Surface Passivation of Bare Boron Nanoparticles Using New Dicyanamide-Based Dicationic Ionic Liquid R. Fareghi-Alamdari,* ,† F. Ghorbani-Zamani, † and M. Shekarriz ‡ † Faculty of Chemistry and Chemical Engineering, Malek-Ashtar University of Technology, Tehran, Iran ‡ Research Institute of Petroleum Industry, West Boulevard, Azadi Sports Complex, Tehran, Iran ABSTRACT: Boron is traditionally used as an additive in energetic systems as a result of the high density of energy. In particular, the existence of the naturally formed boron oxide (B 2 O 3 ) layer retards the reactivity by acting as a barrier if it cannot be efficiently removed. In this study, the new dicationic ionic liquid based on dicyanamide anoins was synthesized and used as a protective ligand for boron nanoparticles. The effects of newly synthesized ionic liquid are investigated by a combination of X-ray diffraction (XRD), energy-dispersive X-ray (EDX), scanning electron microscopy (SEM), dynamic light scanning (DLS), ζ-potential measurements, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). It was found that this ionic liquid binds to boron well enough and protects the boron surfaces from oxidation during air exposure. 1. INTRODUCTION To make fuels more cost-competitive, one of the potential strategies is to increase the energy density by adding higher energy density additives. Nanosized metal powders have been considered as potential fuel additives as a result of their high specific surface area and ability to store energy on the surface. Among various metals considered as additives for fuels and propellants, the highest combustion enthalpies are seen for aluminum, boron, beryllium, magnesium, etc. 1 Of these, boron has the highest volumetric heating value as well as second highest on a gravimetric basis, after the toxic beryllium. 2 Despite the desirable properties of boron, the incorporation of boron nanoparticles into the propellants is not widely practiced, because of its oxidation product B 2 O 3 . The boron particles combustion occurs in two successive steps: the first step involves the removal of the oxide layer, while the second involves the burning of bare boron. 3 The pre-existing oxide layer on the boron surface plays an important role in the ignition and combustion processes; more precisely, it delays the ignition process. Various methods have been conducted to mitigate the effects of the oxide layer and enhance the ignition of boron nano- particles, including the following: 4-11 (1) treating boron particles with TiCl 4 and triethylaluminum followed by the addition of ethylene or even coating boron particles with LiF and trimethylolpropane, (2) sodium naphtalenid reduction of BBr 3 in 1,2-dimethoxyethene followed by n-octanol, (3) coating of boron particles with metals, such as titanium and Mg, (4) utilization of energetic materials, such as glycidyl azide polymer (GAP) and azide polymer (AP), coating on the boron surface, and (5) capping boron particles with an organic oleic acid layer. Recently, ionic liquids (ILs), which are organic salts with a melting point below 100 °C, were used as capping agents for boron particles. 9 ILs have unique properties, such as low-vapor pressure, liquidity over a wide temperature range, high density, high thermal and chemical stabilities, and structural design ability. 12,13 Dicationic ionic liquids (DCILs), a type of IL, in which two monocations are combined into a dication have been recently synthesized and reported. 14-17 These new types of ILs are considered as solid or melting salts near room temperature. Therefore, they could afford new supramolecules that are suitable for various applications as a result of their high thermal properties and broad liquid range. 18-22 These compounds have been studied in different applications, such as high-temperature lubricants, 23,24 a solvent for high- temperature organic reactions, 23-26 an additive in dye-sensitized solar cells, 27 an extraction liquid 28 in chromatography 29-31 and mass spectroscopy, 32 and an electrolyte for secondary batteries. 24 Several studies have been performed on the physical properties (e.g., density and viscosity) of DCILs. Most of these properties are dependent upon the struture of DCIL, as is usually the case in monocationic ILs. 33,34 Furthermore, the functionalization ability of dicationic ILs explored the opportunity to design their structures with respect to cations, anions, and length of linker chains in between two cations. 35-38 Anderson et al. studied the abilities of boron nanoparticles as a high-density additive for dicyanamide-based monocationic IL propellants. 9,39 Furthermore, they show that how cationic and anionic structures of ILs play the important roles in the interac- tion with boron surfaces. 9 Despite much effort, most of the research work has thus far focused on monocationic-type energetic ILs. To gain a further understanding of structure-property relations and extend the applications of ILs as energetic and hypergolic components, it is incumbent to explore new dicationic IL structures. In the current study and in continuation of our interest in the synthesis and application of ILs, 40,41 here, we report the synthesis and characterization of the new dicationic IL and investigate the effect of this IL on preparing oxide-free and air-stable boron nanoparticles. Received: July 10, 2015 Revised: December 4, 2015 Article pubs.acs.org/EF © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.5b01556 Energy Fuels XXXX, XXX, XXX-XXX