In situ ATR FTIR study of dextrin adsorption on anatase TiO2

Surface chemistry considerations of gangue dissolved species in the bastnaesite flotation system

Inefficient flotation of bastnaesite remains a challenge in the production of rare earth elements. This study aimed to investigate the dissolution and adsorption behaviour of species that are commonly released into bastnaesite flotation pulp from Ca/Ba-bearing gangue minerals. The influence and corresponding mechanisms on the bastnaesite mineral surface and collectors, namely sodium oleate (NaOL), were evaluated experimentally based on micro-flotation, zeta potentials, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-Ray photoelectron spectroscopy (XPS) analyses. The flotation recovery of bastnaesite significantly decreased from ∼95% to ∼25%, ∼15%, ∼80%, ∼25% when exposed to calcite, fluorite, barite, and mixed dissolved species, respectively. The zeta potential of bastnaesite was pH sensitive, indicating that H+ and OH- determine the surface potential of bastnaesite. Solution chemistry analyses revealed that the presence of the dissolved species differed at various pH values. In situ ATR-FTIR demonstrated the different effects of the dissolved species from calcite, fluorite, and barite on collector adsorption. The former two dissolved species mainly depressed the chemisorption of the NaOL monomers (RCOO‒), whereas calcite also affected the physical adsorption of the oleic acid molecular dimer (RCOOH·RCOO‒). Moreover, the barite dissolved species only affected the physical adsorption of the NaOL species. The results of XPS analysis revealed that dissolved species from these three gangues could pre-adsorbed onto bastnaesite and affected the interaction with the collector. Density functional theory calculations were employed to provide further theoretical insights into the interactions between the dissolved species from calcite, fluorite, and barite and NaOL.

Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

Characterization of dynamic processes occurring at the nanoparticle (NP) surface is crucial for developing new and more efficient NP catalysts and materials. Thus, a vast amount of research has been dedicated to developing techniques to characterize sorption equilibria. Over recent years, solution NMR spectroscopy has emerged as a preferred tool for investigating ligand-NP interactions. Indeed, due to its ability to probe exchange dynamics over a wide range of timescales with atomic resolution, solution NMR can provide structural, kinetic, and thermodynamic information on sorption equilibria involving multiple adsorbed species and intermediate states. In this contribution, we review solution NMR methods for characterizing ligand-NP interactions, and provide examples of practical applications using these methods as standalone techniques. In addition, we illustrate how the integrated analysis of several NMR datasets was employed to elucidate the role played by support-substrate interactions in mediating the phenol hydrogenation reaction catalyzed by ceria-supported Pd nanoparticles.

Adsorption of serine at the anatase TiO2/water interface: A combined ATR-FTIR and DFT study

Knowledge of the adsorption reactions between serine and minerals is critical to understanding the geochemical processes of amino acids (i.e., mobility, bioavailability, and degradation) in the environment. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) flow-cell measurements were used to distinguish the inner- and outer-sphere complexation and reveal the dynamic adsorption and desorption processes of each surface complex at the molecular level. Density functional theory (DFT) calculations were applied to determine the structures of the surface complexes and to justify the peak assignments of the serine dissolved in solution and adsorbed on TiO₂. The structures of interfacial serine were governed by pH conditions but were not affected by the changes in adsorption time and serine concentration. The ATR-FTIR spectra and the results of DFT calculations resolved two different bidentate inner-sphere coordination, involving the COO^- group of the serine zwitterion at pH 4-8 and the serine anion at pH 10. The dynamic adsorption processes of these two surface complexes conformed to the pseudo-second-order kinetic model. The stable inner-sphere complexation could not be entirely removed from the TiO₂ surface upon serine desorption. In addition to reducing the migration rate in the environment, the bidentate inner-sphere coordination contributes to the potential degradation of the serine NH₃⁺ and NH₂ groups. Our research provides new insights into serine adsorption and desorption, facilitating further understanding of the fate and transport of amino acids in the environment.

Screening of the Chemical Composition and Identification of Hyaluronic Acid in Food Supplements by Fractionation and Fourier-Transform Infrared Spectroscopy

Hyaluronic acid, together with collagen, vitamins or plant extracts, is a part of many cosmetic and food preparations. For example, this polysaccharide is used in formulation of many food supplements due to its protective effects on human health. In this work, the screening of the chemical composition of three chosen dietary supplements (powder, tablets and capsules) containing hyaluronic acid was carried out using Fourier-transform infrared spectroscopy. Because of the low amount of analyte in all these samples, it was isolated or concentrated prior to the analysis using a suitable sequential fractionation protocol. Individual isolation procedures were established for each sample based on their declared composition. Firstly, the major components such as collagen or vitamins were removed to obtain polysaccharide fractions by the enzymatic treatment and/or washing out with the appropriate solvents. In some cases, the water insoluble part was removed from the rest dissolved in water. Then, hyaluronic acid was precipitated with copper(II) cations and thus separated from the other polysaccharides. Finally, the analyte was identified in the enriched fractions by the characteristic vibrational bands. The amount of hyaluronic acid in the purified fractions was determined in three ways: gravimetrically, spectrophotometrically, and using isotachophoresis. The combination of the appropriate preparative and analytical steps led to the successful evaluation of chemical composition, finding and quantification of hyaluronic acid in all the studied samples.

