The effect of silanization assisted nanoparticle hydrophobicity on emulsion stability through droplet size distribution analysis

Abstract

There has been great recent interest in the potential use of Nanoparticles as modifiers or alternatives to surfactants for stabilizing emulsions, among which silica NPs have been commonly used as the most cost effective option. Since silica NPs are hydrophilic with a tendency toward the aqueous phase, their surface should be activated either by the aid of a surfactant or by chemical treatment. The aim of the present work is to assess the effect of two chemically activated silica NPs on stability of dodecane-water emulsions through optical microscopy. Therefore, hydrophilic silica NPs are firstly synthesized with sodium silicate solution and are subsequently silanized with hexamethyldisilazane (HMDS) into different extents to synthesize two different NPs with different wettability states, namely as HSNP1 and HSNP2. The degree of Nanoparticle wettability is quantified by the variation of the silanol group density of bare silica NPs, which has reduced by 49% for HSNP1, resulting in partially hydrophobic silica NPs and by 60% for HSNP2, resulting in hydrophobic silica NPs. The emulsion stability prepared by HSNP1 and HSNP2 has been assessed via both bottle tests and droplet size distribution analysis, which is tracked by fitting proper distribution functions on the experimental data. The log-normal and the bimodal (a mixture of two log-normal distribution) distribution functions are respectively fitted on the droplet size distribution of emulsions stabilized by HSNP1 and HSNP2. This revealed minor changes in the droplet size distribution of emulsions stabilized by partially hydrophobic silica NPs (HSNP1) over a month, while a significant alteration in droplet size distribution of emulsions stabilized by hydrophobic silica NPs (HSNP2) has been observed after a week. Higher emulsion stability stabilized by HSNP1 is also confirmed by higher zeta potential value and higher surface coverage of partially hydrophobic silica NPs (HSNP1) compared to that of hydrophobic silica NPs (HSNP2). The partially hydrophobic silica NPs, wetted to the same extent by both liquid phases has been found to be suitable for stabilizing emulsions for Enhanced Oil Recovery (EOR) applications.