3.1. Nanoparticle suspension stability
In this section, we are mainly concerned with dispersions of colloidal solid particles in liquids. The main point is that if the dispersion remains in dispersed state under the experiment conditions or flocculation and coagulation take place. Two cases of aqueous suspension stability without any additives and with surfactant addition were considered.
While the aqueous suspensions of AEROSIL 300 hydrophilic nanoparticles remained stable for more than one month mainly due to their tiny size, slightly hydrophobic AEROSIL R816 nanoparticle suspensions tended to agglomerate after a short period of two to three days. This phenomenon may be related to the hydrophobic chain attached to nanoparticle surface which makes the aqueous bulk solution to be an unfavorable environment for individual particles. As a result, nanoparticles begin to agglomerate and finally settle due to gravitational forces.
Addition of SDS anionic surfactant to nanoparticle suspensions extended the stability period for both nanoparticles especially in the case of AEROSIL R816 and no agglomeration occurred. Fig. 2 represents both agglomerated and stabilized suspensions for slightly hydrophobic R816 nanoparticles. Flocculated and agglomerated particles were observed in the absence of surfactant (left). By the inclusion of surfactant molecules (right), however, the suspension remained in stable condition for a long period and no coagulation was observed (The suspension was completely clear). The stability improvement of nanoparticle suspensions may be interpreted through the adsorption of SDS on nanoparticle surface, bringing about supercharged system by increasing the effective charge of silica and consequently stabilize the system (Ahualli et al., 2011 and Luo et al., 2009). In the case of AEROSIL R816 nanoparticles, hydrophobic interactions between alkyl groups of silica particle and surfactant chain may be also responsible for suspension stabilization.