ATR FTIR Study of the Interaction of TiO2 Nanoparticle Films with β-Lactoglobulin and Bile Salts

The technique of in situ particle film attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) has been used to probe the adsorption and coadsorption (sequential) of a common food protein (β-lactoglobulin, BLG) and two representative bile salts (taurocholic acid and glycocholic acid, abbreviated as TCA and GCA) onto the surface of titanium dioxide (TiO₂) nanoparticles. Evaluating of binding interactions between commonly used (historically now, in some countries) food additives and food components, as well as the body's own digestion chemicals, is a critical step in understanding the role of colloidal phenomena in digestion and bioavailability. TCA is found to adsorb onto TiO₂ but without any significant ability to be retained when it is not present in the aqueous phase. GCA is also found to adsorb via two distinct binding mechanisms, with one type of adsorbed species being resistant to removal. BLG adsorbs, is irreversibly bound, and has altered conformation when adsorbed at pH 2 (stomach conditions) to the conformation when adsorbed at pH 6.5 (small intestine conditions). This altered conformation is not interface-dependent and is mirrored in the solution spectra of BLG. Sequential coadsorption studies indicate that TCA and GCA adsorb onto TiO₂ nanoparticle surfaces and display similar degrees of reversibility and binding in the presence or absence of preadsorbed BLG.

In Situ Adsorption of Mixed Anionic/Cationic Collectors in a Spodumene-Feldspar Flotation System: Implications for Collector Design

Herein, we investigated the effects of mixed collectors with varying alkyl chain lengths and ligand types on the hydrophobicity of the spodumene-feldspar flotation system. Various collector-mineral interactions were compared using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy with two-dimensional correlation spectroscopy (2D-COS), in situ microcalorimetry, and X-ray photoelectron spectroscopy (XPS). The highest flotation separation performance can be achieved at a molar ratio of 6:1 and pH 8-9. The in situ microcalorimetry results revealed that the difference in the adsorption reaction heat of the mixed collector is larger than that of the single anionic collector. Moreover, the inconformity between the magnitude of adsorption reaction heat and the results observed for flotation recovery indicates that the heat of the reaction presumably involves the adsorption configurations of the collectors and the amounts adsorbed. In in situ ATR-FTIR with 2D-COS, it can be observed that octanohydroxamic acid/dodecylamine (OHA/DDA) is adsorbed much more intensely onto feldspar than onto spodumene due to the availability of more space on feldspar for the subsequent sorption of DDA after the prior bidentate chemisorption of OHA under alkaline conditions, whereas the sodium oleate (NaOL)/DDA adsorption sequence at pH 4-5 was the reverse of that at pH 8-9. Lastly, XPS was employed to provide further supplemental evidence for the bonding between these two minerals and single anionic/mixed collectors at the optimal pH of 8-9. In this study, the powerful in situ detection technologies can establish a new platform for exploring the underlying mechanism of new reagents at the solid-liquid interface. Moreover, the in-depth understanding related to the adsorption behavior of the mixed collector is beneficial for facilitating the selection and design of efficient and environmentally friendly flotation collectors with improved selectivity.

Adsorption of Carboxymethyl Cellulose onto Titania Particle Films Studied with in Situ IR Spectroscopic Analysis

Adsorption of carboxymethyl cellulose (CMC) in aqueous solution onto a titania nanoparticle film has been studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). CMC was adsorbed onto the positively charged titania surface in neutral, partially charged, and fully charged state. The response of the adsorbed polyelectrolyte layer was monitored upon changing the electrolyte pH and ionic strength. The degree of dissociation of the CMC increased upon adsorption onto the titania surface and changed with the surface coverage. Ionic strength change was observed to influence the degree of dissociation of the adsorbed CMC similar as when in solution. No significant peak shifts were observed in the spectrum of the adsorbed CMC during adsorption or in response to changing solution conditions; therefore, inner-sphere complexation between the carboxyl groups and the titania could not be confirmed. The effect of ion identity on the adsorption process was studied using soft and hard cations and mono- and divalent cations. The presence of a divalent counterion was observed to cause changes in the carboxymethyl vibrations, which can be related to formation of intra- or interchain linkages.

