Electroacoustic characterization of conventional and electrosterically stabilized nanocrystalline celluloses Salman Safari a,c,1 , Amir Sheikhi a,c,1 , Theo G.M. van de Ven b,c,⇑ a Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 0C5, Canada b Pulp & Paper Research Centre, Department of Chemistry, McGill University, H3A 2A7 Montreal, Quebec, Canada c Centre for Self-Assembled Chemical Structures, McGill University, H3A 2A7 Montreal, Quebec, Canada article info Article history: Received 29 April 2014 Accepted 28 June 2014 Available online 15 July 2014 Keywords: Nanocrystalline cellulose Dicarboxylated cellulose Dynamic mobility Zeta potential Sound attanuation Electrosonic amplitude abstract Nanoparticles are widely used as drug carriers, texturizing agents, fat replacers, and reinforcing inclu- sions. Because of a growing interest in non-renewable materials, much research has focused on nanocel- lulose derivatives, which are biodegradable, biocompatible, and easily synthesized. Among nanocellulose derivatives, nanocrystalline cellulose (NCC) has been known for half a century, but its utility is limited because its colloidal stability is challenged by added salt. On the other hand, electrosterically stabilized nanocrystalline cellulose (ENCC) has recently been observed to have superior colloidal stability. Here, we use electrokinetic-sonic-amplitude (ESA) and acoustic attenuation spectroscopy to assess NCC and ENCC f-potentials and sizes over wide ranges of pH and ionic strength. The results attest to a soft, porous layer of dicarboxylic cellulose (DCC) polymers that expands and collapses with ionic strength, electrosterically stabilizing ENCC dispersions at ionic strengths up to at least 200 mmol L 1 . Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Cellulose is a natural polymer comprising repeated anhydroglu- cose units linked by b-1,4-glycosidic bonds (C–O–C) [1]. Because of its high abundance and biodegradability, it is a key source of indus- trial scale recyclable materials. Cellulose exists in trees and plants, is produced by photosynthesis, and is synthesized by some bacte- ria and organisms (tunicates) [1]. Green and renewable replace- ments for fossil-derived materials have turned attention to cellulose products [2]. These ‘green’ nanoparticles are from renew- able sources, have large surface to volume ratios, and have other unique properties. Two distinct cellulosic nanoparticulates are nanocrystalline cel- lulose (NCC) and nanofibrillar cellulose (NFC). The former was pro- duced for the first time by a harsh hydrolysis of plant materials, and an ultrasonication post-treatment [3]; and the latter was pre- pared by subjecting a wood-pulp slurry to a high pressure homog- enizer [4]. NCC and NFC particles have widths in the range 3–10 nm [5,6]; however, because of hydrolysis and dissolution of amorphous regions, lengths of NCC are in the range 100–200 nm [5], whereas NFC can be up to several microns long [7]. High crystallinity imparts NCC with a high tensile strength (7.5–7.7 GPa) [8], high elastic modulus (up to 220 GPa) [9,10], and very low thermal expansion coefficient (0.1 ppm K 1 ) [11]. In addition to these mechanical properties, optical properties [12–15], such as birefringence [12], have paved the way for NCC applications in decorative materials, security papers, foods, emul- sions/dispersions (as a texturizing agent and fat replacer), hygiene/absorbent materials, packaging materials, and nanocom- posites (as reinforcing inclusions) [16,17]. Recent studies suggest NCC applications in the pharmaceutical industry as a tablet binder [18] or a bioimaging agent [19], expedited by a high surface charge density [20,21]. In the conventional chemical hydrolysis approach, the cellulose microfibrils are broken into individual glucose units using a strong acid, and the dissolution of amorphous domains produces short nanocelluloses. Mechanical fibrillation of wood fibers produces longer microfibrils, but consumes considerable energy and leaves most NFC bundled. Some groups have proposed a combination of enzymatic and mechanical treatments to reduce the energy demand [22,23]. In contrast to acid-pretreatment, enzyme-pre- treatment decreases the degree of polymerization less, producing high-aspect-ratio nanofibers [24]. In an alternative approach, the C 6 hydroxyl group was selectively converted to carboxyl via NaClO and NaClO 2 oxidation in the pres- ence of 2; 2; 6; 6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a cata- lyst [7]. Following oxidation, a mild mechanical post-treatment http://dx.doi.org/10.1016/j.jcis.2014.06.061 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Centre for Self-Assembled Chemical Structures, McGill University, H3A 2A7 Montreal, Quebec, Canada. E-mail address: theo.vandeven@mcgill.ca (T.G.M. van de Ven). 1 These authors contributed equally to this work. Journal of Colloid and Interface Science 432 (2014) 151–157 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis