The Spectrophotometric Investigation of Nonionic Polymer Adsorption (Polyethylene Glycol (PEG), Polyethylene Oxide (PEO) and Polyvinyl Alcohol (PVA)) Influence on the Alumina Suspension Stability – Effects of Polymer Type,its Molecular Weight and Solution pH

The effect of nonionic polymer adsorption on the stability of alumina (Al₂O₃) in the pH range 3–9 was examined. The influences of polymer type and its molecular weight, as well as solution pH, were studied. The following macromolecular substances were used: polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA). The spectrophotometry method was applied to obtain the stability curves (dependence of suspension absorbance vs. time). The obtained results indicate that the addition of the polymer influences alumina suspension stability. The addition of the polymer at pH 3 improves the stability conditions of investigated systems. At pH = 6 the decrease of Al₂O₃ suspension stability (except PEG 2 000) was obtained. On the other hand, at pH 9 the presence of polymer improves the stability properties of the alumina. The higher the molecular weight of the polymer, the more pronounced effects were observed. Moreover, adsorption of polyvinyl alcohol whose macromolecules contain ionizable acetate groups causes greater changes in alumina suspension stability in comparison to the systems containing polyethylene glycol and polyethylene oxide.     

Temperature effect on adsorption properties of silica-polyacrylic acid interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

The temperature effect on electrokinetic properties of the silica–polyvinyl alcohol (PVA) system

The influence of polyvinyl alcohol (PVA) adsorption on the structure of the diffuse layer of silica (SiO₂) in the temperature range 15–35 °C was examined. The microelectrophoresis method was used in the experiments to determine the zeta potential of the solid particles in the absence and presence of the polymer. The adsorption of PVA macromolecules causes the zeta potential decrease in all investigated SiO₂ systems. Moreover this, decrease is the most pronounced at the highest examined temperature. Obtained results indicate that the conformational changes of adsorbed polymer chains are responsible for changes in electrokinetic properties of silica particles. Moreover, the structure of diffuse layer on the solid surface with adsorbed polymer results from the following effects: the presence of acetate groups in PVA chains, the blockade of silica surface groups by adsorbed polymer and the shift of slipping plane due to macromolecules adsorption.     

Temperature effects on the adsorption of polyvinyl alcohol on silica

The influence of temperature on the adsorption of polyvinyl alcohol (PVA) on a silica surface was studied from 15–35°C. The structure of the polymer adsorption layer was determined from spectrophotometric, viscosity and thermogravimetric measurements. The amount of PVA adsorbed, macromolecules’ conformation in solution, thickness of the polymer adsorption layer, and changes in the heating curve of SiO₂ with adsorbed polymer were determined. Temperature influences the PVA chain conformation in solution and the structure of the polymer adsorption layer. A temperature rise causes relaxation of polymer coils which results in an increase in the linear dimensions of PVA chains in the solution, the creation of a thicker adsorption layer, and an increase in polymer adsorbed. Polymer adsorption on the silica surface also causes changes in the heating curve of these systems. The mass losses due to heating are smallest for the systems obtained at 15°C because the least polymer is adsorbed at this temperature.      

Influence of solution pH on stability of aluminum oxide suspension in presence of polyacrylic acid

The effect of solution pH and molecular weight of polyacrylic acid (PAA) on its adsorption as well as on stabilization-floculation properties of the colloidal Al₂O₃ and electrolyte solution systems was studied. The measurements showed that at pH = 6, the presence of the polymer of molecular weight 2?000 and 240?000 does not change stability of Al₂O₃ suspension. However at pH =3 and 9 the effect of polyacrylic acid is significant. At pH = 3 it creates destabilization of the suspension while at pH = 9 PAA it improves significantly the stability of Al₂O₃. It was shown that the increase in solution pH affects conformation of adsorbed macromolecules which causes the decrease in PAA adsorbed amount and thickness of polymer adsorption layer. By comparing the values of diffusion layer and surface charges, main effects responsible for the decrease in surface charge and ζ potential of the solid in the presence of the polymer as well as suspension stability were determined.     

