The stability and surface activity of environmentally responsive surface-modified silica nanoparticles: the importance of hydrophobicity

In this study, the effect of hydrophobicity of environmentally responsive surface-modified silica nanoparticles on the stability and surface activity of the nanoparticles is examined. To this end, n-propyl, n-hexyl, or n-octyl chains and methoxy poly(ethylene glycol) chains at various quantities were coated covalently on the surface of silica nanoparticles to regulate the hydrophobicity of the responsive nanoparticles. Various experimental tools such as stability analysis, interfacial-tension and contact-angle measurements, and emulsion formation were performed to investigate the effect of hydrophobicity. It became evident that the presence of the hydrophobic agents influences considerably the stability and surface activity of the responsive nanoparticles. To achieve the most stable nanofluids and the largest reduction in the oil-water interfacial tension, the hydrophobicity should be tuned at an optimum value, which would be at a lower amount with longer linear-alkyl chains. While our solely hydrophobic- or hydrophilic-silica nanofluids do not emulsify with n-hexane, the ability of the responsive-silica nanofluids to emulsify with n-hexane is more, with the ability being largely dependent on the length of the linear-alkyl chains. Furthermore, increasing the hydrophobicity has adverse effect on the wettability-alteration ability of the responsive nanoparticles.