Attenuated Total Reflection Fourier Transform Infrared Results Research Articles

Mechanism behind polypropylene surface modifications by OH radicals: An experimental study

We quantitatively investigated the roles and mechanisms of OH radicals in polypropylene (PP) surface modification utilizing a previously proposed vacuum ultraviolet photodissociation reactive species supply method. Water contact angle (WCA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the modifications in surface hydrophilicity and components. The relationship between the WCA decline rate and OH radical density at the PP surface was quantified as (2.1 ± 0.1)[OHs] + (0.5 ± 0.1) degrees/s, where [OHs] is in ppm. The ATR-FTIR and XPS results demonstrated that C–OH, CO, C–O–O, and OCO groups were gradually introduced onto the PP surface at different treatment stages. The formation and behavior of low-molecular-weight oxidized materials (LMWOMs) on the PP surface during OH radical treatment were investigated. A modified layer composed of LMWOMs eventually formed on the PP surface. The physicochemical mechanisms underlying these phenomena at the micro-level were also discussed. A possible surface oxidation model was proposed based on WCA results, surface functional groups, and LMWOMs, indicating that the oxidation of the PP chains by OH radicals was initiated at the topmost surface and advanced in the depth direction.

Possible pathway of zeolite formation through alkali activation chemistry of metakaolin for geopolymer–zeolite composite materials: ATR-FTIR study

Geopolymer–zeolite composite materials are hybrid materials that combine the advantageous properties of both constituents. This study reports the effect of alkali concentration on the production of zeolite in geopolymer materials for synthesizing geopolymer–zeolite composite materials. The effects of the concentrations of activators on organic aluminosilicate raw materials based on metakaolin were studied. The phase changes were characterized by X-ray diffraction with qualitative phase amounts. Scanning electron microscopy was used to confirm the zeolite crystal and geopolymer. In situ and ex situ attenuated total reflectance–Fourier transform infrared (ATR-FTIR) were used to study the changes in the aluminosilicate structure with different curing times from 5 ​min until 112 days. The sodium hydroxide concentration played an important role in controlling the reaction mechanism of the metakaolin-based geopolymer. The Si/Al ratio was fixed at 1.01 with different Na/Al ratios from 1.18 to 3.05. The templating function of sodium hydroxide activators was responsible for the suppression of crystallization. The results show that sodalite zeolite, zeolite-A, and zeolite-X were produced in geopolymer composite materials with different concentrations of sodium hydroxide. Moreover, the increase in sodium hydroxide concentration promoted the higher dissolution and ordering structure that can be seen from the ATR-FTIR results.

Thermal-aging constitutive model for a silicone rubber foam under compression

Silicone rubber foam is a key function component in engineered systems. The long-term effect of temperature and compressive strain can degrade the mechanical properties of the foam. In this work, the temperature and aging time dependence of the mechanical behavior of the silicone rubber foam under long-term compressive strain was investigated by accelerated aging tests and uniaxial compression tests. The Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and crosslinking density were used to evaluate the chemical changes of the foam samples under different conditions. The ATR-FTIR and crosslinking density results indicate that crosslinking reaction and chain scission reaction occurred simultaneously. The crosslinking reaction dominated during the aging process resulting in the increase of the crosslinking density. The mechanical properties results show that the increase in the temperature and/or aging time can lead to the hardening of mechanical behaviors due to thermal aging-induced oxidative reactions. A thermal-aging constitutive model for the compressed foam was established by incorporating the dependence of the model parameters on temperature and aging time into the Hyperfoam model. Two types of exponential models were proposed to describe the dependence of model parameters on temperature and aging time. The performance of the thermal-aging constitutive model was verified by comparing the model predictions with independent testing data. Results show that the model can accurately describe the compressive stress-strain responses of the unaged and the aged foams under long-term compressive strain.

