A. James McQuillan

Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters, Macraes mine, New Zealand

Abstract Processing waters contain up to 10 mg l−1 dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, Kads=(1.9±0.4)×104 M−1, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l−1. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm−1 (alkaline solutions) to 820 cm−1 (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation.

IN SITU INFRARED SPECTROSCOPIC ANALYSIS OF THE ADSORPTION OF RUTHENIUM(II)BIPYRIDYL DICARBOXYLIC ACID PHOTOSENSITISERS TO TIO2 IN AQUEOUS SOLUTIONS

Abstract Internal reflection spectroscopy has been used to obtain the in situ infrared spectra of ruthenium(II) bipyridyl dicarboxylic acid photosensitiser species adsorbed to Degussa P25 and sol-gel TiO 2 films from dilute aqueous solutions. The spectra indicate that adsorption is via the carboxylate groups which bridge surface titanium ions. Adsorption isotherms and binding constants have been derived from the spectral data. The ruthenium(II) cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) complex, which is an efficient sensitiser, has the highest binding constant. This suggests a link between the strength of surface binding and the solar energy conversion efficiency.

An in situ infrared spectroscopic investigation of adsorption of sodium dodecylsulfate and of cetyltrimethylammonium bromide surfactants to TiO2, ZrO2, Al2O3, and Ta2O5 particle films from aqueous solutions

An in situ infrared spectroscopic study is described of the adsorption of the ionic surfactants, sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB), from aqueous solution to metal oxide particle films. Thin porous metal oxide particle films were prepared by drying of aqueous colloidal dispersions on single internal reflection ZnSe prisms. Analysis of the spectra of SDS in both aqueous solution and adsorbed to positively charged TiO2, ZrO2, Al2O3 and Ta2O5 surfaces indicates that adsorption occurs primarily via electrostatic and hydrogen bonding interactions between the sulfate headgroup and the metal oxide surfaces. The adsorption of CTAB to negatively charged TiO2 and Ta2O5 was observed. The pH dependence of CTAB adsorption to TiO2 indicated that both electrostatic and hydrophobic interactions are important. Langmuir adsorption constants from isotherm data were similar for adsorption of CTAB on TiO2 at pH=6.3 and at pH=11.0 indicating that charge effects are not dominant in this pH range. Use of a single internal reflection prism allowed the displacement of interfacial water by surfactant adsorption to be clearly discerned in the spectra.

Attenuated total reflection infrared studies of oleate and trioctylphosphine oxide ligand adsorption and exchange reactions on CdS quantum dot films.

Ligand exchange reactions at the surface of oleate- and trioctylphosphine oxide (TOPO)-capped CdS quantum dots have been studied with attenuated total reflection infrared (ATR-IR) spectroscopy, using thin films deposited from organic solvent suspensions. The oleate and trioctylphosphine capping ligands were found to form highly ordered and densely packed monolayers on the CdS surface. Adsorbed oleate is coordinated to CdS in a chelating bidentate manner through the carboxylate functional group, while adsorbed trioctylphosphine oxide is coordinated though the P=O functional group and appears to have numerous adsorption environments on the CdS surface. Exposure of such films to aqueous solution was found to cause partial delamination of the films from the ATR prism interface which was reversible upon redrying. Ligand exchange reactions on the oleate- and trioctylphosphine-capped CdS films were studied in situ at room temperature by allowing the films to be exposed to dilute aqueous solutions of thiol-containing ligands. Oleate and trioctylphosphine oxide are both strongly adsorbed to the CdS surface, and ligand exchange with monothiol-containing ligands has been found to be highly dependent upon experimental conditions, in particular pH, where exchange is only observed at solution pH where the exchanging ligand is uncharged. This is attributed to the inability of a charged ligand to penetrate the hydrophobic polymethylene layer on the CdS surface.

Infrared Spectroscopic and DFT Vibrational Mode Study of Perfluoro(2-ethoxyethane) Sulfonic Acid (PES), a Model Nafion Side-Chain Molecule

We have used attenuated total reflection infrared (ATR-IR) spectroscopy to study the model compound perfluoro(2-ethoxyethane) sulfonic acid (PES) and the spectral changes induced by humidity variations to improve understanding of the IR spectrum of Nafion. This work was supported by density functional theory (DFT) calculations of the PES molecule and ion complexes to confirm assignments and determine local modes that contributed to specific absorptions in the IR spectrum. The work illustrates the difficulties of interpreting the spectrum of Nafion with several mixed modes being present. However, the loss of degeneracy in the -SO3- asymmetric stretching mode is clearly observed in difference spectra, and the use of DFT calculations provides an insight into changes induced by the variation in humidity.

Adsorption/desorption kinetics from ATR-IR spectroscopy. Aqueous oxalic acid on anatase TiO2.

