1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 DOI: 10.1002/prep.201700097 Modifying the Wettability of Nitramine Explosives using Anionic, Cationic and Nonionic Surfactants Mouhcine Doukkali,* [a] Eric Gauthier, [a] Rajen B. Patel, [a] Victor Stepanov, [a] and Hamid Hadim [b] Abstract: Wetting behavior of energetic materials surface in- cluding cyclotrimethylene trinitramine (RDX), cyclotetra- methylene tetranitramine (HMX) and hexanitrohex- aazaisowurtzitane (CL-20) using nonionic (Triton-X), anionic (SDS), and cationic (TTAB) surfactants has been studied by contact angle tensiometry. Results show that TTAB more sig- nificantly reduces the contact angle and improves wettability as compared to SDS and Triton-X. The liquid-vapor surface tension g lv was measured as a function of TTAB surfactant concentration in aqueous solutions and used to construct a Zisman plot to determine the critical surface tension of RDX, HMX and CL-20. The results show that HMX displays the high- est degree of wettability while RDX is most difficult to wet. The computed values of the work of spreading complement the previously discussed results where contact angle de- creases with increasing surfactant concentration. They also in- dicate that RDX appears most impacted by the addition of TTAB surfactant. However, the addition of TTAB also has a sig- nificant impact on improving the wettability of HMX and CL- 20. This wettability study plays an important role in the for- mation of well-wetted energetic surfaces needed for efficient wet milling, coating and granulation processes. Keywords: Wettabilitty · Nitramines · surfactant · RDX · HMX · CL-20 1 Introduction Wettability plays an important role in numerous technological applications, such as: oil recovery, coating, adhesion, flotation, printing, detergency and the cosmetics industry [1–7]. Wett- ability is also considered one of the primary factors leads to higher mechanical stability of energetic materials due to im- proved wetting of the liquid (polymeric binder) on a particulate explosive [8]. The friction sensitivity of primary explosives is also affected by the wettability [9]. Wettability studies usually involve the measurement of contact angles (q) as the primary data, which indicate the degree of wetting when a solid and liquid interact. Small contact angles correspond to high wettability, while large contact angles correspond to low wettability ( Fig- ure 1). The contact angle is affected by the chemical composi- tion, roughness, the surface charge of the solid, and by the liq- uid properties [10–12]. The sessile drop method is a technique that directly measures contact angle on a solid sample and is used to measure the contact angle between the liquid and the compressed explosive powder in this work. The method is usu- ally used for smooth, homogeneous, impermeable and non-de- formable surfaces. Due to the inherent porous architecture of compressed powder cakes, liquid penetration may occur de- pending on the wettability of the individual particles. This type of liquid penetration would eliminate the potential to utilize the direct measurement method, high surface energy materials re- sist such liquid penetration. An indirect method to measure the contact angle such as the capillary penetration method [8] could be used as an alternative. For our energetic materials the surface energy is high enough that reproducible wetting was made possible by using identical pellet compression conditions resulting in a low porosity sample. As first described by Thomas Young [13] in 1805, the contact angle of a liquid drop on an ideal solid surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions (Figure 2): g lv cos q ð Þ¼ g sv  g sl ð1Þ where g lv , g sv , and g sl represent the liquid-vapor, solid-va- por, and solid-liquid interfacial tensions, respectively, and q [a] M. Doukkali, E. Gauthier, R. B. Patel, V. Stepanov U.S. Army, Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ, USA *e-mail: mouhcine.doukkali.civ@mail.mil [b] H. Hadim Department of Mechanical Engineering, Stevens Institute of Tech- nology, Hoboken, NJ 07030, USA Figure 1. Illustration of contact angle for poor and good wetting. Full Paper These are not the final page numbers! ÞÞ Propellants Explos. Pyrotech. 2017, 42, 1–7 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1