Comparison of adsorption affinity of polyacrylic acid for surfaces of mixed silica–alumina

The influence of solution pH (in the range 3–9) on the adsorption of polyacrylic acid (PAA) on the mixed silica–alumina surface (SA-3: SiO₂ 97 %–Al₂O₃ 3 % and SA-96: SiO₂ 4 %–Al₂O₃ 96 %) was investigated. The following methods were applied in experiments: spectrophotometry, viscosimetry, potentiometric titration, and microelectrophoresis, which enable determination of adsorbed amount of the polymer, thickness of its adsorption layers, surface charge density, and zeta potential of solid particles in the presence and absence of PAA, respectively. The obtained results indicate that rise of solution pH causes the decrease of PAA adsorption and the increase of its adsorption layer thickness on surfaces of both solids. Moreover, significantly higher adsorption of polyacrylic acid was obtained on the SA-96 surface. This is a result of more favorable electrostatic interactions occurring between the adsorbing polymer chains and the SA-96 surface and formation of a greater number of adsorbate-adsorbent connections through hydrogen bridges.     

Studies of temperature influence on adsorption behaviour of nonionic polymers at the zirconia–solution interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

Impact of poly(vinyl alcohol) adsorption on the surface characteristics of mixed oxide Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>–SiO<Subscript>2</Subscript>

The poly(vinyl alcohol) (PVA) influence on the adsorption and electrokinetic properties of the mixed oxide Mn _x O _y –SiO₂/polymer solution system was examined. Three oxides differing with the Mn _x O _y contents were applied (0.2; 1 and 3 mmol/g SiO₂, respectively). The PVA with the molecular weight 100 kDa was characterized with the acetate groups content equal to 14 %. Adsorption, solid surface charge and zeta potential measurements were made as a function of solution pH (3–10). The obtained results showed that the PVA adsorption amount strongly depends on not only the solution pH, but also manganese oxide content on the mixed oxide surface. The higher solution pH value (or Mn _x O _y content) is, the higher polymer adsorption is obtained. The PVA addition to the solid suspension causes minimal changes of the mixed oxide surface charge density, whereas the zeta potential of solid particles increases significantly in the polymer presence.     

CATIONIC POLYMERIC SURFACTANTS

An influence of polyvinyl alcohol molecular weight and acetate groups, present in the macromolecule, on adsorption and electrochemical properties of the TiO₂–polymer solution interface was studied. Calculated thickness of adsorption layers of PVA, on the surface of the oxide, allowed assume that acetate groups may have meaningful influence on the polymer chain conformation at the interface. Structure of macromolecules at titania–polymer solution interface was compared with that of bulk of solution. Obtained data allow determine the changes of the size and shape of polymer coils in the system. The results of experiments let us conclude main factors, responsible for observed zeta potential and surface charge changes of TiO₂. It was proved that change of the ion structure of Stern layer, depends on molecular weight and number of acetate groups (degree of hydrolysis) of PVA macromolecule. Possible mechanism of zeta potential changes was proposed as a function of pH of the solution and molecular weight of the polymer.     

Control on shape, porosity and surface hydrophilicity of hematite particles by using polymers

The shape, porosity, and surface hydrophilicity of hematite particles formed from a forced hydrolysis reaction of acidic FeCl₃ solution were controlled by using a trace of polymers (0.001 and 0.003 wt%). The spherical particles were produced on the systems with polyvinyl alcohol (PVA) and polyaspartic acid (PAS). In the case of polyacryl amide (PAAm), slightly small spherical particles were precipitated at 0.003 wt%. However, polyacrylic acid (PAAc) and poly-γ-glutamic acid (PGA) gave ellipsoidal particles. This morphological change on hematite particles depended on the order of functional groups of polymers as –OH<–CONH₂<–COOH<–COOH and ⟩C=O, corresponding to the order in extent of polymer molecules for complexation to Fe³⁺ ions and adsorption onto particle surface. Accompanying this order, the hematite particles produced were changed from less porous to microporous. On the other hand, only the system with 0.003 wt% of PAAm produced mesoporous hematite particles. Choosing the kinds of polymers also controlled the ultramicroporosity and surface hydrophilicity of the particles.     

Interactions of NO with iron deposited on alumina

The interaction of nitric oxide with single-crystal surfaces of alumina at temperatures of 298, 473, and 673 K, which had been covered by various amounts of iron, was studied using X-ray photoelectron spectroscopy. The iron was deposited onto Al₂O₃ in the Fe⁰ state. At low coverages, iron was partially oxidized due to its interaction with Al₂O₃. Scanning auger mapping analysis showed that the iron was randomly distributed on the Al₂O₃ surface. The amount of adsorbed NO increased with increasing iron coverage. However, at very high iron coverages, there was a decrease in adsorption. This indicated that the aluminum ions may have activated the NO adsorption on the iron atoms. For increasing temperature there was also an increase in adsorption for high iron coverages, but the adsorption decreased with increasing temperature for low iron coverages. Sticking probability calculations indicated that the adsorption was mobile and dissociative. Binding energy of the nitrogen peaks indicated that NO was adsorbed onto the Fe/Al₂O₃ surface as a nitride.     

Control over the self-assembly of polymers and layered molybdenum oxide: from the micrometer scale to the molecular scale

We report adjustment on the self-assembly between polymer of polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) and inorganic molybdenum oxide layers from the micrometer scale to the nanometer scale. Our method is to break the strong interactions between the organic polymers by introducing suitable bridging agents and adjust the reaction speeds of the two competitive reactions in the reaction system. We use I₂ to complex with PVA and break the strong hydrogen interactions between the PVA chains, resulting in a PVA-I₂/(Mo_xO_y)_∞ⁿ⁻ complex, in which the organic and inorganic species self-assemble homogenously on the molecular scale. We also adjust the thickness of the inorganic (Mo_xO_y)_∞ⁿ⁻ layers in the hybrid of PVP/(Mo_xO_y)_∞ⁿ⁻ by controlling the reaction speeds of the two competitive reactions: hydrolysis of Mo₇O₂₄ ⁶⁻ into (Mo_xO_y)_∞ⁿ⁻ and packing into thick inorganic layers on the one hand, and hybridization of (Mo_xO_y)_∞ⁿ⁻ and PVP into layered hybrid on the other hand. Experimental results proved that when the hydrolysis is overwhelming, the inorganic molybdenum oxide chains pack into heavy layers and self-assemble with PVP polymers on the micrometer scale, and when the hybrid reaction dominates, the organic polymer and molybdenum oxide hybridize on the molecular scale. These findings open new routes to disperse organic polymer and inorganic species homogenously and fabricate novel organic/inorganic hybrid nanomaterials in situ.     

Indirect radiotracer study of the adsorption of chromate ions on γ-Al2O3 and direct study of the adsorption of sulfate ions on Cr2O3 using 35S-labelled H2SO4

The specific adsorption of sulfate ions on Cr₂O₃ was studied by a radiotracer technique using ³⁵S-labelled sulfuric acid in low concentrations (c<10^–3 mol dm^–3) in the presence of a large excess of perchlorate supporting electrolyte. The pH and concentration dependence were determined. It was found that the extent of adsorption is determined by the protonation of the surface sites, similar to other oxides studied previously. A comparison of Cr₂O₃ and Al₂O₃ in this respect shows that the protonation of the former takes place at significantly lower pH values than that of the latter. The indirect radiotracer study of the adsorption of chromate on Al₂O₃ was carried out using labelled sulfate ions as indicator species. The results obtained show that the adsorption strength of chromate species is very low in comparison to sulfate ions and a regular Langmuir-like adsorption behaviour can be observed. It is believed that the observations presented may contribute to a better understanding of the behaviour of surface layers with a mixed oxide content. Electronic Publication     

Synthesis,characterization, and catalytic activity in the <Emphasis Type="Italic">n</Emphasis>-heptane conversion over Pt/In–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel prepared catalysts

Platinum catalysts supported on indium-doped alumina were prepared by the sol–gel method. The method allows the incorporation of In³⁺ in the alumina network. The indium-doped alumina supports showed narrow pore size distribution (5.4–4.0 nm) and high specific surface areas (258–280 m²/g). The ²⁷Al NMR-MAS spectroscopy identified aluminum in tetrahedral, pentahedral, and octahedral coordination; however, the intensity of the signal assigned to aluminum in pentahedral coordination diminishes with the increase of the content of indium. Total acidity determined by ammonia thermodesorption diminishes strongly in Pt/In–Al₂O₃ catalysts, suggesting a selective deposit of platinum over the acid sites of the support. The effect of the support in the platinum catalytic activity was evaluated in the n-heptane dehydrocyclization reaction. The selectivity patterns for such reaction were modified substantially in the doped Pt/In–Al₂O₃ catalysts, in comparison with the Pt-In/Al₂O₃–I coimpregnated reference catalyst. As an important result, the formation of benzene was suppressed totally over the indium-doped alumina sol–gel supports with a high content (3 wt%) of indium.     