Coupled effects of temperature and compressive strain on aging of silicone rubber foam

The coupled effects of temperature and compressive strain on aging behaviors and mechanisms of silicone rubber foam were investigated by conducting the accelerated aging tests. The silicone rubber foams subjected to various levels of temperature and compressive strain were characterized by Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, micro hardness, volume fraction, compression set, continuous stress relaxation, mechanical properties, and thermogravimetric analysis (TGA) etc. The ATR-FTIR results indicate that the complex oxidation reactions (crosslinking, chain scission, and oxidation of side chains) occurred simultaneously with the formation of oxidation products. The high temperature facilitated the increase in crosslinking density and enhanced the thermal stability of the foams, implying that the oxidation crosslinking predominated over chain scission leading to the formation of a denser network structure. The physical and mechanical properties of the foams underwent significant changes due to the oxidation reactions and the irreversible collapse of the porous microstructure. The elevated temperature and the compressive stress both have a direct effect on the degradation of the foam. The elevated temperature played a more important role in changes of the physical-mechanical properties and microstructure.

Effect of plant sample preparation and measuring methods on ATR-FTIR spectra results

Attenuated Total Reflectance-Fourier-transform infrared (ATR-FTIR) spectroscopy is an effective tool for analysing the chemical composition of biological material, as well as distinguishing plant species. However, the results obtained from ATR-FTIR spectroscopy for the same plant differ between other researchers. This is related to the methodology used by scientists in the preparation of the material and its measurement. Therefore, it is very important to investigate the influence of sample preparation, as well as measuring methods for the ATR-FTIR spectra. In this work we determined the influence of (i) the preparation methods of homogeneous plant material and (ii) the influence of sample measuring methods on the obtained ATR-FTIR spectra of angiosperms (flowering plants), which were measured by ATR-FTIR spectroscopy. Moreover, (iii) we investigated which studied factor has greater influence on the obtained ATR-FTIR spectra. For this purpose, we measured, using ATR-FTIR spectroscopy, six different plant species representing a wide range of clearly distinctive plant functional groups, leaf phenology, as well as phylogeny. They were prepared by five different methods, and measured by two different methods. ATR-FTIR spectra, as well as Principal Components Analysis (PCA) and Linear Discriminant Analysis (LDA) analyses of attained spectra, showed that both sample preparation, as well as measurement methodology, has an influence on the obtained ATR-FTIR results. Interestingly, the results achieved showed that the chemical composition of plants should be made using fresh plants, while the interaction between the plants should be made using any prepared method. We found surprisingly higher impact of measuring then sample preparation method on ATR-FTIR spectra results. It seems very important finding from researchers point of view because they very rare pay attention and discuss issues connected with the impact of used sample preparation and measuring methods on their results.

Polymorphism and stability of nanostructures of three types of collagens from bovine flexor tendon, rat tail, and tilapia skin

Many types of collagens from different sources have been used in food, pharmaceutics, biomedicine, tissue engineering, etc. Their physicochemical properties have been widely investigated to understand their behaviors and functions. However, the polymorphism and stability of collagen nanostructures have not been systematically studied. In the current manuscript, polymorphism and stability of nanostructures of three types of collagens from bovine flexor tendon, rat tail, and tilapia skin are characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), atomic force microscopy (AFM), and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometry. SDS-PAGE results show loading volumes have no influence on the protein bands of three types of collagens except the intensity, whereas collagen concentrations have obvious effects. AFM results show all the three types of collagens have multiple nanostructures, which are concentration-dependent. AFM results also show collagen nanostructures change with incubation time at 37 °C. According to the ATR-FTIR results, the nanostructures changes are associated with the change of protein secondary structures. These results demonstrate three types of collagens have different nanostructures, stability, protein secondary structures, and SDS-PAGE behaviors. This work also indicates that the nanostructures and secondary structures of collagens can be controlled by adjusting concentration and incubation time for the three types of collagens, which provide simple ways to design and prepared desired nanostructures of collagen-based foods. It will be also beneficial to fundamental understanding of the collagen nanoscale structure formation in different collagen-based foods.

The Effect of Accelerated Aging on the Properties of Nitrile Butadiene Rubber (NBR) O-Rings

Abstract The degradation behaviors and mechanisms of nitrile butadiene rubber O-rings aged in air are investigated by accelerated aging tests at different temperatures. The compressive stress changes of nitrile butadiene rubber O-rings are monitored by using a self-designed test device during the degradation process. The aging samples under different conditions are evaluated by measuring attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, compressive stress relaxation, weight loss, mechanical properties, and fracture morphology. The results demonstrate that the compressive stress declines with aging time, and the higher the temperature is, the faster the stress decreases. The ATR-FTIR results indicate that amide groups and hydroxyl groups are formed during the degradation process. The elevated temperature plays a significant role in the surface chemical changes. The changes in the crosslinking density of the samples could be attributed to the competition between crosslinking and chain scission, and the crosslinking mainly predominates in the aging process. The results of mechanical properties show that the tensile strength, elongation at break, and hardness of samples changes significantly with aging time and temperature. In addition, the fracture morphology observation reveals that the formation of a hardened brittle outer layer, voids, and agglomerates lead to a further decrease in mechanical properties. These results are very important for evaluating the lifetime of rubber O-rings and can be utilized to improve the stability and durability of sealing systems.

