پایداری طولانی مدت نانولوله های کربنی چند جداره در الکترولیت Automatic dispersion, long-term stability of multi-walled carbon nanotubes in high concentration electrolytes

Introduction Nanoparticles are attention-attracting materials in oil and gas industry (Cai et al. 2012; Contreras et al. 2014; Ghanbari et al. 2016), and it have been reported in a wide range of applications such as micronanometer particle plugging (Barry et al. 2015; Kosynkin et al. 2012), enhancing oil recovery (Luo et al. 2016; Luo et al. 2017), oil-based lubricates (Zhang et al. 2017), and downhole hydrocarbon detection (Hwang et al. 2012). Nanoparticles are always exposed to brine environment in most of the applications for the petroleum industry, such as the oil reservoirs, which usually contains high salinity. Nanoparticles are inclined to agglomerate into larger particles in salt solutions, losing the unique properties of nanomaterials. For example, nanoparticles (NPs) as fluid loss additives were added into water-based drilling fluids for plugging micro-nanometer pore and throat, nanoparticles will become larger size and loss them functions. Therefore, nanoparticles cannot keep at nanoscale when it is added into the working fluids containing salts. Dan Luo (Luo et al. 2017) and their group firstly prepared graphene-based amphiphilic Janus nanosheets and investigated the colloidal stability of nanosheets at different salt concentrations. Their studies showed that graphene-based amphiphilic Janus nanosheets could not disperse in salt solution, even in 0.05844 wt% NaCl solution. As shown in Fig. 1, uncoated multi-walled nanotubes aggregated almost immediately after being dispersed in both 1.0 wt% NaCl solutions and the standard API brines. It reveals that multi-walled carbon nanotubes (MWNTs) was not stably dispersed even in low salinity of 1.0 wt% NaCl solutions and the standard API brines at room temperature (Fig. 1). This was attributed to the presence of salts, which could easily  disrupt electric double layers between particles and make nanoparticles unstable (Elimelech and O’Melia 1990; Kharissova et al. 2013). Thus, stabilization of nano-sized materials in aqueous environments with high salinities is an ongoing important issue for petroleum industry. In recent years, surfactants (Duque et al. 2008), polymers (Kadhum et al. 2017), and small ligands (Kotsmar et al. 2010) used to stabilize nanoparticles in brines have been reported. Surfactants, which adsorb onto nanoparticles by hydrophobic components and extend their hydrophilic components into brines, are providing steric stabilization (Jones et al. 1991). Sodium dodecyl sulfate (SDS) is applied to make single-walled carbon nanotubes stable in NaCl concentrations of 200 mM, and MgCl2 concentrations of 10 mM brines for 24 h. Glaura G.Silva (Soares et al. 2014) mentioned that AvanelS 150 CGN surfactant (C12–15(EO)15SO3 − Na+ ) was used as a surfactant for stable dispersions of the modified carbon nanoparticles in saline water. Polyvinyl pyrrolidone is a highly water-soluble polymer, which adsorbed onto the CNTs surface to provide steric stabilization by long chains in water repelling each other.