Colloidal Interactions between Particles with Tethered Nonpolar Chains Dispersed in Polar Media

Colloidal interactions between particles dispersed in a liquid can be suitably tailored by modifying the surface chemistry of the particles. In the case of fumed silica particles, the surface can be systematically altered from hydrophilic to hydrophobic by replacing a portion of the original silanol (Si-OH) groups by nonpolar alkyl chains. In this study, we probe the effect of surface modification of fumed silica on their rheology and microstructure in polar media. Variables of interest include the length of the tethered alkyl chain and the extent of surface coverage. For the continuous phase, we examine a range of polyether liquids comprising different architectures and molecular weights. We find that when the alkyl chains are C8 or longer, and are attached at saturation levels, a dense nonpolar surface layer is formed on each silica unit. Such particles experience strong interactions in polar media, leading to the formation of a volume-filling network (gel). We show that these interactions arise as a result of the negative free energy of mixing between the tethered chains, owing to the mismatch in chemical nature between chains and solvent. In this flocculation process van der Waals interactions between the particles play a negligible role. We also find that the greater the mismatch between particle surface and liquid, the greater the “stickiness” of the surface chains and correspondingly, the higher the elastic modulus (G′) of the fumed silica network. This leads to a unique correlation between G′ and a term comprising the ø parameter for the chain-solvent pair. An approximate but useful form of this correlation can be written as G′ ∼ (δs δm) where the latter expression characterizes the mismatch in solubility parameters between the surface chains (δs) and the liquid medium (δm).