Article pubs.acs.org/Langmuir
Enhanced Dewatering of Polyelectrolyte Nanocomposites by Hydrophobic Polyelectrolytes Joshua D. Kittle,† Holger Wondraczek,§ Chao Wang,† Feng Jiang,‡ Maren Roman,‡ Thomas Heinze,§ and Alan R. Esker*,† †
Department of Chemistry and ‡Department of Sustainable Biomaterials, Virginia Tech, Blacksburg, Virginia 24061, United States Center of Excellence for Polysaccharide Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany
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S Supporting Information *
ABSTRACT: We demonstrate that increasing the hydrophobic environment around the charge center of a polyelectrolyte (PE) not only decreases the water content of an adsorbed PE layer but can even dewater up to ∼50% of an initially hydrated substrate. The results of this work are expected to yield new stratagies to dewater PE systems and have potential applications in mineral recovery, paper manufacturing, and biomedical materials. Adsorption of a series of cationically derivatized dextran polyelectrolytes onto sulfated nanocrystalline cellulose (SNC) has been studied using quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). Synthesized samples of (N,Ndimethylamino)ethyldextran (DMAE-Dex), (N,N-diethylamino)ethyldextran (DEAE-Dex), and (N,N-diisopropylamino)ethyldextran (DIAE-Dex) had degrees of substitution (DS) ranging from 0.05 to 0.82. DMAE-Dex, DEAE-Dex, and DIAEDex all showed decreasing adsorption onto SNC and decreasing water content of the adsorbed film with increasing DS. Additionally, DEAE-Dex and DIAE-Dex films adsorbed onto SNC contained less water than DMAE-Dex films with the same DS. Interestingly, QCM-D results for high DS DIAE-Dex adsorbed onto SNC revealed mass loss, whereas SPR results clearly showed DIAE-Dex adsorbed. These observations were consistent with dehydration of the SNC substrate. This study indicates that the water content of the substrate could be tailored by controlling the DS and hydrophobic character of the adsorbed polyelectrolytes.
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INTRODUCTION Polyelectrolytes (PEs) are synthetic or natural polymers containing functional groups that ionize in polar solvents to yield charged groups along the polymer chain. Natural PEs, such as pectins and DNA, play an important role in the selfassembly and function of living systems,1,2 while both natural and synthetic PEs have increasingly found use in the water treatment,3 cosmetic,4 food,5 and medical industries.6 Several attractive aspects of PEs include water solubility, simple nanoarchitectural control of thin films using layer-by-layer (LbL) assembly of anionic and cationic PEs,7 and tunable functionality via pH, ionic strength, counterion, and solvent effects.8−10 Films of PEs are typically swollen in water, entrapping significant amounts of solvent.11 However, PEs with lower water content are desirable for a variety of applications, such as dewatering coal,12 improving wet paper strength,13 mineral recovery,14 improving flocculation,15,16 and developing water-resistant vapor sensors.17 In this work, we demonstrate that increasing the hydrophobic environment around the charge center of a PE not only decreases the water content of an adsorbed PE layer but can even dewater up to ∼50% of an initially hydrated substrate. Binding between anionic and cationic PEs is an entropically driven process in which PE charge neutralization leads to the release of counterions and water from the hydration layer.18 © 2012 American Chemical Society
Consequently, the release of some water during PE adsorption is expected. However, additional deswelling of PE complexes has been demonstrated by decreasing the ionic strength of the solution,19 reducing the PE charge density,8 or selecting a cosmotropic counterion.20 Generally, altering these parameters affects the PE chain conformation in solution and, as a result, also affects the thickness of the adsorbed PE layer, thereby controlling its swelling.21 Controlling the hydrophobic character of PE system components presents an additional avenue to control water content. Studies of analogous systems of physically cross-linked gels have indicated that increasing the hydrophobic content of those systems leads to reduced water content within the gel.22 Additionally, a recent paper by Illergård et al. described the adsorption of a hydrophobically modified poly(vinyl amine) and poly(acrylic acid).23 Although not explicitly stated by the authors, their data seem to indicate decreasing water content with increasing hydrophobic content within the PE complex. Compared to strong and weak PEs, hydrophobically modified PEs (HPEs) have received little attention in the literature. Often, studies employing HPEs have focused on Received: April 25, 2012 Revised: June 1, 2012 Published: June 5, 2012 11086
dx.doi.org/10.1021/la3016996 | Langmuir 2012, 28, 11086−11094
Langmuir
Article
adsorption and using a PE with primary amino groups as a reference.33,34 The results of this work are expected to yield new strategies to dewater PE systems and have potential applications in mineral recovery, paper manufacturing, and biomedical materials.14,35,36
block copolymers incorporating hydrophobic domains or large pendant hydrophobic groups along a PE backbone.24−26 These materials tend to aggregate in solution, especially as charge density decreases or hydrophobic domain size increases.25 Adsorption of HPEs onto hydrophobic surfaces is often a focus of research,27,28 as is LbL films involving HPEs.29,30 Systems prepared by LbL assembly containing HPEs tend to result in thicker films, as hydrophobic groups may sterically hinder electrostatic effects in a manner analogous to increased ionic strength.31 Collectively, these studies indicate that hydrophobic content plays a significant, and sometimes even a dominant, role within a PE system. However, the role of hydrophobic content in dewatering PE systems has not been established. The aim of this work is to determine how increasing the size of alkyl substituents around the nitrogen atom of a series of amine-functionalized, cationic dextran derivatives (cDex) affects their adsorption onto an anionic substrate and to quantify the water within the PE complex. Toward this end, the adsorption of a series of cDex onto sulfated nanocrystalline cellulose (SNC) thin films was studied via quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) experiments. These complementary techniques enable the determination of both adsorbate surface excess (Γ) as well as water entrapped within the film.32 Three types of cDex having different degrees of substitution (DS): (N,Ndimethylam ino)ethyldextran (DMAE-Dex), (N,Ndiethylamino)ethyldextran (DEAE-Dex), and (N,Ndiisopropylamino)ethyldextran (DIAE-Dex) (Figure 1) were studied. A solvent exchange procedure using QCM-D was used to quantify the amount of water before and after HPE
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EXPERIMENTAL SECTION
Materials. Dextran (nominal MW ∼60 kDa) from Leuconostoc mesenteroides was purchased from Fluka (Neu-Ulm, Germany). 2Chloro-N,N-dimethylethylamine hydrochloride, 2-chloro-N,N-diethylethylamine hydrochloride, and 2-chloro-N,N-diisopropylethylamine hydrochloride were obtained from Sigma-Aldrich (Deisenhofen, Germany). SNC was prepared by sulfuric acid hydrolysis of dissolving grade softwood pulp (Temalfa 93-A-A, Tembec, Inc.).37 SNC had a charge density of 0.293 mequiv g−1 (157 μS cm−1) by conductometric titration and an average height and length of 4.8 ± 1.4 nm and 154 ± 84 nm, respectively. 11-Amino-1-undecanethiol, NH4OH (28% w/w), and H2O2 (30% w/w) were purchased from VWR International, Fisher Scientific, and EM Science, respectively. Poly(allylamine) (PAH, 65 kDa) was purchased from Aldrich. Ultrapure water (MilliQ Gradient A-10, Milli-Q, 18.2 MΩ·cm,
