Green Chemistry Dynamic Article Links Cite this: Green Chem., 2011, 13, 2464 www.rsc.org/greenchem PAPER Surface hydrophobization of bacterial and vegetable cellulose fibers using ionic liquids as solvent media and catalysts Liliana C. Tom´ e, a,b Mara G. Freire, a,b Lu´ ıs Paulo N. Rebelo, b Armando J. D. Silvestre, a Carlos Pascoal Neto, a Isabel M. Marrucho a,b and Carmen S. R. Freire* a Received 19th April 2011, Accepted 2nd June 2011 DOI: 10.1039/c1gc15432j The surface hydrophobization through heterogeneous chemical modification of bacterial (and vegetable) cellulose fibers with several anhydrides (acetic, butyric, hexanoic and alkenyl succinic anhydrides) and hexanoyl chloride suspended in an ionic liquid, tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)imide, [TDTHP][NTf 2 ], was studied. Furthermore, in the reaction with hexanoyl chloride, another ionic liquid, N-hexyl-4-(dimethylamino)pyridinium bis(trifluoro- methylsulfonyl)imide, [C 6 N(CH 3 ) 2 py][NTf 2 ], was used instead of common organic bases as catalyst and to trap the released HCl. The analysis of the ensuing modified fibers by FTIR, XRD and SEM clearly showed that the esterification reactions occurred essentially at the fibers’ outmost layers, not affecting their ultrastructure. The degree of substitution (DS) of the ensuing esterified fibers ranged from less than 0.002 to 0.41; and in all instances, the fibers’ surface acquired a high hydrophobicity. This novel approach constitutes an important strategy in the preparation of modified fibers under greener conditions relaying in the use of non-volatile solvents. Introduction Cellulose, the major component of plant cell walls, is un- doubtedly the most abundant natural polymer. Its remarkable properties (as well as those of its derivatives), have been exploited by mankind for millennia, particularly in paper and textiles, and new potential applications for this ubiquitous renewable resource are being increasingly hunted, 1 mainly in response to the predicted scarceness of fossil resources, their unsustainable prices and consequently to the increasing interest in the develop- ment of biodegradable materials based on renewable resources. 2 In addition to the “conventional” vegetable cellulose fibers, other forms of cellulose have also attracted the attention of the scientific community in the last few years, in particular bacterial cellulose (BC) and also other nanocellulose substrates (nanofibrillated cellulose and cellulose whiskers). BC is an extracellular polysaccharide, produced by several bacteria of the Gluconacetobacter genus, as a three-dimensional network of nano- and microfibrills with 10–100 nm in diameter, which possesses unique physical and mechanical properties, such as its high water holding capacity, crystallinity, tensile strength and Young modulus. 3 a CICECO, Departamento de Qu´ ımica, Universidade de Aveiro, Campus Universit´ ario de Santiago, 3810-193, Aveiro, Portugal. E-mail: cfreire@ua.pt; Fax: +351 234 370 084; Tel: +351 234 401 405 b Instituto de Tecnologia Qu´ ımica e Biol ´ ogica, Universidade Nova de Lisboa, Av. Rep´ ublica, Apartado 127, 2780-901, Oeiras, Portugal; Web: www.itqb.unl.pt Nanocellulose fibers (as well as vegetable fibers) have found application in several domains, 4–6 namely as reinforcing ele- ments in composite materials based on polymeric matrices. 7–10 The major problems faced by researchers in this field are undoubtedly related to the highly polar surface of the cellulose fibers, associated with its OH-rich structure, which entails (i) a very low interfacial compatibility with non-polar matrices like polyolefins (resulting in inadequate mechanical performance of the corresponding composites), (ii) moisture uptake (which results in loss of fiber strength) and (iii) inter-fiber aggregation by hydrogen bonding (leading to poor fiber dispersion in the composite). 11 Several strategies have been explored to overcome these drawbacks, mostly involving specific surface treatments of the fibers aimed at reducing its polar character, which include both physical and chemical modifications. 12 However, most of the chemical approaches described for the surface hydropho- bization of cellulose fibers involve the use of hazardous non- polar organic solvents such as toluene and dichloromethane, among others, 13–15 which represent an important concern when considering the scale-up of these processes. Ionic liquids (ILs) are low-melting-point salts, and apart from their broad definition as compounds with a unique combination of properties such as negligible volatility, high stability, and easy recyclability, their most attractive feature is their tunability which allows the design of ILs for each specific purpose. 16 Although ILs are already very popular in cellulose chemistry, most literature is focused on the dissolution/regeneration and homogeneous derivatization of cellulose. 17–19 For example, Wu 2464 | Green Chem., 2011, 13, 2464–2470 This journal is © The Royal Society of Chemistry 2011 Downloaded by Universidade de Aveiro (UAveiro) on 31 August 2011 Published on 18 July 2011 on http://pubs.rsc.org | doi:10.1039/C1GC15432J View Online