Adsorption behaviour of polyvinyl pyrrolidone on oxide surfaces

Adsorption behaviour of polyvinyl pyrrolidone on oxide surfaces

July 2000 Materials Letters 44 Ž2000. 352–360 www.elsevier.comrlocatermatlet Adsorption behaviour of polyvinyl pyrrolidone on oxide surfaces Manoran...

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July 2000

Materials Letters 44 Ž2000. 352–360 www.elsevier.comrlocatermatlet

Adsorption behaviour of polyvinyl pyrrolidone on oxide surfaces Manoranjan Pattanaik ) , Sumitra Kumar Bhaumik Regional Research Laboratory (CSIR), Mineral Beneficiation DiÕision, Bhubaneswar 751013, India Received 29 August 1999; received in revised form 28 January 2000; accepted 31 January 2000

Abstract Adsorption of polyvinyl pyrrolidone ŽPVP. onto the oxide surfaces, namely, kaolinite, titanium dioxide, iron oxide and alumina has been studied by electrokinetics and measurement of adsorption density. The mechanism of adsorption is suggested to be as the acid–base interaction between the substrate surface and polymer segments. The presence of surface hydroxyl groups, with the tendency to lose protons acting as Bronsted acid sites in the crystal lattice of these metal oxides, can interact with the binding sites of PVP segments considered as Lewis base in aqueous solution. Electrokinetic and adsorption studies of individual oxide system has been carried out to comprehend the mechanism of interaction and qualitative investigation were sketched out incorporating the Stern–Graham and Langmuir adsorption isotherm equations. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Polyvinyl pyrrolidone; Electrokinetics; Kaolinite; Titanium dioxide; Iron oxide; Alumina

1. Introduction Adsorption of polymers on particles have been investigated to understand the mechanism of interactions at the solidrliquid interfaces in different solvent media w1x. In industrial processes and products, polymers used as surface modifiers by critically adsorbing them onto the particle surfaces has a greater significance than using them just as other organic and inorganic surfactants. The oxide particulater composites, which are used in different filler industries, can be successfully fabricated either by dispersion or by aggregation through flocculation with suitable adsorption of polymers at the particle–ma)

Corresponding author. Fax: q91-674-581-637. E-mail addresses: [email protected], [email protected] ŽM. Pattanaik..

trix interface. Surface modification of those particles can also be tactfully handled by using suitable polymers to achieve the desirable mechanicalrphysicochemical properties of the composites on use. In the areas of adhesives, coatings and polymer composites, adsorption strength of polymerrsegments are as relevant as that of the polymer onto the solid substrate w2x. Polymers can adsorb by electrostatic, covalent, hydrophobic and hydrogen bonding mechanisms w3x. Hydrogen bonding is assumed to be the predominant mechanism for nonionic polymer adsorption on oxide surfaces w4x. This may be due to the acid–base mechanism between the rich hydroxyl groups available on the surfaces and polymers having a functionality such as with ether oxygen, alcohol, ester, amides, etc. capable of forming hydrogen bonds at the interface w5x. In investigations of starch–haema-

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 5 8 - 6

M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360 Table 1 Particle size distribution Sample

Kaolinite Rutile Hamatite Alumina

Particle size distribution, mm d10

d50

d90

Specific surface area, m2 rg

0.235 0.114 0.325 0.123

0.429 0.329 0.516 0.216

1.163 1.0124 2.231 1.537

15.634 11.982 6.731 9.627

tite systems, it was believed that the bonding may be due to the specific interactions of Fe sites with the functional groups of starch w6–8x. It has been evidenced from the reports on the interaction of oxide materials with polyethylene oxide ŽPEO. that hydrogen bonding is the means of interaction between the substrate and polymer w9x. PEO, a nonionic polymer is generally used as flocculant for clays and silica, but not for oxides like hematite and alumina w8–10x.

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The aim of the present study is to understand the adsorption mechanism of polyvinyl pyrrolidone ŽPVP. on oxide surfaces as there is a possibility of acid–base mechanism operating among the substrate and the reagent due to the presence of nitrogen atom along the pyrene ring. The elucidation of the adsorption mechanism of PVP is expected to provide a scientific approach to select the polymers for their applications as dispersant, flocculants, coagulant or as modifiers dealing with oxide particles.

