Surfactant-Based Dispersant for Multiwall Carbon Nanotubes to Prepare Ceramic Composites by a Sol-Gel Method Patrícia R. Silva, † Voltaire O. Almeida, † Geraldo B. Machado, ‡ Edilson V. Benvenutti, ‡ Tania M. H. Costa, ‡ and Ma ́ rcia R. Gallas* ,† † Instituto de Física, Universidade Federal do Rio Grande do Sul, PO Box 15051, CEP 91501-970 Porto Alegre, RS Brazil ‡ Instituto de Química, Universidade Federal do Rio Grande do Sul, PO Box 15003, CEP 91501-970 Porto Alegre, RS Brazil ABSTRACT: A dispersant for multiwall carbon nanotubes (MWCNTs) is proposed that fulfils the requirements of creating a uniform dispersion in the matrix and obtaining a good interface between CNTs and the matrix, and is soluble in generic nonpolar solvents. This dispersant is based on a long chain surfactant, called in this work dabcosil stearate, containing a stearate-based 18-carbon alkyl chain as an anion, and a silsesquioxane containing a bridged, positively charged 1,4-diazoniabicyclo[2.2.2]octane group. It provides not only a very good dispersion medium for the MWCNTs, but also a very good interface between MWCNTs and ceramic matrices, such as alumina and zirconia, prepared by the sol-gel method. 1. INTRODUCTION Carbon nanotubes (CNTs), observed for the first time at 1991, 1 have attracted considerable attention, because of their unique set of extraordinary properties, which allows wide range of applications. It is well-known, however, that for any advanced applications, ranging from the medical sciences 2,3 to electronics, 4 and to composite materials, 5-9 it is required a homogeneous and stable dispersion of isolated CNTs in both organic and aqueous solutions before the preparation of the final material. Recent advances in nanomaterials, particularly, CNT/polymer 5,6 and CNT/ceramic composites, 7-9 reported that not only the dispersion is important, but also a strong interfacial bonding of the CNTs with the host matrix components is fundamental to have an efficient load transfer from host matrix to CNT. Achieving that, it is possible to obtain high performance materials with multifunctional proper- ties. During the past decade, ceramic matrices composites reinforced by CNTs have been extensively studied, 7 aiming to improve the intrinsic brittleness of these materials. 10-13 Among ceramics, alumina and zirconia are of great interest because of their numerous applications in several fields as guide wires in textile industry, catalyst, electric and dielectric materials, refractory materials, and others. 14,15 However, to produce these composites, the CNTs must be processed in such a way to ensure that a homogeneous dispersion is obtained within the matrix, while developing an appropriate degree of interfacial bonding. It is worth to mention that in the case of poor or absent interfacial bondings, CNTs may even act as a source of microcracks, leading to failure. 7 A fair amount of research has been conducted on dispersion of CNTs based on both physical and chemical approaches. 16-23 Chemical functionalization was proposed as a promising method to improve the dispersion of CNTs in organic solvents as well as in aqueous media. However, this method showed that covalent surface functionalization can affect inherent electrical, mechanical, and optical properties of CNTs. 17,18,22 Hence, several studies were concentrated on noncovalent modifica- tions, and the use of surfactants to stabilize CNTs suspension becomes an efficient approach. Different kinds of surfactants, classified as anionic, cationic, nonionic or zwitterionic, depending on their head groups, have been investigated. 18-20 The surfactant choice basically depends on the kind of solvent that is used to disperse the CNTs, the type of matrix (polymeric or ceramic) the CNTs will be incorporated, and what properties you want to improve. The most used anionic surfactant for the preparation of dispersed single wall carbon nanotubes is the sodium dodecyl sulfate (SDS) and the simulation described by Xu et al 23 provides comprehensive direct evidence for SDS self-assembly structures on carbon nanotube surfaces, which can help to clarify the relevant debate over the exact adsorption structure. In general, surfactants can interact with CNTs through several types of interactions, for example, hydrophobic interaction between hydrophobic chain of surfactant and sidewalls of CNTs, or π-π interaction of benzene rings on surfactants with the surface of CNTs. Such interactions lead to noncovalent adsorption of surfactants onto CNTs, providing a net positive or negative charge on the tube Received: August 5, 2011 Revised: December 1, 2011 Published: December 6, 2011 Article pubs.acs.org/Langmuir © 2011 American Chemical Society 1447 dx.doi.org/10.1021/la203056f | Langmuir 2012, 28, 1447-1452