Adsorption and desorption kinetics at the solid/solution interface have been monitored using attenuated total reflection infrared (ATR-IR) spectroscopy to evaluate this approach as an alternative to equilibrium (adsorption isotherm) measurements of adsorption affinity. The adsorption and desorption kinetics of oxalate ion to anatase TiO2 have been measured by using aqueous 1 x 10(-4) mol L(-1) oxalic acid solutions at pH 4 and thin films of TiO2 particles deposited on an internal reflection prism. The adsorption kinetics were obtained from the absorbance versus time behavior of major adsorbed oxalate infrared absorptions with flow of oxalic acid solution followed by flow of solution not containing oxalic acid to measure the desorption kinetics. Regression analysis of the desorption data based on Langmuir kinetics yielded three distinct pseudo-first-order rate constants with desorption half-lives of 300, 14, and 2 min, indicating the presence of three adsorbed oxalate species of different adsorption affinities. The most slowly desorbing and most strongly bound adsorbate species is likely to be a bidentate chelating oxalate ion from comparisons with the IR spectra of coordination compounds involving oxalate ligands. Regression analysis of the adsorption data was unable to yield the corresponding pseudo-first-order adsorption constants and prevented the calculation from kinetics data of Langmuir adsorption affinity constants. Measurement of adsorption and desorption kinetics by ATR-IR spectroscopy is expected to provide a relatively rapid means of assessing the presence of species of different adsorption affinities in systems in which their spectra are not well differentiated.

Influence of ionic strength and pH on the first 60 min of Pseudomonas aeruginosa attachment to ZnSe and to TiO2 monitored by ATR-IR spectroscopy

Establishing the factors which influence the attachment of bacteria to surfaces is important in both preventing and enhancing biofilm formation. The initial hour of attachment of Pseudomonas aeruginosa to ZnSe and to TiO2 from solutions of different ionic strength and pH was studied using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The TiO2 surface was prepared by dip-coating a ZnSe internal reflection element, which produced a ∼50 nm thick, continuous flat film. At pH 6.3 attachment was found to increase with ionic strength up to 0.03 mol l−1 but to decrease at 0.15 mol l−1. At an ionic strength of 0.003 mol l−1 attachment increased with pH from 4 to 6.3 to 10, but at ionic strength of 0.03 mol l−1 attachment was greater at pH 6.3 than at pH 10. The influence of ionic strength appears to be due to charge factors and/or related changes in the degree of extension of bacterial surface polymers. The complex trends in the influence of pH on attachment can not be explained solely in terms of bacterial and substrate charge, bacterial surface polymer extension or bacterial metabolic activity.

Mechanism of oxalate ion adsorption on chromium oxide-hydroxide from pH dependence and time evolution of ATR-IR spectra

Abstract A chromium (III) oxide-hydroxide colloid film has been used to model the passive surface of stainless steel in in situ ATR-IR studies of oxalate ion adsorption from aqueous oxalate solutions over a wide pH range. Studies of time and pH dependence of adsorption have been used to reveal a mechanism of adsorption proceeding through ionic, hydrogen bonded and coordinated oxalate species. A Langmuir adsorption constant of 4.5×10 4 M −1 was determined for the surface coordinated oxalate from an adsorption isotherm at pH=3.

The discovery of surface-enhanced Raman scattering

This article contains a personal account[1][1] of the sequence of events leading to the first observation of surface-enhanced Raman scattering (SERS)[2–4][2] in 1973 in the Chemistry Department at Southampton University. The article begins with a concise explanation of SERS as understood today and

Salt modulates bacterial hydrophobicity and charge properties influencing adhesion of Pseudomonas aeruginosa (PA01) in aqueous suspensions.

The influence on cell hydrophobicity of differential extension with ionic strength of lipopolysaccharide molecules (LPS), which exist as charged and uncharged polymers at the surface of the gram-negative bacterium Pseudomonas aeruginosa (PA01), has been investigated. Attenuated total reflection infrared (ATR-IR) spectral absorptions from a single layer of cells adsorbed to ZnSe increased in intensity with increasing NaCl concentration up to 0.1 mol L(-1). Dynamic contact angle measurements (Wilhelmy plate tensiometry) made with a ZnSe plate having an adsorbed cell layer and the adherence of the cells to hexadecane suggest that PA01 cells were most hydrophobic in contact with 0.1 mol L(-1) NaCl solutions. These data indicate a charge screening induced compression of the charged LPS polymers decreasing the cell-surface approach distance and increasing the cell hydrophobicity due to the greater surface predominance of the uncharged LPS polymers. Interestingly, adsorbed cell layers in 0.3 mol L(-1) NaCl had a lower IR absorption intensity, and PA01 cells suspended in 0.3 mol L(-1) were found to be more hydrophilic, indicating that other factors influence the cell-surface approach distance and hydrophobicity. The examination of cell electrophoretic mobility variation with NaCl concentration suggests that the compression of charged polysaccharides increases the polysaccharide charge density and may also reduce the flow of liquid through the polysaccharide layer affecting the effective potential at the interface, the cell hydrophobicity, and the cell-surface approach distance.