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We present a study of four monoterpene isomers (limonene, γ-terpinene, terpinolene, and α-pinene) that are prevalent in indoor environments and their interaction with the hydroxylated SiO2 surface, a model for the glass surface, by combining infrared spectroscopy and computational simulations. These isomers are molecularly adsorbed onto SiO2 through π-hydrogen bonds with surface hydroxyl groups. However, experimental results suggest that the strength of interaction of these compounds with the SiO2 surface varies for each isomer, with α-pinene showing the weakest interaction. This observation is supported by molecular dynamics simulations that α-pinene adsorbed on the SiO2 surface has lower free energy of desorption and a lower mass accommodation coefficient compared to other isomers. Additionally, our ab initio molecular dynamics simulations show lower π-hydrogen bonding probabilities for α-pinene compared to the other three constitutional isomers. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces and, thus, provide a thorough framework for comparing the interactions of organic molecules on indoor relevant surfaces. [ABSTRACT FROM AUTHOR]

The structures of cis-1,2- and trans-1,2-dichloroethene (DCE) adsorbed on the Pt(111) surface have been examined by electron energy loss and thermal desorption spectroscopies over the temperature range 110 to 300 K. For monolayer and submonolayer coverages, the DCEs form sigma bonds to the platinum surface and assume staggered, alkane-like structures. Cis-DCE gives the gauche isomer with its most intense EEL peaks at 650 and 2985 cm-1, while the trans-DCE gives the trans conformer with its most intense peaks at 720 and 3000 cm-1. Isomerization does not occur. On warming to 160 K, the multilayer desorbs and above 200 K, dehalogenation occurs. A prominent EEL peak around 290 cm-1 shows that the chlorine atoms remain bonded to the surface. For both cis- and trans-DCE, the remainder of the EEL spectrum is that of C2H2 bonded to the surface in an olefinic structure signaled by prominent peaks at 770–785 and 3010–3020 cm-1. [ABSTRACT FROM AUTHOR]

Given the importance of nanoparticle surface composition in nanotoxicology, analytical tools that can probe nanoparticle surfaces in aqueous media are crucial but remain limited. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy is a technique capable of in situ characterization of the liquid–solid interface to probe surface adsorption on nanoparticle surfaces in environmentally and biologically relevant media. Furthermore, given that the interfacial region in these media is dynamic, ATR-FTIR spectroscopy facilitates monitoring these dynamics by interrogating a layer of immobilized nanoparticles coated on the ATR element while changing the overlying aqueous phase. The molecular information acquired from this technique allows for the determination of the adsorption mode, including conformational and structural changes of the coordinating ligand, and can directly measure ligand displacement reactions. Furthermore, in some cases, ATR-FTIR spectroscopy can be used as a quantitative surface analytical tool. In this article, we briefly review the fundamentals of the technique and then provide several examples of using ATR-FTIR spectroscopy to probe nanoparticle surfaces in general with respect to: (i) the adsorption of different environmentally and biologically relevant coordinating ligands; (ii) competitive ligand adsorption and; (iii) the determination of kinetic and thermodynamic parameters. We have also investigated surface adsorption of TiO2 nanoparticles in different biological media typically used for toxicity studies and show that the surface composition of TiO2 nanoparticles depends to a large extent on the composition of the medium due to surface adsorption. This result has important implications for the interpretation of toxicity data as well as inter-comparisons between toxicity studies. [ABSTRACT FROM AUTHOR]