Preparation and characterization of nanocomposite films based on gum arabic, maltodextrin and polyethylene glycol reinforced with turmeric nanofiber isolated from turmeric spent

Turmeric nanofibers (TNF) were used as reinforcement in the gum arabic (GA), maltodextrin (MDX) and polyethylene glycol (PEG) matrices to enhance the physicochemical properties. The TNF were prepared from turmeric spent by acid hydrolysis accompanied by high pressure homogenization. The thermal and mechanical properties, structure morphology and antimicrobial activities of the prepared nanocomposites were investigated. Differential scanning calorimetry (DSC) data indicate that the addition of TNF significantly increased the onset temperature (T_o), peak temperature (T_p) and conclusion temperature (T_c) of the melting peaks of nanocomposites, but considerably decreased the enthalpy change values. The tensile properties showed that the addition of TNF enhanced mechanical properties due to the formation of networks within the GA, MDX and PEG. The scanning electron microscopy (SEM) images revealed the films of GA-TNF and MDX-TNF show smooth, homogenous surface due to intermolecular hydrogen bonding, and the film of PEG-TNF shows good dispersion of TNF with PEG matrix with rough surface because of strong interfacial adhesion between TNF and PEG and strong hydrogen bonding, which are further confirmed by the FT-IR spectroscopy. XRD results exhibited the disappearances of peaks of TNF indicating the reinforcement of TNF in the prepared nanocomposite matrices. The antibacterial tests show the prepared nanocomposites exhibited excellent antibacterial performance against Bacillus cereus, Escherichia coli, Staphylococcus aureus and Salmonella typhimurium.

Degradation and Mineralization of Benzohydroxamic Acid by Synthesized Mesoporous La/TiO₂

Rare earth element La-doped TiO₂ (La/TiO₂) was synthesized by the sol-gel method. Benzohydroxamic acid was used as the objective pollutant to investigate the photocatalytic activity of La/TiO₂. The physicochemical properties of the prepared materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, specific surface area and porosity, scanning electron microscopy and transmission electron microscopy. As a result, the doping of La could inhibit the crystal growth of TiO₂, increase its specific surface area and expand its response to visible light, thus improving its photocatalytic activity. La/TiO₂ with the doping ratio of 0.75% calcined at 500 °C, showing the highest photocatalytic activity to degrade benzohydroxamic acid under the irradiation of 300 W mercury lamp. About 94.1% of benzohydroxamic acid with the original concentration at 30 mg·L^-1 was removed after 120 min in a solution of pH 4.4 with an La/TiO₂ amount of 0.5 g·L^-1. Furthermore, 88.5% of the total organic carbon was eliminated after 120 min irradiation. In addition, after four recycling runs, La/TiO₂ still kept high photocatalytic activity on the photodegradation of benzohydroxamic acid. The interfacial charge transfer processes were also hypothesized.

An Underwater Surface-Drying Peptide Inspired by a Mussel Adhesive Protein

Water hampers the formation of strong and durable bonds between adhesive polymers and solid surfaces, in turn hindering the development of adhesives for biomedical and marine applications. Inspired by mussel adhesion, a mussel foot protein homologue (mfp3S-pep) is designed, whose primary sequence is designed to mimic the pI, polyampholyte, and hydrophobic characteristics of the native protein. Noticeably, native protein and synthetic peptide exhibit similar abilities to self-coacervate at given pH and ionic strength. 3,4-dihydroxy-l-phenylalanine (Dopa) proves necessary for irreversible peptide adsorption to both TiO2 (anatase) and hydroxyapatite (HAP) surfaces, as confirmed by quartz crystal microbalance measurements, with the coacervate showing superior adsorption. The adsorption of Dopa-containing peptides is investigated by attenuated total reflection infrared spectroscopy, revealing initially bidentate coordinative bonds on TiO2, followed by H-bonded and eventually long-ranged electrostatic and Van der Waals interactions. On HAP, mfp3s-pep-3Dopa adsorption occurs predominantly via H-bond and outer-sphere complexes of the catechol groups. Importantly, only the Dopa-bearing compounds are able to remove interfacial water from the target surfaces, with the coacervate achieving the highest water displacement arising from its superior wetting properties. These findings provide an impetus for developing coacervated Dopa-functionalized peptides/polymers adhesive formulations for a variety of applications on wet polar surfaces.

