solutions prepared in 0.1 M KClO 4 provided coverages of 3.0×10 −10 and 2.5×10 −10 mol cm −2 for the l and the d– l species, respectively as evaluated from the desorption peaks. Desorption of cystine in the same medium yielded coverages of 1.2×10 −10 mol cm −2 for both l and d– l solutions (or 2.4×10 −10 mol cm −2 in cysteine equivalents). Surface coverages obtained from Au(1 1 1) in 0.1 M NaOH corresponded to 3.9×10 −10 mol cm −2 for l-cysteine, and 1.2×10 −10 mol cm −2 (or 2.4×10 −10 mol cm −2 cysteine equivalents) for l and d–l cystine.