Article pubs.acs.org/Langmuir
In Situ ATR FTIR Study of Dextrin Adsorption on Anatase TiO2 Audrey Beaussart,†,§ Luigi Petrone,‡ Agnieszka Mierczynska-Vasilev,† A. James McQuillan,‡ and David A. Beattie*,† †
Ian Wark Research Institute, ARC Special Research Centre for Particles and Material Interfaces, University of South Australia, Mawson Lakes SA 5095, Australia ‡ Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand S Supporting Information *
ABSTRACT: The adsorption of two dextrin-based polymers, a regular wheat dextrin (TY) and a carboxymethyl-substituted (CM) dextrin, onto an anatase TiO2 particle film has been studied using in situ attenuated total reflection (ATR) FTIR spectroscopy. Infrared spectra of the polymer solutions and the polymer adsorbed at the anatase surface were acquired for two solution conditions: pH 3 and pH 9; below and above the isoelectric point (IEP) of anatase, respectively. Comparison of the polymer solution spectra and the adsorbed layer spectra highlighted a number of spectral differences that were attributed to involvement of the carboxyl group of CM Dextrin interacting with the anatase surface directly and the adsorption of oxidized dextrin chains in the case of regular dextrin (TY) at high pH. The adsorption/desorption kinetics were determined by monitoring spectral peaks of the pyranose ring of both polymers. Adsorption equilibrium was not established for Dextrin TY for many hours, whereas CM Dextrin reached equilibrium in its adsorption within 60 min. The extent of desorption of Dextrin TY (observed by flowing a background electrolyte dextrin-free solution) was extensive at both pH values, which reflects the poor affinity and binding of the polymer on anatase. In contrast, CM Dextrin underwent almost no desorption, indicating a high affinity between the carboxyl groups of the polymer and the anatase surface.
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silica.9 Likewise, this technique has been used extensively to follow adsorption on titania for species such as proteins,10 amino acids,11 and oxalic acid.12,13 A number of studies have highlighted the involvement of carboxylic acid groups in the adsorption mechanisms of some molecules on hematite14−16 and clays.17 Similar conclusions have been drawn regarding the adsorption of sodium polyacrylate on titania.18 There have only been a limited number of in situ spectroscopic investigations of the binding of polysaccharides on mineral surfaces. Recently, the adsorption of alginic acid on an anatase TiO2 particle film was investigated by ATR FTIR spectroscopy over a pH range,19 revealing the conversion at low pH of protonated carboxyl groups to deprotonated/complexed carboxyl groups when bound to the surface of anatase. ATR FTIR has been used to study the adsorption of CMC on talc20 which forms platelike particles with two dominant surface types: hydrophilic edges and hydrophobic basal plane faces. CMC was determined to bind through two distinct binding mechanisms: complexation of carboxyl groups with surface sites on the edge of the talc particles (talc) and hydrophobic interactions with the nonpolar basal plane surface of the talc
INTRODUCTION Titania pigments are used in industrial applications, such as paints and papermaking. The desired properties of paint and paper, such as opacity, and gloss or color distribution largely depend on the pigment dispersion within the surrounding medium. A number of publications have therefore been devoted to pigment dispersion by addition of polymers,1 such as the studies of sodium polyacrylate 2 and modified polyacrylamide3 adsorption on TiO2 suspensions. Polysaccharides have also seen some use in the dispersion of titania pigments and in altering the surface properties of the treated pigment.4−6 The application of such polymers has primarily been in the papermaking industry where carboxymethyl cellulose (CMC) and starch are widely used to influence the properties of the final product. More widespread use of such polymers could possibly be obtained through controlled structural modification of the base polymer. However, this would rely on the acquisition of detailed information on how structural modifications affect the polysaccharide adsorption mechanism and adsorption kinetics. ATR FTIR has proved to be a powerful tool for probing binding mechanisms and the rate of adsorption of molecules on surfaces7 and has seen a rapid emergence in the past decade of studies probing adsorption phenomena on mineral surfaces in liquid media, such as polymers on clay8 and surfactants on © 2012 American Chemical Society
Received: November 25, 2011 Revised: January 17, 2012 Published: January 24, 2012 4233
dx.doi.org/10.1021/la204652f | Langmuir 2012, 28, 4233−4240
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Article
The anatase, purchased from Dyesol, Australia (batch BB19), was a 20 wt % aqueous suspension of anatase containing HNO3 and no other components. The crystalline size measured by X-ray crystallography was 16 nm, and the particle size obtained by dynamic light scattering (DLS) was 18 nm. The BET surface area was measured at 67.0 m2 g−1. An SEM picture of the ∼100 nm thick dry film of anatase particles deposited on a glass slide (using the same procedure as used for the ATR FTIR experiments) is shown in Figure 2.
particles. Dextrin adsorption has also been studied for the same mineral surface, with the lack of alteration of the adsorbed polymer spectrum relative to the solution spectrum being indicative of binding through hydrophobic interactions,21 a conclusion supported by molecular dynamics simulations performed by other researchers.22,23 Dextrin adsorption on a range of other minerals has also been studied using FTIR, albeit using dried samples analyzed with the diffuse reflectance sampling methodology.24 The inability of an ex situ methodology to accurately determine binding mechanisms is reflected in the debate that these measurements create in the literature.25 The study presented here consists of an investigation of dextrin adsorption on anatase titania using ATR FTIR spectroscopy. Two dextrins were chosen for the study: a regular dextrin (Dextrin TY) and a carboxymethyl substituted dextrin (CM Dextrin). Although dextrins are of lower molecular weight than starch or CMC (polymers currently used in pigment dispersion applications), their selection increases the potential for identifying binding mechanisms due to the higher expected fraction of shorter polymer chains being bound. Spectra have been acquired for the two polymers in solution and adsorbed on the anatase surface at pH values below (pH 3) and above (pH 9) the isoelectric point (IEP) of anatase (pH ∼5.526). The range of experimental conditions used in this study (in terms of polymer chemistry and pH of the background media) has provided a broad picture of dextrin adsorption behavior on anatase TiO2.
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Figure 2. SEM picture of anatase particles film deposited on glass slide. All solutions were prepared using high-purity water, produced using an Elga UHQ unit, with a conductivity of