The adsorption of nortriptyline on montmorillonite, kaolinite and gibbsite

Abstract Nortriptyline (NT) is a tricyclic antidepressant in common use in many countries. Its presence in drinking water indicates that current wastewater treatment practices fail to remove it effectively from effluents with consequent adverse effects on aquatic organisms. Since mineral surfaces readily remove many organic substance from effluent streams, this research sought to study the adsorption of NT on montmorillonite, kaolinite and gibbsite. Adsorption edge and isotherm experiments were performed to determine the relative effectiveness of the three substrates for NT removal, and X-ray diffraction (XRD) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used in order to gain an understanding of the mode of adsorption. Adsorption of NT on all three substrates was effectively independent of pH in the range from 3 to 12. Both montmorillonite and kaolinite proved to be excellent adsorbents of NT with maximum adsorption capacities of 19.3 and 6.4 μmol m−2 respectively at pH 6.0, but gibbsite adsorbed relatively little. The XRD and ATR-FTIR results indicated that NT formed as a multilayer on montmorillonite with no evidence of intercalation. Both electrostatic and hydrophobic interactions appeared to play important roles in the adsorption.

Degradation Behavior of Nitrile Rubber O-rings in Hydraulic Oil

The degradation behaviors of nitrile rubber O-rings immersed in hydraulic oil were investigated by adopting the artificial accelerated aging test. The chemical structures and mechanical properties of the aged samples under different conditions were evaluated by measuring the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results indicate that the amide groups and the hydroxyl groups were formed in the surface of the aged samples. The changes in the crosslinking density of the samples attributed to the competition between crosslinking and chain scission. The mechanical test results show that the tensile strength of samples immersion in oil changed remarkably for the different temperatures. In addition, the fracture morphology results demonstrate that the formation of the defects also led to the decrease in properties of the aged samples.

Influence of hydraulic oil on degradation behavior of nitrile rubber O-rings at elevated temperature

Abstract The degradation of nitrile rubber O-rings exposed to hydraulic oil and air were investigated by adopting accelerated aging test. The chemical structures and mechanical properties of the aged samples were evaluated by measuring attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results indicate that chemical structure changed significantly due to loss of additives and oxidation reactions. Hydraulic oil and temperature played a critical role in surface chemical changes. The crosslinking density changes were attributed to the competition between crosslinking and chain scission. The mechanical test results show that there existed large differences in changes of mechanical properties for samples under different conditions, which correlated to loss of fillers, the changes in crosslinking density and the formation of defects. Additionally, the fracture morphologies demonstrate that the formation of hardened brittle outer layer, voids and agglomerates also promoted the decrease in mechanical properties.

Colon-specific pulsatile drug release provided by electrospun shellac nanocoating on hydrophilic amorphous composites.

BackgroundColon-specific pulsatile drug release, as a combined drug controlled-release model, is a useful drug delivery manner for a series of diseases. New nanomedicines and related preparation methods are highly desired.MethodsWith diclofenac sodium (DS) as a model drug, a new type of structural nanocomposite (SC), in which composite polyvinylpyrrolidone (PVP)–DS core was coated by shellac, was fabricated via modified coaxial electrospinning. For comparison, traditional PVP–DS monolithic hydrophilic nanocomposites (HCs) were generated using a traditional blending process. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), water contact angle (WCA), and in vitro dissolution and ex vivo permeation tests were conducted to characterize the composites.ResultsSEM images demonstrated that both composites were linear nanofibers with smooth surface morphology and cross sections. TEM disclosed that the SCs had a thin shellac sheath layer of approximately 12 nm. XRD and ATR-FTIR results demonstrated that the crystalline DS was converted into amorphous composites with PVP because of favorable secondary interactions. WCA and in vitro dissolution tests demonstrated that the sheath shellac layers in SC could resist acid conditions and provide typical colon-specific pulsatile release, rather than a pulsatile release of HC under acid conditions. Ex vivo permeation results demonstrated that the SCs were able to furnish a tenfold drug permeation rate than the DS particles on the colon membrane.ConclusionA new SC with a shellac coating on hydrophilic amorphous nanocomposites could furnish a colon-specific pulsatile drug release profile. The modified coaxial process can be exploited as a useful tool to create nanocoatings.