Adsorption of Lysine on Na-Montmorillonite and Competition with Ca(2+): A Combined XRD and ATR-FTIR Study

Lysine adsorption at clay/aqueous interfaces plays an important role in the mobility, bioavailability, and degradation of amino acids in the environment. Knowledge of these interfacial interactions facilitates our full understanding of the fate and transport of amino acids. Here, X-ray diffraction (XRD) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) measurements were used to explore the dynamic process of lysine adsorption on montmorillonite and the competition with Ca(2+) at the molecular level. Density functional theory (DFT) calculations were employed to determine the peak assignments of dissolved lysine in the solution phase. Three surface complexes, including dicationic, cationic, and zwitterionic structures, were observed to attach to the clay edge sites and penetrate the interlayer space. The increased surface coverage and Ca(2+) competition did not affect the interfacial lysine structures at a certain pH, whereas an elevated lysine concentration contributed to zwitterionic-type coordination at pH 10. Moreover, clay dissolution at pH 4 could be inhibited at a higher surface coverage with 5 and 10 mM lysine, whereas the inhibition effect was inconspicuous or undetected at pH 7 and 10. The presence of Ca(2+) not only could remove a part of the adsorbed lysine but also could facilitate the readsorption of dissolved Si(4+) and Al(3+) and surface protonation. Our results provide new insights into the process of lysine adsorption and its effects on montmorillonite surface sites.

In situ particle film ATR FTIR spectroscopy of poly (N-isopropyl acrylamide) (PNIPAM) adsorption onto talc

The adsorption of poly(N-isopropyl acrylamide) (PNIPAM) onto talc from aqueous solutions has been studied using the in situ methodology of particle film attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. PNIPAM was observed to adsorb significantly onto the talc particle film at a temperature below its lower critical solution temperature (LCST). Peak shifts were seen in the adsorbed layer FTIR spectrum that match those observed when PNIPAM solution is heated above its LCST. This observation indicates that adsorption causes a conformational re-arrangement similar to that seen when PNIPAM undergoes a coil-to-globule transition, in this case presumably induced by hydrophobic interactions between PNIPAM and the talc basal plane surface. The kinetics of adsorption are seen to be complex, with potential influences of conformational rearrangement and differential adsorption kinetics for the two dominant surface regions of talc particles. The adsorbed PNIPAM was seen to be exceptionally resistant to removal, with no desorption occurring when a background electrolyte solution was flowed over the adsorbed layer. Spectra acquired of the adsorbed polymer layer heated above the LCST reveal that a further conformational rearrangement takes place for the adsorbed layer, finalizing the transition from coil-to-globule that was initiated by the interaction with the mineral surface.

Modeling the antisymmetric and symmetric stretching vibrational modes of aqueous carboxylate anions

The infrared spectra of six aqueous carboxylate anions have been calculated at the M05-2X/cc-pVTZ level of theory with the SMD solvent model, and validated against experimental data from the literature over the region of 1700 cm(-1) to 1250 cm(-1); this region corresponds to the stretching modes of the carboxylate group, and is often interrogated when probing bonding of carboxylates to other species and surfaces. The anions studied here were formate, acetate, oxalate, succinate, glutarate and citrate. For the lowest energy conformer of each anion, the carboxylate moiety antisymmetric stretching peak was predicted with a mean signed error of only 4 cm(-1) using the SMD solvent model, while the symmetric peak was slightly overestimated. Performing calculations in vacuum and scaling was found to generally over-predict the antisymmetric vibrational frequencies and under predict the symmetric peak. Different conformers of the same anion were found to have only slightly different spectra in the studied region and the inclusion of explicit water molecules was not found to significantly change the calculated spectra when the implicit solvent model is used. Overall, the use of density functional theory in conjunction with an implicit solvent model was found to result in infra-red spectra that are the best reproduction of the features found experimentally for the aqueous carboxylate ions in the important 1700 cm(-1) to 1250 cm(-1) region. The development of validated model chemistries for simulating the stretching modes of aqueous carboxylate ions will be valuable for future studies that investigate how carboxylate anions complex with multivalent metal cations and related species in solution.

In situ attenuated total reflectance fourier-transform infrared study of oxytetracycline sorption on magnetite

Adsorption of antibiotics on the surfaces of common mineral sorbents plays a major role in determining their fate in soils and sediments. The mechanisms of these reactions are, therefore, important for understanding and predicting the environmental fate of antibiotics. We used in situ attenuated total reflectance Fourier-transform infrared spectroscopy to elucidate the binding mechanisms of oxytetracycline (OTC) onto the surface of magnetite [FeO], a common Fe oxide mineral in soils and sediments, as a function of pH (3-9) and aqueous OTC concentration (5-150 μmol L). Comparison of dissolved OTC spectra to those of OTC-magnetite surface complexes indicated strong interactions of OTC molecules with the FeO surface via carbonyl (C=O) and amine (-NH) moieties of the amide group (-CONH) and the N atom of the dimethyl amino group [-N(CH)]. Increasing the aqueous OTC concentration led to increased OTC adsorption but did not notably alter the OTC binding mode at the magnetite surface. The results of this study would help to assess the importance of Fe oxide minerals in determining the environmental fate of OTC in soils and sediments.