n-Alkane isomerization on heteropolyacids

Acid-base properties of bulk 12-tungstophosphoric heteropolyacid (HPWA) and HPWA supported on alumina and fluorinated alumina were studied by DRIFT spectroscopy using adsorption of probe molecules. Catalytic properties of Pt-containing catalysts carried by these supports were studied inn-pentane isomerization. It was shown that the Pt/HPWA/Al₂O₃ system is almost inactive in isomerization as a result of the interaction of HPWA with basic sites of the alumina support. On the contrary, the presence of acidic sites in fluorinated alumina prevents HPWA destruction and favors the uniform distribution of HPWA on the support surface. As a result, enhancement of the activity and selectivity of the Pt/HPWA/Al₂O₃-F catalyst inn-pentane isomerization is observed. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1744–1750, October, 2000.     

Factors affecting drug adsorption on beta zeolites

The adsorption behaviour of three commonly used drugs, namely ketoprofen, hydrochlorothiazide and atenolol, from diluted aqueous solutions on beta zeolites with different SiO₂/Al₂O₃ ratio (i.e. 25, 38 and 360) was investigated by changing the ionic strength and the pH, before and after thermal treatment of the adsorbents. The selective adsorption of drugs was confirmed by thermogravimetry and X‐ray diffraction. The adsorption capacity of beta zeolites was strongly dependent on both the solution pH and the alumina content of the adsorbent. Such a remarkable difference was interpreted as a function of the interactions between drug molecules and zeolite surface functional groups. Atenolol was readily adsorbed on the less hydrophobic zeolite, under pH conditions in which electrostatic interactions were predominant. On the other hand, ketoprofen adsorption was mainly driven by hydrophobic interactions. For undissociated molecules the adsorption capability increased with the increase of hydrophobicity.     

Sorption—desorption studies of anionic dyes on alumina pretreated with acids

Sorption—desorption behaviour of the anionic dyes naphthol blue-black (NB) and lissamine green ‘BN’ (LG) on chromatographic alumina, pretreated with mineral acids, is described. Alumina samples of surface-phase pH 1.0–8.0 were prepared and studied for their sorption behaviour. The sorption was found to vary with surface pH of the substrate and acid used for pretreatment. Quantitative sorption was shown at pH ⩽ 4.0 (NB) and pH ⩽ 3.5 (LG) on Al₂O₃ treated with HNO₃ [Al₂O₃(n)], and maximum sorption occurred at pH 5.0 (NB) and pH 5.5 (LG) on Al₂O₃ treated with H₂SO₄ [Al₂O₃(s)] and pH 2.5 (NB) and pH 3.0 (LG) on Al₂O₃ treated with H₃PO₄ [Al₂O₃(p)]. Variation in sorption with time (10 min-72, h), temperature (30–60°C) and regeneration of the substrates with aqueous electrolytes is also reported. Desorption efficacy of anions was in the order: PO³⁻₄ > SO²⁻₄ > NO⁻₃. The acid-treatment, and hence the specifically adsorbed anions (NO⁻₃, SO²⁻₄, PO³⁻₄), appears to modify the sorption properties of alumina significantly. It appears that the controlling force for adsorption is predominantly ion exchange. A few synthetic mixtures of the dyes were separated by column chromatography, using inorganic electrolytes as eluents.     