2. Experimental 2.1. Materials The powdered minerals were procured from different process industries produced by a synthetic route. Those samples were washed thoroughly with

Fig. 1. PZC of kaolinite, titanium dioxide, iron oxide and alumina Ž10y4 M NaNO 3 ..

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M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360

doubly distilled water Žconductance- 1.0 mmhor cm. till a constant pH was attained and then air-dried at room temperature. All samples were more than 95% pure as per the specifications supplied by the respective manufacturer as well as chemical characterisation by analytical methods. PVP of three different molecular weights of 2000, 40 000 and 360 000 were taken for this experiment. These were procured from Sigma polymers, Bombay ŽIndia., and used as received . The polymer solutions were prepared freshly everyday to avoid the possibility of degradation. Sodium nitrate was used as an electrolyte for electrokinetic studies. pH was adjusted with buffer solutions of nitric acid and sodium hydroxide.

2.2. Methods 2.2.1. Physical characterisation Physical characterisation Žspecific surface area and particle size distribution. of the solid samples were carried out by BET surface area analyser Žmicromeritius. and Horiba laser particle size analyser. 2.2.2. Electrokinetics studies Electrophoretic mobility of the oxide samples were measured by Zeta metre 3.0, Zetametre USA, to determine the point of zero charge ŽPZC. of the individual oxide sample. Conditioning period in each case were experimentally determined to attain equilibrium in sodium nitrate solution as an electrolyte of

Fig. 2. Electrokinetic behaviour of kaolinite in the presence of PVP, exhibiting the characteristic shift in IEP for adsorption through acid–base interactions.

M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360

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known strength and at constant pH of the solution. For each individual suspension, 0.005% of the solid suspension was prepared in a container at its specified working conditions of reagents. Electrophoretic mobility of each suspension were measured after allowing sufficient conditioning time to attain equilibrium pH.

withdrawn for determination of residual reagent concentration by UV-VIS spectrophotometre. The amount of adsorption were estimated by the solution depletion method.

2.2.3. Adsorption studies Adsorption experiments were conducted with the polymer solution. About 0.05 g of individual solid powder in 50 ml of polymer solution at a particular pH in 100 cm3 conical flask were set for conditioning in a gyratory shaker for 24 h. After having attained equilibrium, those samples were centrifuged for 10 min and the clear supernatant liquid was

The size distribution and specific surface area of the individual oxide samples are given in Table 1. All the sample have a wide range of size distribution and differ from each other. Fig. 1 shows the PZC of the different oxides. These are at pH 2.5, 5.9, 7.8 and 8.5 for kaolinite, titanium dioxide, iron oxide and alumina, respectively. The PZC of a material is the condition of the system where the surface ap-

3. Results and discussion

Fig. 3. Electrokinetic behaviour of titanium dioxide in the presence of PVP Ždifferent molecular weight., exhibiting no significant shift in IEP for no adsorption.

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M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360

pears to be uncharged. This is an important and unique property of fine particles and can be characterised through electrokinetics using different methods of experiments w16x. This can be expressed in terms of pH condition of the system containing the material. Iso-electric point ŽIEP. is the pH condition of the material at which the surface is at zero charge in presence of any specifically adsorbed ions. Below the pH PZC of a material, the surface is positively charged and above it is negatively charged. Direct evidence of various adsorption mechanism can be obtained through electrokinetic investigations w15x. For physisorbing reagent system, adsorption takes place only when the adsorbate and adsorbent are oppositely charged, but in chemisorption, similar charged ions can interact with a specific site on the

surface. The IEP shifts as a result of reagent adsorption in case of chemisorbing systems whereas physisorption does not lead to any shifts in IEP w14,15x. The PZC of oxide minerals are determined by the potential determining ions ŽPDI. as Hq and OHy ions present onto the surface at equilibrium. So, the PZC values of most of the oxides can be comparatively translated with their relative strength of Bronsted acidity w6x. The oxide surface that donates protons relatively easier than the other at any given pH is more acidic and have a lower PZC. So, kaolinite, having the lowest PZC of all, has rich hydroxyl groups, which can easily donate protons facilitating active Bronsted acid sites. The PZC of titanium dioxide is at pH 6.0 has lower Bronsted acidity than that of kaolinite. Iron oxide and alumina with PZC

Fig. 4. Electrokinetic behaviour of iron oxide in the presence of PVP Ždifferent molecular weight., exhibiting no shift in IEP for no adsorption.