Citric acid plays an important role as a stabilizer in several nanomaterial syntheses and is a common organic acid found in nature. Here, the adsorption of citric acid onto TiO2 anatase nanoparticles with a particle diameter of ca. 4 nm is investigated at circumneutral and acidic pHs. This study focuses on both the details of the surface chemistry of citric acid on TiO2, including measurements of surface coverage and speciation, and its impact on nanoparticle behavior. Using macroscopic and molecular-based probes, citric acid adsorption and nanoparticle interactions are measured with quantitative solution phase adsorption measurements, attenuated total reflection-FTI R spectroscopy, dynamic light scattering techniques, and zeta-potential measurements as a function of solution pH. The results show that surface coverage is a function of pH and decreases with increasing pH. Surface speciation differs from the bulk solution and is time dependent. After equilibration, the fully deprotonated citrate ion is present on the surface regardless of the highly acidic solution pH indicating pKa values of surface adsorbed species are lower than those in solution. Nanoparticle interactions are also probed through measurements of aggregation and the data show that these interactions are complex and depend on the detailed interplay between bulk solution pH and surface chemistry. [ABSTRACT FROM AUTHOR]

The surface chemistry of monoethanolamine (MEA) on oxide surfaces is investigated by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy under different environmental conditions of pH, concentration, and ionic strength. [Display omitted] Chemical contaminants are becoming an increasingly greater concern for water quality and it is well known that interactions with geochemical interfaces impact the fate and transport of these contaminants in the environment. In this study, we investigated the interactions of one such chemical contaminant, monoethanolamine (MEA), with oxide surfaces, particularly titanium dioxide (TiO 2) and iron oxide (α-Fe 2 O 3). Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy was used to probe the adsorption behavior of MEA on titanium dioxide (TiO 2) and iron oxide (α-Fe 2 O 3) nanoparticles as a function of pH and other environmental conditions including concentration and ionic strength. Both the extent and initial rates of adsorption of MEA on these oxide surfaces increases with increasing pH. Adsorption on these oxide surfaces increases with solution concentration until saturation occurs and MEA adsorbs more readily at higher pH. Furthermore, adsorption decreases with increasing ionic strength, demonstrating the importance of electrostatic interactions to this process. Based on these results, a mechanistic picture emerges for the interaction of MEA with titanium dioxide and iron oxide across a range of pH values. Overall, this study provides important insights into the surface chemistry and interactions between an alkanolamine and geochemical oxide interfaces. [ABSTRACT FROM AUTHOR]

TiO 2 nanoparticles (NPs) are widely used in different applications, and potential exposure to these NPs raises concerns about their impact on human health. In contact with biological fluids, proteins adsorb onto NPs to create a protein corona. Protein adsorption is highly dependent on the affinity between exterior amino acid residues and the NP surface. Thus, studying amino acids adsorption onto NPs can provide insight into protein corona formation. Herein, the pH-dependent adsorption of α-amino acids onto TiO 2 NPs in buffered solutions is described. Methods include attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy to analyze molecular interactions and dynamic light scattering (DLS) to measure changes in size and zeta-potential upon adsorption. Depending on the predominant speciation and TiO 2 NP surface charge, adsorption involves a combination of carboxylate and amine group interactions. Gly and Lys reveal a similar trend of higher adsorption with increasing pH. In contrast, Glu adsorption decreases with increasing pH. Ser adsorption onto TiO 2 NPs surfaces is the highest around pH IEP. These differences are attributed to the different speciation of the functional groups within the amino acids and the TiO 2 surface charge at each pH. Under our experimental conditions, multiple surface species coexist at different pH values. Protonated surface species are present for all amino acids at pH 2. At pH 9, Lys and Glu adsorbate spectra have new peaks at 1740 cm−1 and 1744 cm−1, respectively. This is a possible result of surface-induced deprotonation of the amine group and proton transfer to the carboxylate. Analyzing the pH-dependent adsorption of amino acids can provide a better understanding of biomolecule-surface interactions in in vivo and different biological milieu. [ABSTRACT FROM AUTHOR]

