Surface activity and flocculation behavior of polyethylene glycol-functionalized silica nanoparticles

https://doi.org/10.1016/j.jcis.2015.04.043Get rights and content

Abstract

Colloidal silica nanoparticles have been functionalized with methyl polyethylene glycol silane (mPEG silane) and the PEGylated particles have been characterized with focus on exploring their surface chemical properties. The degree of surface functionalization was quantified using NMR diffusometry, and the measurements showed that the silane binds covalently to the silica surface. Samples with surface coverages ranging from 0.068 to 0.315 μmol silane/m2 have been analyzed. The functionalized particles proved to be surface active and showed a significant reduction in surface charge and zeta potential with increasing degree of PEG functionalization. All samples showed colloidal stability at neutral pH and above within the range studied. At lower pH, the samples with low surface coverage displayed a reversible flocculation behavior, while samples with a high surface coverage and samples without functionalization remained stable. This suggests that steric stabilization is effective at low pH when the surface coverage is high enough; electrostatic stabilization is effective for samples without functionalization; and that inter-particle PEG–silica interactions cause flocculation of particles with too low degrees of PEG functionalization.

Introduction

Dispersions of colloidal silica, also referred to as silica sols, are utilized in numerous industrial processes and commercial products, such as retention aid in paper making, as additives in paints, as binder in foundry applications, for beverage clarification, and in polishing applications [1]. In all these applications the nature of the silica surface and the ability to control the aggregation of the colloidal particles are of great importance. Studies concerning the surface chemistry of amorphous silica have previously been undertaken [2], [3], [4]. The surface chemistry is complex and further characteristics remains to be understood. To further expand the uses of silica, and to improve the use in existing applications, development of new composite silica materials has been extensively explored. Surface modification is a common approach to customizing the silica material properties, which requires further investigations of the material in terms of characterization and evaluation methods. Publications addressing the grafting of poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO) onto silica surfaces are frequently appearing, often with uses in biomedical applications due to the biocompatibility of the material [5], [6], [7], [8], [9]. PEG-containing coatings is another field of interest, where for example Malmsten et al. [10] found that a high enough interfacial density of PEG on flat silica surfaces results in efficient protein rejection. Leckband et al. studied intermolecular forces and show that grafted PEG chains can exist in both protein-repulsive and protein-attractive states, depending on factors such as compression and polymer chain length [11]. The grafting of PEO onto the surface of silica particles was reported already in the 1980s by Bridger and Vincent [12]. They investigated two methods for terminal grafting of PEO chains of which one was suitable for aqueous conditions, using in situ grafting of isocyanate-capped PEO during particle formation (by Stöber method [13]). However, the modified particles obtained had a low colloidal stability in water [12]. Other examples are Zhang et al. [14] who prepared PEGylated silica particles in methanol addressing both chemical and colloidal stability issues in water and Xu et al. [15] who obtained PEG-coated silica through synthesis in a methanol–ammonia mixture. Joubert et al. [16] prepared PEO-grafted silica through graft polymerization; the polymerization of EO was initiated from the silica surface, onto which alcohol groups had been attached. Still, the grafting of PEG onto silica particles where the synthesis is carried out by a simple process in purely aqueous conditions is not as recurrent.

In this paper we report on the characterization of colloidal silica particles functionalized with methyl end-capped tri-methoxy poly(ethylene glycol) silane (mPEG silane) prepared via a simple water-based route. A direct measurement of the mPEG silane attached to the silica particles has been employed using NMR diffusometry, providing quantitative information of the grafting efficiency. The aggregation behavior of the PEGylated particles has been studied with dynamic light scattering (DLS) and UV–Vis spectroscopy. Further, the modified silica surface has been characterized through polyelectrolyte adsorption and zeta potential measurements. In addition, surface activity of the particles has been assessed, since the use of particles as stabilizers for emulsions has recently gained much interest [17], [18]. This full study and characterization provides both quantitative information concerning the degree of surface functionalization and surface activity as well as qualitative information concerning flocculation behavior of surface modified colloidal silica.

Section snippets

Materials

Sodium ion-stabilized colloidal silica particles with the trade name Bindzil 40/130 were provided by AkzoNobel Pulp and Performance Chemicals. These anionic particles have a surface area of 130 m2/g, as measured by Sears titration [19], corresponding to an equivalent spherical diameter of 21 nm [4]. The suspension has a native pH of 9.1 and a concentration of 40 wt% silica. Silquest A-1230 (from Momentive), a tri-methoxy poly(ethylene glycol) silane end-capped with a methyl group and denoted mPEG

Functionalization procedure

Various syntheses were conducted, in order to investigate the effect of the reaction conditions on the yield in terms of bound mPEG silane in relation to the added amount of silane, and also to acquire particles with varying degree of surface coverage. The synthesis could be conducted at room-temperature. A slow addition rate, preferably below 0.25 μmol/(m2 h), was required to minimize self-condensation of the mPEG silanes. The maximum surface coverage obtained at the reaction conditions in this

Conclusions

In this article we have presented a simple, water-based route through which PEGylated silica particles of varying surface coverage can be obtained. The covalent attachment of the mPEG silane to the silica surface has been confirmed using NMR diffusometry, a method allowing quantification of the degree of functionalization. Although significant amounts of free mPEG silane were present in the as-prepared samples, purification allowed studies of the effects on the surface properties of silica

Acknowledgments

The authors would like to acknowledge the Swedish Research Council and AkzoNobel Pulp and Performance Chemicals (PPC) for financial support of this project. Krister Holmberg and Romain Bordes, Chalmers University of Technology and Andreas Sundblom, AkzoNobel PPC are acknowledged for fruitful discussions. The NMR diffusion measurements were carried out at the Swedish NMR Centre, Göteborg, Sweden.

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