M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360

much higher than that of the kaolinite are weak acids. From Fig. 2, it is evidenced that PVP adsorption is taking place at a pH region where the surface charge of kaolinite is negative and at that condition, there is no charge on the polymer segment. The IEP shift observed there is independent of molecular weight of the polymers, but not of their concentrations. Similar studies with polyacrylic acid ŽPAA. and modified polyacrylamide ŽPAMX. shows no shift in the IEP indicating minimum adsorption on the surface. It is noteworthy from Figs. 3–5 that with titanium dioxide, iron oxide and alumina, the IEP shifts are absent with respect to all the molecular weights and concentrations of PVP taken in these experiments. Similar studies with polyacrylic acid ŽPAA. and modified polyacrylamide ŽPAMX. shifts

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of IEP have been noticed in the following order: alumina) iron oxide ) titanium dioxide. In the case of kaolinite, the IEP shifts observed are due to the interactions of PVP and kaolinite surface and which could be an acid–base mechanism as a result of hydrogen bonding. The dissociable hydroxyl ions present with accessible orientation on the surface of kaolinite can easily facilitate the acid–base interaction, whereas it is insignificant in case of titanium dioxide, iron oxide and alumina. Fig. 6 is the adsorption isotherms of PVP of various molecular weights from aqueous solution on kaolinite, titanium dioxide, iron oxide and alumina at 258C. The adsorption isotherms show that these are of Langmuir type and independent of molecular weight of PVP. In the case of kaolinite, the amount of PVP adsorbed is significantly higher as compared

Fig. 5. Electrokinetic behaviour of alumina in the presence of PVP Ždifferent molecular weight., exhibiting no shift in IEP for no adsorption.

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Fig. 6. Adsorption isotherms of PVP ŽMW 360 K. with kaolinite, titanium dioxide, iron oxide and alumina at room temperature.

to the other three. The interacting site of the PVP segment in solution can be due to the negatively charged carbonyl group within itself as shown in the resonance structure of the pyrene ring of PVP segment ŽFigs. 7 and 8.. The driving force for adsorp-

Fig. 7. Resonance structure of pyrene ring within the PVP.

tion of PVP on kaolinite is due to the interaction between carbonyl groups of pyrene ring along the PVP segment and silanol groups present in the basal plane of kaolinite. But in the case of titanium dioxide, PVP adsorption is not prominent in aqueous solution. The reason may be due to the weaker Bronsted acid sites and the Lewis acid sites Žexposed metal ions on the surface. are crowded by water molecule. In the medium of solvents like methanol and ethanol, there is a significant adsorption of PVP on titanium dioxide w11x. It has been pointed out there that Lewis acid sites are surrounded by undissociable hydroxyl ions from water. The hydroxyl groups present in Bronsted acid sites are dissociable and, hence, can form hydrogen bonds with suitable groups or ions. The competitive adsorption of hydroxyl groups of water molecule and

M. Pattanaik, S.K. Bhaumikr Materials Letters 44 (2000) 352–360

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Fig. 8. Interaction between silica and PVP through hydrogen bond with acid–base mechanism.

alcohol on titanium dioxide make a difference to the dissociation constants for which a better adsorption of PVP with methanolrethanol solution can occur. In the case of iron oxide and alumina, there is no significant adsorption of PVP from aqueous solution. The specific interaction as proposed by Pradip et al. could not facilitate the acid–base interaction due the structural inconsistency or incompatibility among the active functional groups present in the polymer segment and Bronsted acid sites available on the crystal lattice of alumina and iron oxide. The Lewis acid sites are not accessible due to the undissociable hydroxyl groups covalently bonded with metal ions in the crystal lattice w12,13x. The Lewis acid sites in alumina are hindered sterically from the adsorption of PEO segments as explained by Moudgil et al. since in the fixed positions of those sites, they cannot optimally orient themselves for acid–base interaction.

4. Conclusions The specificity of hydrogen bonding of PVP on oxide surfaces has been studied. The pH PZC, which is a measure of the strength of Bronsted acidity, are characterised through electrokinetic studies. PVP, a non-ionic polymer adsorbs onto the acidic oxide like kaolinite through hydrogen bonds in the acid–base mechanism, but not on titanium dioxide, iron oxide and alumina from aqueous solution.

Acknowledgements The authors are grateful to Prof. H.S. Ray for his constant guidance, encouragement and support for this work and permission to publish this paper.

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