Here we compare the thermostability of different proteins adsorbed onto titanium dioxide nanoparticle surfaces using ATR-FTIR spectroscopy. Adsorbed BSA protein displayed a different thermostability compared to solution phase whereas Fib protein did not. This is attributed to the very different interactions between BSA and Fib protein with the titanium dioxide nanoparticle surface. Protein adsorption on metal oxide nanoparticle surfaces is crucial to the behavior of oxide nanoparticles in biological systems. In this study, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used for the first time to probe differences in the temperature-dependent conformational changes of two plasma proteins, bovine serum albumin (BSA) and fibrinogen (Fib), in solution and adsorbed on titanium dioxide nanoparticle (ca. 20 nm in diameter) surfaces. ATR-FTIR spectra and autocorrelation moving window two-dimensional correlation (MW2D) spectroscopy were used to identify the denaturation temperature range of these two proteins in aqueous solution and adsorbed. Generalized two-dimensional correlation spectroscopy (2DCOS) was done to investigate the protein unfolding pathway. The results show that the thermostability of BSA changed when adsorbed on the surface whereas there was no difference for solution phase Fib and adsorbed on TiO 2. The most notable fact was that solution BSA underwent significant structural changes upon adsorption whereas adsorbed Fib did not. Upon heating adsorbed BSA, no further changes occurred while BSA in solution thermally denatured. In contrast, solution and adsorbed Fib had identical denaturation temperatures and similar sequential secondary structural changes with increasing temperature. These studies show quantitatively, for the first time, that there are differences in the temperature-dependent structural changes of proteins adsorbed on nanoparticles surfaces and these changes depend on the initial interaction with the nanoparticle surface. Overall, the results of this study provide new insights into understanding the effects of temperature on the structure of adsorbed protein on nanoparticle surfaces and the range of different behaviors that can occur. [ABSTRACT FROM AUTHOR]

The adsorption of nitric (HNO3) and formic (HCOOH) acids on silica particle surfaces and the effect of adsorbed water have been investigated at 296 K using transmission FTIR spectroscopy. Under dry conditions, both nitric and formic acids adsorb reversibly on silica. Additionally, the FTIR spectra show that both of these molecules remain in the protonated form. At elevated relative humidities (RH), adsorbed water competes both for surface adsorption sites with these acids as well as promotes their dissociation to hydronium ions and the corresponding anions. Compared to HNO3, the extent of dissociation is much smaller for HCOOH, very likely because it is a weaker acid. This study provides valuable insights into the interaction of HNO3 and HCOOH with silica surface on the molecular level and further reveals the complex roles of surface-adsorbed water in atmospheric heterogeneous chemistry of mineral dust particles-many of these containing silica. [ABSTRACT FROM AUTHOR]

Abstract: In this study, FTIR spectroscopy and flow reactor measurements are used to investigate the adsorption, desorption and thermal oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (2-CEES), on nanocrystalline zeolites. Adsorption, desorption and thermal oxidation of 2-CEES on nanocrystalline NaZSM-5 are compared to that on nanocrystalline silicalite-1, the purely siliceous form of ZSM-5. It is shown here that NaZSM-5 is more reactive toward 2-CEES compared to silicalite. Furthermore, it is shown that the total 2-CEES adsorption capacity increases as the nanocrystalline particle size decreases suggesting that adsorption occurs on the external surface as well as in the internal pores of zeolite. Both zeolites effectively adsorb 2-CEES, suggesting that 2-CEES adsorption and subsequent thermal oxidation may be a viable method for the decontamination of mustard gas. [Copyright &y& Elsevier]

Abstract: Environmental molecular surface science is an important and expanding area of current research. Here we give a brief review of the chemical nature of two representative oxide and carbonate surfaces, MgO(100) and CaCO3(104), under ambient conditions of temperature and relative humidity. Studies using a variety of spectroscopic techniques have shown that water readily dissociates on these surfaces under ambient conditions leaving a surface truncated with hydroxyl groups. For MgO, the surface stoichiometry is best represented as Mg(OH)2 whereas for CaCO3, it is Ca(OH)(CO3H). Water readily adsorbs on these hydroxylated surfaces. This adsorbed water layer plays an important role in the reactivity of MgO(100) and CaCO3(104) under ambient conditions. The adsorption of nitric acid, a trace gas found in the troposphere, on MgO(100) and CaCO3(104) surfaces under dry (<1% RH) and wet conditions (20–25% RH) demonstrates this point. Adsorption at higher relative humidity results in the formation of nitrate thin films that are on the order of 10–100 layers thick. Water uptake on these thin nitrate films results in several phase transitions as a function of relative humidity. Thus the results presented here are meant to demonstrate the importance of adsorbed water in the surface composition and surface reactivity of oxide and carbonate surfaces. [Copyright &y& Elsevier]