Physical Mixture Interaction of Acetaminophenol with Naringenin

The interaction of Active Pharmaceutical Ingredient (API) with other compounds will affect drugs stability, toxicity, modified dissolution profiles or may form a new compound with the different crystal structure. Acetaminophenol (APAP), the most common drug used widely (also known as Panadol) was mixed with Naringenin (NR) to glance for a new phase of interactions leading to new compound phase. The amide-acid supramolecular heterosynthon; N-H…O interaction between acid and the respective base were observed in the APAP-NR mixture blends. The interaction was prepared by the binary interaction from neat grinding and liquid-assisted grinding techniques at a different stoichiometry of binary mixture ratio of APAP-NR which were 1:1, 1:2 and 2:1 molar ratio. The interaction was estimated using Group Contribution Method (GCM) and physicochemical properties were characterized by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR), powder X-ray diffraction (PXRD) and Differential Scanning Calorimetry (DSC) analysis. The GCM calculation gave good interaction strength at 212.93 MPa1/2. The ATR-FTIR, DSC and PXRD results obtained revealed an interaction with new phase formed.

Insights into Cocrystal Polymorphic Transformation Mechanism of Ethenzamide–Saccharin: A Combined Experimental and Simulative Study

In this work, the solvent-mediated phase transformation of the metastable form (Form II) to the stable form (Form I) of ethenzamide–saccharin cocrystal in isopropanol was investigated. Solubility of Form I and Form II in pure isopropanol was also measured. The transformation mechanism from Form II to Form I was analyzed by some online and off-line tools including attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, and polarizing microscopy. The results demonstrate that the transformation process consists of three steps, involving the dissolution of Form II, the nucleation of Form I, and the following growth of Form I. The ATR-FTIR and Raman results show that the polymorphic transformation from Form II to Form I is controlled by the nucleation and growth of Form I. Furthermore, the microscope photographs clearly reveal that the Form I preferably nucleates and grows on the (100) surface of Form II. The molecular simulation results indicate that higher adsorpt...

Effect of backing films on the transdermal delivery of cyclobenzaprine patch

The aim of this study was to investigate the effect of backing films on transdermal delivery of cyclobenzaprine patch. Different backing films were chosen to prepare the cyclobenzaprine transdermal patch. The cumulative amount of cyclobenzaprine released from different patches was evaluated in vitro. To investigate the interaction between cyclobenzaprine and backing films, the partitioning experiments and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were performed. The cumulative amount of cyclobenzaprine released from the patch with Cotran™ 9700 as backing film was less than that of other patches with different backing films. Furthermore, the cumulative amount of cyclobenzaprine released from the patch with Cotran™ 9700 as backing film decreased significantly after 7 d storage at room condition. The partitioning experiments indicated a strong adsorption of cyclobenzaprine onto the Cotran™ 9700, which could explain the decrease of cumulative amount of cyclobenzaprine released from the patch with Cotran™ 9700 as backing film. According to the ATR-FTIR results, there was no interaction between Cotran™ 9700 and cyclobenzaprine. The effect of backing films on the release behavior of cyclobenzaprine transdermal patch was attributed to the adsorption of cyclobenzaprine onto the Cotran™ 9700.

Spectroscopic Characterization of Dust-Fall Samples Collected from Greater Cairo, Egypt.