Probing Catalytic Solid‐Liquid Interfaces by Attenuated Total Reflection Infrared Spectroscopy: Adsorption of Carboxylic Acids on Alumina and Titania

The adsorption of carboxylic acids (formic, acetic, and pyruvic acid) from corresponding solutions in CH₂Cl₂ solvent on Al₂O₃ and TiO₂ thin films has been studied by attenuated total reflection infrared spectroscopy. The metal‐oxide films were vapor‐deposited on a Ge internal reflection element, which was mounted into a specially designed flow cell. The system allowed in situ monitoring of the processes occurring at the solid‐liquid interface. The metal‐oxide films were characterized by X‐ray photoelectron spectroscopy, ellipsometry, and atomic force microscopy. Formic acid and acetic acid adsorbed predominantly as bridging species on alumina surfaces. Adsorbed free acids were not observed under a flow of neat solvent. Based on the position of the ν_AS(COO) and of the keto‐group stretching vibration of the pyruvate ion, pyruvic acid is proposed to coordinate to the Al₂O₃ surface in a monodentate fashion, whereas, on TiO₂, a bidentate species is preferred. Comparison of the adsorption behavior on the vapor‐deposited alumina film and on an α‐Al₂O₃ layer deposited from a water suspension of the corresponding metal‐oxide powder indicated that pyruvic acid adsorbs in a similar mode, irrespective of the metal‐oxide deposition technique.     

Surface properties of catalytic aluminas modified by alkaline-earth metal cations: A microcalorimetric and IR-spectroscopic study

Lewis acidic properties of transition aluminas whose surfaces have been doped with alkaline-earth metal cations (Ca²⁺ and Ba²⁺) were studied by means of the room temperature adsorption of carbon monoxide. The vibrational features of CO adsorbed at the surface of doped aluminas were investigated by IR spectroscopy in comparison with pure parent aluminas, while the quantitative and energetic features were studied by adsorption microcalorimetry. Various CO adspecies were found to form at the surface of both pure and doped-alumina, owing to the structural heterogeneity of the Al₂O₃ surface and to the presence of alkaline-earth metal cations. The surface heterogeneity was revealed by different v_co stretching frequencies, namely v_co≈2230, 2218 and 2205 cm⁻¹ for coordinatively unsaturated tetrahedral Al³⁺ cations in different crystallographic configurations, and v_co≈2186 and 2172 cm⁻¹ for coordinatively unsaturated Ca²⁺ and Ba²⁺ cations, respectively. Heats of adsorption of ≈80, 70 and 55 kJ/mol were assigned to the formation of Al³⁺/CO complexes, ≈45 kJ/mol for Ca²⁺/CO and ≈30 kJ/mol for Ba²⁺/CO complexes. The latter value was estimated through a correlation curve existing between v_co stretching frequencies and adsorption enthalpies. This correlation, already proposed in the past for CO adsorbed on non-d/d⁰/d¹⁰ metal cations, has been revisited and confirmed here, by including Al₂O₃ data for which an apparent lack of correlation between the two parameters was first observed. With respect to pure alumina, the population of Lewis acidic sites was found to be significantly depressed by the presence of alkaline-earth cus metal cations. These acidic sites are intrinsically weaker than tetrahedral cus Al³⁺ cations, as witnessed by smaller upward shifts of the v_co stretching frequencies with respect to CO gas and lower heats of adsorption, in accordance with expectations from the charge/ionic radius ratios. Ca²⁺ cations were found to compete in adsorbing CO with Al³⁺ cations more efficiently than the larger Ba²⁺ cations. In the case of CaO/Al₂O₃ systems outgassed at 1023 K, a thin surface layer of calcium aluminate, not detected by XRD or HRTEM, was suggested to form.     

The displacement of adsorbed polymer from silica surfaces by the addition of a nonionic surfactant

The adsorption of polyvinyl alcohol and Synperonic NP8 (nonyl phenol ethoxylate with an average of eight ethylene oxide groups per molecule) on fumed silica has been studied at various pH values. This was followed by an investigation of the competitive adsorption of NP8 and PVA. It was shown that NP8 can displace the polymer from the silica. This was attributed to a higher adsorption energy for the NP8 molecule compared with the value of the individual adsorbed PVA segments. Sediment volume experiments showed that the addition of NP8 to a colloidally stable silica dispersion with adsorbed PVA can induce flocculation as a result of displacement of some or all of the PVA chains from the surface. Initially the adsorption of the NP8 molecules caused an increase in the hydrophobic interaction (resulting in the flocculation) between the alkyl phenol groups which are oriented towards the bulk solution (since the PEO chains preferentially adsorb on the silica surface). Restabilisation at higher NP8 concentrations occur through the formation of bilayers with the PEO chains now dangling in solution.