The interaction of a model protein, bovine serum albumin (BSA) with two different metal oxide nanoparticles, TiO 2 (∼22 nm) and SiO 2 (∼14 nm), was studied at both physiological and acidic pH. The pH- and nanoparticle-dependent differences in protein structure and protein adsorption were determined using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis (TGA). The results indicated that the surface coverage of BSA decreases with decreasing pH on both TiO 2 and SiO 2 surfaces, and BSA coverage is higher by a factor of ca. 3–10 times more on TiO 2 compared to SiO 2 . The secondary structure of BSA changes upon adsorption to either nanoparticle surface at both pH 7.4 and 2. At acidic pH, BSA appears to completely unfold on TiO 2 nanoparticles whereas it assumes an extended conformation on SiO 2 . These differences highlight for the first time the extent to which the protein corona structure is significantly impacted by protein-nanoparticle interactions which depend on the interplay between pH and specific nanoparticle surface chemistry. [ABSTRACT FROM AUTHOR]

Abstract: Iron-containing oxide nanoparticles are of great interest from a number of technological perspectives and they are also present in the natural environment. Although recent evidence suggests that particle size plays an important role in the dissolution of metal oxides, a detailed fundamental understanding of the influence of particle size is just beginning to emerge. In the current study, we investigate whether nanoscale size-effects are observed for the dissolution of iron oxyhydroxide under different conditions. The dissolution of two particle sizes of goethite, α-FeOOH in the nanoscale and microscale size regimes (herein referred to as nanorods and microrods), in aqueous suspensions at pH 2 is investigated. It is shown here that in the presence of nitrate, nanorods shows greater dissolution on both a per mass and per surface area basis relative to microrods, in agreement with earlier studies. In the presence of carbonate and phosphate, however, dissolution of α-FeOOH nanorods at pH 2 is significantly inhibited, despite the fact that these anions result in a three- to fivefold enhancement of the dissolution of microrods relative to the nitrate anion. Light scattering techniques and electron microscopy show that nanorod suspensions are less stable compared to microrod suspensions resulting in nanorod aggregation under conditions where microrods stay more dispersed. Furthermore, spectroscopic studies using ATR–FTIR spectroscopy show distinct differences in phosphate and carbonate adsorption on nanorods compared to microrods. These results demonstrate that aggregation and the details of surface adsorption are important in the dissolution behavior of nanoscale materials. [Copyright &y& Elsevier]

Recent studies have shown that organics can alter the water adsorption and cloud condensation nuclei (CCN) activity of common deliquescent species in the Earth''s atmosphere. However, very little is known about the effect of organics on water adsorption and CCN activity of insoluble nuclei, such as mineral dust aerosol. A large fraction of unidentified organic material in aerosol particles is composed of poly-acidic compounds resembling humic substances. The presence of these humic-like substances (HULIS) can alter the water adsorption and CCN activity of mineral dust aerosol. We have measured the CCN activity of model humic and fulvic acids and of mineral dust particles coated with these substances in the laboratory. We find that coatings of humic and fulvic acids on calcite particles significantly increases water adsorption compared to uncoated particles. CCN measurements indicate that humic- or fulvic acid-coated calcite particles are more CCN active than uncoated calcite particles. Additionally, thicker coatings of humic or fulvic acids appear to result in more efficient CCN activity. Thus, mineral dust particles coated with high molecular weight organic materials will take up more water and become more efficient CCN in the atmosphere than uncoated mineral dust particles, potentially altering the effect of mineral dust on the Earth''s climate. In addition to the experimental results, we have explored a newly modified Köhler theory for predicting the CCN activity of insoluble, wettable particles based on multi layer water adsorption measurements of calcite. [Copyright &y& Elsevier]