This work aimed to characterize dust-fall samples collected from street's trees in Greater Cairo (GC), Egypt, and its surroundings by different spectroscopic techniques, namely; X-ray diffraction (XRD), attenuated total-reflection Fourier transform infrared (ATR-FTIR), particle-size analyzer, and scanning electron microscopy (SEM) combined with energy dispersive X-ray measurements. Samples were collected from 19 different locations inside and outside of GC. Quantitative phase analysis of the dust-fall samples was performed using the Rietveld method. Results showed that the most frequently observed phases in the dust-fall samples were calcite (CaCO3), dolomite (CaMg(CO3)2), gypsum (CaSO4·2H2O), and quartz (SiO2) with average concentrations of 39 ± 16, 8 ± 7, 22 ± 13, and 33 ± 14 wt%, respectively. The occurrence of these constituents referred to a combination of different anthropogenic and natural sources. The ATR-FTIR results are in good agreements with XRD data of the different observed phases. Based on the SEM and particle-size measurements, quantitative determination of the particle-size distribution was described. It was found that not only the large-sized particles are deposited but also the small-sized ones (PM10 and PM2.5). In addition, the particle size of the collected dust-fall samples varied from 0.1 to 200 µm with an average particle size of 17.36 µm; however, the particle size ranged from 2.5 to 40 µm predominated in all of the dust-fall samples.

Adsorption of lactate and citrate on montmorillonite in aqueous solutions

Lactate and citrate were adsorbed on montmorillonite surface from aqueous solutions from pH values 2 to 12 in the presence of 10mmolL−1 KNO3. The adsorbed molecules were characterized using Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy at pH values 4, 6 and 8. Macroscopic adsorption measurements indicate that the adsorption of both ligands is highly pH dependent. ATR-FTIR results indicate that lactate is likely adsorbed by nonspecific electrostatic interactions, since no changes were found in peak position and shape of carboxyl group stretches. Nevertheless, ATR-FTIR results suggest that citrate is adsorbed in the >AlOH groups as an inner-sphere surface complex at low pH. The macroscopic adsorption behavior of the ligands was modeled as a function of pH by using the Diffuse Layer Model (DLM) for lactate and the Triple Layer Model (TLM) for citrate. Surface complexation of lactate and citrate was described with the complex >AlLac (logK=11.25) and the complex >AlCit2− (logK=10.58) respectively.These findings are of relevance for the evaluation of the catalytic effect that takes place during the dissolution of clay minerals in the presence of low-molecular-weight organic ligands.

Real-Time Monitoring of in Situ Polyethyleneimine-Silica Particle Formation

Silica particles are traditionally made via the hydrolysis and condensation of tetraalkoxysilanes with the use of methanol and ammonia as a basic catalyst. More recently, bioinspired polyamines have been used in place of ammonia. Particle formation via the use of tetraalkoxysilanes typically occurs extremely quickly with cloudy precipitates forming immediately, making it practically impossible to characterize the reaction in real time. Our study uses trimethoxymethylsilane (TMOMS) and the polyamine polyethyleneimine (PEI) to form PEI-silica particles via a reaction that takes place over several minutes, allowing us to study the reaction in real time. The acidic hydrolysis of TMOMS and basic polymerization condensation of TMOMS via PEI to form solid PEI-silica particles were observed in situ over time using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and dynamic light scattering (DLS) for the first time. The ATR-FTIR data suggest that dimer formation occurs during acidic hydrolysis followed by PEI-catalyzed condensation to form silsesquioxane structures. The results for the particles formed in situ were then compared with those for particle samples that had been washed to remove excess reactants. The ATR-FTIR results were corroborated via scanning electron microscopy and DLS, which suggest that the growth of PEI-silica particles occurs by aggregation of smaller particles to larger ones, because the data show the presence of small particles and much larger particles at the same time throughout the whole particle growth process.

Degradation of the elastomeric gasket material in a simulated and four accelerated proton exchange membrane fuel cell environments

Long-term stability and durability of gaskets in Proton Exchange Membrane (PEM) fuel cell are critical to both sealing and the electrochemical performance of the PEM fuel cell. In this paper, the time-dependent chemical degradation of the silicone rubber, which is one of the potential gasket materials for PEM fuel cells, is studied in a simulated PEM fuel cell environment and four accelerated durability test (ADT) media for short-term aging tests at 70°C. The weight loss of samples is monitored. The optical microscopy is used to show the topographical changes on the sample surface. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) are employed to study the surface chemistry of the samples before and after exposure to the test environments over time. The results show that the weight loss increases with the exposure time. The optical microscopy reveals that the surface conditions of the samples changes from initially smooth to rough, crack appearance and finally crack propagation. The ATR-FTIR and XPS results show that the surface chemistry changes significantly via de-crosslinking and chain scission in the backbone for the samples exposed to the environments over time.

Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

The adsorption and conformation of bovine serum albumin (BSA) on gold nanoparticles (AuNPs) were interrogated both qualitatively and quantitatively via complementary physicochemical characterization methods. Dynamic light scattering (DLS), asymmetric-flow field flow fractionation (AFFF), fluorescence spectrometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy were combined to characterize BSA-AuNP conjugates under fluid conditions, while conjugates in the aerosol state were characterized by electrospray-differential mobility analysis (ES-DMA). The presence of unbound BSA molecules interferes with DLS analysis of the conjugates, particularly as the AuNP size decreases (i.e., below 30 nm in diameter). Under conditions where the γ value is high, where γ is defined as the ratio of scattering intensity by AuNPs to the scattering intensity by unbound BSA, DLS size results are consistent with results obtained after fractionation by AFFF. Additionally, the AuNP hydrodynamic size exhibits a greater proportional increase due to BSA conjugation at pH values below 2.5 compared with less acidic pH values (3.4-7.3), corresponding with the reversibly denatured (E or F form) conformation of BSA below pH 2.5. Over the pH range from 3.4 to 7.3, the hydrodynamic size of the conjugate is nearly constant, suggesting conformational stability over this range. Because of the difference in the measurement environment, a larger increase of AuNP size is observed following BSA conjugation when measured in the wet state (i.e., by DLS and AFFF) compared to the dry state (by ES-DMA). Molecular surface density for BSA is estimated based on ES-DMA and fluorescence measurements. Results from the two techniques are consistent and similar, but slightly higher for ES-DMA, with an average adsorbate density of 0.015 nm(-2). Moreover, from the change of particle size, we determine the extent of adsorption for BSA on AuNPs using DLS and ES-DMA at 21 °C, which show that increasing the concentration of BSA increases the measured change in AuNP size. Using ES-DMA, we observe that the BSA surface density reaches 90% of saturation at a solution phase concentration between 10 and 30 μmol/L, which is roughly consistent with fluorescence and ATR-FTIR results. The equilibrium binding constant for BSA on AuNPs is calculated by applying the Langmuir equation, with resulting values ranging from 0.51 × 10(6) to 1.65 × 10(6) L/mol, suggesting a strong affinity due to bonding between the single free exterior thiol on N-form BSA (associated with a cysteine residue) and the AuNP surface. Moreover, the adsorption interaction induces a conformational change in BSA secondary structure, resulting in less α-helix content and more open structures (β-sheet, random, or expanded).

Surface Complexation of 2,5-Dihydroxybenzoic Acid (Gentisic Acid) at the Nanosized Hematite−Water Interface: An ATR-FTIR Study and Modeling Approach

In this study, characteristic interactions of 2,5-dihydroxybenzoic acid (or gentisic acid, GA) with the surface of 15-nm-sized hematite (α-Fe2O3) were studied by combining batch macroscopic experiments, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic investigations, and surface complexation modeling. A correlation between the pH, the amount of adsorbed GA, and the amount of Fe(III) released from the hematite surface was observed, whereas the dissolution of hematite nanoparticles became significant only at low pH and high ligand loading. From the ATR-FTIR results, two aqueous complex structures have been identified depending on pH. At the hematite-water interface, the occurrence of one deprotonated inner-sphere "bidentate" complex and one outer-sphere complex was suggested through all of the investigated pH range. At high surface coverage, variations of vibrational band intensities were observed, suggesting the occurrence of nonspecific molecular interactions. The macroscopic results (i.e., GA batch sorption and the ligand-promoted dissolution of hematite) obtained under a wide range of experimental conditions corroborated the ATR-FTIR microscopic findings. GA adsorption was described by a surface complexation model fitted to pH-adsorption curves with 1 mM sorbate concentration in the pH range of 3-9. Two surface complexes (one outer-sphere species (≡FeOH2)2···H2L((1+,1-)) and one inner-sphere species (≡Fe)2H2L) were proposed using the three-plane model. The inner-sphere complexes were predominant at low pH values, and the relative concentrations of the outer-sphere species increased with the pH increase. The formation of inner-sphere complexes at acidic pH values can promote the dissolution of nanosized hematite. At high solute loading, GA oxidation into carboxybenzoquinone compounds by ferric species was suspected, suggesting the occurrence of a redox reaction analogous to that of hydroquinone compounds.